REBEKA is a software tool that has been developed to assess impacts of urban drainage to receiving waters based on immission criteria. The program calculates the average number of critical events per year for NH3 pollution and riverbed erosion caused by urban storm water overflows for a given catchment. It is already widely used by Swiss practitioners in processing the receiving water status report of the drainage master plan, obligatory for each community in Switzerland.
REBEKA is now extended to REBEKA 2 by (1) a model which simulates input, transport and emission of total suspended solids (TSS) by the drainage system and their immission in the receiving water and (2) by stochastic modelling in general: each model parameter is treated as a random variable with a specific mean value, variation and probability distribution. Different probability distributions are available (uniform, normal, lognormal, triangle). The probability distribution of model outputs (i.e. number of critical events for ammonia toxicity and riverbed erosion, TSS load and time of critical TSS immission) is calculated by means of a Monte Carlo simulation. Number of runs and sampling method can be chosen by the user. To determine the importance of the model parameters to the output variables a local sensitivity analysis is implemented.

Receiving Water Impacts

The following table shows which impacts are taken into account by the program. The requirements are defined according to new receiving water standards developed and proposed by the project STORM. These standards will be used as a basis for a new technical guideline in Switzerland.

Impact Threshold Requirements
Hydraulic stress Critical shear stress 0.5 – 10 events per year
NH3 toxicity Critical NH3 conc. for given duration t
( ccrit = 0.025 mg/l + 1.5 mg/l·min/t )
0.2 – 1 critical events per year
Turbidity from TSS Crit. TSS conc. for given duration t
depends on local cond.
Colmation of riverbed from TSS Crit. TSS density on river bottom: 625 g/m2 < 20 % of time
Accum. of toxic substances in riverbed Crit. TSS density on river bottom: 25 g/m2 < 5 % of time
Oxygen depletion in riverbed 5 g/m2 (16 g/m2) per day < 10 % of time

Hydraulic stress

The increase in discharge in rivers due to CSOs causes not only a pollution effect but also a direct physical impact in the form of an increased bottom shear stress. If the shear stress exceeds a certain threshold that is determined predominately by the properties of the river bottom, erosion starts. It is clear that erosion events in rivers are not only caused by anthropogenic impacts but also by extreme flooding from the natural catchment area. However, urban drainage systems usually lead to a significant increase in the number of events. Accordingly, Frutiger et al. (2000) propose a certain number of events (between 0.5 and 10 events per year, depending on the morphological and biological state of the river) whose exceedance indicates that the river is affected. REBEKA uses the empirical model of Meyer - Peter to calculate the critical bottom shear stress for erosion for the given situation. The actual bottom shear stress is then computed dynamically in each timestep by assuming uniform flow conditions. Key parameters in this model are the mean (dm) and the 90% (d90) value of the grain size distribution of the riverbed material.

NH3 toxicity

For calculating critical situations with respect to NH3 pollution REBEKA applies the approach proposed by Frutiger et al. (2000). Based on the investigations of Whitelaw and de Solbé (1989) they define a critical 'dose' (concentration · period of exposure) equal to the LC10 (lethal concen-tration for 10% of a trout population) threshold level. REBEKA computes, for each overflow event, the resulting 'dose' (based on actual conditions in the river) and compares the value against the defined threshold value for the specific event.

Impacts caused by suspended solids (SS)

Suspended solids play an important role during wet weather conditions. Most of the pollutants problematic for the environment (organic materials, heavy metals etc.) are bound to particulate matter. Two different impacts of TSS on the receiving water are considered: (1) accumulation of sediments that may lead to colmation, oxygen depletion or toxic impacts on fauna and flora of the river bottom and (2) turbidity of the receiving water by TSS that may cause physiological stress on fish.

(1) REBEKA II models accumulation and erosion of TSS and degradation of organic matter by exponential approaches (Rossi et al. (2004), Rossi et al. (2005)). Model parameters are TSS settling velocity and minimum shear stress for accumulation, erosion coefficient and minimum shear stress for erosion and TSS degradation rate for degradation. It is important to mention that the model does not account for a longitudinal sedimentation but assumes sedimentation at one virtual point. The resulting TSS density [g/m2] in the riverbed is calculated for each time step due to the described processes. This density is compared to defined thresholds for critical colmation (density > 625 g/m2) and for toxicity (density > 25 g/m2). For each run the percentage of time is calculated while these thresholds are exceeded. No impacts to the receiving water are expected if the thresholds are exceeded less than 20% of the time for colmation and less than 5% for toxicity.

(2) Turbidity of receiving waters due to CSO discharges can not be avoided even during small rain events. The criterion for impacts on fish is based on TSS 'concentration-exposure duration' curves according to Fischnetz (2004) and Newcombe and Jensen (1996). For certain TSS concentration and exposure durations different effects are expected, e.g. a concentration of 50 mg/l during 60 minutes or a concentration of 300 mg/l during 10 minutes causes little to medium physiological stress. If the critical concentration is less than 25 mg/l (for longer exposure durations) this value is taken as limit. A security factor of 10 is used in these functions to account for toxic effects of adsorbed matter. All rain events for which the impact - calculated as concentration times exposure duration - is lower than the threshold for behavior change of fish are tolerable. REBEKA II calculates the probability that a certain number of critical events per year is not exceeded for a chosen level of impact (low, medium and high physiological stress (threshold for lethality)).

Limitations and drawbacks of the program

The main limitation of the program is the coarse and fixed representation of the system (combined and separate sewer system and natural catchment).
Therefore we are developing a new program which allows the interactive creation of a network system by choosing elements (rainfall or discharge sources, catchments, pipes, overflow structures, receiving water branches etc.) and connecting them.


Fischnetz (2004): Schlussbericht des Projekts «Netzwerk Fischrückgang Schweiz», EAWAG, 8600 Dübendorf,
Frutiger A., Engler U., Gammeter S., Luedi R., Meier W., Suter K. and Walser R. (2000). Zustandsbericht Gewässer (Teil Gewässerschutz) Empfehlungen zur Bearbeitung. Report of VSA, Zürich.
Newcombe C.P. and Jensen J.O.T (1996): Channel suspended sediment and Fisheries: a synthesis for quantitative assessment of risk and impact. North American Journal of Fisheries and Ma-nagement, 16:693–727.
Rossi L., Kreikenbaum S., Gujer W., Fankhauser R. (2004). Modélisation des matières en suspension (MES) dans  les rejets urbains en temps de pluie. Gas, Wasser, Abwasser 10(2004):753-761.
Rossi L., Krejci V., Rauch W., Kreikenbaum S., Fankhauser R., and Gujer W. (2005). Stochastic modeling of total suspended solids (TSS) in urban areas during rain events. Water Research 39(17): 4188-4196.
Whitelaw K. and de Solbé J.F. (1989). River catchment management, an approach to the derivation of quality standards for farm pollution and storm sewage discharges. Wat. Sci.Tech., 21:1065 –1076.