Guido Vaes

The effect of rainwater storage tanks on design storms

Abstract The effect of source control measures on the design of combined sewer systems can in most cases only be correctly assessed using the intrinsic temporal rainfall variability, because long antecedent periods can have an important influence. A conceptual model was built to assess the effect of rainwater tanks on the rainfall runoff using long term historical rainfall series. The outflow of the rainwater tank model is converted to equivalent rainfall series. Based on intensity/duration/frequency-relationships (IDF-relationships) for this equivalent flattened rainfall, modified design storms are developed.

Rainfall input requirements for hydrological calculations

Rainfall is the most important input for many hydrological and hydraulic design calculations. Ideally, long historical rainfall series should be used and a statistical analysis should be performed on the hydraulic results afterwards. In combination with the detailed models that are commonly used nowadays, this leads to huge calculation times. This research was set up in order to verify which kind of simplifications can be made with respect to the rainfall input. One must find an optimum between accuracy of the modelling results and calculation effort. This optimum can be different for different applications. The different types of rainfall simplifications which are considered here are composite design storms, short selected rainfall series and modified single storm events. In many cases the optimum is more likely a simplified model in combination with continuous long term simulations. Well-calibrated (physically based) simplified models can reach almost the same accuracy as the corresponding detailed models within a fraction of the calculation time. Furthermore, these simplified models are very useful in order to select or compose the proper rainfall input for detailed modelling.

In Sewer Sediment and Pollutant Transport Models

Several commercial software providers offer in addition to their hydrodynamic urban drainage modeling software a tool for water quality modeling. Yet these quality modeling tools are not common practice in urban drainage design. Prior to start using these quality modules it is of paramount importance to have an insight into these models in order to know what these models (can) do (and cannot do) and how they could be applied. The paper shows the results of a thorough analysis of the quality components of two widely spread hydrodynamic models, namely Mouse (DHI Software, Denmark) and HydroWorks / InfoWorks CS (Wallingford Software, United Kingdom). The focus will be on how both models deal in a different way with the same problem, namely in sewer transport of sediments. The analysis reveals that the adequacy of the quality modules is not as high as compared with the hydraulic model. Care should be taken when using these models. Especially in the case of insufficient field data, the generated results must be analyzed with care and attention. A case study illustrates how the InfoWorks CS software can be used to model the transport of ammonia nitrogen.

Influence and modelling of urban runoff on the peak flows in rivers

Surface waters and urban drainage systems are usually studied separately. However there are important interactions between both systems. Urban drainage systems can have an important impact on the surface waters, mainly at combined sewer overflows. On the other hand during periods of high water levels in a river, the runoff from the urban drainage system can be significantly influenced by backwater, which increases the probability of flooding in is not obvious, because the modelling tools for both systems are often hard to combine properly. To properly assess the probability of flooding for this kind of integrated water systems, different submodels are needed for both subsystems. In practice often one single model is used to describe the runoff to rivers despite the presence of urban catchments. The main objective of this study is to show the limits of this simplified approach. Furthermore, it is necessary to use continuous long term simulations, because of the differences in runoff behaviour. Detailed hydrodynamic models do not really fit for this purpose because of long simulation times and high demands in memory and disk space. Therefore simplified conceptual models are more useful.

Chapter 4.2:WISE-RTD – A Portal for Science & Technology Transfer to Policy Making & Implementation in Integrated Water Resources Management

Bridging the science–policy gap has been of major concern to the European Commission since the beginning of this century and has led to several activities to support the issue. For instance, in the FP5 project Harmoni-CA (Chapter 2.4) workshops have been held for water managers and policy makers to ...

The use of continuous long term simulations for the design and impact assessment of source control measures

Since the new Flemish guidelines for urban drainage were introduced in 1996 [1], more emphasis has been put on source control measures in order to reduce the peak runoff from urban areas during wet weather conditions. The keyword is ‘disconnecting’ impervious areas from the combined sewer system. This involves the construction of upstream storage and infiltration facilities, rain-water tanks for reuse in households and the revaluation of ditches [2,3]. However, one must realise that the runoff discharges from the ‘disconnected’ areas still have to be taken into account, because during heavy rainfall periods not all rain-water can be stored locally. Therefore, urban runoff plans with respect to extreme events and flooding risks are very important.