Storm water source control

Since the new Flemish guidelines for urban drainage in 1996 were introduced, more emphasize was 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. 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. This new approach brings along new modelling requirements. Because of the relatively long emptying times of source control facilities, a long antecedent period of rainfall influences the design. Moreover these facilities most often have an outflow which is not linearly varying with the storage. Because of the high variability of the rainfall, the storage volumes can therefore only be assessed well if continuous simulations with long historical rainfall series are performed.

Full paper 'Design rules and impact assessment for source control measures based on continuous long term simulations' (2002) in Water Science & Technology.

Full paper 'The Use of continuous long term simulations for the design and impact assessment of source control measures' (2001), in 'Advances in Urban Stormwater and Agricultural Runoff Source Controls', eds. Marsalek J. et al., Kluwer.

Full paper 'Source control design using continuous long term modelling' (2004) at 6th international conference on Urban Drainage Modelling, Dresden, Germany.

Full paper ' Design of source control facilities based on continuous long-term modelling' (2004) at the international conference on hydrology : science & practice for the 21st century, London, UK.

It is important to design storage and infiltration facilities based on long term simulations. The long ‘memory’ of the storage facilities requires the use of the complete time variability of the rainfall. Based on long term simulations in a conceptual model a design table was produced which gives the required storage volumes as a function of the return period of the overflow and the outflow discharge. This outflow discharge can be either infiltration, throttled outflow, evaporation, etc... The storage volumes are based on the assumption of a constant outflow and are rounded off for practical use. In case of a more linear outflow relationship (e.g. throttle pipe with free outflow) the required storage volumes are slightly underestimated. For large storage basins (larger than 10 ha contributing area) a specific modelling is recommended, because it can lead to further optimisation. When the antecedent rainfall (or initial storage) is not taken into account and the storage facility is designed based on single design storms, it is found that the frequency of the overflow can be down to half of the frequency found with the continuous simulation.

DESIGN TABEL (Pdf)

Full paper 'Design rules and impact assessment for source control measures based on continuous long term simulations' (2002) in Water Science & Technology.

Full paper 'The Use of continuous long term simulations for the design and impact assessment of source control measures' (2001), in 'Advances in Urban Stormwater and Agricultural Runoff Source Controls', eds. Marsalek J. et al., Kluwer.

Full paper 'New Flemish design guidelines for source control measures' (2004) at 5th international conference on sustainable techniques and strategies in urban water management (Novatech), Lyon, France.

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The revaluation of ditches is one of the main topics in the source control policy in Flanders. The main function of the ditches shifts from runoff to storage and infiltration, but the runoff function at peak flows must be assured without flooding. Therefore, for the design of ditches a two step approach is proposed. First, the storage volume in the ditch must be large enough to store and infiltrate the water from locally connected areas. This means that the ditches have to be designed using the design values for the storage volumes. In the second step, a control calculation is performed for the storm drainage system. For this control simulation no flooding may occur for a return period of 5 years. The infiltration flow is neglected as compared with the peak flow in the drainage system and the initial conditions must be chosen according to the outflow conditions of the ditches. A simultaneous evaluation of the storage and runoff function is very difficult or practically impossible with the currently available modelling tools.

Full paper 'Design rules and impact assessment for source control measures based on continuous long term simulations' (2002) in Water Science & Technology.

Full paper 'The Use of continuous long term simulations for the design and impact assessment of source control measures' (2001), in 'Advances in Urban Stormwater and Agricultural Runoff Source Controls', eds. Marsalek J. et al., Kluwer.

The Flemish guidelines prescribe that for new large impervious areas (starting from 0.1 ha) the downstream flow must be limited. However, limiting the throughflow to too low values for a small return period does not always lead to the optimal flow attenuation. It is more important to use a high return period for the overflow of the storage facility, than to limit the throughflow. In most cases a better attenuation of the peak flows is obtained using a larger throughflow limit. This shows that the throughflow limit only is not sufficient to optimise the attenuation of the downstream flow and that a case specific long term simulation can have a large benefit. Therefore, for sensitive receiving waters it is better to increase the return period of the overflow than to further limit the throughflow.