PH: S0273-1223(96)00554-9

• Pergamon . Waf. Sci. Tech. Vol. 34, No. 3-4, pp. 41-48.1996.Copynght © 1996 IAWQ. Published by Elsevier Science Ltd

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NEW CRITERIA FOR STORMWATERDISCHARGES INTO URBAN STREAMS

Andreas Wagner and Wolfgang F. Geiger

Institute ofEnvironmental Engineering, University ofEssen, Universitiitsstraj3e }5,45114 Essen, Germany

ABSTRACT

The Gennan state of the art for designing stonnwater discharges and combined sewer retention tanks isemission oriented. This approach does not adequately reflect the needs of urban brooks. Literature on thetopic shows the need for further research investigations. Firstly, a detailed study is used to define thehydraulic impact of discharges and overflows on an urban stream. This paper attempts to relate the hydraulicimpact in the fonn of shear stress with substrate movement. The correlation between the extent ofstonnwater impacts and ecomorphological properties is considered to be the significant parameter forevaluation of runoff. Finally a new approach for assessing stonnwater discharges into urban brooks ispresented. This approach is divided into hydraulic and pollutant components and takes the ecomorphologicalproperties into consideration. Copyright @ 1996 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS

Combined sewer overflow; environmental impact assessment; receiving water; stormwater discharge; streambed erosion; water quality impact.

INTRODUCTION

In densely populated areas, streams have to satisfy a wide range of requirements. The use of urban streamsas receiving waters for stormwater discharges is incompatible with recreational and ecological requirements.In the past the first objective of urban drainage was to ensure the quick and safe disposal of waste- andstonnwater. If the capacity of urban streams was insufficient, they were straightened and the cross-sectionsenlarged. In recent years however, urban streams have been expected to fulfil recreational needs and meetecological standards. The procedures for designing overflows and discharges as well as for retention basinsare not sufficient to meet water quality objectives for brooks (Geiger et ai" 1994). Most urban streams aresmall waters with a low self-purification capacity and with a plurality of urban runoff sites. Stormwatermanagement has to develop new approaches to ensure the objectives of storm runoff and water protectionare met.

POSSffiLE APPROACHES FOR EVALUATION OF URBANSTORMWATER DISCHARGES

At present in Germany for combined sewer overflows only a guideline exists, which aims to size necessarydetention volumes (ATV, 1992). Although the goal of the guideline is based on COD load calculations, the

41

42 A WAGNER and W F GEIGER

procedures in principle are emission based. This paper aims to develop emIssIon parameters reflectingecological requirements in design procedures.

A working group of the ATV proposed simple screening techniques to quantify limiting values. These havebeen developed to ensure water quality standards for pollutants and hydraulic impacts. Critical values, whichdepend on the velocity, depth and pH-value in the water body, are developed for the quotient of thepopulation equivalent and the average low water flow. Keeping below the critical values should reduce thematerial impacts (ATV, 1993). The quotient of drainage area and river basin is used for a basic evaluation ofhydraulic impacts (Krier, 1995). A comparable guideline developed at Dundee Institute of Technology forassessing CSOs uses a similar quotient called "Drainage Area Ratio" as a value for dilution. The guidelines,which are in a preliminary stage, will simplify decisions as to whether improvements to a particular over•flow are necessary (Jefferies et ai., 1993).

A more detailed approach is described by Warg (1991), who evaluates the single runoff event. An impactvalue is assessed to be the product of functions containing the significant parameters such as duration of theevent or increasing NH4-N concentration in the water body. The difficulty with this approach is to estimatedeterministic functions for the parameters, which up till now have only empirical functions as their basis.The approach can be used in combination with stormwater modelling; a matrix of impact values and numberof occurring runoff events can be used to compare design variants.

The present approaches emphasise the need for further research investigations especially in the field ofhydraulic impacts for CSOs. Several monitoring programs have been carried out over recent years (Winterand Borchardt, 1989; Pitt, 1991; Krejci et ai., 1993; Lamrnersen, 1993; Fuchs et ai., 1994; Jokiel et ai.,1994), but since each has dealt with a different type of watercourse, the monitored effects are site specific.

INVESTIGAnON ON THE SCHONDELLE BROOK

Urban brooks originating within the urban area itself are used at source for discharge of stormwater. Such acase has been investigated at the Schondelle in Dortmund, from where this paper reports on first findings.

Figure 1. River basin and drainage area.

Characterisation of the Brook and the Monitoring Station

The Sc~ondelle creek is a small urban stream with a total length of 5.2 kIn and a catchment area of 1175 halocated In the urban area of Dortmund (Fig. 1). The morphology is characterised by a varying stream profile,

Stormwater discharges into urban streams 43

a meandering course, and riffles and pools. Three CSOs discharge into the stream; the headwater areasconsist of two natural springs and two sources, which are formed by separate sewer runoffs. During dryweather conditions the average low water flow is about 10 lis in summer and 70 lis in winter; whereas thedischarges rise to 400 lis during wet weather, caused by the separate sewer runoffs.

Since January 1992 the Institute for Environmental Engineering at the University of Essen has operated agauging station to monitor the impacts of the first overflow in the sewer system of a residential area with10,865 inhabitants and 61.45 ha built-up area. The throttle has been dimensioned for a critical rainfall persecond and area of 12.8 lI(s*ha).

The gauging station comprised three monitoring points. The location of the first monitoring point was 70 mupstream from the overflow; at this point the water quality was influenced by the CSO. The secondmonitoring point downstream from the overflow recorded the direct impact of the CSO. To capture theretention of the hydraulic wave and the longitudinal dispersion, the third monitoring point was established350 m downstream from the overflow.

Structure of the monitoring station "Schondelle"

DIIa proceulng

I~egendI I Transducer box

Dissolved oxygen

I~..=pH·vaIue

I ~ Ul1raaound meter

~ InducllYe flow meier

[] Rain gauge

['lEI Mea.uring ampllfter

Ia fmpulae oountar

~ Sampler

AnaIoga data

D1g11a1 data

Contrallmpulae

Remole contnlla

Figure 2. Structure of the monitoring station.

With the help of two rectangular measuring weirs and two inductive flow meters, the hydraulic conditionswere defined. Sensors for dissolved oxygen, temperature, pH-value and conductivity were installed in thewater body at each measuring point. The automatic samplers incorporated a cooling system to store thesamples till they were transported to the laboratory. After monitoring 37 runoff events over 21 months, therange of parameters analysed was reduced to the following significant parameters:

Five days biochemical oxygen demand (BODs)Chemical oxygen demand (COD)Arnmonia-N (NH4-N)Nitrite-N (NOrN)Nitrate-N (NOrN)Total-NPhosphate-P (dissolved P04-P, particulate Ppart.-P)

The data logging saved all values at five minute intervals. At the start of a runoff event the automaticsamplers were triggered and the interval for data saving was reduced to 30 seconds.

44 A. WAGNER and W. F. GEIGER

-- Mean daily flow in the brook

Impacts of the CSO on the Schondelle Brook

Between 1 November 1992 and 31 October 1995, 59 runoff events were monitored. Different weath~rconditions occurred in the monitored hydrological years with an annual height of precipitation of 986 rnm,In1993, 830 mm in 1994, and 864 mm in 1995. In addition, 27 runoff events occurred in 1993 comp~ed wIth14 in 1994 and 18 in 1995. Nevertheless, figure 3 shows an accumulation of maximum flood dIschargesduring spring and summer and a longer runoff duration with lower flood discharge:s in winter. These .effectscorrelate to the seasonal weather conditions of Central Europe with thunderstorms In summer and penods oflonger rainfall with lower intensity during winter. A comparison with a ~ive-year monitoring of a CSO inMunich-Har1aching (Geiger, 1984) and the results at Maisenbach brook In the Black Forest (Fuchs et al.,

1994) confirms these trends.

r~=============,,"__2~5~16~7~0~------12500 ,I

: I• Maximum esa overflow discharge • I

2000 ~c:::~============--'--~--I.-I--------'.'--~-___1I

•

.. • 4. ..

. :l' ~lIJ\,: ~;~ ~_M_

VI 1500 . I=.IIIe>nl •.c0 • :

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500 •.~ "~

..•

0 -l-~

01.11.92 31.10.93

•I

1,

I' •

31.10.94

--~--

... •.

.•

I•

31.10.95

Date

Figure 3. Annual discharge curve and discharge peaks of run off events.

The sudden variation in the mean daily flow of the Schondelle brook was caused by the two separate seweroutlets in the headwater. They were responsible for the hydraulic flood routing in the winter period becauseof the lack of storage in the drainage areas. The combination of separate sewer outlets and combined seweroverflows caused a continuous disturbance over the year.

The two-dimensional hydrodynamic model RMA 4.1 (Ruland and Rouve, 1992) has been used to quantifythe hydraulic impacts on the Schondelle brook. The longitudinal distribution of shear stress was calculatedfor discharges from 70 lis to 3000 lis. Up to 800 lis sediment movement occured only in some reaches. Themacroinvertebrates could find shelter in the unmoved sediment, when the shear stress was below 15 N/rn2.

The peaks of shear stress during runoff events reached much higher values of up to 70 N/m2 at single pointson the river bed.

Figure 4. Sediment trap.

Stonnwater discharges into urban streams 45

For the evaluation of stream erosion, sediment traps (figure 4) with a volume of five litres were installed atseveral points on the stream bottom. The total weight and the grain size distribution were determined in thelaboratory by wet sieving.

0, 0,

I IIIIII1

-I-I-H+I-+·I- •I IIIIIII

_ 1_LUl.t.i.l __I IIIIIII

I IIIIII1-I-I-,-.T,T,- -

t I lilt/I-t-r-I,TITr- •

I I II till-1- .... '-l .. t+l __

I 1IIItii_,_ 1_1_11111 __

I 1111111I 1111111

-1-j-nT,T,- -I 1111111

-1- t-1""1 TIT"I- -

I t 111111_1_ L-I...J J.IJ.I __

I 1111111, , ,0, 0, I 0

I IIIIII1----1-1-4 HI-+--

I 1111111___ .J_I..,J Ul.l __

I IIIIII1

I IIIIII1- - - I -'-I I (1'1 - •

I I IIIIII---'-1-' 1'/1-•

I I 1111/'--- .... -1-l1414. __

I 1111111__ !J_IJ U'J. __

I 1'11111I I I I 1111

---1-1111111--I 11111'1

---;-1""1 Hh"-•I I 1IIIlt

___ ..J_I...J UU __

I 1111111I III lilt

100

l 10•~ 10"2

J 70

10

50

40

30

20

10

00,

Silt Sand Gravel

line coarse line medium coarse fine meillum coarse

I L~ Location~=--=--=-1~G;;:ra:::v:::e:01 tra=p:-:-u=p::;str=e=em=th:::e"":o-::ut"'leC;-t-+~~.::..::.:~

Gravel trap downstreem the outlet

Grain size d (mml

Figure 5. Size distribution of grain.

The stream erosion correlated with the runoff events as expected. In the first gravel trap upstream of theoverflow the size distribution tended towards smaller diameters compared with the other traps downstream(Fig. 5). After a seven-day period without wet weather it was impossible to carry out a size distributionanalysis, because less than 30 g sediment was caught in the traps.

The chemical impacts were not as serious as presumed. After three years of monitoring it has been shownthat the hydraulic impacts were significant. A long-term oxygen depletion occurring in impounded reachesof a river (Winter and Borchardt, 1989) could not have been monitored in the Schondelle brook. A typicalcurve of dissolved oxygen in the Schondelle during a runoff event is shown in figure 6.

12,0

11,0

• 10,0

• I • • • • . • • •• • • • • • • - 9,0

900

800

700

600

~ •Q) 500·e'III

'5 400.!!l0

300

200

100

.............

,._._-------''----':>......-_------~

I __ Overflow discharge i......Discharge of the brook at the first monitoring point

• Dissolved Oxygen at the first monitoring point

i • Dissolved Oxygen at the second monitoring pointl~ • _ __ ...-J

~oS

8,0 0

7,0

6,0

0.... .... .... .... .... .... 5,0

on 0 on 0 on 0 on 0 on 0 on 0 on 0 on 0 on 0 on 0 on 0N l') l') ~ ~ on on 0 0 - - N N l') l') .... .... on on 0 0 -Ii; Ii; Ii; on on Ii; Ii; ~ ~ ~ ~ ~ ~ ~ ~ ~ iD iD iD ;..:. i- ;..:.- - - - - - -

Time

Figure 6. Dissolved oxygen during the runoff event of 12 May 1993.

During the event of 12 May 1993, the discharged wastewater from the CSO with a flood peak of 820 lisreached the monitoring station before the flood wave of the headwater. The dry weather flow of theSchondelle brook and the wastewater discharge coincided over a period of 30 minutes. Owing to theturbulence in the stream the physical rearation led to an increased dissolved oxygen concentration

46 A. WAGNER and W. F. GEIGER

downstream of the overflow. Only in two cases of the 59 monitored runoff events did the concentration ofdissolved oxygen decrease below 5 mg O2/1 over a duration of 15 and 55 minutes.

The chemical oxygen demand (COD) was not affected by discharged wastewater. Organic material carriedwith the flood wave of the headwater during wet weather has caused average COD-values of 380 mgn. Thecoincidence between the runoff discharge and the NH4-N concentration is shown in figure 7.

·2,0

·2,5

0,5

i1,5 Z

~Z

• • •.... '.0• •• •

. .

• • • •• • •.: .

•

0,0It) 0 It) 0 It) 0 It) 0 It) 0 ltl 0 ltl... N N C'l C'l ~ ~ It) It) 0 0 ... ...cD cD ~ ~ ~ ~ ~ cD ~ ~ ~

;...:. ;...:.... ... ... ...Time

- Overflow discharge

...... Discharge 01 the brook at the first monitoring point

• NH4·N at the first monitoring point

• NH4·N at the second monitoring point.,------------~

• ••

• •••••••••• 0 •••••••••••••

r-------:-.=_-=._=-=-=-=::._::-::_=_:=~-===:=:===-=-=-==-=-=--=--=-=i3.0900

800

700

600';i':::.-; 500~III'5 400.!!l0

300

200

100

0It)N,;;...

Figure 7. NH4-N concentration during the runoff event of 12 May 1993.

The NH4-N concentration downstream of the runoff reached its maximum at the beginning of the dischargewith 1.94 mgn, afterwards it decreased slowly to the concentration upstream. The leap in the NH4-Nconcentration upstream of the runoff was caused by the smaller CSO in the headwater. The maximum NH4•N concentration reached during 59 monitored storm events was 3.26 mgn on 08 June 1994. Owing to thelow NH4-N concentration a detrimental effect of NH3-N toxicology on the stream ecology is not expected.

In contrast to most of the monitoring sites on lowland rivers (Winter and Borchardt, 1989; Lammersen,1993) the hydraulic impacts of the CSO dominate at the Schondelle brook. The superimposition of CSOsand separate sewer outlets frequently causes sediment erosion.

DEVELOPING A NEW APPROACH FOR ASSESSING STORMWATERDISCHARGES INTO URBAN BROOKS

The ecomorphological conditions are responsible for significant stormwater impacts in urban brooks. Inaddition to the water quality mapping a new classification of the stream morphology has been tested inGermany. The structure of the river valley, the regime, the longitudinal section, the river bottom, the cross•section, and the stream banks are quantified with the help of a specified scheme. A classification has beenestablished with seven steps from minor influenced to totally affected ecomorphology (LAWA, 1993).Based on the nationwide use of this scheme it is useful to combine the classification of the streammorphology with a criterion for stormwater discharges. The new approach adopts this classification as a firststep and combines the results of the emmission based evaluation with the morphological classification.

For the protection of river habitats a division into hydraulic and pollutant impacts seems to be useful (ATV,1993; Fuchs et al., 1994). To maintain the correlation between hydraulic effects and the variety ofecomorphological properties, the hydraulic approach relates the frequency of bed-load transport with theclassification of the stream morphology as shown in figure 8.

Stonnwater discharges into urban streams 47

------ -------------, ---- -.J

r .- .... -- L ~

, I

!Critical discharge of the outlet I

---- J

7total affected

3 5

Ecomorphological classification

minorInfluenced

~ 7 r-----.,- ..

8. 61::

&. 5..~ 4..,~ 3

~'i5 2~iii 1

I 0 ~--I----J~~----l-_ __l_-_L_

Classification of streammorphology

i-size distribution of streamli sediment sample for ILevaluation of mean gain sizeJ

!

,------~._---i Deduction of the criticalI stream power !L , ---.J

Figure 8. Frequency of bed-load transport dependent on classification of stream morphology.

The flowchart of figure 8 describes the procedure to evaluate the critical discharge of an outlet. The relationbetween shear stress and discharge of the cross-section can be found with the simple Manning-Stricklerequation or from flow calculation. Both CSOs and separate sewer outlets can be dimensioned with thehydraulic approach.

Whether or not the pollutant impacts are significant depends on the stream morphology. The pollutantapproach takes this correlation into consideration. To reduce the risk of oxygen depletion under 4 mgll, amixing rate of 1: 1 between wastewater and mean water flow must not exceed the annual frequency as shownin figure 9. The procedure finds a critical discharge according to which structures will be designed.

i-- Classification of streammorphology

r------ -I Evaluation of the mean water :, flow II II. I

1 _

7

~... 6·Olc::8 5E'0 4.B-&i 3:>

I 2

~ 1c:

~Ol---4---I-----I---4----l---l-~

ICritical discharge and annual i! frequency of storm events !I I1._._-------

...5

~

>.~~ Iii lii >- ..,-~

..,.!! .., >. > .!! c\ij '01i c.!! :SCQ c ::s J!I> .c > :'iU .. >

~~ ~£ E >'5 ..c

Stream classification

Figure 9. Annual frequency of mixing rate between wastewater and mean water flow.

This approach relates the ecomorphological structure of the brook to CSO and stormwater discharge impact.The criterion should be a proposal for forthcoming activities. In addition to the new classification of streammorphology it could be easily applied and tested because of the low data requirement. With a range ofstream classification the criterion could be specified. The criterion will be tested with the data from otherriver sites where CSOs discharge to validate the simplifications applied.

ACKNOWLEDGEMENT

The authors are grateful for the financial support of the Graduiertenkolleg "Verbesserung desWasserkreislaufs urbaner Gebiete zum Schutz von Boden und Grundwasser" by the DeutscheForschungsgemeinschaft (Bonn, Germany). Parts of the monitoring instrumentation are financed by a projectof the Ministerium fUr Umwelt, Raumordnung und Landwirtschaft des Landes Nordrhein-Westfalen(DUsseldorf, Germany), grant no. IV B 6 - 041077.

48

REFERENCES

A. WAGNER and W. F. GEIGER

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GFA.ATV (1993). ATV-Arbeitsberichte: Weitergehende Anforderungen an Mischwasserentlastungen. I. Arbeitsbericht der

Arbeitsgruppe 2.1.\.: Grundlagen und Vorpriifung. Korrespondenz Abwasser 40(5),802-806.Fuchs, S. T., Haritopoulou, T., Xanthopoulos, C. and Hahn, H. H. (1994). Results and conclusions of research activities on

impacts of CSOs on small freshwater ecosystems: Evaluation of hydraulic and chemical parameters. Proceedings of thespecialty conference of the water environment federation: A global perspective for reducing CSOs: Balancingtechnologies, costs, and water quality. Louisville, Kentucky, USA, 15-26 May.

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Geiger, W. F., Schuhmacher. H., Grauenhorst, V., Podraza, P. and Redder, A. (\ 994). Auswirkungen von Regenentlastungen aufdie Biozonose von Stadtbachen. Schluj3bericht eines Forschungsvorhabens des Ministers fur Umwelt, Raumordnung undLandwirtschaft des Landes Nordrhein Westfahlen. Dusseldorf, Germany.

Jefferies, c., Langlands, E. and Dugard, P. (1993). Developing a strategy for assessing combined sewer overflows. Proceedings ofthe sixth international conference on urban storm drainage. Niagara Falls, Ontario, Canada, Vol. 1,42-47.

Jokiel, c., Wagner, A. and Kongeter, J. (\ 994). Water quality modelling using GIS. Presented at the IA WQ 18th biennialinternational conference: Water quality international '94, Budapest, Hungary.

Krejci, V., Schilling, W. and Gammeter S. (\993). Receiving water protection at wet weather Proceedings of the sixthinternational conference on urban storm drainage. Niagara Falls, Ontario, Canada, Vol. 1,506-511.

Krier, H. (1995). Hydraulische Wirkungen von Mischwassereinleitungen auf Flie13gewasser. Proceedings of the ATV workshop:Weitergehende Anforderungen an Mischwassereinleitungen, Essen, Germany.

Lammersen, R. (1993). The effect of an urban drainage system on receiving water quality. Proceedings of the sixth internationalconference on urban storm drainage. Niagara Falls, Ontario, Canada, Vol. 1,206-21 I.

LAWA (1993). Die Gewasserstrukturgutekarte der Bundesrepublik Deutschland. Teil I: Verfahrensentwurf flir kleine undmittelgroBe Flie13gewasser in der freien Landschaft Entwurfder Liinderarbeitsgemeinschaft Wasser, Germany.

Pitt, P. R. (1991). Biological effects of urban runoff discharges. Proceedings of the engineering foundation conference: Urbanrunoff and receiving systems: An interdisciplinary analysis of impact, monitoring, and management. Mt. Crested BUlle,Colorado, USA.

Ruland, P. and Rouve, G. (1992). Risk assessment of sediment erosion in river basins. Proceedings of the fifth internationalsymposium on river sedimentation. Karlsruhe, Germany, 357-364.

Warg, G. (l99\). Auswirkungen unterschiedlicher Mischwasserruckhaltung auf den Vorfluter - Ansatze flir Bewertungskriterien.Wasser Abwasser Abfali, 7, 169-183.

Winter, 1. and Borchardt, D. (1989). Die Stickstoffbelastung eines Flie13gewassers durch stoBartige Einleitungen aus stadtischenEntwasserungssystemen und ihr Einflu13 auf die Gewasserbiozonose. Korrespondenz Abwasser 36(3), 316-322.