The effects of climate change on the robustness of the urban drainage system of the Soesterkwartier.
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Many urban areas are facing pluvial flooding problems due to extreme rainfall events that are too intensive for the local drainage network. One of these urban areas is the Soesterkwartier, a district in the city of Amersfoort which is located in the central part of the Netherlands. A large part of the sewer system of this district is a so-called combined sewer system, discharging wastewater as well as rainwater. In 2014, two extreme rainfall events on respectively the 11th and 28th of July caused water nuisance in this district whereby also sewage came out of the sewer system. This resulted inter alia in increased health risks and odour problems. The KNMI expects that the frequency and intensity of these extreme rainfall events will increase in the future. If nothing is done with the current urban drainage system of the Soesterkwartier, this will lead to more water nuisance, increased health risks and more economic damage in the future. The main objective of this research project is to investigate how the current drainage system of the Soesterkwartier could be made more robust to prevent the effects of extreme rainfall events. Hereby, climate change is taken into account. The aim is to find practical, sustainable and area specific solutions that are in line with the vision of the municipality of Amersfoort and increase the robustness of the drainage system of the Soesterkwartier. To achieve this objective, information about the extent and degree of the water nuisance due to the heavy rainfall events in July 2014 is obtained via a district survey and an investigation in the municipal archives. Besides, a rainfall runoff model of the Soesterkwartier is made using the openLISEM model, whereby the sewer system is modelled as a buffer with an inflow (runoff water) and outflow (pumpovercapacity and overflow). With the help of the results of the district survey it is tested if this model corresponds well with reality. Subsequently, the best measures to reduce the consequences of extreme rainfall events are analysed using the model. The district survey identified two vulnerable areas in relation to pluvial flooding in the Soesterkwartier. These areas were flooded during both heavy rainfall events in July 2014 and are respectively the Soesterweg, Hulststraat and Palmstraat (1) and the Spaarnestraat and Dollardstraat (2). The nuisance consisted mainly of streets that were flooded with rainwater that also contained sewage. The events caused no significant economic damage because the district is in general well-designed. The maximum water levels on the surface seems to have approximately been the same during both rainfall events, even though the return periods of the events on the 11th and 28th were quite different, respectively 5 and 70 years. The high water levels on the 11th of July were in contrast to the 28th of July largely because of the failure of the functioning of the sewer system. Increasing the robustness of the drainage system of the Soesterkwartier is not a short-term straightforward process, as it takes time and requires cooperation between the different stakeholders. Because of this, it is important to have a flexible long term vision whereby necessary measures are adapted to other developments in the district. This way, projects to improve the drainage system will be integrated and costs of the measures could be reduced when their proceedings are combined with other work. On the short term, it is important to increase the awareness of the civilians of the Soesterkwartier in relation to pluvial flooding. In the course of time more water retention areas and infiltration systems should be constructed to increase the robustness of the drainage system. Besides, more use should be made of the potential buffering capacity and infiltration capacity of green areas if the groundwater levels are favourable. Furthermore drainwave (permeable pavement) could be constructed in both problem areas. On the long term the combined sewer system must be replaced by a separated system. If this replacement is considered it should be analysed if an expensive rainwater sewer is really necessary everywhere in the district or that the money could better be invested in a dry weather system only and the remaining funds be used for retention and infiltration measures that increase the robustness of the system.