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Combined sewer system design

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Why does flooding occurs in urban drainage systems ?

When flooding occurs, combined sewer systems are often immediately blamed. However, drainage systems are complex systems with large uncertainties on the processes and a highly variable input. The main causes of the failure of the drainage systems (for extreme runoff) can be divided into the following categories : extreme rainfall input, insufficient maintenance and a design with an insufficient safety.
It is clear that nowadays more flooding with damage occurs. It is however doubtful that this is caused by an increase in rainfall. Mainly the human impact on land use and drainage systems are leading to more flooding problems. Large progress is made in recent years with respect to knowledge acquisition on drainage systems and modelling tools. The practical implementation of it is however a huge effort. It is obvious that all sewer system owners have to take their responsibility as well for the design and maintenance as in case of failure of the drainage system. A sustainable improvement can only be obtained using source control measures, which includes technical measures as well as a harmonisation with urban planning.

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Influence of rainfall input and model simplification on the design of combined sewer systems

A wide range of computer models and different types of rainfall input are used in practice for combined sewer system calculations. Driven by rapidly improving computer technology, modelling techniques, tools and possibilities are changing equally fast. The question can arise whether there is an unambiguous relationship between more detailed models and more accurate modelling results. Therefore, models and rainfall input were investigated and compared in order to balance their degree of detail with the accuracy of the results.
Two types of rainfall simplifications are studied in detail and the simulation results are compared with the results obtained with the original historical rainfall series. First, the most extreme simplification into mean rainfall (i.e. IDF-relationships) is discussed, together with the single design storms (i.e. composite storms) which are derived from these IDF-relationships. Secondly, the selection of critical short time series from the historical rainfall series is discussed.
Concerning the model simplifications, a rather extreme simplification into a reservoir model has been studied. With this approach the minimum requirements can be addressed, which are necessary to obtain results within a specific range of accuracy.
These different types of simplification are then applied to different types of calculations : design of combined sewer systems versus impact calculations.

POSTER 'Rainfall input requirements for urban drainage modelling' (2000)

Full paper 'The effect of changing technology on combined sewer system design' (1999), 8th International conference on Urban Storm Drainage, Sydney, Australia.

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  Guidelines for the design of sewer systems ( Raf Bouteligier, Patrick Willems)

Currently the Flemish administration (Aminal Water) has asked the Hydraulics Laboratory to rewrite the Flemish guidelines for urban drainage. This project is carried out togheter with the Ecosystem Management Research Group of the University of Antwerp.

More on guidelines for the design of Dry Weather Flow systems.

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Software Evaluation (Raf Bouteligier)

The Hydronaut users group is the Flemish user group of sewer system modellers and member of the Integrated Modelling User Group (IMUG). New software is tested by the Hydraulics Laboratory and Aquafin before they are used in Flanders.

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Urban flood modelling (Raf Bouteligier)

Combined sewer systems and storm water sewers are designed to guarantee the rainfall runoff with a certain design return period. At extreme rainfall conditions flooding can occur at specific locations because of the insufficient capacity of the sewers. In order to assess the flood risk the currently available modelling practice fails, because in the current modelling software it is assumed that the water, that raises above the ground level and causes flooding, is remaining at the specific location where it leaves the sewer system. The local topography is neglected. This does not correspond to the reality. It is not only a problem of correct estimation of the local water levels, but in practice it can occur that the water flows over the streets to lower locations and leads to damage at completely other locations than predicted with this kind of models. This paper describes how the street runoff can be incorporated in the model as a second layer using existing commercially available hydrodynamic software and what the effect might be.

Full paper 'Modelling of floods caused by urban drainage systems' (2004) at 6th international conference on Urban Drainage Modelling, Dresden, Germany.

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