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Modelling the management of street surfacesediments in urban runoffModélisation de la gestion des dépôts sur les voirieset de son impact sur les rejets urbains

Ana Deletic (**), Cedo Maksimovic (*) , Fouad Loughreit (*),David Butler (*)

(**) Dept of Engineering, University of Aberdeen,Aberdeen AB9 2UE, UK, [email protected]

(*) Dept of Civil Eng’g Imperial College of Science Technology andMedicine, London SW7 2BU, UK & ∞CUW-UK,[email protected] , [email protected], [email protected],

SUMMARYContrary to common wisdom, water quality of runoff is comparable in certain as-pects with that of sewage. Although the regular sweeping of streets is by far themost common form of (indirect) pollution abatement practised, little is known aboutits real effect on pollution control. The paper presents the initial results of a studywhich aims to assess the influence of the street sweeping during dry weather(various devices and management techniques) on mass of solids wash-off bystorms. This mode is backed by an advanced model of suspended solids wash-offwhich was developed and tested successfully for two small urban catchments [3,13]. A new block for street sweeping efficiency was recently developed and addedto the existing wash-off model. The performance of several different street sweepingequipment were taken from the Sutherland’s work [12] and have been analysedfor their long term efficiency in sweeping small particles during dry weather, whichwould otherwise be eroded during wash-off. The results obtained by modellingthe combined effect of sweeping and wash-off are used for the assessment of thepossible sweeping scenaria in full scale applications. The first results presented inthis paper indicate that there is a need for development of an appropriate mana-gement procedure in which the pollution of receiving water would be reduced bymore prudent combination of sweeping and cleaning of gully pots.

RESUMELa qualité des eaux de pluies est comparable à celle des réseaux d’égout, et plu-sieurs techniques sont alors utilisées pour améliorer sa qualité. Le balayage desrues est la technique la plus répandue bien que son rôle dans le contrôle de laqualité des eaux soit encore mal connu. Ce papier présente les premiers résultatsd’une étude qui tend à évaluer les effets du balayage des rues par temps sec (dif-férents équipements et différentes techniques) sur les masses solides transpor-tées par les eaux de pluie. Basée sur un modèle de ruissellement et de transportsolide en suspension, ce modèle est développé et validé avec succès pour deuxsites urbains [3, 13]. Récemment un nouveau module traitant l’efficacité du ba-layage des rues a été créé et ajouté au modèle existant. La performance des diffé-rents équipements de balayage est issue du travail de Sutherland [12] et leur effi-cacité est analysée pour les particules fines. Les résultats obtenus suite à la modé-lisation des deux effets: le balayage et le transport, sont utilisées. Les premiersrésultats montrent la nécessité du développement d’une procédure de gestion ap-propriée dans laquelle le réduction de la pollution des eaux de pluie doit être vueavec prudence quant à la combinaison du balayage et du curage des regards.

KEYWORDSStreet Sweeping, Sediment wash-off, Equipment effiency, Management tool

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1. INTRODUCTION

The pollution problems of urban runoff are gradually being recognised in the UK[5] and the importance of environmental control of polluted storm waters highlightedby British Environmental Protection Agencies [6]. Currently, the most commoncontrol measures of storm runoff pollution are traditional street and gully-pot cleaningmethods, such as: manual, mechanical or vacuum street sweeping, street flushingand washing, and gully emptying. Butler and Clark’s survey showed [2], despite itsvery common practice that there is little evidence on the efficiency of these methods,because no comprehensive scientific studies have been carried out in the UK sofar. Results in the USA have revealed the weakness of existing methodologies inpredicting and quantifying storm runoff pollution in a way that would enabledevelopment of a rugged operational management tool suitable for application indaily practice. The efficiency of the various street sweeping methods has beensomewhat studied in the connection with build-up and wash-off of pollutants.

The first street cleaning and pollutant source identification projects were carriedout by USA EPA in the mid 1970s [8, 10, 11]. They were extended throughout theeighties within Nationwide Urban Runoff Program (NURP), in which over $ 30million was spent in the intensive investigation at 28 urban locations throughoutthe United States [4]. The main conclusion from these studies was that the streetsweeping is generally ineffective as a technique for improving the quality of urbanrunoff. They found that street sweepers are very effective in removing litter andlarge debris, but do not reduce the event mean concentrations (EMC) of mainpollutants in storm runoff. It was also found that the efficiency of street sweepingdepends on type of equipment, as well as on type and condition of the surface.However, very recent papers published in the USA and Canada suggest that streetsweeping programs can be optimised to significantly reduce pollutant input intodrainage systems [9, 12]. They argue that several newly developed sweepers canreduce annual total suspended solids from paved areas by up to 80%. Few studieson street sweeping efficiency exist in Europe [2, 7]. These studies only highlightthe problem and emphasise the needs for further research, since the efficiencyfor the sweeping equipment which is used in practice is not properly studied.

In this paper a first step is presented towards creation of comprehensive manage-ment rules for more rational street sweeping.

2. ASSUMPTIONS AND WASH-OFF MODELLING CONDITIONSThe procedure for modelling of sweeping is based on the assumption that therelease of sediment from paved areas is properly represented by the modeldeveloped by Deletic (nee Tomanovic) et al. [3, 13]. The model contains two majorblocks: (1) the model of solids build-up at an impervious surface, and (2) themodel of suspended solids wash-off from an impervious surface. These processesare simulated continuously for a series of rainfall events.

The model assesses only fine sediment particles of median diameter, d50=100 mm,because it is observed median diameter of particles that enter a typical roadside gullyduring storms [3]. This is a serious drawback of the model, however it is not difficult toadapt the model for assessment of sediment with complex particle distribution.

Solids build-up at impervious surfaces

Sartor and Boyd [10, 11] suggested an exponential relationship between theamount of solids available on the surface, M, and the duration of antecedent dryweather period, tdry. This equation was adopted in the model (Fig. 1):

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M(T) - M0 (1-e-k(tdry+t')) (1)

where M [g/m2] is the amount of solids available on the surface, T [day] is the timeelapsed from the start of the first rainfall in the series, tdry [day] is the duration ofantecedent dry weather period, and t’ [day] is the virtual time. The virtual time iscalculated by assuming that deposition is zero at t’ days before the start of theantecedent rainfall, as indicated in Fig. 1. There are two calibration parameters inEq. 1 which have been determined for each particular catchment; M0 [g/m2] whichis the maximum amount of solids expected at the surface, and k [day-1] the accu-mulation constant.

A spatial distribution of solids is modelled, based on records from the literature [7,8, 10, 11], a different from prior models, which all assume that sediment isdistributed evenly over the street surface.

Suspended solids wash-off from impervious surfaces

The solids wash-off model contains the following sub-blocks: 1. overland flow; 2.solids entrainment; 3. suspended solids transport by overland flow. One dimensionalapproach was adopted. Overland flow is modelled using kinematic wave equation,which has been used before for the modelling of surface runoff. Solids entrainmentis assessed by a new method, developed by the first author, which considersindependently rainfall and overland flow effects on amount of material lifted fromthe surface [3, 13]. Rainfall effect is assessed by means of the kinetic energy ofrain drops, while the effect of flow is expressed by shear stress. One calibrationcoefficient is needed for this method. However, according to tests carried out sofar, this calibration coefficient has the same value for similar paved surfaces (itdoes not depend on other catchment characteristics) .

Suspended solids transport by overland flow is modelled bytransport equations written for road surface and gutter respectively.

3. COUPLING OF STREET SWEEPING WITH WASH-OFF MODELLING

In order to model the combined effect of wash-off and sweeping, a new block forstreet sweeping modelling was developed and added to the existing wash-off model.The emphasis have been placed on modelling the small diameter particles (d50 ª100mm) to which most of the pollutants tend to be attached.

In the sweeping module of the model it is assumed that:

a) The amount of material removed by sweeping is assessed according to Equa-tion 2 (see below);

b) Sweeping is performed by each of the five types of equipment tested by Suther-land as defined below;


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c) Sweeping is carried out during dry weather periods only at regular time intervalswhich were varied between one per day to one per month;

d) The solid material left on the surface after sweeping or wash-off is used as aninitial value for the next period.

The amount of removed material by sweeping, Msweeped [g/m2] is calculatedusing the following equation [12]:

Msweeped = Eff (Mini - Mmin) for Mini > Mmin (2)

where Eff is efficiency of the sweeping method, Mini [g/m2] is initial amount ofmaterial before sweeping, Mmin [g/m2] is base residual. The base residual is amountof material which, in any case, can not be removed from the surface by the usedsweeping method. Eff and Mmin depend on the equipment type and sweepingmethod. In the absence of the original data on performance of street sweepers,the values for these coefficients were taken from Sutherland’s work [12] for thetested equipment and fine particles (63-125mm).

The equipment tested by Sutherland, and then modelled in this work, ranges frommechanical sweepers to newly developed vacuum-assisted dry sweepers whichare claimed to have high potential in picking-up the small diameter particles (allmanufactured in the USA). Five type of equipment are compared;

• Equip. 1: Older Mechanical Sweepers: broom sweepers such as presented inthe Nationwide Urban Runoff Program [4],

• Equip. 2: Newer Mechanical Sweeper: broom and conveyor belt,

• Equip. 3 Tandem Sweeping: involves two successive cleaning passes, first by amechanical sweeper, then immediately followed by a vacuum-assisted sweeper,

• Equip. 4: Regenerative Air Sweeper: blow air into the pavement and immediatelyvacuum it back in order to entrain and filter out accumulated sediments,

• Equip. 5: Enviro-Whirl: stand alone use of a new, highly effective, vacuum-assisteddry sweeper.

4. RESULTS AND DISCUSSION OF MODELLING THE COMBINEDEFFECTS OF WASH-OFF AND SWEEPING

The model has been applied to the experimental catchment Miljakovac in Bel-grade (Fig. 2) for which continuous rainfall, runoff and water quality data havebeen collected [3, 13]. The catchment covers a part of an asphalt street fromwhich runoff water flows into a single road-side inlet. No sweeping was recordedduring the measuring period (from May to November 1993).

Figure 2: The experimental catchment Miljakovac, Belgrade


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Modelling was performed by continuous simulation of sediment build-up betweensuccessive storms, wash-off during each of the 52 recorded storm events, andsweeping at regular time intervals.

From the results obtained it was concluded that there are two main parametersthat effect sweeping efficiency; (1) equipment type, and (2) sweeping frequency.These parameters are closely interconnected.

The variation of the amount of solid material on the surface left over after sweepingand wash-off is presented in Figure 3, for sweeping every day by Equip. 1 (on theleft), and by Equip. 3 (on the right). For very frequent sweeping the amount ofmaterial on the surface oscillates around one constant figure which is different foreach type of equipment (for example for Equip. 3 it oscillates around 0.44 g/m2).This figure is strongly related to parameter Mmin. It was also observed that moresediment is left on the surface after using Equip. 1 than any other type. This iseven more pronounced in the mass curves of the material washed-off and sweptby different types of equipment but with the same sweeping frequency (Fig. 4).


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Figure 6 : Reduction of seasonal wash-offfor different equipment types (% ofMeasures)

Results obtained by integration of themass of solid material washed-off andswept during the modelled season(Fig. 5) are used for study of the in-fluence of the sweeping frequency onoverall sweeping performance. Thesweeping efficiency is also calculatedfor each equipment type, as thedifference between measuredsediment wash-off during whole

season and simulated wash-off while sweeping with the chosen equipment. In Fig6 the sweeping efficiency is presented against sweeping frequency for all equipmenttypes. From these graphs (Fig. 5 and Fig. 6) it was noticed that sweeping efficiencydepends exponentially on sweeping frequency. The efficiency increasesinsignificantly when the number of cleanings exceeds twice per week. This couldimply that investing money in sweeping on a very frequent base (every day) wouldnot result in much better pollution control.

From Figure 6 it is clear that old mechanical sweepers are the least efficient, whilethere is no big difference in performances of equipment type 3, 4 and 5.

The advantage of more advanced (more efficient) sweeping equipment is morepronounced when the sweeping frequency is higher, as Figure 7 shows. This meansthat if a surface is to be cleaned on a very infrequent base, there is no need ininvesting in very expensive, high-tech equipment. This is even more pronouncedin Figure 8, which presents the ratio between the amount of material swept andwashed-off for each sweeping frequency. From this graph it is obvious that thereis no need to use expensive Equip. 5 if sweeping is going to be less frequent thantwice per week.


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Figure 8: Ratio between the total material swept and washed of during the model season

The methodology applied and the results obtained in the present study, can serveas a tool for further analysis on other locations for the evaluation of the pollutionabatement and economical consequences of street sweeping method, providedthat the efficiency of the used equipment is known. At the present stage ofdevelopment, the model can be used for small sample areas, the result of whichcan be extrapolated to the ones of the similar topological and hydrological condi-tions. These can be used for planning of operational management of streetsweeping as a complementary measure in reduction of pollution of receiving wa-ters by surface runoff.

5. CONCLUSIONS

The following conclusions can be drawn based on the results obtained by coupledmodelling of wash-off and sweeping process.

1. Physically based wash-off process modelling [3, 13] allows the realistic im-pact of storm characteristics to be taken into account and, enables quantifica-tion of the effect of street sweeping provided that the efficiency characteristicsof the sweepers are known for each of solid material fraction.

2. The efficiency of sweepers, depending on the method by which the initiationof motion and picking up of solid material is performed as well as sweepingfrequency plays a crucial role in the overall amount of the material being sweptof washed-off.

3. The modelling procedure enables operational management strategies in pol-lution management to be assessed in a quantitative fashion.

4. For a wider application of the method presented here, it is essential that:

• the performance characteristics of the equipment are known for the fractionsbetween 10-250 mm

• advanced sweepers technology further improved.

6. REFERENCES

1. Bertrand-Krajewski J. L., Scrivener O., Briat P. (1993). Sewer Sediment Pro-duction and Transport Modelling: A Literature Review, Journal of HydraulicResearch, 31(4), 435-460

2. Butler D, and Clark P. B., (1995). Sediment Management in Urban Catchments,Report RP134, CIRIA (London).


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3. Deletic A., C. Maksimovic and M. Ivetic, (1997). Modelling of Storm wash-off ofSuspended Solids from Impervious Areas, J. of Hydraulic Research (IAHR),35 (1), 99-117.

4. EPA (USA Environmetal Protection Agency), (1983), Final Report fromNationwide Urban Runoff Program, Water Planning Division, Washington, D.C.

5. Forth River Purification Board, (1994). A Clear Future in the Forth Catchment,Edinburgh

6. Forth River Purification Board, (1995). A Guide to Surface Water Best Mana-gement Practice, Edinburgh

7. Grottker M., (1987). Runoff Quality from a Street with Medium Traffic Loading,J. of the Science of the Total Environment, 59, 457-466.

8. Pitt R., (1979). Demonstration of Nonpoint Pollution Abatement ThroughImproved Street Cleaning Practical, U.S.A. Environ. Prot. Agen. Rep., EPA-600/2-79-161.

9. Pitt R. and Dee P.E., (1996) Unique Features of the Source Loading and Ma-nagement Model (SLAMM), Proceeding of Stormwater Management ModelingConference, Toronto, Canada.

10. Sartor J. D., Boyd G. B., (1972). Water Pollution Aspects of Street SurfaceContaminants, USA. EPA Report, EPA-R2-72-081.

11. Sator J. D., Boyd G. B., Agardy F.J., (1974). Water Pollution Aspects of StreetSurface Contaminants, Journal WPCF, 46(3), 458-467

12. Sutherland R. C., Jelen S. L., (1996), Contrary to Conventional Wisdom: StreetSweepers can be an Effective BMP, Advances in Modelling the Managementof Stormwater Impacts - Volume 5, CHI, Canada.

13. Tomanovic A., Maksimovic C., (1996). Improved Modelling of Suspended SolidsDischarge from Asphalt Surfaces During Storm Event, Water Science andTechnology, 33(4-5), 363-369.

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