REBEKA 2

Deterministic Simulation

Model Description

REBEKA2 uses a ﬁxed catchment configuration (Rauch et al., 2002). It consists of each a combined and a separate sewer system that both discharge into the same river. For consideration of the upstream dynamics in the river discharge a third natural catchment is included. Additional discharges (e.g. from industrial sources or from an upstream sewer system) may be included by means of an external inﬂow (constant or constant during rainfall). Both urban catchments have a detention basin installed, prior to the discharge into the receiving water.
model configuration in REBEKA2

To analyse a given situation by REBEKA2 it is therefore necessary to simplify the existing combined and separate sewer systems and the natural catchment and describe them by linear reservoirs. The most difficult part of the simplification is the handling of multiple overflow structures in the sewer system because REBEKA only allows for one overflow device per sewer system. Solutions to this problem are (1) either to make a separate analysis of the importance of each overflow structure and choose the most important one or (2) model the sewer system progressively from the uppermost to the lowermost overflow structure taking the outflow from the first subsystem as constant inflow to the second subsystem etc.

Rainfall data

For simulating the systems dynamics, long time series of precipitation data are required as model input. In order to simplify the use of the software, REBEKA offers a choice of ﬁve rain series. The series have been selected to represent the main hydrological characteristics of Swiss regions. All ﬁve series cover the same time period from 1986 to 1995. User specific rain data (in KMD-format) of arbitrary length can be read. Time resolution must be 10 min. To investigate seasonal effects it is possible to choose only specific months of the year within the 10-year rainfall data.

Runoff simulation

The generation of stormwater is calculated by considering the initial losses and only that part of the catchment area that actually produces runoff. Surface ﬂow is computed by means of the linear reservoir model. These models have been used for all three types of catchments (see figure). The effluent from both the combined and the separate system is subsequently discharged via a CSO (or via a retention basin respectively) into the receiving water. As superposition and instantaneous mixing of the three discharges is assumed, hydrodynamic effects are conveniently neglected. The type of CSO and retention basin can be chosen as on- or off-line.

Water quality simulation

Ammonia concentration is computed by means of a simple mixing model, where constant concentrations are assumed in the base ﬂow, the stormwater and the wastewater.
Total suspended solids (TSS) load is calculated by a stochastic approach (Rossi et al., 2004, Rossi et al. 2005). The model predicts the probability of TSS loads arising from CSOs in combined sewer systems as well as from stormwater in separate sewer systems, and the amount of TSS that is retained in treatment devices on both sewer systems.

Results of deterministic simulation

This screenshot from the program shows an example of the output from the deterministic simulation.

results of deterministic simulation

If the user needs more detailed results, he can analyse the file *_det.csv and *_step.csv which are generated during the simulation.
*_det.csv contains a summary of the results per rainfall event. Each line represents the results for an event.
*_step.csv contains results for each time step (10min) of the simulation. Each line represents a time step. By default, only the first 100 events are output.

REFERENCES

Rauch W., Krejci V. and Gujer W. (2002). REBEKA - a software tool for planning urban drainage on the basis of predicted impacts on receiving water. Urban Water, 4: 355-361.
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.