Could Porous Pavements be Part of the

Urban Water Solution?

A transfer report submitted in fulfillment of the upgrade from a

Masters of Engineering Science (Research) to Doctor of Philosophy

by

Chui Fern, Yong

Bachelor of Environmental Science Honours

Institute for Sustainable Water Resources

Department of Civil Engineering

Supervisor: Dr. Ana Deletic

Associate Supervisors: Dr. Tim Fletcher

Dr. Mike Grace

February 2008

Summary

As water use in Australia is fast approaching, and in some cases, exceeding the limits of sustainability, urban stormwater management is therefore a high priority at the national, state and local government levels. One of the greatest challenges lies in achieving the required level of stormwater treatment within established, densely populated urban areas (such as inner city areas). Most of the available stormwater management measures are difficult or impossible to implement on a wide scale within developed urban areas due to infrastructure, space and/or cost constraints, with porous pavements being one technique, which can be deployed in a wide range of space-constrained situations. Despite being a promising stormwater management technology that is widely used abroad, Australia is lagging behind in the development and application of porous pavement, mainly due to a low level of confidence in its stormwater treatment performance under Australian conditions, and early perceptions relating to clogging. This project will thus fill the gap in our knowledge with respect to the clogging and treatment processes of porous pavements. The first part of this report examines the primary issues associated with the impacts of urbanization, with particular reference to the role of impervious areas and the generation of stormwater runoff. This is followed by a comprehensive review of the relevant literature on porous pavements, in which the hydrologic and pollutant removal processes contributing to the effectiveness of porous pavement in meeting the primary objectives of urban stormwater management are discussed. Knowledge gaps relating to the hydrological and pollutant removal performances of porous pavements are then identified. By reviewing the relevant literature, the aims and objectives of this research are formed. Preliminary results from a pilot study are subsequently discussed, in which a series of calibration tests and material testing is performed prior to the commencement of the porous pavement comparison experiments. Lastly, details of the proposed research, which consists of two laboratory studies and one modelling activity is explained. Given that stormwater management requires consideration of a wide range of issues across multiple disciplines, it is impossible for a single study to address all of the relevant concepts and processes. Conclusions on the effectiveness of porous pavement for stormwater management derived from this study are based on hydrologic considerations. Other important issues relating to stormwater management effectiveness such as the structural adequacy of the selected pavement configuration and economic considerations have not been addressed.

Table of Contents

Abbreviations............................................................................................................................. i Glossary.....................................................................................................................................ii 1. Introduction .......................................................................................................................... 1

1.1 Stormwater Runoff and Urbanization .............................................................................. 1 1.2 Managing Stormwater Runoff ......................................................................................... 1 1.3 Porous Pavement Systems ............................................................................................... 2 1.4 General Aim of the Research ........................................................................................... 2

2. Literature Review................................................................................................................. 3 2.1 Urbanization..................................................................................................................... 3 2.2 Stormwater Pollutants ...................................................................................................... 4 2.3 Porous Pavement Principles of Operation ....................................................................... 5 2.4 Porous Pavement Components ........................................................................................ 8 2.5 Types of Porous Pavements ........................................................................................... 10

2.5.1 Porous asphalt (PA) ............................................................................................... 12 2.5.2 Hydrapave (HP) ..................................................................................................... 13 2.5.3 Permapave (PP) ..................................................................................................... 15

2.6 Porous Pavement Design Consideration ........................................................................ 16 2.7 Hydrological Performance ............................................................................................. 17 2.8 Pollutant Removal Performance .................................................................................... 20 2.9 Clogging......................................................................................................................... 22 2.10 Summary of Key Gaps and Research Needs ............................................................... 25

3. Research Aims and Objectives .......................................................................................... 26 4. Preliminary Research......................................................................................................... 26

4.1 Development of Rig 1 .................................................................................................... 27 4.1.1 Housing of the Three Pavements ............................................................................ 28 4.1.2 Stormwater Distribution System ............................................................................. 30 4.1.3 Monitoring Equipment............................................................................................ 39

4.2 Development of Rig 2 .................................................................................................... 39 4.2.1 Housing of PP Pavements (Columns) .................................................................... 41 4.2.2 Stormwater Distribution System ............................................................................. 41 4.2.3 Monitoring Equipment............................................................................................ 42

4.3 Stormwater Preparation.................................................................................................. 42 5. Proposed Research ............................................................................................................. 44

5.1 Activity 1: Clogging and Treatment Process of 3 Porous Pavement Types .................. 44 5.1.1 The Experimental Programme................................................................................ 44 5.1.2 Method .................................................................................................................... 45 5.1.3 Research Progress .................................................................................................. 51

5.2 Activity 2: Laboratory Trials of Alternative Monolithic Pavement Designs................. 59 5.3 Activity 3: Modelling and Development of Guidelines for the Design, Construction and Maintenance of Porous Pavements under Australian Conditions........................................ 61 5.4 Activity Timeline ........................................................................................................... 62

5.4.1 General Research Project Timeline over 3 years................................................... 62 5.4.2 Achievements in 2007 and Research Targets for 2008 and 2009. ......................... 62

6. References ........................................................................................................................... 64 7. Appendix ............................................................................................................................. 70

Appendix 1: Front, Side and Top View of Activity 1 Experimental Rig. ........................... 70 Appendix 2: Calculation of Tubing Diameter based on Settling Velocity. ......................... 74 Appendix 3: Calculation of Particle Settling Velocity......................................................... 76 Appendix 4: TDN and PON Measurements on Five Different Days. ................................. 77 Appendix 5: FRP and Particulate Bound Phosphorus Measurements on Five Different Days. .................................................................................................................................... 79

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Abbreviations

ARI Average Recurrence Interval c.a. approximately CBR California Bearing Ratio Cd Cadmium Cr Chromium Cu Copper EIA Effective Impervious Area FRP Filterable Reactive Phosphorus HP Hydrapave i.d. internal diameter k Hydraulic conductivity LID Low Impact Development MUSIC Model for Urban Stormwater Improvement Conceptualisation NH4+ Ammonium NO2- Nitrite NO3- Nitrate NOx Oxidized nitrogen (Nitrate and nitrite) N2 Nitrogen gas PA Porous Asphalt Pb Lead PP Permapave SE Standard Error SUDS Sustainable Urban Drainage Systems TIA Total Impervious Area TKN Total Kjeldahl Nitrogen TN Total Nitrogen TP Total Phosphorus TSS Total Suspended Solids UPS Uninterruptible Power Supply WSUD Water Sensitive Urban Design Zn Zinc

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Glossary

Terms Definition Adsorption The adhesion of a substance to the surface of a solid or liquid

Aggregate Any mass of particulate material

Asphalt Dark coloured bituminous cement

CBR California Bearing Ratio, a measure of bearing value in soil or

other material, a test of strength for granular materials

Darcy’s law Relationship between flow rate through a porous medium, pressure gradient, and the medium’s hydraulic conductivity

Denitrification The biological reduction of nitrate (NO3-) to ammonium (NH4

+), molecular nitrogen (N2) or the oxides of nitrogen, resulting in the loss of nitrogen into the atmosphere

Filterable Reactive Phosphorus (FRP)

A measure of inorganic phosphate and some organic phosphorus in a water body. Samples are filtered through a filter of 0.45µm pore size that separates particulate P from “dissolved” P fractions. The term reactive refers to the fraction of phosphorus that responds to colorimetric test without preliminary hydrolysis of digestion (reacts with the molybdate ion under acidic conditions to form a molybdenum blue complex)

Fines Small (“fine”) soil particles such as clay and silt

Gravel Rounded river stone, or generically, any aggregate containing particles larger than sand

Groundwater Water found underground as a result of rainfall, ice and snow melt, submerged rivers, lakes, and springs. Some groundwater may be found far beneath the earth surface, while other groundwater may be only a few inches from the surface

Head Difference in elevation between two points in a fluid, producing pressure

Hydraulic conductivity, k

Permeability under a given head or pressure of water

Infiltration Movement of a fluid into the surface of a porous substance

Mineralization Decomposition of organic matter into its inorganic elemental components by microorganisms.

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Monolithic Referring to a type of porous pavement, which is continuous (not made of individual blocks), and consisting of bound granular material such as concrete or asphalt, without the finer aggregate grain sizes; porosity derives from the gaps in between particles.

Modular Individual concrete or pavers, constructed with a gap in between each paver. Porosity comes from gap in between blocks, rather than gaps within blocks.

Nitrification The oxidation of ammonium (NH4+) to nitrite (NO2

-) and from nitrite (NO2

-) to nitrate (NO3-) by microorganisms

Nutrient Elements of compounds essential for the growth of organisms

(most common nutrient species include phosphorus (P) and nitrogen (N)).

Open-graded Single-sized or of a narrow range of sizes, such that the voids between particles are not to be filled by relatively small particles

Open-jointed Having joints wide enough to produce or permit production of significant porosity and permeability

Organic matter Elements or material containing carbon, a basic component of all living matter

Particulates Solid particles borne in water, as distinct from dissolved constituents

Permeability The rate at which a fluid flows through a porous substance, under given conditions

pH A measure of hydrogen ion activity (the acidity or alkalinity of water)

Runoff Water that flows on or near the ground surface during storms

Sediment Soil particles, sand, colloidal and biotic minerals washed from the land into aquatic systems as a result of natural and human activities

Sedimentation The removal, transport, and deposition of detached soil particles by flowing water, wind and gravity. Sediment includes decaying algae, weeds, soil and organic matter eroded from a water body

Water table Top of the saturated zone in a porous material

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1. Introduction

1.1 Stormwater Runoff and Urbanization

Stormwater runoff is a term that describes rainwater or snowmelt that originates from precipitation events, along with pollutants that it may carry from these events. Prior to urbanization, natural systems such as forests and wetlands act as sponges for excess rainwater to replenish the groundwater table or evaporate off plants and trees. However, as development progressed, land that was once capable of absorbing precipitation was replaced by huge areas of impervious surfaces. A study by Klein, (1979) estimated that an increase of impervious surfaces by 30% could potentially double the size of a 100 year flood event and lead to frequent flood events of an even greater extent. To deal with flood events, as a direct consequence of urbanization, stormwater management measures that include the development of drainage systems that connect efficiently from impervious land cover to rivers and streams were designed. Historical stormwater management systems are typically designed with an emphasis on engineered flood control measures such as dams and pipe network systems for “efficient conveyance” (France 2005). Unfortunately, this solution to quickly channel stormwater away from impervious areas via concrete paved channels and stormwater pipes, has only added to the increasing amount of impervious surfaces present, thus creating more stormwater runoff. Instead of providing retention and infiltration facilities that mimic pre-development conditions, the creation of more efficient conveyance systems has only worsened the flooding and water quality problems of today (Ferguson 2005). Impervious surfaces increase runoff volume by preventing infiltration and by reducing the opportunity for evapotranspiration between rainfall events. Peak discharges are increased by the greater efficiency of the urban drainage network, which reduces flow attenuation associated with storage on the catchment surface. The downstream impacts of urbanisation affects stream hydrology, water quality as well as stream morphology (Paul and Meyer 2001). Accumulated deposits on impervious surfaces, which are easily washed off during rainfall events, are also a significant pollutant source. Pollutants may be deposited on impervious surfaces by wet and dry precipitation, abrasion of roads and transportation materials, by commercial, industrial and residential activities and by illegal dumping (Dempsey, Tai et al. 1993).

1.2 Managing Stormwater Runoff

The main problem with traditional stormwater management may be the perception of stormwater as a problem to solve, or as a waste product to be eliminated as quickly as possible, rather than as a valuable resource. Given the negative impacts of urban stormwater, systems which can detain and treat stormwater are now relatively common with the adoption of the Water Sensitive Urban Design (WSUD) philosophy in Australia, which aims to reduce the potential of flooding on new and existing urban developments (Boyd, Bufill et al. 1993; Walsh, Fletcher et al. 2005). Unlike traditional urban stormwater drainage systems, they also protect and enhance

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groundwater quality. In the United Kingdom (UK), the term WSUD is better known as Sustainable Drainage Systems (SuDS) while in the United States, the terminology Best Management Practice (BMP) or Low Impact Development (LID) is adopted. The usage of the term SuDS is however more constrained as it is restricted to structural measures only.

Porous pavements are one of the many WSUD measures. Porous pavements infiltrate stormwater at the point source and could therefore be used to restore pre-development hydrology even in highly paved catchments. Compared to other WSUD measures, porous pavement has a particular advantage of not requiring additional land area and is easily retrofitted in existing developments (Newton, Jenkins et al. 2003). Porous pavement is also capable of controlling stormwater quantity as well as quality, making it potentially effective in meeting the key stormwater management objectives of peak discharge control, pollutant removal and runoff volume reduction.

1.3 Porous Pavement Systems

Porous pavements, as their name implies, are a pavement type that promote infiltration, either to the underlying soil, or to a storage reservoir below them. Porous pavements come in several forms and are either monolithic or modular. Monolithic structures consist of bound granular material such as concrete or asphalt, without the finer aggregate grain sizes, while modular structures are constructed from individual concrete or pavers, constructed with a gap in between each paver. Porous pavements are usually laid on sand or fine gravel, underlain by a layer of geotextile, with a layer of coarse aggregate below. Porous pavements have two main advantages over impervious pavement, in terms of stormwater management (Pratt, Mantle et al. 1995): (a) flow attenuation, through infiltration and storage, and (b) improvement to water quality, through filtering, interception and biological treatment. Compared to the level of porous pavement adoption in Europe, Japan and USA, porous pavements have not been widely implemented in Australia (Fletcher, Duncan et al. 2005), despite being one of the most progressive countries with respect to the adoption of the WSUD philosophy. This is mainly due to the lack of understanding of how porous pavement technology will perform under Australian conditions. A conflict seems to exist between the widely recognised potential of porous pavements and the perception of porous pavements being expensive and prone to clogging, derived from numerous negative experiences in field installations. With clogging being the main cause of porous pavement failure, an in-depth discussion on clogging is warranted. This will be addressed in Section 2.9.

1.4 General Aim of the Research

With urban stormwater runoff being one of the major causes of pollution, the use of porous pavements could benefit Australia’s urban water management, with their potential to detain and treat stormwater. Given the need to decrease urban flooding, stormwater pollution and potable water demand, and the potential of porous pavement to assist in these objectives, the aim of this research project is to understand the

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long-term performance - focussing on treatment efficiency and clogging behaviour - of a range of porous pavements used in stormwater management.

2. Literature Review

Based on a thorough review on the available literature on porous pavements, the potential of the many types of porous pavements to achieve improved stormwater management outcomes are identified. The primary issues associated with urbanization, with particular reference to the role of impervious areas and the generation of stormwater runoff are first discussed. This is followed by a brief description of the types of porous pavements available on the market as well as their principles of operation and design considerations. The various installations of porous pavements and their advantages and disadvantages are then discussed and compared. Lastly, knowledge gaps relating to the physical behaviour of certain types of porous pavement are then identified to provide the basis for the investigations described in later chapters.

2.1 Urbanization

Urbanization is defined as the development of land into residential, commercial or industrial properties. The process of urban development causes significant modifications to the natural vegetation and soil properties by introducing impervious areas such as pavements, buildings as well as drainage and flood control infrastructure (Schueler 1994). A subsequent increase in impervious area is closely associated with profound changes in stream hydrology, water quality and stream morphology. These changes include an increased frequency of flooding and peak flow volumes, decreased base flow (Novotny and Olem 1994), increase stormwater runoff and stream temperature, increased pollutant (sediment and nutrient) loading (Hatt, Fletcher et al. 2004) as well as a loss of aquatic and riparian habitat. The degree of urbanisation of a catchment is commonly quantified by the proportion of impervious area, termed “imperviousness”. Imperviousness, which can be further divided into total impervious area (TIA) and effective impervious area (EIA) consists of two principal components, namely roofs, and transportation facilities (roads, car parks and driveways) (Booth and Jackson 1997). While TIA represents the sum of all constructed impervious surfaces within a catchment, EIA is defined as impervious surfaces with direct hydraulic connection to the downstream drainage system. A common drawback of TIA is that it often ignores nominally pervious areas that behave as impervious, such as exposed rock or highly compacted soils, as well as impervious areas that have no measurable effect on hydrology, such as isolated roof areas that are not connected to the drainage system (Booth and Leavitt 1999). A miscalculated increase in TIA could result in a decreased volume of precipitation during a storm event, thus giving an inaccurate increase in the volume of surface runoff. As such, a more meaningful description of imperviousness is provided for by the use of EIA. EIA not only addresses the deficiencies of TIA but is also a more accurate parameter to be used in modelling rainfall runoff from a catchment, particularly with drainage connectivity being an important explanatory variable for pollutant concentrations in urban streams.

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To minimize and mitigate the effects of urbanization, as mentioned above, urban stormwater management techniques that incorporate the WSUD philosophy have been adopted. Urban stormwater management measures may be divided into structural and non-structural measures (Taylor and Fletcher 2004). Structural measures are engineered facilities or design practices that modify the hydrologic and water quality response of an urbanised catchment, while non-structural measures are those which aim to influence design practices and community behaviour and attitudes such as land-use planning, policy, education and enforcement. These measures may be further divided into those that treat stormwater near its source and those located further downstream along the drainage system. Examples of source controls include rainwater tanks and porous pavements, while downstream or “end of pipe” controls include gross pollutant traps, detention basins and constructed wetlands (Wong, Allison et al. 2006). Any effective urban stormwater management system will include a range of structural and non structural measures. The ideal approach to urban stormwater management is to reduce or eliminate the problem at the source by preventative measures. One source control WSUD measure is porous pavement. Porous pavements, which are easily retrofitted in existing dense urban environments, not only reduce the TIA and the hydraulic connectivity of the drainage system, but also resolve the main problem of urbanization by providing detention, treatment and the reuse of stormwater (Bond, Pratt et al. 1999).

2.2 Stormwater Pollutants

Stormwater is sometimes defined as runoff from a precipitation event that contains materials in concentrations greater than that which would occur naturally (Chiew, Mudgeway et al. 1997). A number of studies have investigated the primary sources of pollutant constituents from impervious surfaces (Sartor and Boyd 1972; Shaheen 1975; Ellis and Revitt 1982; Barrett, Zuber et al. 1993; Batley, Brockbank et al. 1994; Sansalone 1996; Ball, Jenks et al. 1998). The range of contaminants typically found in stormwater runoff, along with their possible sources is presented in Table 1. These contaminants in stormwater runoff could be present in both the particulate and dissolved forms, but are most commonly associated with particulates. Sartor and Boyd (1972) and Boyd, Bufill et al. (1993) however showed that pollutant constituents tend to be sorbed to the finer rather than larger particulates in urban stormwater. This is particularly true for metals (Walker and Hurl 2002). Media reports frequently focus on the ‘wastage’ of large quantities of stormwater into urban streams, and stormwater has since been recognised as a valuable resource. More recent implementations of water restrictions across Australia’s major urban cities (Sydney, Melbourne, Perth and Adelaide) have also increased the public awareness of stormwater harvesting. However, analyses of stormwater quality impacts associated with urbanization by the Environmental Protection Agency (EPA) have found high concentrations of heavy metals, organic pollutants, faecal coliform bacteria, elevated nutrients as well as suspended solids. If stormwater is to be harvested and recycled, these pollutants, which could be highly toxic and detrimental to human, terrestrial and aquatic life, should be removed to an adequate level.

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Table 1: Typical Road Runoff Contaminants and Their Sources (Ball, Jenks et al. 1998).

Contaminant Primary Source Sediment Pavement wear, vehicles, maintenance activities Nitrogen Roadside fertiliser applications Phosphorus Roadside fertiliser applications Lead (Pb) Auto exhaust, tyre wear, lubricating oil and grease,

bearing wear Zinc (Zn) Tyre wear, motor oil, grease Iron Auto rust, steel highway structures (eg.guard rails),

moving engine parts Copper (Cu) Metal plating, bearing and brush wear, moving engine

parts, brake lining wear, fungicides, insecticides, pesticides

Cadmium (Cd) Tyre wear, insecticide application Chromium (Cr) Metal plating, moving parts, brake lining wear Nickel (Ni) Diesel fuel and petrol exhaust, lubricating oil, metal

plating, brush wear, brake lining wear, asphalt paving Manganese (Mn) Moving engine parts, auto exhaust Cyanide Deicing compounds Sodium/Calcium Chloride Deicing salts Sulfate Roadways surfaces, fuels, deicing salts Petroleum Hydrocarbons Spills, leaks, or blow-by of motor lubricants, anti-freeze

and hydraulic fluids, asphalt surface leachate PCB PCB catalyst in synthetic tyres, spraying of rights-of-

way PAH Asphalt surface leachate

2.3 Porous Pavement Principles of Operation

Permeable paving is a paving system that allows stormwater infiltration and storage, thus having the potential to provide a number of beneficial functions as a stormwater management measure. There are essentially two main types of permeable paving, those which use the pavers themselves to infiltrate stormwater (porous pavements), and those that use gaps between impermeable pavements to infiltrate stormwater (permeable surface) (Diyagama, VanHuyssten et al. 2004). As described by Ferguson (2005), porous (having pores or voids) is a description or characteristic of the material’s physical structure, while permeable (allowing fluids to pass through) is a description of the behaviour of fluids after a structure is built, the voids of which are connected throughout the material. In this report however, the term “porous pavement” will be adopted to refer to either types of pavements. The surface layer of porous pavement may either be “monolithic” or “modular”. Monolithic porous pavement consists of bound granular material, such as asphalt or concrete without the finer aggregate grain sizes. Modular porous pavement is

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constructed from individual concrete, clay or plastic paving blocks, such as Hydrapave (HP) and Rocla, which may act as a structural matrix for unbound gravel or soil which is exposed at the surface. The traditional design of porous pavements consists of a porous surface overlaying a bedding material (base-course) placed on top of a sub-base (usually divided by geotextile). The pavements are usually installed at pedestrian areas, car parks, low-traffic use roads, residential driveways and are aesthetically pleasing (U.S Environmental Protection Agency (USEPA) 1999). To enhance their structural performance and reduce the costs, they may be combined with non-permeable surfaces. As such, porous pavements are often used to cover only a certain percentage, rather than the whole catchment. The open-graded aggregates used in the base course of many porous pavements are commonly coarser (of much larger particle size) than either the sub-grade below the base, or the surface layer above. Filter layers are installed in some pavements to separate the various layers, and thereby maintain the porosity and structural integrity of each layer. They are made of intermediate- size aggregate, or of permeable geotextile (Ferguson 2005). Geotextile mitigates the migration of fines from the underlying sub-grade into the sub-base layer. In some countries (UK, Sweden, Japan and the USA), porous pavements have been widely used for the control of stormwater. They have infiltration capacities usually upwards of 4500 mm/hr when new. Studies show that after 15-20 years of operation, they still provide high infiltration rates (100-1000 mm/hr) (Bond, Pratt et al. 1999; Pratt 1999). Water may be removed from the pavement structure either by infiltration to the underlying soil or by collection in a formal drainage system beneath the pavement surface. There are three most common ways of applying porous pavements in WSUD practice, as outlined below:

a) lined with an under-drain for collection (Figure 1 (a)) b) unlined without an under-drain (Figure 1 (b)) c) unlined with an under-drain. (Figure 1 (c))

Figure 1 (a): Lined with an under-drain for collection.

Lined

gravel

Impervious Area (IA)

300-500mm

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Figure 1 (b): Unlined without an under-drain.

Figure 1 (c): Unlined with an under-drain.

While the modular pavement with infiltration is perceived to be more effective in water quality control and volume reduction than the drained modular pavement, the pavement with infiltration is more sensitive to site conditions, has a higher potential for failure, and introduces the possibility of groundwater contamination (Newton 2005). Meanwhile, the operation and maintenance needs and potential for failure for monolithic pavement are considered to significantly reduce its effectiveness potential. Porous pavement is not just a pavement; it is also part of a site’s drainage and stormwater management system. As such, the total effect of porous pavement on site development costs should be ascertained by taking into account both pavement and drainage costs. The main advantages and disadvantages of porous pavement systems are listed in Table 2.

Un-Lined

Impervious Area (IA)

gravel 300-500mm

Un-Lined

Impervious Area (IA)

gravell

300-500mm

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Table 2: Advantages and disadvantages of porous pavement systems (Ferguson 2005).

Advantages Disadvantages Construction costs of some systems are less than traditional paving

Construction costs of some systems are more expensive than traditional paving

Improves stormwater quality by pollutant removal

Could result in more subsurface pollution and groundwater contamination if improperly installed

Improves road safety because of better skid resistance

Not recommended for high traffic areas because of durability concerns

Increases groundwater recharge Many pavement engineers and contractors lack expertise with this technology

Reduces or even zeroes peak stormwater discharges from paved areas

Porous pavement has a tendency to become clogged if improperly installed or maintained

Reduces the need for curbs and storm sewer installation or expansion

Porous pavement has a high rate of failure

Retains natural vegetation and drainage patterns

Use depends on infiltration rates of underlying soils

2.4 Porous Pavement Components

Porous pavements are assembled from several types of components. The basic features of porous pavements are listed in Table 3 and the main components are further described and shown (Figure 2) below. Each type of porous pavement is usually constructed with a specific combination of components to meet its own requirements, rather than contain all the listed components. Further details can be found in the porous pavement textbook by (Ferguson 2005).

Figure 2: Main components of a porous pavement system (Source: (Ferguson

2005).

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Table 3: Typical terms with particular application to porous pavement components (Ferguson 2005).

Term Definition Base Course Layer placed below a surface course to extend

pavement thickness; also known as base Course Layer in a pavement structure Filter layer Any layer inserted between two other layers, or

between a pavement layer and the sub-grade to prevent particles of one from migrating into the void space of the other; also known as choker course

Geomembrane Impermeable manufactured fabric, also known as liner Geotextile Permeable manufactured fabric, also known as filter

fabric Pavement Any treatment or covering of the earth surface to bear

traffic Pavement Structure A combination of courses of material placed on a sub-

grade to make a pavement Reservoir Any portion of a pavement that stores or transmits

water; a reservoir may overlap or be combined with other pavement layers such as base and sub-base; also known as reservoir base, drainage layer or drainage blanket

Sub-base Layer of material placed below a base course to further extend pavement thickness

Sub-grade The soil underlying a pavement structure and bearing its ultimate load

Surface course Pavement layer that directly receives the traffic load; this layer presents a pavement, surface qualities such as accessibility, travel quality, appearance and resistance to direct traffic abrasion

Filter layers Filter layers are layers inserted between two other layers, or between a layer and the sub-grade to segregate their materials. Segregation is needed in some pavements to maintain the porosity and structural integrity of each layer. Filter layers can be made of intermediate-size aggregate. Alternatively, they can be made of geotextiles, which are fabrics that are permeable to water but that inhibit the movement of small particles. They are informally called filter fabrics in some pavements. For example, if the base course aggregate is much larger than the aggregate in the pavement surface, an aggregate filter layer or “choker course” may be added between the two main courses. It prevents the aggregate in the surface course from collapsing into the base’s large voids. Lateral outlet A pipe or any other lateral outlet can discharge excess water from a pavement reservoir safely and limit the depth and duration of ponding in the upper segment of the pavement. The capacity of the outlet controls the rate of discharge. Specific outlet configurations may be chosen for reasons of maintenance or cost.

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Liners Some porous pavements are lined at the bottom to prevent the infiltration of water into the sub-grade. The technical term for plastic lining sheets is ‘geomembranes’, which refers to manufactured impermeable fabrics. The use of liners prevents moisture fluctuations, thus stabilizing the underlying soil, and prevents infiltrating water containing toxic chemicals from leaching into the environment. It is also potentially a means for turning the base course into a reservoir of harvested water. Reservoir A reservoir is any portion of a pavement that stores or conveys water while it exits through a drainage pipe or into the soil. A reservoir includes all pavement materials where stored or flowing water occurs with any frequency, even though the same materials also have a structural function. The storage volume is in the void space between particles of material. The reservoir is also called a drainage layer or drainage blanket. Surface and base courses The construction of a pavement in two or more courses (layers) is common. Differentiating the courses allows each layer to be optimized for the special purpose it serves in the pavement and the structure as a whole to be built with the least possible expense. The surface course directly receives the traffic load and the disintegrating effects of traffic abrasion. It is likely to be made of special, relatively expensive material to resist abrasion and provide qualities such as appearance and accessibility. A distinct base course builds up the thickness of a pavement with comparatively inexpensive material in order to spread out the traffic load over the sub-grade or to protect the sub grade from frost penetration. If necessary, a sub-base can be added to further thicken the pavement structure or to store more water as it discharges to a pipe or infiltrates the soil.

2.5 Types of Porous Pavements

There are seven general types or families of porous pavements. They are introduced here with a brief summary of their performance and efficiencies. Further details can be found in the porous pavement textbook by Ferguson (2005). Porous Aggregate Aggregate is any mass of particulate material such as gravel, crushed stone, crushed recycled brick or decomposed granite. It is by far the most ubiquitous material in pavement construction. It is also the most common material in pavement base courses, thus making it a major component in most kinds of pavements. Single-size particles create an aggregate mass with 30% to 40% void space, thus making it an “open graded” material, which can be extremely permeable to air and water. Single-size aggregate is also the principal component of porous asphalt and porous concrete, and is used as porous fill in the open cells and joints of paving blocks, grids and geocells. In most regions, aggregate is the least expensive of all firm surfacing materials. At the same time, the high porosity and permeability of single-size aggregate make it the most favourable of all pavement materials for restoring watershed hydrology and tree rooting habitat.

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Porous Turf A turf surface is a “green” open surface. Turf’s permeability is positive as long as it is not compacted by excessive traffic. The transpiration of living grass actively counteracts potential urban heat islands as well as maintains soil permeability by building soil aggregation. Locations for turf use must be selected to avoid compaction by frequent traffic. All turf must be regularly maintained with mowing and some degree of fertilization and irrigation is normally required. As regular maintenance must be scheduled, turf should be used only where traffic can be controlled or is predictably scheduled such as in an office, church or in event parking. Although porous turf is also a pavement structure that supports pedestrian or vehicular traffic, its grass component makes it more characteristic of a biofilter, rather than a porous pavement. Plastic geocells Plastic geocells are manufactured lattice-like products that hold aggregate or topsoil in their cells, inhibiting displacement and compaction. They extend the use of aggregate and turf into more demanding traffic settings than they could bear alone. Most plastic geocells are flexible, so they are adaptable to sites with swelling or freezing soil. In most models, the plastic ribs occupy a very small portion of the surface area, so the surface permeability, temperature and visual appearance are essentially those of the grass or aggregate fill. Open-jointed paving blocks Paving blocks are solid units of concretes, brick or stone laid side by side to bear traffic loads. The models that can be used to make porous pavements are shaped to produce open joints or gaps between adjacent units. Porous aggregate or turf in the joints gives the pavement its porosity and permeability. Many block products are remarkably durable, giving their installations long life-times and low life-cycle costs. They can bear very heavy traffic. However, block pavements are more expensive to construct than some other types of pavement due to high labour costs. They are sensitive to deformation in the base or sub-grade. This type of pavement needs maintenance due to clogging. Open-celled paving grids Open-celled paving grids are units of concrete or brick, which are designed with open cells that can be filled with porous aggregate or turf. The units are laid side by side and the resulting surface is a grid work of solid ribs or pedestals commonly an inch or more wide, alternating with cells of aggregate or grass. Many models are durable and long-lived. They are made of concrete or plastic containing open cells. The cells may be filled with free-draining soil, which allows for the growth of grass, or with aggregate laid on a recommended sub-base. The solid portion is intended to transmit structural loads for generally light vehicular loads. Porous Concrete Porous concrete is made of single-sized aggregate bound together by Portland cement, cast in place to form a rigid pavement slab. They are moderately high in initial cost but the long life of properly installed material can make its life-cycle cost low. Properly installed porous concrete is appropriate for both the low traffic loads of

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driveways and walkways, and moderate traffic loads such as those of commercial parking lots and residential streets. Porous Asphalt Porous asphalt is made of single-size aggregate bound together by bituminous asphalt binder. Asphalt is a familiar and inexpensive paving material, but porous asphalt is a subtle variation of conventional dense asphalt, requiring a special grade specification. They provide additional permeability through the surface of the paver itself and are generally manufactured using no-fines asphalt, creating many small, interlinked internal voids. Numerous installations have proven that porous asphalt’s permeability can be high. However, some installations have suffered from clogging by the asphalt binder (Cahill, Adems et al. 2003). To develop a better understanding of the similarities and differences amongst the seven types of pavements discussed above, a detailed review, focussing on three systems (Figures 3 (a), (b) and (c)) that were thought to best represent the range of available pavement systems was performed. These three systems were: a) monolithic porous pavement, Porous Asphalt (chosen for being the most widely used conventional monolithic pavement), b) modular open-jointed paving blocks, Hydrapave (chosen for being the most widely used modular system, particularly in Europe), and c) monolithic porous aggregate, Permapave (chosen as one of the relatively new porous aggregate product in Australia)

Figure 3(a) Porous Asphalt. Figure 3 (b) Hydrapave. Figure 3 (c) Permapave.

2.5.1 Porous asphalt (PA)

First developed in the 1970s at the Franklin Institute in Philadelphia (Diniz 1980), PA consists of standard bituminous asphalt, in which the fines have been screened and reduced, creating void space to make it highly permeable to water. PA has been installed in the field as much as any other type of porous material. Over the years, some of the installations have suffered from clogging and a decline in infiltration rate, due to mediocre construction or the immaturity of the technology (Cahill, Adems et al. 2003). Newly installed PA can have a porosity of 15% to over 20% (depending on the aggregate gradation and variations in components), as opposed to 2% to 3% in conventional asphalt (Cahill, Adems et al. 2003). Increasing void content is associated with a higher infiltration rate and less clogging potential.

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The design of PA consists of at least four layers; a two to four inch layer of asphalt, a one to two inch filter layer of half-inch crushed aggregate, a 12 inch minimum reservoir layer of one to three inch aggregate, and a layer of geomembrane. The PA layer itself provides for some pre-treatment of runoff. The crushed aggregate filter layer aids with pollutant removal and provides stability for the stone reservoir layer during application of pavement. Treated runoff is stored in the reservoir bed, a highly permeable layer of open-graded clean-washed aggregate with at least 40% void space. Non-woven geomembrane placed between the reservoir bed and uncompacted subsoil prevents the migration of fines into the stone reservoir, which could clog the system. With proper maintenance, including regular vacuuming of the surface to prevent clogging by sediment, PA can have a minimum service life of 20 years. With proper installation and maintenance, porous paving allows for infiltration of up to 80% of annual runoff volume (depending on local climatic conditions). Additionally, studies indicate that porous paving systems can remove between 65% and 85% of undissolved nutrients from runoff and up to 95% of sediment from runoff (Cahill, Adems et al. 2003). PA pavement installations in practice Since the late 1970s, PA has been used for numerous parking lots across the US, and Europe. Numerous installations of PA parking lots, streets and highways can also be found in France, Germany, Britain, Japan, Singapore, America, UK and Australia (Ferguson 2005). In Melbourne, Australia, they are used partially on the Tullamarine Freeway, the Monash Freeway and the Eastern Freeway. Figure 4 (a) is a typical design of PA, while Figure 4 (b) is a comparison between porous and standard asphalt.

Figure 4 (a) Figure 4 (b)

Figure 4 (a): Typical installation of PA (Dauphin County Conservation District

2007); Figure 4 (b): Comparison between PA and standard asphalt (Adams 2003).

2.5.2 Hydrapave (HP)

HP is an 80 mm thick concrete paver with a unique edge chamfer and bevel which permits butt joining and eliminates the need for jointing sand (Boral Clay and Concrete 2005). It is laid on a 50 mm course of 5 mm clean stone. The laying course

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is separated from the sub-base by a layer of geotextile. The sub-base is comprised of 2 layers, namely the upper and the lower sub-base. The upper sub-base consists of a 100 mm thick layer of 5-20 mm stone while the lower sub-base typically consists of 10-63 mm stone of up to 250 mm thickness. HP pavers must be laid in a 90 degree herringbone pattern, tightly butt jointed to ensure that a close fit is achieved. The sub-grade must have a minimum California Bearing Ratio (CBR) of 5 (Boral Clay and Concrete 2005). The HP system allows stormwater to infiltrate through small channels formed in the ends of the concrete pavers and through a filter treatment as it passes into a prepared sub-base. The water can then pass through a secondary filter into suitable sub-grade or be stored for controlled release or harvested for non-potable use. There are two main types of installation of HP; infiltration where stormwater is allowed to infiltrate back into the water table; and tanked where stormwater is collected for re-use. HP is most suitable for light to medium duty applications such as car parks and domestic driveways. The sub-base has a reservoir capacity of approximately (c.a.) 30%. As a quick rule of thumb, 10 m2 of HP system with a sub-base depth of 350 mm will accommodate 1 m3 of water. Where it is proposed to shed stormwater from impermeable surfaces onto areas of HP, it is recommended that a maximum ratio of 2:1 impermeable to HP be used (Boral Clay and Concrete 2005). HP pavers can drain up to 2.5 litres /m2/ sec, while the overall system is capable of draining up to 1.25 litres/ m2/ sec due to the effect of the geotextile beneath the laying course. Flow rate through an aged system is 0.22 litres/ m2/ sec (35 years of average sediment build up). Organic matter and loam are caught on the filter fabric and held in the layer of 5 mm aggregate. Oils and heavy metals coat the surface of the organic matter and loam. Natural microbial filament growth in the sub-base digests low level hydrocarbon pollution. The surface of the pavers should be brushed at least twice a year with a mechanical suction brush. This is recommended to be done in the spring and after leaf fall in autumn. Under normal conditions, a working life of 25 years can be expected. HP system can retain all particulate matter coarser than 20 um and remove up to 95% of sediments. Therefore, over 90% of the surface permeability of the paving can be lost through silting, and the system will still have the capacity to deal with rainfall intensities well in excess of the most extreme Australian urban design conditions (Darwin, 100 year storm, and 10 minute duration)(Rommel, Rus et al. 2001). Ultimately, perhaps after 25 years or more, areas of the laying course may become filled with silts and toxins. If this occurs, the surface pavers should be lifted and the affected areas of laying-course material and filter fabric disposed of. HP pavement installations in practice HP technology, previously known as Formpave, has had many applications in the UK and Europe for over ten years. In Australia, HP installations can be found at the parking bays at Victoria Road oval, Adelaide as well as the residential driveways at Yerrabi 2 Estate, Yerrabi Ponds, Gungahlin ACT. Figure 5 (a) and 5 (b) are 2 typical HP installations.

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Figure 5 (a): Typical HP installation, Figure 5 (b) lined with tanked system.

unlined with infiltration system.

2.5.3 Permapave (PP)

PP is a new type of porous pavement that has been developed in Australia. The pavers are made from gravel (crushed stones) that is bound using a PET based glue. A typical PP system consists of pavers (made of 10 mm-12 mm aggregate) of 50 mm thickness, placed over a layer of 5 mm-20 mm screen crushed rock, to allow water to flow freely under the pavers (Dymon Industries 2007). If required, geomembranes may be used to direct the flow of water to storage points. They are used in different applications, ranging from being an integral part of paved areas (i.e. used as any other type of porous pavement) to being used as stormwater pit covers (as part of stormwater drainage outlets). PP is usually placed on top of a gravel sub-base that is compacted for structural soundness. Usually, the sub-base gravel has a similar particle size as the PP gravel, effectively forming a 300-500 mm deep porous surface (Dymon Industries 2007). PP can be used to cover total catchment impervious surface (as is the case with other porous pavement types), but is used more in combination with some type of impervious surface and therefore makes up only a certain percentage of the total impervious surface area (as is the case with other WSUD systems). PP pavers have a flow through rate of up to 30 litres/ sec depending on stone size chosen. This facilitates meeting the rainfall expectancy of any given site. Cleaning is required every 2 years or more frequently if required, by mechanical vacuum. PP is suitable for domestic applications, most light traffic and commercial pedestrian applications. The sub-grade must comply with the minimum standard requirement of CBR 5. According to the manufacturer, water exiting the system has had 100% of gross pollutants, and up to 70% phosphorus, 80% of heavy metals, and 98% of hydrocarbons filtered from it (Dymon Industries 2007) PP pavement installations in practice Being a relatively new product in the market, PP has not had as many installations as PA, which has been around for decades. Some of the local installations in Australia include Pentridge Village in Coburg, Melbourne; Maroochydore Queensland Council Parking Lot, Queensland and Aldi Stores in Ballina and Grafton, New South Wales. Figure 6 shows some PP installation pictures around Queensland.

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Figure 6: Various PP pavement installations in Queensland.

2.6 Porous Pavement Design Consideration

The effectiveness of porous paving systems is a function of various factors. This includes the geological make up of a region and the intended purpose, structural and hydraulic requirements, sediment retention capacities and clogging of the sediment retaining layers. Furthermore, factors such as economics (useful life, costs of installation and maintenance) and social considerations (aesthetic qualities and public approval) must also be recognised when proposing the implementation of such a system. Key parameters to consider for porous paving placement include depth, land use, pre-treatment, site conditions, slope, sub-grade, stability and traffic volume, as described below (Diyagama, VanHuyssten et al. 2004). Depth The depth of the stone reservoir should be such that it drains completely within 48 hours to 72 hours, depending on the type and nature of system installed. This allows the underlying soils to drain out between storms (improving pollutant removal) and also preserves capacity for the next storm (Ferguson 2005). Pre-treatment Some sediment removal measure is required prior to runoff entering a porous pavement system, to prolong the life and performance of the system. To remove oil, dirt and grit from off-site facilities, a pre-treatment facility such as a sand filter or water quality inlet should be installed to prevent clogging of the pavements. Site Conditions A common cause of porous pavement failure is clogging, due to excessive sediment discharge on to the pavement surface. As such, the installation of porous pavement should be avoided for catchments with a high potential for sediment generation, such as access roads with construction sites and the beach. Extra precaution should also be taken during the construction phase of these systems to prevent sediments from entering the stone reservoir. Contributing runoff from offsite should be limited to a 3:1 ratio of impervious area to pervious pavement area (Diyagama, VanHuyssten et al. 2004). Slope Slopes under porous pavement should be as flat as possible, with maximum grade being less than 5%.

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Stability Porous paving should not be placed on wind blown or loose sands, clay soils that collapse in contact with water and soils with hydraulic conductivity, k of less than 0.36 mm/hour. Existing soil base should be level to prevent ponding under the system. The design of the system should include the expected type and frequency of usage as well as overflow drainage to remove excess stormwater. Clean-washed aggregate should be used to prevent clogging from pre-existing sediment. The sub-grade should be able to sustain traffic loading without excessive deformation. A minimum CBR of 5 has been selected for designs under this guideline. Traffic conditions Porous pavement is typically used in low traffic areas including overflow parking areas, emergency vehicle lanes, residential driveways and pedestrian areas. Moderate to high traffic areas with significant truck traffic should be avoided. Heavy trucks and equipment should be diverted from areas with porous pavement.

2.7 Hydrological Performance

Some of the main benefits of porous pavement systems are in their ability to restore hydrology towards the pre-development state. They are able to attenuate peak discharges, which results in smaller flow rates, as well as to promote infiltration and evaporation, resulting in volume reductions. The attenuation of peak stormwater flows in porous pavements is provided by the storage within the underlying structure or sub-base. This, together with the ability to reduce runoff volume by infiltration to the underlying soil or evaporation from the pavement structure, is an important aspect of the role of porous pavement in managing stormwater quantity and quality (Scholz and Grabowiecki 2007). In pavements designed for infiltration, the thickness of the reservoir course may be chosen to store a certain depth of rainfall for a sufficient time to allow infiltration. It is thus possible to design the structure to produce no runoff for storms up to a given magnitude. In pavement structures without infiltration, the degree of attenuation is a function of the infiltration rate through the pavement surface, the hydraulic properties of the sub-base material and the type of outlet (Niemczynowicz 1990; Pratt 1997; Rambault 1997). In this case, there may be a tradeoff between attenuation and the infiltration rate necessary to prevent localised flooding. The ability of porous pavements to reduce peak flood discharges has been one of the major reasons for its adoption in several countries. However, a lack of information on hydrological performance is also one reason for the lack of adoption of porous pavement. Although new porous pavements, of any type, can show impressive infiltration results in the first few months of service, it is the long-term infiltration performance of a pavement that determines their ultimate success or failure. Many studies on the performance of monolithic and modular porous pavements for stormwater treatment have been performed over the years, the majority of which are field-scale systems. A review of the hydraulic performances of monolithic and modular pavements is described in the following section.

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Monolithic systems Laboratory studies of monolithic systems by Pratt, Mantle et al. (1995) have shown that trapping efficiency tend to increase with increased deposits, thus leading to lower infiltration rates. Pratt, Mantle et al. (1995) observed that the accumulation of material affects the rate of infiltration of stormwater and eventually causes the ponding of surface waters over the monolithic pavements. For example, a field study of four monolithic pavements by Pratt, Mantle et al. (1995) also showed reduced discharge volume of stormwater over time. Pratt, Mantle et al. (1995) discovered that monolithic porous pavements tend to show more significant reductions in outflow volume, outflow rate and pollutant concentrations, so much so that the pollutant loadings in discharges were much reduced as compared with the traditional impermeable asphalt surfaces. The effective trapping of sediments in the upper layers of the construction minimised the throughput of pollutants, a process which was estimated to continue for c.a. 5 years. Following a study by Booth and Leavitt (1999), which showed no measurable surface runoff from monolithic pavement areas, Brattebo and Booth (2003) also examined the long-term effectiveness of four commercially available monolithic pavement systems for their ability to infiltrate precipitation and its impacts on infiltrate water quality after 6 years of daily parking usage. All four permeable pavement systems infiltrated virtually all precipitation, even during the most intense storms experienced during the study period. While this study demonstrated long-term success for infiltration, it does not necessarily assure uniformly good performance at other locations. The experimental site has particularly favourable soil conditions, and rainfall intensities in the Pacific Northwest United States are typically quite low, overshadowing any potential consequences of reduced infiltration of the surfaces over time. In a separate study, the hydraulic behaviour of pavements and the pollutants transported by runoff water were studied using two 1-year campaigns of measurements. Firstly, on a conventional asphalt, and secondly, after the replacement of the conventional asphalt by a porous surfacing. Pratt, Mantle et al. (1989) found that the response times are longer in the case of the porous pavement. On average, they are twice as long as standard conventional asphalt. This can be explained by the storage capacity of the porous pavement, which delays the evacuation of water into the outlet. This delaying effect also makes the evacuation a more gradual process, as reflected by a reduction in the maximum flow rates and by the increase in the time required for discharge (Pratt, Mantle et al. 1989). Modular systems The reduction of peak discharges by modular systems has been investigated in a number of studies. The effectiveness of porous pavement in attenuating peak discharges has been confirmed in the field by Pratt, (1999), who measured the hydrologic response of a modular porous pavement car parking area at Trent Polytechnic University. The results showed peak effluent discharge to be only about 30% of peak rainfall intensity (Pratt 1999). Whilst numerous numerical and field investigations of the impacts of porous pavement on peak stormwater discharges have been undertaken, none have considered behaviour under the high rainfall intensities characteristic of a subtropical climate. In addition, the effectiveness of porous pavement in attenuating impervious area inflows has not been assessed.

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Tests have also shown that evaporation, drainage and retention within the permeable structures are mainly influenced by the particle size distribution of the bedding material, and by the retention of water in the surface blocks (Andersen, Foster et al. 1999; Scholz 2006). In comparison to monolithic conventional asphalts, modular porous pavements provide more effective peak flow reductions (up to 42%) and longer discharging times (Booth and Leavitt 1999; Pagotto, Legret et al. 2000; Abbott and Comino-Mateus 2003; Scholz 2006). Results from another preliminary field evaluation of modular porous pavements, which includes both grass and gravel surface, conducted in the first year following the construction of the site by Booth and Leavitt (1999) showed a dramatic decrease of surface runoff and attenuation of peak discharge. They also found that although different pavement systems have different mechanical properties and ranges of suitable applications, there were no significant differences in the hydrologic behaviour of the different types of paver. Another study on four permeable pavement sections by Collins (2007) also showed dramatically reduced surface runoff volumes, similar to results found by Hunt, Stevens et al. (2002), Bean, Hunt et al. (2007) and, Brattebo and Booth (2003) on unclogged pavement sites. The key to the low infiltration performance of HP as found by Rommel, Rus et al. (2001) is undoubtedly the restriction to flow which takes place at the geotextile layer. When new, the infiltration capability of geotextile fabric in such a situation is around 4000 mm/h and depends on the type of geotextile used. Over time however, an accumulation of sediment occurs at the geotextile barrier. The thickness of this layer and its effect on flow rate (infiltration) as well as the time taken for the build-up to occur, both depend on the size of contributing catchment and the concentration of sediment carried in the runoff. The process of build-up is also dependent on the magnitude of local annual rainfall. Results of a similar HP study in Adelaide, South Australia suggest a clear and systematic departure from the ‘as constructed” value of permeability, as expected. The results suggested long term infiltration might be about 20% of the “new system” performance (Rommel, Rus et al. 2001). Results obtained from the field installation showed that the slots and spaces between the blocks became filled with sediment and organic matter derived from local native vegetation. The very poor permeability results are not only consequences of blockage at the geotextile barrier, but also caused by the almost complete blockage occurring in the ‘first line” of paving blocks receiving runoff directly. These tests showed that a “build up” occurred at the geotextile layer below the concrete blocks; dissection of the test specimens carried out at the conclusion of the programme (35 years) indicated that this blockage layer was about 10 mm thick. Although the infiltration rate was only 20% of the original performance, the system would still perform up to twice the greatest design storm intensity in Australia, after 35 years of operation (Rommel, Rus et al. 2001). From this hydrological performance review of both monolithic and modular systems, it may be concluded that a vast amount of information is available on benefits that these systems can bring to urban hydrology. However, no single study has been done on the effects of ageing and clogging on the hydrological performances of each of

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these systems. Neither has any study been done to compare PA, HP and PP under the same experimental conditions.

2.8 Pollutant Removal Performance

Many studies (Niemczynowicz 1990; Rushton 2001; Vaze and Chiew 2002) suggest that the majority of stormwater pollutants are attached to suspended solids and these solids are therefore regarded as the principal transport mechanism of pollutants in stormwater. However, significant proportions of some contaminants, including nitrogen and heavy metals such as Zn and Cu, may be present in dissolved form (Drapper, Tomlinson et al. 1999). Pollutants attached to particles will generally remain in particulate form during transportation by runoff, especially if the pH remains above 7. However, any reduction in pH will subsequently increase the proportion of dissolved trace metals. Some of the stormwater pollutant removal mechanisms include sedimentation, filtration, adsorption and the biological assimilation of pollutants by microbial activity within the porous media. A summary of each of these major removal mechanisms (Niemczynowicz 1990; Pratt 1990) are discussed below. a) Sedimentation The different courses in porous pavements reduce flow velocities and cause particles and suspended solids to settle out. Sedimentation is one of the main mechanisms for the removal of sediment and its associated pollutants. b) Filtration Mechanical straining of pollutants occurs as water percolates down the various courses in a porous pavement system. Along with sedimentation, filtration is a main mechanism for the removal of sediment and its associated pollutants. c) Adsorption Adsorption is the adherence process of chemical ions to the surfaces of solid particles as a result of electrostatic attraction and/or physical attraction. This is the principal mechanism for the removal of dissolved pollutants. The adsorption process is further enhanced by the presence of iron, organic matter and fairly neutral pH. d) Nitrification Nitrification is the oxidation of ammonium (NH4

+) to nitrite (NO2-) and from nitrite

(NO2-) to nitrate (NO3

-) by microorganisms. e) Denitrification Denitrification is the biological reduction of nitrate (NO3

-) to molecular nitrogen (N2) gas, resulting in the loss of nitrogen into the atmosphere. Denitrification happens under low-oxygen conditions. f) Microbial uptake and mineralization Microbial uptake is the assimilation of nutrients to be used for growth and other biological processes while microbial decomposition is defined as the decomposition and degradation of organic matter into its inorganic elemental components by microorganisms. Mineralisation is the conversion of organic nitrogen into ammonia

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by bacteria. The presence of bacteria within the sub-base or sub-grade will reduce phosphorus, nitrogen, metals and hydrocarbons. A review of the pollutant removal performance of monolithic and modular pavements is described in the following section. Monolithic systems Previous studies (Legret, Colandini et al. 1996; Legret and Colandini 1999), which compared the mean pollutant concentrations at the outlets of a reservoir structure of a nearby reference catchment over thirty rain events, have shown that the pollution level of such a catchment is rather low. The reductions of total suspended solid (TSS) and Pb concentrations were 64% and 79% respectively; and 72% and 67% respectively for Zn and Cu. Analyses carried out on materials taken in 1992 from the reservoir structure and the soil underneath showed that heavy metals (Pb, Cu, Cd and Zn) accumulate, for the most part, on the surface of the porous asphalt and a small amount is found at the level of the geotextile separating the reservoir structure from the soil, which did not appear significantly contaminated after the four year period, during which the structure has been in operation. A comparison of pollution loads TSS, Pb, Cd, Cu and Zn in stormwater at the outlets of both a porous pavement with reservoir structure and a nearby catchment drained by a separate sewer system demonstrated that the quality of the waters was significantly improved by the passage through the porous pavement (Legret, Colandini et al. 1996; Legret and Colandini 1999). A study conducted in the Netherlands by Berbee, Rijis et al. (1999) compared the quality of runoff from impervious highways with runoff from roads with a porous asphalt surface course. For nearly all the parameters, runoff from the porous asphalt road had lower concentrations of pollutants than runoff from the conventional, impervious highway. Other authors have also arrived at the same conclusions. With regards to various pavement systems, Booth and Leavitt (1999) showed that the water quality of the resulting infiltrate was significantly different from, and generally much better than the surface runoff from a normal asphalt parking area. For both copper and Zn, the infiltrated stormwater usually had concentrations below detectable levels. Motor oil was also consistently much lower in the infiltrate than in the surface runoff. Over a five year period, concentrations of some infiltrated constituents have increased while others have stayed the same or decreased. Zn concentrations in both infiltrated and surface runoff exhibited marked increases; Cu concentrations decreased substantially in both systems. Brattebo and Booth (2003), and Rushton (2001) reported that monolithic pavements exfiltrate contained lower Zn and Cu concentrations than asphalt run-off. In France, a bridge paved with impervious asphalt was monitored for runoff pollutants and then repaved with porous asphalt and monitored again. Loadings of TSS, total Kjeldahl nitrogen (TKN), nitrite and nitrate, ammonium, Cu and Zn from porous asphalt were lower than the impervious asphalt loadings. Pagotto, Legret et al. (2000) reported that the main phenomenon responsible for the retention of pollutants by the porous asphalt appears to be the filtering function performed by this pavement (Stotz and Krauth 1994). Although a lot is known about the pollutant removal performance of porous pavement, particularly when the system is “new”, no similar study has been

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performed with the effects of clogging and ageing considered. The mechanisms responsible for the retention of those dissolved forms by the porous pavement are also poorly understood; the mechanisms involved might actually be adsorption either onto the clogging materials or onto the pavement materials (due to higher pH and the longer contact time). Modular systems Dierkes, Holte et al. (1999) found that modular pavements appear to be effective at trapping dissolved heavy metals in runoff. Most metals are precipitated in the upper 2 cm of the porous concrete. However, pH in effluent shows that the buffering capacities of the concrete are very high, so that there is no danger of mobilisation. In the sub-base, higher concentrations of metals were found to a depth of 20 cm for Cd and Pb, and the depth decreased to 10 cm after simulating 50 years of operation. Metal concentrations in the effluent only reached the permissible limits for Cd and Cu, when very coarse material for the sub-base was used. Most structures showed no danger of a possible groundwater contamination during the tests, with the authors concluding that porous pavements made of concrete blocks could be used without fear of groundwater contamination for about 50 years. Legret, Nicollet et al. (1999) showed that suspended solids and lead can be reduced by up to 64% and 79% respectively, similar to the results found for monolithic pavements. According to Pratt, Mantle et al. (1989), runoff water quality is improved by modular porous pavement, for the main pollutants of runoff water; heavy metal loads discharged into the environment are reduced from 20% (Cu) up to 74% (Pb); solids are detained at a rate of 87% and hydrocarbons are intercepted at an even higher rate (90%). It is basically the retention of fine particulate pollution (not subject to settling) by filtration that explains the reduction in the amount of hydrocarbons and metals. Sediments, suspended solids, particulate metals and hydrocarbons are significantly detained by the porous structure (hydrocarbons contained in runoff water are most often bound to particles and seldom in a free-moving form). Some dissolved forms of metals could also be retained in the porous media. In summary, it may be concluded that porous pavements are highly efficient infiltration systems that have the capacity to remove stormwater contaminants via physical, chemical and possibly biological mechanisms. While some of these pollutants are transformed, some of them are accumulated on the filter media or sub-base. However, the long-term performance and the effect of wetting and drying on these systems after years of operation have not been investigated, and remain an important knowledge gap.

2.9 Clogging

The use of porous pavement is often associated with the issue of clogging due to pollutant accumulation. Clogging is a process that develops over time, due to the deposition of sediments from stormwater. It is defined as the process of reducing porosity and permeability, and hence decreasing the infiltration rate of a system due to physical, biological, chemical or a combination of all three processes (Bouwer 2002). These processes depend to a certain extent, on the characteristics of water sources used in the system. It is evident from the literature that stormwater contains less

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organic particles than waste water (Mitchell, Mein et al. 2002). Therefore, clogging of stormwater systems is more likely to be of a physical nature, although the extent to which biological activities influence the clogging is still uncertain. Although clogging has been studied for several decades for other systems such as sand filters (Reddi, Ming et al. 2000), recharge basins (Goodrich, Phipps et al. 1990) and injection wells (Pfeiffer, Ragusa et al. 2000), clogging of porous pavements has not been extensively studied. This section is a review on the literature available for monolithic and modular pavements with regards to clogging and its processes. Monolithic systems For monolithic pavement structures such as porous asphalt, the consequences of clogging are severe as the pavement surface must be mechanically removed and replaced. Even with vigorous maintenance, such as high-pressure washing and vacuuming, clogging of these types of pavements is a common occurrence. Previous studies by Galli (1992), Nozi, Mase et al. (1999), Raimbault, Nadji et al. (1999) and Warnaars, Larsen et al. (1999) have all found clogging to be the main reason for the failure of stormwater infiltration systems. This was further supported by Lindsey, Roberts et al. (1992) who observed that two of the three porous pavement facilities studied had failed due to sediment accumulation and the lack of maintenance. A field study conducted by Ishizaki, Masahiro et al. (1996) found that the time taken for a system to clog depends on the type of contributing impervious areas. However, results from such field studies are difficult to interpret or generalise due to the lack of control of the large number of variables. A small scale laboratory study on the clogging of stormwater infiltration systems by Pokrajac and Deletic (2002) showed that clogging occurs at the interface between the filter media and the surrounding soil. Siriwardene (2007) studied physical clogging under both constant and variable water levels, and found that a clogging layer forms at the interface between the filter and underlying soil, irrespective of the inflow regime of both water and sediment. It was also found that clogging is much slower if the water level is kept at a constant level than if it varies within the column, due to a sediment plug that ‘shields’ the filter/soil interface. Most importantly, Siriwardene (2007) found that physical clogging is mainly caused by migration of sediment particles less than 6 µm in diameter, because they are more likely than large particles to reach the filter/soil interface. As such, sediment particles less than 6 µm are the main driver in the development of the clogging layer and hence causing system failure. Some of the effects of clogging on the fate of stormwater pollutants have been studied, but not to a large extent. Marsalek and Chocat (2002) found that clogging could increase the likelihood for contaminant release over time, thus causing secondary impacts to the environment. Cu, Pb, Zn, and to a lesser extent Cd, were retained in the porous asphalt by clogging particles (Legret, Nicollet et al. 1999). Wilde (1994) found that porous pavement retained all the sand, most silt and the largest clay particles, but smaller clay particles and colloids were apparently not filtered out. Despite the wide availability of data on the clogging phenomenon on porous pavements, not much has been done to address clogging and its impacts on the

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hydraulic behaviour over time. Neither has any study been conducted to investigate the long term impacts of clogging on the treatment efficiency of porous pavements. Modular systems Most modular pavement systems are constructed with a geotextile layer so as to reduce the flow of pollutants beyond this barrier as well as create an effective filter whose removal efficiency improves with age. This fabric also provides favourable sites for colonization by microbial populations. Studies from the USA, Australia and Europe (Shaheen 1975; Thompson 1995; Legret and Colandini 1999; Shackel, Ball et al. 2003; Ferguson 2005; Newton, Jenkins et al. 2003) have shown evidence that water quality will improve when it flows through a concrete block porous pavement, without the use of a geotextile layer. However, there is also evidence that the use of geotextile between the laying course and the sub-base, may assist in enhancing pollution removal and the degradation of pollutants, although this has never been proven. Clogging of the geotextile layer is an important mechanism for particulate removal (Niemczynowicz 1990; Pratt 1990). Numerous studies have identified filtration through the surface layer and the geotextile layer (Hogland, Niemczynowicz et al. 1987; Pratt 1990) as the primary mechanism for the removal of particulates. Although this effective removal of particles by the geotextile is rather desirable, it is also the same mechanism which leads to the reduction in infiltration capacity which is the principal operational concern for all forms of porous pavement (Newton, Jenkins et al. 2003). Clogging can occur during or immediately after construction, or through long-term use. Pratt et al. (1995) found that clogging can result from fine particles accumulating in the void spaces of porous pavements. As smaller particles trap larger particles, the rate of clogging will therefore increase as more fines are trapped (Balades, Legret et al. 1995). Two mechanisms of clogging which operate to reduce the effective permeability of the pavement are blockage of the slots between adjacent bricks and blockage of the geotextile fabric which separates the upper structure from the gap graded (gravel) sub-structure (Ferguson 2005). Over time, the system tends to show a gradual reduction in permeability, explained by clogging at the geotextile layer. Tests on modular systems have shown that the ultimate decrease in permeability is likely to be 80%, thus giving a long-term permeability of only ca. 20% of the new product value (The Environmental Protection Group (EPG) 2007). The time taken to reach this level of permeability depends of the nature and rate of sediment supply to a site and its geometry in terms of the ratio paved/permeable area. In conclusion, there is conflicting evidence that the upper geotextile is critical to the biodegradation and filtration performance of porous pavements. There is also a lack of research that has been done on the processes of clogging as well as the impact of clogging over time on the treatment performance of porous pavements, despite the importance of clogging for pollutant removal. The decision to use a geotextile between the laying course and the sub-base is ultimately a balance between durability and structural performance of the pavement and possible improvements in water

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quality. Although it is known that the clogging of the geotextile would eventually lead to a decrease in hydraulic and treatment performance, very little research has been done on stormwater infiltration systems to understand the clogging process and how clogging might be predicted and or prevented. No comparative testing has also been carried out on porous pavements that do not have an upper geotextile in order to demonstrate the benefits of including one. Due to a lack of understanding of the clogging process, many existing design guidelines and methodologies do not address the clogging issue adequately. Instead, they tend to make unrealistic assumptions about the dynamics of clogging.

2.10 Summary of Key Gaps and Research Needs

A promising approach focuses on the design of pavement systems that utilize both porous and impervious pavements. This allows the use of impervious pavement in areas of high traffic and shear loads, with runoff from these areas discharging onto areas of porous pavement. The porous pavement thus acts as a treatment device for impervious area runoff. However, there are still significant knowledge gaps with respect to porous pavement designs and performance. The following key gaps in the literature and guidelines have been identified from this review:

a) Clogging is still not well understood. The key mechanisms for clogging need to be investigated further before any predictions can be made on the life span of these systems.

b) Limited information currently exists to quantify the stormwater management performance of combined impervious/ porous pavement systems.

c) Most studies report values for the removal of a wide range of pollutants immediately after installation (while the system is still “new”), but there are not many studies on installations after years of operation.

d) A large number of studies have reported pollutant removal rates and hydraulic performance values but there is a paucity of research being conducted on the specific impact of clogging on these two factors, and their subsequent interaction.

A list of critical questions affecting the adoption of porous pavements in Australia is outlined below. This research will aim to provide answers to some of these questions.

a) What are the key mechanisms that govern clogging in different porous pavements, when exposed to typical Australian operational conditions?

b) Given that the systems are effective in accumulating sediment conveyed to them in stormwater runoff, what “lifespan” can be expected before complete or partial replacement is required?

c) What effluent water quality improvement can be achieved in situations where outflow passes to sensitive environments such as aquifers, urban waterways or estuaries?

d) What is the impact of clogging and other ageing processes on the treatment performance of these systems?

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3. Research Aims and Objectives

The aim of this research project is to understand the long-term performance (treatment efficiency and clogging behaviour) of a range of porous pavements used in stormwater management, and to provide guidance on their design. However, there are two main problems that need to be resolved before porous pavements can be implemented as a reliable stormwater treatment and reuse technology, particularly for retrofitting in existing urban areas:

a) Lack of research on the clogging processes of porous pavements in Australia. Early perceptions of clogging have hindered the adoption of porous pavements in Australia. While some systems still provide infiltration rates far above their design requirements after 15-20 years of operation, some systems have clogged below acceptable levels within relatively short periods.

b) Very little is known about the impact of clogging and other ageing processes on treatment performance of these systems. Although clogging in porous pavements is eventually inevitable, no large scale study has been conducted to answer this important question. The treatment performance of porous pavements is particularly under-researched in Australia.

To address these problems, the three porous pavement systems, PA, HP and PP will be studied in detail, since they represent the most common porous pavement technologies available on the market (as discussed in Section 2.5.1, 2.5.2 and 2.5.3). The specific objectives of this research project will be to:

1 Gain an understanding of the treatment efficiency and clogging processes within three major porous pavement types used in Australia, by conducting an extensive laboratory investigation.

2 Develop new insight into the nature of porous pavement clogging, whether physical clogging is the only process that is important in the overall clogging, or whether biological growth also plays a significant role.

3 Optimize the design of an Australian monolithic porous pavement, for multiple applications including stormwater collection, treatment and reuse, through detailed laboratory exploration.

Results of this research project will fill the gap in our knowledge with respect to the clogging and treatment processes in porous pavements, and provide guidance on porous pavement design for stormwater harvesting. Acquisition of such knowledge will accelerate the adoption of the technology in urban areas, and therefore assist stormwater treatment and water conservation projects in Australian cities.

4. Preliminary Research

To achieve objectives 1 and 2 (as listed in Section 3), an experimental rig that would allow the comparative study of the three porous pavement types was developed. As it was imperative that each pavement system received the same composition of stormwater, the constructed rig not only had to be large enough to house all three systems but it also had to be equipped with a reliable and efficient delivery system.

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To achieve objective 3 (refer to Section 3), a second rig, consisting of three separate columns was constructed. Each of these columns will be used for the study of alternative monolithic Permapave systems. The design of rigs 1 and 2 are shown below, along with a brief description of their development so far.

4.1 Development of Rig 1

The aim was to develop a laboratory rig that will house the following three pavement types:

a) a traditional monolithic porous pavement (PA) b) a typical modular porous pavement (HP) ; and c) a new type of monolithic porous pavement (PP).

Rig 1 should allow for the investigation of clogging (i.e. monitor the reduction in flow rates) and treatment efficiency, over time. As these two factors need to be investigated in parallel, it is vital that the same stormwater and sediment inflow is delivered to all three pavements. The design of the rig is presented in Figure 7. It consists of three main components:

a) housing of the three pavements (Section 4.1.1) b) stormwater distribution system, that consists of (Section 4.1.2):

• peristaltic pump • tank • connection tubes, and • stormwater inflow distributor (pneumatic system)

c) monitoring equipment (for flow and pressure measurements and sample extraction) (Section 4.1.3)

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Figure 7: Overall design of Rig 1 for the comparison of PA, HP and PP.

The completed rig is shown in Figure 8, while the details are explained further below.

4.1.1 Housing of the Three Pavements

In this study, the three different pavements were installed in a rig of 2700 mm by 450 mm by 1950 mm in dimension. This rig was further divided into three separate vertical compartments of 680 mm in length and 239 mm in width, overlaying a sub-base that is typical of current practice (Figure 8). Detailed top, side and front view diagrams of these systems are shown in Appendix 1. The installation of each pavement was based on the recommended guidelines and the first step involved the thorough washing of the gravel to be used in the sub-base to ensure that they are free of fines and sediment. The detailed design of each of the three pavements with information on the gravel specifications as recommended in the guidelines are shown in Appendix 1. Prior to the filling of the rig with the sub-base, the inside surface of the rig was roughened with sand to prevent preferential flow paths down the sides of the rig. Each individual sub-base layer was then constructed accordingly and compacted with a hand tamper before the pavement was placed firmly over the sub-base.

Computer

Semi-Synthetic Storm Water In 550 L tank

Sediment

Distributor Head (Located at the

base of the tank)

Data Taker

Outflow

Rain Gauge

Inflow

Peristaltic Pump

2700mm

PA HP PP

Tubings Electrical Lines Pavements Perforated Pipe Samplers PressureSensor

Pneumatic Distributor 1950mm

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Figure 8: The experimental rig for the comparison of PA, HP and PP.

As the investigated pavement areas, relative to the real system, was very small, it was crucial that the scales of the constructed pavements were not compromised. This was particularly so for modular HP (but not for monolithic PA and PP) as the basis for the operation of HP is the infiltration of water through the small channels formed at the ends of the concrete pavers, and the ratio of channel-to-paver area must be maintained as would be typical in field application. As the HP guidelines recommend an installation of 41.3 pavers (~82.6 channels) per m2, the experimental area of 0.16 m2 thus requires c.a. 6.7 pavers (~13.4 channels). In order to replicate the exact number of pavers and channels in the rig, a cardboard window frame that was smaller by a factor of ten of the experimental area was made. This frame was subsequently moved inch by inch over a large A3 paper, onto which the herringbone pattern of the pavement layout had been printed according to scale. This step enabled the appropriate cut-out of the pavers to be determined, without losing its herringbone layout pattern (Figure 9).

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Figure 9: Herringbone pattern of HP pavers.

4.1.2 Stormwater Distribution System

The stormwater distribution system consists of 4 separate components: (a) peristaltic pump, (b) tank, (c) connection tubes, and (d) sediment distributor system (pneumatic system). The development of each of these components is described in the following section. Peristaltic Pump Acquisition and Design To determine the type of pump to be acquired for this study, the range of flow rates to be simulated in Rig 1 first had to be calculated. The flow rates should be reflective of at least Melbourne and Brisbane climates, in order to cover the possible Australian conditions. The calculations consisted of two steps: (1) determination of initial losses, and (2) modelling flow. This was followed by the calibration of the peristaltic pump. (1) Initial Loss Determination The determination of initial loss is necessary as the first step to obtain the experimental flow rates for Rig 1. Initial loss is defined as the amount of precipitation that does not appear as direct runoff (I.E. Aust. 1998; Ilahee 2005). It is an input parameter, amongst many other parameters for rainfall runoff models and is used widely in rainfall based design flood estimation. Despite its importance, there seems to be a paucity of initial loss data in flood design, particularly in the case of PP. Being a new product in the market, there is currently no design loss values for PP that is recommended by the Australian Rainfall and Runoff (ARR) (I.E. Aust. 1998) thus making this field test necessary. Although a range of simplified rainfall runoff models that approximate catchment runoff behaviour are easily available for design flood estimation (I.E. Aust. 1998), they often have many inadequacies in the recommended loss values for catchments. These inadequacies include the underestimation of initial loss values or they could be biased towards the wet antecedent conditions of the systems, which could potentially lead to losses that are too low (Waugh 1991). Rainfall runoff models also frequently fail to account for the interception losses by vegetation, depression storage due to retention on the surface as well as infiltration losses into the soil (Hill, Mein et al. 1998).

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Results from previous studies of porous pavements have shown that a minimum of 1 mm of rain is needed to create runoff. In other words, an event of less than 1 mm will only be sufficient to create surface wetting of a pavement, but insufficient to observe a flow of water either above or below the surface as runoff. As mentioned before, with PP being the first of its kind of porous pavement in the market, especially in Australia, no such data is available that can help determine the volume of rain required to see runoff. As such, a PP focussed field study on initial loss was undertaken, rather than adopting any of the available models. In this field test, the initial loss was estimated from two storms, in which the total volume of rainfall prior to the commencement of surface runoff was determined using a stop watch and a tipping bucket. Based on the weather forecast by the Australian Bureau of Meteorology (BOM), the experimental equipment was prepared in anticipation of two heavy rain events on the 28th March 2007 and the 21st April 2007. Twenty-four hours prior to the anticipated rain event, three individual PP pavements with a dimension of 290 mm by 290 mm by 50 mm were dried in the oven at 105˚C for 3 hours. This kept the moisture content within each pavement to a minimum and ensured that the pavements had a standardized ‘clean state’ condition. The set-up of each experiment was performed in a well-sheltered area to prevent the pavements from coming in contact with rain. This was done in a garage, where all the preparation could be done conveniently and transported easily out to a 2 m by 2 m open area upon the first sign of rain. The set-up consists of placing a pavement securely over the top of a laboratory stool that has had its cushion removed. This rectangular metal frame of 270 mm by 270 mm by 620 mm had three perspex panels and a single plywood panel, taped firmly on each side of the frame to provide extra support and protection from rain (Figure 10).

Figure 10: Various snapshots taken from the determination of initial loss field

test. To ensure that the observation of runoff came only from the infiltration of rainwater through the surface of the pavement, and not from preferential flow through the sides of the pavement, a suitable stretchable, waterproof, and self-adhering material, Parafilm was used to wrap the sides of the pavement. This material was stretched around the sides of the pavement to form a strong seal against moisture. As the

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perspex and plywood panels were not joint or welded at the sides, the area on the inside of the frame could also be susceptible to rain water, potentially causing a preferential flow or edge effect of rain from the side gaps, through to the centre of the frame. A small gap of 5 mm between the base of the pavement and the surface of the frame also caused a similar problem. As such, cling wrap was used to wrap the pavement from the highest point down to the mid-section of the frame. Paper towels were also laid horizontally on the inside of the frame, directly under the pavement to aid in the observation of the first drop of runoff from the pavement. The complete set-up of pavement and frame was then covered with a plastic sheet for further protection from rain, before being placed out in the open. The volume of rainfall that infiltrated through the pavement to produce the first drop of runoff was determined through the use of a 0.2 mm tipping bucket and a stop watch. At the sound of the first tip, the plastic sheet cover was removed from the pavement to commence the experiment. A stop watch was used to measure the time taken for the first drop of runoff to be seen. This experiment was subsequently repeated for the other two pavements in the same rain event into the night. Due to a lack of resources and funds, it was only possible to build a single set-up, rather than three separate experimental set-ups, which would have been a more accurate method of measuring initial loss (although simultaneous observation of the three pavements may have been very difficult, and created inaccuracies). Initially, the idea of using a sprinkler to simulate drops of tap water as raindrops was considered, but the idea was eventually abandoned as it was near impossible to re-create the natural dynamics of rain, not to mention simulate an equal distribution of rainfall on a specific area of pavement. If this method had been employed in this experiment, the results would have been non-representative, unrealistic and biased to a certain extent. On the second occasion of rain event on the 21st of April 2007, the same experiment was repeated using the same pavements, to minimise the uncertainties and errors involved. However, the pavements were first flushed with water and dried in the oven once again before being used. Based on the results from these two experiments, a surface runoff threshold value of 0.85 mm was established. (2) Calculation of Experimental Flow Rates To determine the flow rates to be used in this study, the calculated initial loss was then applied as input data in a modelling programme, Model for Urban Stormwater Improvement Conceptualisation (MUSIC). This programme was used to generate a frequency curve of a typical 30 year rainfall event for both Melbourne and Brisbane. The average annual rainfall in Melbourne and Brisbane is 650 mm and 1150 mm respectively. The frequency curve of rainfall shows the frequency of each rain event of a particular intensity over a certain period of time. In MUSIC, the initial loss of 0.85 mm was applied as the rainfall threshold value in the classification of rainfall runoff parameters. By setting 0.85 mm as the threshold value in the frequency curve, any rain event of 0.85 mm or less, which will not contribute to any surface runoff, was eliminated from the frequency curve. From each of the constructed frequency curves, four different flow rates that corresponded to the mean, median, 90th and 99th percentile were subsequently chosen as suitable flow rates for the experiment. These flow rates are tabulated in Table 4 (Melbourne) and Table 5 (Brisbane). With these frequency curves, it was thus possible to mimic the distribution of flow at a particular percentile, and also conduct the experiment at the desired flow rate.

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Table 4: 30 year Melbourne Rainfall Time Series.

Flow rate per ha. Velocity Pavement Area Flow rate

Frequency m3/sec m/sec mm/hr m2 m3/sec ml/sec

Median 0.0017 1.7E-07 0.6 0.1564 2.6E-08 0.026

Mean 0.0045 4.5E-07 1.6 0.1564 7.1E-08 0.071

90% 0.0108 1.1E-06 3.9 0.1564 1.7E-07 0.169

99% 0.0375 3.8E-06 13.5 0.1564 5.9E-07 0.587

Table 5: 30 year Brisbane Rainfall Time Series.

Flow rate per ha. Velocity Pavement Area Flow rate

Frequency m3/sec m/sec mm/hr m2 m3/sec ml/sec

Median 0.0031 3.1E-07 1.1 0.1564 4.8E-08 0.048

Mean 0.0088 8.8E-07 3.2 0.1564 1.4E-08 0.137

90% 0.0208 2.1E-06 7.5 0.1564 3.3E-07 0.325

99% 0.0900 9.0E-06 32.4 0.1564 1.4E-07 1.500

Upon determining the experimental flow rates, the next step was to acquire, test and calibrate a peristaltic pump that could distribute sediments and stormwater homogenously to the three systems. (3) Peristaltic Pump Set-up For the purpose of delivering a constant flow of semi-synthetic stormwater to the pavements, a Gilson Minipuls 3 low-flow head peristaltic pump was used. Although a flow rate chart was provided by the supplier, a manual calibration of the peristaltic pump was performed, to ensure accuracy of the results. This was achieved by taking the average of three flow rate readings at every increment of 2 rpm, using a measuring cylinder and a stop watch. The flow rates as determined from the calibration test (using a 1.65 mm internal diameter (i.d.) tubing) ranged from 0.1 ml/min (1 rpm) to 10.44 ml/min (48 rpm). The desired flow rate of 10.2 ml/min (0.169 ml/sec), as determined in the previous section, was subsequently achieved by setting the pump at 47 rpm, the value of which was obtained using the equation shown in Figure 11. The selection of peristaltic tubing is a compromise between minimizing the pulsations and maximizing the life-time of the tubing. Damaged tubing can cause excessive pulsations and erratic flow, thus leading to unnecessary expenses. To ensure that consistent and accurate flow rates are maintained, the peristaltic tubing should be

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replaced as soon as signs of wear such as flatness or cracking appear. By doing so, the performance of the pump is not only maximised but any losses associated with the use of sediments is also minimised, thus ensuring the integrity and quality of results obtained. This is especially important for the experiments to be simulated in Rig 1, as a constant flow of 10.2 ml/min is expected to be delivered throughout the duration of the experiment for ca. 4 months. Based on this flow rate, the equivalent of “one year Melbourne runoff” is achieved through running the rig for seven days.

Figure 11: Flow Rate Chart showing the Average Flow (ml/min) vs. Revolutions

per minute (RPM) ± Standard Error (S.E.) Tank Specifications Based on the flow rate of 10.2 ml/min, as calculated for the 90% Melbourne flow in the previous section, c.a. 45 L of stormwater was estimated to be drained in 24 hours by all three pavements. In order to run experiments continuously for several days without having to drain more than 200 L of stormwater, a suitable tank of a capacity of 550 L that had previously been used by another student was employed. The tank was fitted with two coiled aerators, situated in the middle and the base of the tank to ensure a constant and homogenous mixing of stormwater. Connection Tube Specifications To connect the outlet of the stormwater tank to the peristaltic pump, a suitable one-to-three way sediment distributor head was necessary to split the single outlet tubing into three separate tubings, which feed into each of the three systems. The appropriate internal diameter of these three parallel tubings also had to be determined to avoid the settling of sediments during the operation of the rig. The following section describes the steps taken to calculate the settling velocity of sediments in order to determine the appropriate tubing specification as well as the design process of the appropriate sediment distributor head. (1) Determination of Connection Tube Diameter based on Settling Velocity Settling velocity of a particle is the rate at which the particle settles to the bottom of a liquid to form a sediment. It is dependent on grain size, grain shape (roundness and sphericity), the density of the grains as well the viscosity and density of the fluid (Cheng 1997). The settling velocity of natural sediment particles is an important

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factor in determining the appropriate diameter of connection tubes to be used, particularly for Rig 1, the tubes of which needed to be connected vertically to a height of 2 m. Initially, 3 mm internal diameter tubes were used, as they were easily available. However, their diameter proved to be too large to overcome the sediment settling velocity, thus failing to support the transport of sediments up the 2 m height. In an attempt to resolve this problem, velocity data from a previous experiment performed on the same rig by another student was evaluated. Since this velocity had worked well in delivering the sediments up the 2 m height, a suitable tube diameter was then back-calculated, using this velocity and the desired flow of 10.2 ml/min (Refer to Appendix 2). This calculation showed that a tube with a diameter of less than 1.55 mm would be able to overcome the particle settling velocity, which was calculated using the particle settling velocity formula (Refer to Appendix 3). Following this, trial runs conducted using a 1.52 mm internal diameter tube, proved successful. (2) Design of Sediment Distributor Head For the efficient delivery of semi-synthetic stormwater consisting of sediments to the three separate porous pavements, a suitable one-to-three way sediment distributor head was necessary. Upon connecting this one-to-three way distributor to the single outlet of the stormwater tank, equal amounts of sediments as well as synthetic stormwater should be delivered via the 1.52 mm internal diameter connection tubes to each of the three porous pavements. A total of three prototypes were trialled before reasonable success was found with the final prototype. Prototype A Prototype A (Figure 12 (a)) was a simple design, in which 3 tubes of 3 mm internal diameter each were fitted snugly in a single 8 mm internal diameter tube. The extra gap surrounding the smaller tubes was filled with silicone glue and allowed to dry overnight. As mentioned before, the connection tubes require a minimum internal diameter of 1.55 mm to overcome the sediment settling velocity. Failure to do so would result in a velocity that is lower than the sediment settling velocity, which will cause sediments to settle and clog in the tubes. In the first prototype, the use of the 3 mm internal diameter tubes failed to provide the high velocity needed to transport the sediments up to the pump (Figure 12 (b)). This resulted in very small amounts of sediments reaching the pavements. Extensive clogging was also observed, causing the originally opaque tubes to appear black before the pump, and clear after the pump (Figure 12 (c)). To overcome these problems, Prototype B was designed.

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Figure 12 (a) Figure 12 (b) Figure 12 (c)

Figure 12 (a): Prototype A consisting of three snugly fitted 3 mm internal

diameter tubings in a single 8 mm internal diameter tube; Figure 12 (b): The inflow tubings to the pump partially clogged with sediments; Figure 12 (c): The inflow tubings to the pump are partially clogged while the outflow tubings are

visibly clearer. Prototype B Following the failure of Prototype A, a new one-to-three way sediment distributor head, Prototype B was designed (Figure 13 (a), (b) and (c)). Prototype B was made from a 20 mm length brass tube of 10 mm diameter, in which three holes were drilled at equal distances away from the mid-point of the brass tube. Three capillary tubes of 1.3 mm internal diameter were then pushed into the drilled holes and the gaps sprayed with a heavy duty ‘Loc-tite’ sealant. Subsequently, 1.52 mm internal diameter connection tubes, rather than 3 mm, were individually sleeved over the capillary tubes, to transport the synthetic stormwater up towards the porous pavement systems. Upon using Prototype B for a week, a visual inspection of the surface of the brass tube showed a layer of sediment that had settled over time. Elongated shaped twigs would also occasionally get stuck perpendicular to the drilled holes of the brass tube, thus restricting the flow. However, the use of the 1.52 mm internal diameter tubes managed to overcome the problem of the particle settling velocity, as sediments were observed to flow smoothly to and from the pump. Other than these small occurrences of blockage, Prototype B performed significantly better than the first prototype, but an attempt was made to refine the distribution of holes on the brass tube. Hence Prototype C was designed.

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Figure 13 (a) Figure 13 (b) Figure 13 (c)

Figure 13 (a): Top view of the brass tube surface; Figure 13 (b): Bottom view of

the brass tube, with the 1.3 mm internal diameter capillary tubes protruding outwards; Figure 13 (c): Front view of Prototype B, with holes drilled at equal

distances away from the mid-point. Prototype C The design of Prototype C was similar to Prototype B, the only difference being the holes were drilled towards, rather than away from the centre, at a distance of 1.5 mm away from the mid-point (Figure 14 (a), (b) and (c)). A visual observation of the brass tube surface after a week of operation did not show any settling of sediment. In fact, sediments were seen getting sucked into the brass tube towards the pump. In the long run however, settling and clogging of the distributor head may occur, as this observation (made in the early days of operation) does not take into account the temporal effect of this experiment. Nevertheless, Prototype C was the best design amongst all three prototypes. Based on its satisfactory performance, and with consideration of time constraints, no other prototypes were designed and a decision was made to use prototype C as the sediment distributor head.

Figure 14 (a) Figure 14 (b) Figure 14 (c)

Figure 14 (a): Top view of the brass tube surface; Figure 14 (b): Bottom view of

the brass tube, with the 1.3 mm internal diameter capillary tubes protruding outwards; Figure 14 (c): Front view of Prototype C, with holes drilled towards, rather than away from the centre, at a distance of 1.5 mm away from the mid-

point.

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Stormwater inflow distributor (pneumatic system) One of the most important steps in completing the stormwater distribution system is the acquisition of a rainfall distributor that was capable of distributing random and equal drops of water as well as sediments. Two attempts were made to design this distributor, the process of which required a significant amount of time, but none proved to be fruitful. The first design was a plastic rod with approximately twenty irrigation drippers installed at equal distances across the horizontal length of each pavement, to facilitate the delivery of stormwater (Figure 15 (a). This rod was attached onto a rotating motor, which allowed the rod to sway forwards and backwards uniformly at a 45˚ angle. Stormwater was introduced at three equally distributed points across the length of the rod, via three tubes that were connected to a flat-base cup, situated above the rotating motor (Figure 15 (b). Unfortunately, the presence of sediments in stormwater meant that this horizontal rod design was unable to cope with the equal distribution of sediments - most sediment were drained together with stormwater from the irrigation drippers that were closest to the location of the three inlets, where the inflow tubes were located. The concept of using a flat-base cup to facilitate the non-biased flow of stormwater through either holes into the rod also failed, as preferential flow was always observed through two of holes drilled on the cup (Figure 15 (c). A second design was attempted to overcome the two issues encountered but halfway through the process, the prototype design broke in half.

Figure 15 (a) Figure 15 (b)

Figure 15 (c) Figure 15 (a): Prototype 1 with irrigation drippers installed on to the horizontal rod; Figure 15 (b): Flat-base cup with 3 holes drilled on the base; Figure 15 (c):

Overall design of Prototype 1.

Due to time constraints, the time and labour costs in repeating the whole process were weighed against the costs of purchasing a completely automated system, and a

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decision was subsequently made to purchase an automated dripper system from SMC Pneumatics (Figure 8). This pneumatic distributor may be adjusted to travel in three directions (to the front, back and the sides) at various speeds and pressure and is very efficient in distributing stormwater (plus sediments) at random locations on the pavement surface, at rainfall intensities that are consistent with typical Melbourne climate. Since installation, the system has performed well.

4.1.3 Monitoring Equipment The monitoring equipment used in Rig 1 includes rain gauges (tipping buckets), pressure sensors, sampling ports, a data taker, a computer and an uninterruptible power supply (UPS). Three tipping buckets were used to measure flow rates in this study, each receiving outflow from a pavement system (Figure 8). Tipping buckets were more suitable than v-notches, as their degree of accuracy was sufficient to cope with the low flows. Pressure sensors were also installed below the pavement structure of each system, to capture the progress of the clogging process (Figure 8). However, the collected data will only be used when the system becomes clogged (which has not occurred to date) Attached to each pressure sensors are sampling ports, which allow water samples to be extracted (the process of which only happens if the surface of these systems get clogged). All electrical equipments (i.e. the rain gauges and pressure sensors) were connected to the UPS to prevent a loss of data upon the event of a blackout. The UPS was then connected to the computer via a data taker, which was programmed to log data continuously every 1 minute.

4.2 Development of Rig 2

The aim was to develop a rig that will house three different monolithic PP systems, to allow the study of various PP designs. PP can be made from three different aggregate sizes (10 ml-12 ml, 12 ml-14 ml, and 16 ml-20 ml aggregate sizes), and placed either on void, sand, gravel or soil. PP can also be constructed at two different pavement heights (50 mm or 100 mm). Thus, at least 15 configurations could be studied. Due to the high infiltration rates and costs of these systems, they often only represent 10% to 20% of a catchment, with the remaining paved area draining into it. The design of the rig is presented in Figure 16, while the completed rig is shown in Figure 17. It consists of three main components:

a) housing of three PP pavements (in individual columns) (Section 4.2.1) b) stormwater distribution system, that consists of (Section 4.2.2):

• peristaltic pump • tank • connection tubes, • stormwater inflow distributor (rotating sprinkler and funnel)

c) monitoring equipment (for flow measurements and sample extraction) (Section 4.2.3)

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Figure 16: Overall design of Rig 2 for the study of alternative monolithic PP designs.

Figure 17: Experimental rig for alternative PP design study.

Semi-Synthetic

Storm Water In 550 L tank

Peristaltic Pump

Rotating Sprinkler

Funnel

PerspexColumns

Outflow

Inflow

Rain Gauge

Computer

Data Taker

Tubings Electrical Lines Pavements Perforated Pipe Samplers

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4.2.1 Housing of PP Pavements (Columns)

In this study, three different PP pavements were installed in a rig that consisted of three identical round detachable perspex columns (each 1890 mm in diameter, and 100 or 200 mm high). The installation of each pavement in each column involved the vertical stacking of one 100 mm column above two 200 mm columns. They were subsequently tightened with screws and o-rings to prevent any possible leakage (Figure 17). Prior to the installation of pavements in the columns, the individual pavements must be washed and dried in the oven at 104˚C to obtain the dry weight in its “new” condition.

4.2.2 Stormwater Distribution System

The stormwater distribution system consists of 4 separate components: (a) peristaltic pump, (b) tank, (c) connection tubes, and (d) sediment distributor system (rotating sprinkler and funnel). The development of each of these components is described in the following section. Peristaltic Pump Acquisition and Design The flow rates to be used in this study are dependent on the percentage of PP that is to be represented as the total catchment. When these systems represent 2%, 5%, 10% or 20% of a catchment, the corresponding flow rates could range from five to fifty times higher than the flow rates in Rig 1 (as shown in Table 4). As such, the peristaltic pump from Rig 1 could not be used for this study. For the purpose of delivering a constant flow of semi-synthetic stormwater to the pavements in Rig 2, three Heidolph Standard SP 5006 peristaltic pumps were purchased. Although a flow rate chart was provided by the supplier, a manual calibration of the peristaltic pump was performed nevertheless, to ensure accuracy of the results. This was achieved by taking the average of three flow rate readings at every increment of 100 rpm, using a measuring cylinder and a stop watch. The flow rates as determined from the calibration test (using a 3 mm internal diameter tube) ranged from 85 ml/min to 1024 ml/min. Tank Specifications Similar to Rig 1, another existing 550 L tank was employed. This tank was also fitted with 2 coiled aerators, situated in the middle and the base of the tank to ensure a constant and homogenous mixing of stormwater. Connection Tube Specifications Unlike Rig 1, Rig 2 will be run at higher flow rates, thus eliminating the need for a specific tube internal diameter or a complex sediment distributor head. The outlet of the stormwater tank, was connected to the peristaltic pump via a simple one-to-three way sediment distributor head, which consists of three 3 mm internal diameter tubes, fitted snugly in a single 8 mm internal diameter tube. The extra gap surrounding the smaller tubes was filled with silicone glue and allowed to dry overnight before use. This splitter allowed the single outlet tube to split into three separate tubes, which feed into each of the three systems. Due to the higher flow rates of Rig 2, particle settling velocity was not a problem and the use of a 3 mm tube was sufficient to deliver sediments smoothly into the pavements.

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Rotating Sprinkler and Funnel In completing the stormwater distribution system, a funnel and a rotating sprinkler was used to deliver random and equal drops of water as well as sediments. The use of the funnel helps to reduce splashing as well as guide the smooth delivery of stormwater down the rotating sprinkler.

4.2.3 Monitoring Equipment

The monitoring equipment used in Rig 2 includes rain gauges (tipping buckets), sampling ports, a data taker, a computer and an uninterruptible power supply (UPS). Three tipping buckets were used to measure flow rates in this study, each receiving outflow from a pavement system (Figure 17). Tipping buckets will be used to cope with the low discharge of this study, while manual measurements of flow will be done for the higher flow rates in this study. Sampling ports were also positioned below each pavement to allow water samples to be extracted (the process of which only happens if the surface of these systems gets clogged). The rain gauges were connected to the UPS to prevent a loss of data upon the event of a blackout. The UPS was then connected to the computer via a data taker, which was programmed to log data continuously every 1 minute.

4.3 Stormwater Preparation

One of the most important steps of the preparation for the laboratory work was to determine the source and type of stormwater to be used for this study. Upon the complete construction of the rig as well as ensuring that all the accompanying parts were working efficiently, the last and final step was to prepare a stormwater mixture that was to be used as the standard inflow throughout the whole project. In determining the source of stormwater to be used in this experiment, the advantages and disadvantages of using real stormwater and synthetic stormwater were considered. While the collection of real stormwater would maintain the integrity of the physical, chemical and biological characteristics of the stormwater, it would be difficult to maintain the required consistency of particles and pollutants present for each sampling session. Having to top-up the tank twice a week would also make it unrealistic and impractical to have a fresh supply of real stormwater every time. Although the use of synthetic stormwater may ensure a steady consistency for the experiment, the composition of chemicals may not be a true representation of the complexity that exists in real stormwater. As such, a compromise was made to use a semi-synthetic stormwater mixture, the term of which refers to the use of real stormwater sediment from a wetland but tap water, instead of stormwater. This method has been previously used by Hatt, Siriwardene et al. (2006), and Hatt, Fletcher et al. (2007). Real stormwater sediments were collected from Huntingdale Wetland, Melbourne and were subsequently filtered through a 300 µm sieve. Upon filtration, the solid fraction of the slurry was then determined through a series of drying tests. Based on a careful calculation of the desired target pollutant concentrations for Melbourne stormwater

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(Table 6), known amounts of slurry was then added into a constantly mixed 550 L tank of tap water. The constant mixing was provided for, by two separate aerator coils located around the perimeter of the tank, one at the base, and the other in the middle. This constant mixing ensures that the sediments are kept in a homogenous suspension and also oxygenates the water to remove any residual chlorine from the tap water. Based on a slurry recipe, known concentrations of nutrients and metals were then added to achieve the desired stormwater quality concentrations. Table 6: Typical Melbourne stormwater pollutant concentrations (Duncan 1999).

Pollutant Concentration (mg/L) Total Suspended Solids (TSS) 150 Total Nitrogen (TN) 2.6 Total Phosphorus (TP) 0.35 Copper (Cu) 0.05 Lead (Pb) 0.14 Zinc (Zn) 0.25 Cadmium (Cd) 0.0045 A recent comparison of stormwater quality data from catchments around Melbourne, Australia by Francey, Fletcher et al. (2007) has shown that the Event Mean Concentrations (EMC) of TSS is c.a. half of that reported 20 years ago. On average, TSS was around 80 mg/L, which is c.a. half of that reported in the Fuchs’ (2004) world wide study and the US Nationwide Urban Runoff Programme (URP) study dating back 20 years ago. These results were confirmed by the more recent findings by Smullen, Shallcross et al. (1999) who reported an average TSS concentration of 78 mg/L. Despite these findings, a decision was made to prepare the slurry, based on the higher TSS concentration as it was better to investigate a worst case scenario value of TSS, rather than underestimate the probability of sediments found in stormwater. Additionally, it was preferable that the study considered worldwide data, rather than results specific to Melbourne. In order to determine if the recipe was delivering the calculated target pollutant concentration, several TSS tests were performed over a week. TSS was chosen, amongst all the other pollutants as the basis for comparison because it is the pollutant with the quickest laboratory testing time (of less than 24 hours), and is typically the most variable (Francey, Fletcher et al. 2007). Despite following the stormwater recipe, measured TSS concentrations were 50% lower than the target concentration of 150 mg/L, indicating a settling or deposition of larger particles at the bottom of the tank. Two possible reasons for this could be an inefficient aerator in keeping the sediments in suspension or a possible clogging in the sediment distributor head, over time. Subsequently, a decision was made to increase the dosing concentration of pollutants, rather than refine the design of the system as the former solution was more cost and time efficient. As such, more TSS tests were performed at four other pollutant concentrations (2x, 3x, 4x and 5x the normal TSS concentration). Finally, the most representative dosing concentration of 2.5x the normal dosing recipe was chosen as the best recipe.

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5. Proposed Research

This study will comprise of three activities, each designed to achieve one of the objectives listed in Section 3 of this report;

• Activity 1: Clogging and treatment processes of 3 porous pavement types; • Activity 2: Laboratory trials of alternative monolithic pavement designs; • Activity 3: Modelling and development of guidelines for the design,

construction and maintenance of porous pavements under Australian conditions

The first two activities will be carried out in the Hydraulics Laboratory of Monash University, using the rigs developed above. Based on the data and results from these activities, a clogging model for infiltration systems will then be developed in Activity 3. The progress achieved so far, including future plans for the next two years are presented in a time line in Section 5.4. Details of each activity including the background, setup, methodology, sampling procedure and results are further explained in the following section. Amongst the three activities, only Activity 1 has commenced and the results obtained so far will be discussed.

5.1 Activity 1: Clogging and Treatment Process of 3 Porous Pavement Types In Activity 1, the performance of three different porous pavement types will be studied and compared with the aim of understanding how treatment efficiency changes over time. The three types of porous pavements being investigated are

d) a traditional monolithic porous pavement (PA) e) a popular modular porous pavement (HP) ; and f) a new type of monolithic porous pavement (PP).

These pavements will be examined for the following factors: a) rate of clogging under a range of conditions (i.e. hydraulic and sediment

loadings) b) treatment efficiency of key stormwater pollutants (i.e. TSS, TP, TN and

total and dissolved metals) for different applications of the pavements c) type and nature of clogging and treatment processes, and how they

interact (i.e. the role of biological and physical processes)

5.1.1 The Experimental Programme

Two groups of experiments will be carried out to achieve this, as explained below. A: Experiments with a Compressed Time Scale where wetting periods are exaggerated to achieve clogging within a short time. These experiments will enable us

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to develop an understanding of the physical processes of clogging and stormwater treatment. They will be performed at a constant hydraulic loading rate for Melbourne and Brisbane climates, using a typical concentration of stormwater pollutants. In the second part of this experimental programme, drying as well as varied inflow rates will be simulated, to mimic natural variability in runoff (these will be done in compressed time to achieve many years of operation).

B: Experiments with a Real Time Scale where the pavements will be dosed, but at stochastic intervals which mimic the natural pattern of wetting and drying for Melbourne or Brisbane. This pattern will also be adjusted to account for seasonal differences. Biological growth will be monitored on the pavement by scraping small samples from the surface at monthly intervals, and analysed for chlorophyll. The experiment will either run for approximately 12 months or until the pavement is clogged (defined as when the hydraulic conductivity diminishes to less than 10% of its initial value). The data collected from the first compressed time scale experiment will be analysed to determine the physical clogging rates for each porous pavement system. The following factors will be examined; decrease in hydraulic conductivity of both the pavement and the sub-base, increase in their hydraulic resistance and accumulation rates of sediments in the pavements. The main variables that govern the clogging process will also be determined. The same data sets will then be used to examine the physical treatment within the systems. This approach will ultimately allow us to determine whether clogging increases or decreases the treatment efficiency of the pavement and how this changes over time. The data collected from the second real time scale experiment will be used to determine whether physical clogging is the only process that is important in the overall clogging, or whether biological growth also plays a significant role. It will also be possible to assess how biological growth affects treatment and whether it helps, for example, in the removal of TN in stormwater. The ratio of biological to physical clogging, as well as biological versus physical treatment efficiency for typical loading conditions will also be determined, by comparing the results of the compressed time experiment and the real-time experiment. Using this ratio, the physical clogging and treatment performance obtained from the first experiment will be corrected to account for biological processes.

5.1.2 Method

The three different pavements will be installed in Rig 1 as shown in Figure 8 and outlined in Section 4.1. Prior to the commencement of any experiments, a series of hydraulic conductivity (k) measurements will be performed to determine the porosity of the individual pavement systems when the systems are in their “new” condition. Clean tap water will be used to flush through each system, and water quality of this water would be recorded as the reference. Each of the pavements will then be dosed with stormwater of identical quantity and quality, as determined in the pilot study (Section 4.3). Samples will be collected and analysed for key water quality parameters. Outflow rate will be recorded continuously, while at regular intervals, a large storm will be simulated to study reduction in infiltration rate during ‘flood’

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condition. Each experiment will last until the system shows signs of clogging (outflow during floods is approx 10% of the ‘clean’ infiltration capacity). Hydraulic Conductivity Tests Hydraulic conductivity (k) is defined as the rate of movement and the ease with which water can move through a porous medium. Hydraulic conductivity can be measured by applying Darcy’s law on the material to be investigated, by creating a hydraulic gradient between two points and measuring that flow rate. Among the many methods available for measuring hydraulic conductivity in the laboratory, two of the most common are the constant head and the falling head method. While the constant head method is most suitable for determining the hydraulic conductivity of unsaturated mediums such as coarse sands and gravels (due to the high permeability), the falling-head method is more appropriate for fine silt and clay like soils (Wanielista, Kersten et al. 1997). As the systems studied in this project were mainly gravel-like, the constant-head method was thus used to determine the hydraulic conductivity of the three pavements, in their “clean state”, using a typical Darcy’s apparatus set-up. This procedure delivers a constant supply of water to the pavements under a steady state pressure head while the volume of water flowing through the system is measured over a period of time. A simple application of Darcy’s law leads to the expression K =QΔL/AΔH, where Q is the steady volumetric discharge through the system, A is the cross-sectional area of the system and the term ΔH/ΔL is referred as the hydraulic gradient, which is a ratio of the difference in head between two points and the length separating them (ΔL) (Freeze 1979). As discharge (Q) is proportional to the change in head pressure and inversely proportional to the length of the column containing the porous media, hydraulic conductivity, which estimates the rate of flow through a porous medium per unit hydraulic gradient and cross-sectional area was determined. The basic device for the measurement of hydraulic head is a tube or pipe in which the elevation of a water level can be determined. When this device is used in the laboratory, as in this test, it is known as a manometer, but when used in the field, it is known as a piezometer. Manometers are used to measure the hydraulic head or pressure of a fluid at a specific location in a column. They are designed to allow the inflow of water, but not the sand and grains that will affect the accuracy of measurements. They are also open to water flow from the bottom and open to atmospheric pressure from the top. In this study, two manometers were placed under each of the pavements. Each of these manometers had extensions of different lengths and was attached to rulers either at the constant head tank, or above the pavement to measure the head difference, which would then determine the hydraulic conductivity. In this study, the supply of water to the pavement system was done through both the top and the bottom, thus making the hydraulic conductivity measurement an anisotropy property. For each pavement type, two hydraulic conductivity measurements were performed, one for each direction of water flow. As such, a total of six hydraulic conductivity measurements were taken. In the bottom to top experiments, a constant head tank was used to introduce water at a constant head pressure from the bottom to the top of the pavements (Figure 18 (a),

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(b) and (c)). Prior to the measurement of hydraulic conductivity for each pavement, the constant head tank was adjusted to match the approximate height of each pavement. Any excess water was subsequently purged through an overflow weir in the constant head tank. The pavement system was then left to equilibrate and achieve saturation overnight to remove any air from within the system and the surrounding medium before any measurement was taken. To check if the system had achieved a steady state, manual calibrations of the outflow was taken using a measuring cylinder and a stop watch until a constant value was obtained. Once this was achieved, the hydraulic conductivity of the sub-base (kb), the pavement (kp) and the overall system (ke) was determined through the measurement of three different head readings (H1, H2 and ΔH) and length readings (Lb, Lp and Le). As can be seen in Figure 18 (a), 18 (b) and 18 (c), the difference in head measured between ruler 2 and manometer 2 gives H2, which determines kp, while the overall k of each pavement system was determined by measuring ΔH, the difference in water level in the constant head tank and above the pavement. Instead of using manometers however, adjustable pin gauges were used for increased accuracy in the measurement of ΔH. As the difference between ruler 1 and manometer 1 resulted in a small H1 value that could be inaccurate, kb was determined by subtracting H2 from ΔH, thus giving a more accurate value of H1.

Figure 18 (a): Bottom to top k test for PA.

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Figure 18 (b): Bottom to top k test for HP.

Figure 18 (c): Bottom to top k test for PP.

In the top to bottom experiment, water was introduced using a hose from the top of the pavement until a constant head was achieved above the pavement, the excess water of which was allowed to drain freely through an overflow point (Figure 18 (d)). Constant flow readings from manual measurements performed, further confirmed that a constant head had been achieved.

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Figure 18 (d): Top to Bottom k test for PA.

A: Experiments with a compressed time scale In the first experiment, the hydraulic rates to be used corresponds to the intensity of the 90th percentile of the average recurrence interval (ARI) storm in Melbourne or the 50th percentile of the ARI storm in Brisbane. This corresponds to a flow of 10.2 ml/min through each pavement system. The stormwater, which contains average sediment concentrations of 80 mg/L to 200 mg/L (see Section 4.3) will be mixed constantly in the tank and distributed homogenously over the three surfaces (at the same rate). The tank will be topped-up twice a week (on Monday and Friday mornings) before the water level drops down to 300 L. Initially, water quality samples will be collected daily to determine the required sampling frequency for the duration of the experiment. The sampling frequency will be reduced subsequently to composites of several days. During each sampling session, samples will be taken at the inflow and outflow as well as from the bottom of the pavements (using existing samplers) and analysed for concentrations of TSS, sediment particle size distribution (PSD), TP, TN, TDN, FRP and total and dissolved metals. Chlorophyll samples will also be taken from within the pavement and sub-base. The flow rate, pressure as well as temperature in and just underneath the pavement will also be monitored continuously. The collection of every sample is accompanied by the measurement of its pH to enable early predictions to be made on the behaviour of heavy metals in the system. This is particularly useful as the turnaround time for the metal results to be obtained is approximately three months. A flood that corresponds to a 1 in 5 year storm in Brisbane will be simulated at equal time intervals (after several years of operation) in this experiment. This flood size was chosen since traditional urban drainage systems in Australia are designed to operate for this extreme situation (I.E. Aust. 1998). The intensity of this event is 191 mm/hr (4.6 m/d), which is just above the rainfall intensity of a 5 minute duration, 1 in 100 year storm for Melbourne (which has an intensity of 183 mm/hr). This translates into a flow rate that is c.a. 50 times higher than the current flow, through each system. Therefore, a possible storm in tropical Queensland and a major storm in Melbourne is replicated, under which the systems may start to malfunction and cause flooding.

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This storm was simulated every 6 weeks of the experiment, the period of which represents 6 years of the system’s operation in Melbourne or 3 years of operation in Brisbane. The experiment will conclude when the hydraulic conductivity diminishes to less than 10% of its initial value during the flood simulation, or if no difference in flow rate is observed in the inflow and outflow. At the end of each experiment, the pavement systems will be dismantled and the mass of accumulated sediment and pollutants will be determined. These measurements will provide an indication of the amount of biological accumulation that has occurred over time. The second experiment will be conducted in the same rig (after it is rebuilt using clean materials in the same way as shown in Figure 8, and after hydraulic conductivity tests are performed as above), by simulating varied inflows to mimic natural rainfall frequency. Five flow rates will be randomly applied, each representing 1/5 of the rainfall distribution curve for Brisbane. The drying cycles will also be simulated in between the wet periods using a drying lamp. Once again, an attempt will be made to mimic the distribution of Brisbane dry periods. Brisbane was selected since it undergoes more extreme drying/wetting cycles than Melbourne. The preparation of the rig, sample collection and sample analyses will commence upon the completion of the first experiment. B: Experiment with a real time scale. Only one experiment is envisaged to last over a year. After rebuilding the rig as shown in Figure 8, the front panel of the rig will be covered using a black plastic sheet (to prevent light from entering the system). Upon the completion of the hydraulic conductivity tests (as above), the natural wetting/drying cycles will be mimicked. The sampling will be done in the same way as above, but at smaller intervals. Floods will also be simulated to study clogging. At the end of the experiment, the material collected in the system will be analysed to determine the growth of biological material. Sample Analyses The chemical analyses of all the analytes sampled in this project were performed using the standard methods of the NATA accredited Water Studies Centre (WSC) Analytical Laboratory. The Lachat QuickChem 8000 Flow Injection Analyser was used to analyse the TP, TN, TDN, PON, TKN, NOx and FRP samples, while the Beckham Coulter LS100Q Laser Diffraction Particle Size Analyser was used to analyse PSD. TSS analyses were performed in the Hydraulics Laboratory of Civil Engineering using the standard methods of the NATA accredited WSC laboratory, while heavy metals were analysed using an Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) in the Australian Sustainable Industry Research Centre (ASIRC) of Monash University, Gippsland. Chlorophyll samples will be scraped off the surface of the pavements at monthly intervals and analysed in the WSC Analytical Laboratory using a UV/Vis spectrophotometer. To maintain the integrity and confidence level of the results obtained, quality control methods, with the exception of spikes were employed where appropriate. The WSC QA/QC methods used in this research project included the analysis of blanks and standard checks after every 10th sample, column efficiency, standard reference materials, duplicate as well as triplicate readings.

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5.1.3 Research Progress Experiment A1: The compressed time scale experiment with constant inflow rate and no drying commenced on 2nd Nov 2007. As the running of the experiment for a week is equivalent to one year of Melbourne rainfall at the 90th percentile or Brisbane 50th percentile rainfall, the experiment has since been in progress for almost fifteen weeks. The intensive daily sampling regime conducted in the initial two weeks has since been reduced to three separate composites per week. While the first composite is collected every Monday, Wednesday and Friday, the second composite is collected every Friday and Monday and they are both analysed for TSS, TP and TN. A third composite is collected every fortnight on Wednesdays and is analysed for TSS, TP, TN, TDN, PON, FRP, PSD as well as total and dissolved metals. To study the rate of clogging, a 1 in 5 year Brisbane storm (equivalent to more than a 1 in 100 year Melbourne storm) was simulated in the sixth week of the experiment to investigate the effects of a higher load of sediments and flow rate on the pavement systems. Another similar storm was also simulated after 10 years of operation, but was not completed due to pump failure. The pump has since been repaired, and the tests will be resumed in the coming weeks. Results on hydraulic performance Results on hydraulic conductivity of clean systems While all three bottom to top experiments were conducted successfully, it was only possible for valid top-to bottom experiments to be conducted for PA (i.e. ponding of constant depth only accrued on this pavement type during the experiment, and outflow rate was stable over time). It was not possible to perform top-to bottom experiments for HP and PP as the flow through capacities of both systems was higher than the capacities of their corresponding drainage systems. This was evident from the fact that ponding was never achieved on the surface of these two systems; instead water started to accumulate and pond just above the drainage pipes. The hydraulic conductivity results for both experiments are shown in Table 7, with clear indication that the values obtained for the top to bottom experiment was only valid for PA. It was encouraging that both experiments showed similar hydraulic conductivity values of c.a. 100 m/day for the PA surface. It was also clear that this system was limited by its surface, which was 10 times less porous than its sub-base. Therefore it may be hypothesised that clogging will occur on the surface itself of this system. On the contrary, HP was limited by its sub-base; while its pavers were highly porous (by a factor of more than 10). This was not surprising as the HP sub-base contains the geotextile layer, which acts as the choking element. It could be hypothesised that the geotextile will play a crucial role in subsequent clogging events. PP had a surface that was 4 times more porous than its sub-base. This was not quite expected since the PP surface and sub-base is constructed from aggregates of similar sizes. However, it may be possible that the compaction of stones during the installation process as well as their non-uniformity have caused the observed

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differences. Nevertheless, as there were no any other limiting layers, it may be hypothesised that clogging of this system will be the slowest. Table 7: The hydraulic conductivities of the overall system (ke), pavement (kp) and sub-base (kb) obtained for PA, HP and PP from the two experiments.

Bottom to Top Experiment Top to Bottom Experiment Pavement Type ke(m/d) kp(m/d) kb(m/d) ke(m/d) kp(m/d)

Average 544.3 101.7 1188.6 96.0 PA Std. dev. 15.9 8.9 110.6 4.2

S.E. 5.0 2.8 35.0 0.8   Average 564.9 5156.1 485.5 141.9*

HP Std. dev. 13.5 7573.4 12.0 2.5 S.E. 3.7 2100.5 3.3 0.55   Average 810.0 2888.8 734.5 190*

PP Std. dev. 6.9 24.5 6.2 0.88 S.E. 1.9 6.8 1.7 0.25

* the results are governed by the capacity of the rig drainage design, and are not an inherent consequence of the pavement systems. If we compare the three systems, it is clear that PP is the most porous system, although not overwhelmingly. The other two systems are of similar capacity. With regards to their surfaces, it appears that HP has the highest infiltration capacity (but this is only true at the initial stage). This is not surprising since water can seep freely through the gaps on the sides of the pavements. PP also has a highly porous surface while PA has considerably lower capacity (20-50 times less than the other two). The sub-bases of all three systems are of a similar capacity, with the railway ballast sub-base of PA being the most porous. Results from the top to bottom experiments are interesting in its own right. From Table 7, it appears that both HP and PP had overall porosity values that were approximately four times lower than that measured from the top to bottom experiment. These low values obtained were attributed to the inherent drainage design of this rig (constricted outlet) and were not representative of the maximum flow that could flow through the system. However these figures are far higher than a 1 in 100 year, 5 minute Brisbane event, leading us to conclude that these systems will easily cope with any major flood in most major Australian urban cities. It could therefore be concluded that all three systems, when in their “clean” state, can easily cope with the maximum 1 in 5 year Brisbane flood, as well as the maximum 1 in 100 year Melbourne flood. Results on clogging development In the first 5 weeks of running the 90th percentile Melbourne rainfall, manual flow measurements of the inflow and outflow showed no losses. However, upon the introduction of the 1 in 5 year Brisbane storm (the storm size of which is almost equivalent to that of a 1 in 100 year Melbourne flood) in the 6th week, a ponding layer of approximately 6 cm was observed, only for HP, above the geotextile layer. This ponding layer of 6 cm reached approximately half the height of the HP pavements, but

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did not reach the surface of the pavements. This observation was accompanied by pressure readings, which also increased slightly along with the introduction of the flood. This is clear evidence of clogging happening in the HP system, as hypothesised previously. However, no ponding or malfunction of the other two systems (PA and PP) was observed. It could be concluded that even after 5 years of continuous operation (without cleaning) under Melbourne climate conditions, both PA and PP will be able to cope with a 1 in 100 year event. The same PA and PP system under Brisbane conditions will also be able to cope with a 1 in 5 year event after 2.5 years of operation. However the modular HP system will start to experience some clogging problems in both climate conditions. The results from future ‘storm simulation’ tests should reveal more on the development of clogging over time. Results on Pollutant removal TSS - Prior to the introduction of the 1 in 5 year Brisbane storm, the inflow sediment concentrations for all three pavements ranged from 200 mg/L to 250 mg/L (Figure 19) but upon the introduction of the storm in the 6th week (6th year), the inflow concentration dropped down to 110 mg/L. This sudden drop in TSS concentration could be explained by the use of a different tank set-up, which may have inefficient aerators to maintain the sediments in suspension. After the storm however, the inflow concentration increased slightly to 150 mg/L and remained consistent, matching the desired pollutant load. Despite the variable inflow concentrations, the outflow sediment concentrations recorded for all three pavements up till today (12 years), have been less than 5 mg/L. This low concentration indicates a high removal rate of c.a. 100% (PA; 97%, HP; 98% and PP; 97%) and is of valuable importance from a stormwater reuse point of view. Thus, all three pavements are performing fairly well even after 12 years of operation. TP - At the start of the experiment, all three pavements had a removal rate of 97%. This was not surprising considering the pavements were all in their “clean and new” state. Within 2 weeks however, the removal rates for all three pavements dropped consistently from 97% to 70%. Throughout the duration of the experiment, the inflow and outflow concentration has maintained fairly steady around 0.55 mg/L and 0.17 mg/L each (Figure 20). A similar drop in inflow concentration with the introduction of the storm as observed for TSS was also seen in TP. After 12 weeks, the TP removal rates for PA, HP and PP have managed to remain fairly consistent at 64%, 75% and 71% respectively. As most phosphorus compounds are particulate bound, the removal of sediments would also remove most particulate forms of phosphorus, thus indicating that the three pavement systems are fairly efficient in removing not only TSS, but TP as well.