Waterfront Quarter, Huddersfieldwaterfronthuddersfield.co.uk/wp-content/themes/waterfront/documents... · Issue 2 April 2008 Document Verification Page 1 of 1 Job title Waterfront

Strategic Sites Waterfront Quarter, Huddersfield Level 2 Flood Risk Assessment Black

Strategic Sites Waterfront Quarter, Huddersfield Level 2 Flood Risk Assessment

April 2008

This report takes into account the

particular instructions and requirements

of our client.

It is not intended for and should not be

relied upon by any third party and no

responsibility is undertaken to any third

party

Ove Arup & Partners Ltd

New Oxford House, 30 Barkers Pool,

Sheffield S1 2HB

Tel +44 (0)114 272 8247 Fax +44 (0)114 275 9553 www.arup.com Job number 119046

J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC

Ove Arup & Partners LtdIssue 2 April 2008

Document Verification

Page 1 of 1

Job number Job title Waterfront Quarter, Huddersfield

119046

File reference Document title Level 2 Flood Risk Assessment

Document ref

Revision Date Filename

Description First draft

Prepared by Checked by Approved by

Name Richard Wren Ian Drabble Matthew Lovell

Draft 1 19/03/08

Signature

Filename HWQ 3.1 Level 2 Flood Risk Assessment.doc

Description Initial comments from the EA incorporated into the text including notes on requirement for access to the river bank for maintenance


Name Richard Wren Ian Drabble Matthew Lovell

Issue 02/04/08

Signature

Filename

Description


Name

Signature

Filename

Description


Name

Signature

Issue Document Verification with Document �

Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment


Ove Arup & Partners LtdIssue 2 April 2008

Contents

Page

1 Introduction 1

1.1 Site location and characteristics 1

1.2 The proposed development 1

1.3 Previous work 1

1.4 This commission 1

2 Risk of flooding on this development 3

2.1 Fluvial Flooding from the River Colne 3

2.2 Pluvial flooding 4

2.3 Flooding from the Huddersfield Narrow Canal 4

2.4 Flooding from groundwater 5

2.5 Existing flood defences 5

3 Implications for the proposed development 6

3.1 Flooding from the River Colne 6

3.2 Flooding from the Huddersfield Narrow Canal 7

3.3 Flooding from groundwater sources 7

3.4 Other constraints 7

4 Surface water management 9

4.1 General planning considerations 9

4.2 Existing management of surface water 9

4.3 Proposed management of surface water 10

4.4 Ground conditions – suitability for infiltration techniques 11

4.5 Use of attenuation systems 11

5 Conclusions and recommendations 12

Appendices

Appendix A

Current development proposals by DLG Architects (February 2008)

Appendix B

Environment Agency Flood Map for Waterfront Quarter and surrounding areas

Appendix C

Excerpts from Holme & Colne Flood Mapping Study Summary Sheets



Page 1 Ove Arup & Partners LtdIssue 2 April 2008

1 Introduction

1.1 Site location and characteristics

The Waterfront Quarter development is located approximately 0.5 km south of the centre of

Huddersfield at national grid reference SE 140160. The existing site has a total area of

approximately 30,395 m2 (3.04 Ha) and can be divided broadly into two long and narrow

strips of land.

The northern part of the site is occupied by Kirklees Metropolitan Council and comprises

mainly of offices. Located at an elevation of between 76 and 82 m AOD, this part of the site

is characterised by changes of level, retaining structures and steep slopes. Until the 1960s

this area was residential, with houses fronting onto Manchester Road and Dale Street.

The southern part of the site is occupied by Sellers Engineers Ltd, with a part of the site

sub-let to Acacia Timber Products. The area has historically been used for industrial

purposes and is currently used for manufacturing and storage, with a small office block

located in the south eastern corner of the site. This area is lower lying, at between 70 and

76m AOD, and is more gently graded than the northern part of the site.

The site is located on the northern bank of the River Colne. The Colne is classed as a

“main river”, which means that the Environment Agency (EA) has powers to maintain and

improve the watercourse under the Water Resources Act 1991. The Yorkshire Land Drainage

Byelaws 1980 will also apply.

The Huddersfield Narrow Canal, which was reinstated during 2000-2001, passes beneath

the site in a tunnel.

The site is predominantly covered by buildings and hardstanding. These impermeable

areas are positively drained by means of below ground gravity drainage systems.

1.2 The proposed development

A mixed use development is currently proposed, comprising of a major new development for

Huddersfield Technical College and seven other buildings, some for commercial use and

some for residential accommodation.

A major component of the masterplan for this development is the reinstatement of the

Huddersfield Narrow Canal at surface level. The existing tunnel structure will be opened up

and lock 3E will move downstream within the site. This will allow the canal to return broadly

to the original vertical alignment as it passes through the site, and will allow users of the site

to enjoy the proximity of the canal as well as the riverside.

The overall scheme architect is DLG Architects of Leeds. A plan showing the current

development layout is included in Appendix A.

1.3 Previous work

In January 2006 Arup issued a Preliminary Drainage Report. Reference to the Environment

Agency Flood Mapping available at that time stated that the site was located predominantly

in flood zone 1, i.e. the risk of flooding from the river is less than 1 in 1000 (or 0.1%) in any

year. It was noted that, given the statutory requirements in place at that time, no further

action was required.

1.4 This commission

Planning Policy Statement (PPS) 25, “Development & Flood Risk”, was published in

December 2006. PPS25 changes the framework within which flood risk is considered when

developing, or redeveloping, a site. It states that:




“Planning applications for development proposals of 1 hectare or greater in Flood Zone 1

and all proposals for new development located in Flood Zones 2 and 3 (see Table D.1,

Annex D) should be accompanied by a FRA. This should identify and assess the risks of

all forms of flooding to and from the development and demonstrate how these flood risks

will be managed, taking climate change into account. For major developments in Flood

Zone 1, the FRA should identify opportunities to reduce the probability and consequences

of flooding.”

Furthermore, the EA have procured new flood modelling studies for the River Colne

catchment, which have seen the boundaries of the flood zones redrawn. Inspection of the

current EA flood map for the area around the site (see Appendix B) shows that the EA

consider parts of this site adjacent to the river to be at risk of flooding.

Arup have been commissioned by Strategic Sites to provide a Level 2 Flood Risk

Assessment (FRA) report for the Waterfront Quarter development. This is a scoping study,

which aims to confirm all the sources of flooding that may affect the Waterfront Quarter

development site. In this document we will:

• Identify the pathways taken by floodwater in and around the site.

• Assess the vulnerability of the receptors, based on consideration of the site layout, land

uses and materials.

• Determine whether sufficient quantitative information is already available to complete an

assessment that is appropriate to the scale and nature of the risk.

• Take into account the impact of climate change as described in Annex B of PPS 25.

A Level 2 FRA will not involve any detailed hydrological or hydraulic modelling of the site,

but if necessary it will propose in outline the scope of any further investigation that is

required to complete the flood risk assessment process.




2 Risk of flooding on this development

2.1 Fluvial Flooding from the River Colne

The site is adjacent to the River Colne. The Colne is classed as a “main river” which means

that the Environment Agency (EA) have powers to maintain and improve the watercourse under

the Water Resources Act 1991. Responsibility for the river and river bank lies with the riparian

owner.

We made an enquiry to the EA in January 2008, requesting information relating to flooding

and flood risk at the development site. The response to this enquiry was received on 25

February 2008 and included the following information:

• Excerpt from the EA Flood Map - “Flood Zone Data For England” – showing the site

and surrounding area (see Appendix B)

• Extracts from the ISIS Hydraulic Model Summary Sheets produced as part of the “Colne

and Holme Flood Mapping Study” (see Appendix C).

The EA Flood Map defines areas as being within one of the following flood “zones”. (Note –

different criteria apply for areas subject to coastal flooding; these have been ignored in this

case.)

Zone 1 – Low Probability – This zone comprises land assessed as having a less than 1 in

1000 annual probability of river or sea flooding in any year (<0.1% Annual Exceedence

Probability, or AEP).

Zone 2 – Medium Probability – This zone comprises land assessed as having between a 1

in 100 and 1 in 1000 annual probability of river flooding in any year (1% – 0.1% AEP).

Zone 3 – High Probability – This zone comprises land assessed as having a 1 in 100 or

greater annual probability of river flooding in any year (>1% AEP).

Planning Policy Statement (PPS) 25, “Development and Flood Risk” provides guidance on

the types of development that are permitted in each of these flood zones. In order to

develop a site located within zones 2 or 3 for higher risk uses as defined in PPS25, it is

necessary to undertake the “sequential” and “exception” tests.

An initial review of the flood map shows that, broadly speaking, two small areas of the

Waterfront Quarter site are defined as being within flood zones 2 and 3.

• The area around the Sellers office buildings at the south eastern corner of the site.

• A strip along the bank of the river along the length of the site.

The information in the extracts from the hydraulic model has been considered in conjunction

with the topographical survey data for the site produced by Powers & Tiltman on behalf of

DLG Architects, to confirm the extents of zones 2 and 3 within the development area.

There are a number of control features in the river channel (e.g. weirs, bridges) and hence

there are a number of changes in water level along the length of the river. As a result we

have considered flood water levels at each of the modelled sections of the river.

2.1.1 Extent of flood zone 3

In Table 1 we have presented the modelled flood water level and the lowest surveyed

existing bank top level (as shown on the topographical survey). This relates to the area

near the cross section and may not be exactly at the cross section, thus allowing

consideration of low spots along the bank that may occur between section locations. These

levels do not take account of any walls that may be providing additional protection. The 1%

AEP water level data has been taken from the Environment Agency’s Colne & Holme Flood

Mapping Study ISIS Hydraulic Model (see Appendix C). Please note that the chainages

quoted are taken from the ISIS model – refer to Appendix C.




It can be seen that along the length of the river as it passes the Waterfront Quarter site, the

existing bank top is always higher than the 1% AEP water level. Therefore, the extent of

flood zone 3 should not extend beyond the top of the river bank into the development.

Chainage (m) Section description 1% AEP water

level (m AOD)

Lowest existing bank

top level (m AOD)

07369 Upstream face of Chapel

Hill highway bridge

70.434 70.48

07441 Immediately upstream of

lower weir

70.697 71.84

07468 Midway between weirs 70.633 72.48

07513 Immediately upstream of

upper weir

72.071 72.39

07630 72.003 74.04

07741 Upstream extent of

development

73.103 73.68

Table 1 – 1% AEP water levels (defining extent of flood zone 3)

2.1.2 Extent of flood zone 2

Comparison of the 0.1% AEP water levels given in the Flood Mapping Study with the

existing surveyed levels leads to the conclusion that the extent of flood zone 2 is broadly as

shown in the EA Flood Map (Appendix B). Development in this area should not include any

of the types defined in PPS25 as “highly vulnerable” unless the sequential and exception

tests have been applied.

2.2 Pluvial flooding

The site surface water drainage systems will be designed to current standards as defined in

BS EN 752 and other best practice guidance. The following minimum design standards will

be applied.

• No surcharge in pipework during the 1 in 2 year (50% AEP) event.

• No surface flooding (in pipework, chambers and balancing systems) during the 1 in 30

year (3.3% AEP) event.

• No flooding to buildings during the 1 in 100 year (1% AEP) event.

The development will, therefore, be protected from pluvial flooding up to and including a 1%

AEP event on the development site itself.

2.3 Flooding from the Huddersfield Narrow Canal

Upstream of the existing lock 3E at the western end of the development, the Huddersfield

Narrow Canal has a water level of approximately 73.25m AOD. The canal continues at this

level for some distance before the next change in level at lock 4E, upstream of Longroyd

Bridge. We note from the flood map (Appendix B) that in the area upstream of the

development site, flood zone 3 extends over and beyond the canal. We are not in

possession of surveyed level data to verify the extent of zone 3 beyond the limits of the

development. However, the profiles given on the Hydraulic Model Summary Sheets for

cross sections at chainage 07879 and 07946 (see Appendix C) indicate that the 1% AEP

water level crosses the towpath and would inundate the canal. This could generate a flow

path for floodwater along the canal and subsequently into the development.




It is noted that at the upstream (western) end of the development site, at chainage 07741,

the Hydraulic Model Summary Sheets indicate that the 1% AEP water level has fallen

significantly and is contained within the banks of the river.

We are not aware of any historical occurrences of flooding within the Waterfront Quarter site

as a result of water using the canal as a flood path. Currently, the canal passes in tunnel

beneath the development and this would provide an opportunity for any flood waters

entering the canal from the river to pass safely beneath the site with minimal impact to the

site.

2.4 Flooding from groundwater

We are not aware of any historical records of groundwater flooding on the Waterfront

Quarter site. Ground investigation activities in the site occupied by Kirklees MC, carried out

on behalf of the Huddersfield Technical College design team, found that the groundwater

level in cohesive deposits was between 1.0 – 2.5m below ground level. This implies that

groundwater may be encountered during activities involving deep excavations and design of

permanent cuttings may need to take account of the need to collect and discharge

groundwater.

The Geo-environmental Desk Study produced by Arup in December 2006 notes that the

sheet pile walls of the canal tunnel are likely to locally affect the groundwater regime. This

may be further exacerbated by raising the water level in the canal.

2.5 Existing flood defences

The Environment Agency does not maintain any flood defences in the area around the site.

The existing river bank levels provide protection in excess of the 1% AEP water levels.




3 Implications for the proposed development

3.1 Flooding from the River Colne

Building levels should be located above the water level occurring during the 1% annual

exceedence probability plus climate change (AEP+CC) event. For fluvial flooding, the

current guidance on modelling the allowance for climate change is given in Annex B of PPS

25. For a design horizon beyond 2025, PPS 25 recommends consideration of the effect of

an increase of 20% on the discharge in the watercourse. The modelled 1% AEP flow for the

River Colne adjacent to the development, taken from the Hydraulic Model Summary Sheets

provided by the EA, is 69.0 m3/s (see Appendix C). When an increase of 20% is applied this

equates to a discharge of 82.8 m3/s. The 0.1% AEP flow is 196.7 m

3/s.

The Hydraulic Model Summary Sheets give water levels for the 1% AEP and the 0.1% AEP

events. In order to estimate the water level during the 1% AEP+CC event, we have

interpolated between the water levels given for the 1% and 0.1% AEP events. Although

linear interpolation does not provide a precise assessment of the water level corresponding

to a particular discharge, this simple assumption provides the best estimate available in the

absence of modelled flows.

Chainage 1% AEP

water level

(m AOD)

0.1% AEP

water level

1% AEP+CC

flow (m3/s)

1% AEP+CC water

level

07369 70.434 72.821 82.8 70.691

07441 70.697 72.949 82.8 70.940

07468 70.633 72.954 82.8 70.884

07513 72.071 73.027 82.8 72.174

07630 72.003 73.071 82.8 72.118

07741 73.103 74.284 82.8 73.231

Table 2 – 1% AEP + climate change allowance water levels

The implications for the current proposed floor levels are as follows:

3.1.1 Basement car parking level beneath buildings L and M

The sections appropriate to this structure are at chainage 07369, 07441 and 07468. The

minimum proposed floor level is 71.000m AOD and the maximum 1% AEP+CC water level

is 70.940m AOD. A minimum freeboard of 60mm is therefore achieved above the 1%

AEP+CC water level.

The entrance to the basement car park should be designed so that rainwater runoff and

overland flood flows from external areas of the development do not flow into the basement.

3.1.2 Ground floor level in buildings A, B and C

The sections appropriate to this structure are at chainage 07513, 07630 and 07741. The

minimum proposed ground floor level is 73.350m AOD and the maximum 1% AEP+CC

water level is 73.231m AOD. A minimum freeboard of 119mm is therefore achieved above

the 1% AEP+CC water level.

3.1.3 Ground floor levels in all other buildings

All other buildings proposed on the Waterfront Quarter site have proposed ground floor

levels at or above 74.00m AOD. These buildings are, therefore, above the highest 1%

AEP+CC water level of 73.231m AOD.




3.2 Flooding from the Huddersfield Narrow Canal

As noted in section 2.3, the relative levels of the River Colne and the Huddersfield Narrow

Canal some distance upstream of the development site are such that a possible mechanism

exists for flood water to pass from the river into the canal during the 1% AEP event. This

mechanism could potentially result in a flood path along the canal and into the development.

Currently, the canal passes in tunnel beneath the site. However, a major component of the

masterplan for this development is the reinstatement of the Huddersfield Narrow Canal at

surface level. Lock 3E is to be relocated and the normal water level in the canal will

subsequently match that in the upstream reach, at approximately 73.25m AOD. Any

significant variations in the normal water level in the canal could, therefore, pose a risk of

flooding to the adjacent buildings and / or external areas and is thus undesirable.

As discussed in section 2.3, at the upstream (western) end of the development site the

hydraulic model summary sheet for chainage 07741 indicates that the 1% AEP water level

is contained within the banks of the river. The estimate in Table 2 above gives the 1%

AEP+CC water level at 73.231m AOD, which is below the normal water level in the canal

(approximately 73.25m AOD). This suggests that the provision of an overflow would allow

flood waters to pass back to the river, thus minimising the risk of flooding of the

development. This could be achieved by locally reducing ground levels between the canal

and the river, providing a preferential route for floodwaters. The details of these protection

measures, including the responsibilities for any maintenance, should be considered as part

of the design of the vertical realignment of the canal.

It is noted that the proposed minimum floor level for buildings A, B & C are 73.350m AOD

and 71.000m AOD for buildings L & M. The latter is below the normal water level in the

canal and the former is only marginally higher. Provided that the canal water level is

controlled by means of the protection measures described above to prevent the canal from

overtopping its banks, the risk of buildings being flooding from the canal will be minimised.

3.3 Flooding from groundwater sources

Shallow groundwater (at between 1.0 and 2.5m below existing ground level) has been

recorded on parts of this site. The basement/ground floor car parking levels beneath

buildings A, B & C and L & M could potentially be below the surrounding groundwater levels.

Further assessment is required to determine the groundwater levels in these locations and

to assess any potential implications of the increased water level in the canal. To establish

the groundwater regime in detail across the site, it is recommended that piezometers or

standpipes are installed within boreholes located across the site. These installations should

subsequently be monitored for changes in groundwater level over a representative period.

If possible, this monitoring should extend beyond such time as the canal is realigned to

monitor any changes in the groundwater level as a result of the realignment.

Depending on the results of these tests, it may be necessary to design the below ground

levels of affected buildings as waterproof structures and to provide appropriate drainage

systems.

3.4 Other constraints

The EA have stated that there should be no new buildings, structures (including gates, walls

and fences) or raised ground levels within 6 metres of the top of the river bank. This is to

allow access to the watercourse for maintenance and provide for overland flood flows.

Where existing buildings currently encroach on this “stand off” zone, but these buildings are

to be demolished and replaced, the EA would require that improved access is made

available.




An application for land drainage consent must be made to the EA for any development

taking place within 8 metres of the bank top. This process is subject to a statutory

consultation period of eight weeks.




4 Surface water management

4.1 General planning considerations

PPS 25 states that:

“Site layout and surface water drainage systems should cope with events that exceed the

design capacity of the system, so that excess water can be safely stored on or conveyed

from the site without adverse impacts.

“The surface water drainage arrangements for any development site should be such that

the volumes and peak flow rates of surface water leaving a developed site are no greater

than the rates prior to the proposed development, unless specific off-site arrangements

are made and result in the same net effect.

“For new development, it may be necessary to provide surface water storage and

infiltration to limit and reduce both the peak rate of discharge from the site and the total

volume discharged from the site.”

4.2 Existing management of surface water

The Baseline Drainage Report by Arup issued in March 2008 considers in detail the

implications of the management of surface water on the Waterfront Quarter site. It presents

the findings of a survey of the existing site drainage, carried out during December 2007,

which is used to determine the likely catchments and discharge capacities of the receiving

sewers or waterbodies. A summary of the conditions imposed by the various organisations

is given below.

4.2.1 British Waterways (BW)

Approximately 44% of the existing site discharges surface water to the canal. BW have

stated verbally that they would be prepared to accept unattenuated discharge from the

development to the canal. However, BW also expressed concerns about the capacity of the

overflow from the canal at Aspley Basin, approximately ½ mile downstream of the site. This

may place limits on the capacity of the canal to accept surface water and further discussions

are required to determine how this may impact on the permitted discharge from the site.

The Baseline Drainage Report assumed that the proposed discharge would not increase

beyond existing levels, which is in line with the second statement in section 4.1 above.

4.2.2 Yorkshire Water (YW)

Approximately 32.5% of the existing site discharges surface water to the public combined

sewer system in Chapel Hill. Yorkshire Water have stated that they will accept a discharge

of surface water to the existing combined sewers, restricted to the level of runoff from the

existing/previous use of the site. If it can be demonstrated by means of investigation that

the existing or previous use of the site has (or had) a surface water connection to sewer,

then YW would accept a discharge with the same rate of flow and/or contributing area.

4.2.3 Environment Agency (EA)

Approximately 6% of the existing site discharges surface water to the River Colne. Where

an existing site can be shown to drain to a watercourse, the EA have stated that they will

accept discharge of surface water from the redeveloped site to the watercourse subject to

the discharge being reduced by a minimum of 20% from the existing rate.

The EA have also stated that areas of the site not currently draining to a watercourse may

be allowed to discharge to the watercourse subject to discharge being attenuated to the

equivalent greenfield runoff rate for a 1 in 1 year rainfall event, which is considered to be 5

litres per second per hectare. The attenuation facility should be designed to accommodate

the discharge from a 1 in 30 year rainfall event. The discharge associated with a 1 in 100

year rainfall event should be retained within the site, without causing flooding to buildings. It




is noted that these requirements correlate well with the proposed performance standards

outlined in section 2.2.

4.2.4 Landscaped and other “permeable” areas

The remainder of the site area, approximately 17.5%, is landscaped or derelict and

therefore considered not to contribute to the rainfall runoff for the purposes of these studies.

4.3 Proposed management of surface water

4.3.1 Anticipated conditions on discharge of surface water

The Baseline Drainage Report has presented evidence and calculations to determine the

existing peak rates of discharge of surface water from the development site. These

calculations have been used to determine the permissible peak rate of discharge, based on

the requirements of the various authorities. The results are presented in Table 3 below. For

more information on this, and the derivation of the assumed rainfall intensity, please refer to

the Baseline Drainage Report.

Authority Conditions of discharge Conditions on

additional flows

Estimated permissible

discharge

YW (sewers) Like-for-like

(assumed 40mm/hr)

Not accepted 110 l/s

BW (canal) Like-for like

(assumed 50mm/hr)

Not accepted 185 l/s

EA (river) 20% reduction on existing

(assumed 87.5mm/hr)

Greenfield runoff

(max 5 l/s/Ha)

36 l/s

Anticipated permissible surface water discharge from

proposed development

331 l/s

Table 3 – Restrictions on surface water discharge, imposed by maintaining authorities

4.3.2 Surface water discharge generated by the proposed development

The overall site area, calculated using the proposed development plan in Appendix A,

(excluding existing off-site highways) is 30,395 m2. The proportion of the site area that is

proposed to be landscaped (permeable) is estimated to be 10%. This gives a total

proposed impermeable area of 27,356 m2, which is acknowledged to be greater than the

existing scenario.

The standard to which drainage systems are required to perform has become more onerous

over time. In particular, the EA have stated that the site must be designed so that buildings

do not flood in a 1 in 100 year storm event. When a site is redeveloped it is therefore

inevitable that it will be required to convey surface water more effectively than the

development it replaces.

The predicted surface water run off for the total impermeable area has been calculated

using the Rational Method, with a rainfall intensity of 75mm/hr. We estimate that the peak

discharge from the site will be as follows:

Q = 27,356 x (1/3600) x 75

= 570 litres per second

It can be seen that the proposed development is predicted to generate a significantly greater

peak discharge of surface water than the limits that may be imposed by the various

authorities. Additional discharges from this site will be severely restricted. In order to

reduce the peak discharge to acceptable levels it will be necessary to provide some form of

on-site attenuation, or use sustainable drainage systems, or a combination of these

techniques. The volume of attenuation required can only be calculated once a detailed




hydraulic model of the proposed network has been built, and the magnitudes of the

allowable discharges have been agreed with the controlling authorities.

Refer to the Baseline Drainage Report for recommendations relating to the permissible peak

surface water discharge from individual plots and the suggested connection/outfall locations.

4.4 Ground conditions – suitability for infiltration techniques

A ground investigation was carried out by Soil Mechanics on behalf of Huddersfield

Technical College (HTC) and WML Consulting during June 2007. This investigation

focussed on the proposed HTC plot, in the area currently occupied by KMC.

Four water inflow tests were performed in boreholes to examine the potential for use of

infiltration techniques. More detail relating to the analysis of the results is presented in the

Baseline Drainage Report. The analysis found that the permeability of the cohesive

deposits and underlying bedrock was approximately 4 x 10-6

m/s or less. For infiltration

techniques to be considered viable, the required minimum permeability is generally in the

region of 1 x 10-5

m/s.

Measurements taken in the cohesive deposits also found groundwater levels at between 1.0

and 2.5m below ground. This occurrence of shallow groundwater, when considered with the

relatively poor permeability values, indicates that infiltration is unlikely to be suitable within

the HTC site. This rules out the use of a number of sustainable drainage techniques.

Investigation of existing ground conditions throughout the remainder of the site has not yet

been undertaken. Further tests to identify the permeability should be carried out as part of

the wider ground investigation to establish if and where surface water infiltration is possible.

4.5 Use of attenuation systems

Given the likely increase in the total peak surface water discharge from the development,

the requirement to limit the peak discharge and the poor permeability of the underlying soils

and rock, it is likely that some form of attenuation will be required. This could be provided

on a plot-by-plot basis or by means of one or more attenuation tanks serving the wider

development.




5 Conclusions and recommendations

The Environment Agency (EA) have provided us with an excerpt from mapping of “Flood

Zone Data For England” and extracts from the ISIS Hydraulic Model Summary Sheets

produced as part of the “Colne and Holme Flood Mapping Study”. This information has

allowed us to make an assessment of the risk of flooding to this development based on

actual surveyed levels on the site.

The Waterfront Quarter development site is located entirely within flood zones 1 and 2 as

defined by Planning Policy Statement (PPS) 25. Parts of the site adjacent to the river are

designated as flood zone 2 – that is, the risk of flooding from the river in any year is between

1 in 100 (1% annual exceedence probability, or AEP) and 1 in 1000 (0.1% AEP). These

areas can be developed for all uses except those defined in PPS 25 as “highly vulnerable”,

whereupon the sequential and exception tests must be considered. It is not currently

proposed to locate development types classed as “highly vulnerable” within these areas.

The remainder of the site is located in zone 1 and it is therefore development of any type is

permitted under the guidance given by PPS 25.

Water levels have been estimated for the 1% AEP plus climate change (+CC) flood event,

using data supplied by the EA. This report has recommended that floor levels in the

proposed development be located at or above these levels.

To minimise the risk of flood water entering the site along the canal during extreme events,

the provision of some form of protection should be incorporated as part of the design of the

vertical realignment of the canal. This could comprise of a flood gate and overflow from the

canal to the river at the upstream extent of the development, or other protection measures.

The basement car parking levels beneath buildings A, B & C and L & M could potentially be

below the groundwater level, particularly if groundwater is affected by the vertical

realignment of the canal. Further assessment is required to determine the groundwater

level in these locations and to assess any potential implications of the increased water level

in the canal. Depending on the results of these investigations, it may be necessary to

design the below ground levels of affected buildings as waterproof structures and to provide

appropriate drainage systems.

The surface water drainage systems on this development will be managed so as to not

increase the peak discharge of surface water to any of the existing receiving watercourses

or other drainage assets, when compared to the existing scenario. In some cases a

reduction in the peak discharge will be achieved. The initial ground investigation studies

have determined that the ground is not suitable for the use of infiltration techniques

commonly associated with sustainable drainage systems. Other techniques, including

below ground attenuation, will be required to achieve the discharge conditions.

The management of surface water on the site will be such that the buildings will not flood

during a 1% AEP (1 in 100 year) rainfall event.

Appendix A

Current development proposals by DLG Architects (February 2008)

Appendix B

Environment Agency Flood Map for Waterfront Quarter and surrounding areas

Appendix C

Excerpts from Holme & Colne Flood Mapping Study Summary Sheets

Documents

Waterfront Quarter, Huddersfieldwaterfronthuddersfield.co.uk/wp-content/themes/waterfront/documents... · Issue 2 April 2008 Document Verification Page 1 of 1 Job title Waterfront