River-sewer interactions

In recent years more and more river models are built in Flanders for the assessment of extreme flow and flooding. Detailed hydrodynamic models are built, while the rainfall runoff is modelled in a conceptual way. This makes long term simulations (i.e. multiple decades) possible for the runoff model. From these long term simulations a best selection can be made of critical events for river simulations. However, when a simplified river model is calibrated to the detailed hydrodynamic model, also long term simulations for the river can be carried out. The use of long term simulations enables the modeller to assess the probability of the extreme events more accurately.
In a river catchment with important urban development, combined sewer overflows may have an important impact on the river discharges. The runoff in an urban catchment however cannot be modelled accurately using the rainfall runoff model for the river basin. The responses of the rural and urban catchments differ strongly. For that reason a separate model can be used to predict the combined sewer overflow discharges and use this as point input in the river model. In this way the impact of combined sewer overflows can be investigated and a better estimation of the extreme discharges can be made. When a detailed hydrodynamic model is used for the combined sewer system, the calculation times are very long, certainly in case of long term simulations. With a physically based conceptual model that is well-calibrated to the detailed hydrodynamic model, simulation results with almost the same accuracy can be obtained within a fraction of the calculation time.
This approach is tested for the Molenbeek brook in Belgium. For the current and future sewer system state, a set of (non-linear) reservoir models is built (calibrated against an existing detailed hydrodynamic model) and the effect of the emissions on the discharges in the river is investigated. Especially the summer discharges can be estimated more accurately with this approach. From this project it can be concluded that not the models (even not the simplified ones) cause the largest uncertainty on the results, but the (temporal and especially the spatial) variability of the rainfall. For that reason it is important to perform long term simulations using appropriate rainfall input. Simplified conceptual models then have to be used for the simulation of the sewer system. These conceptual models can be calibrated accurately to the (existing) detailed sewer system models and used in a complementary way to these detailed models.

This research was funded by Aminal Water.

Full paper 'The use of reservoir models for the assessment of the input from combined sewer overflows into river models' (2002) at 9th International Conference on Urban Drainage, Portland, US.

Sewer networks and river systems are most often studied in a separate way. The main reason is that different authorities are responsible for the water management of both systems. Sewer and river systems may, however, interact in two directions. The first direction is straightforward: sewer effluents and overflows have an impact on the receiving river. In mathematical modelling applications, this influence can be described in an easy way by using the output of the sewer model as input for the river model (see The use of reservoir models for the assessment of the input from combined sewer overflows into river models. In the opposite direction, during storm conditions the high water level in the river may avoid water to be discharged into the river. This will cause an increase in the flood risk of the sewer system. To model this second influence, the numerical schemes of the simulation models for the sewer network and the river system have to be linked. This task has been done by means of the SOBEK software of Delft Hydraulics (The Netherlands), for the case of the Molenbeek river in Belgium. This river interacts with the sewer system of the village of Erpe-Mere. In the proposed study, the two-directionally linked model of this case will be applied to simulate both historical and synthetic storm events, and to analyse the influence of the river-sewer interactions on the flood risk.