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.

Impact |
Threshold |
Requirements |

Hydraulic stress | Critical shear stress | 0.5 – 10 events per year |

NH_{3} toxicity |
Critical
NH_{3} conc. for given duration t( c _{crit} = 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/m^{2} |
< 20 % of time |

Accum. of toxic substances in riverbed | Crit.
TSS density on river bottom: 25 g/m^{2} |
< 5 % of time |

Oxygen depletion in riverbed | 5 g/m^{2}
(16 g/m^{2}) per day |
< 10 % of time |

(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/m

(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)).

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.

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.