The effect of the imperviousness on the hydrological response time of sewer districts in Rotterdam: Study based on new monitoring system by municipality of Rotterdam

Additional thesis - Due to an expected increase in rainfall intensity in the future because of climate change for the area of Rotterdam, the amount of storm water runoff will increase as well, resulting in higher stress on the sewer system. But until now we don’t fully understand the behavior of a sewer system. This additional thesis should help in a better understanding of the system, as it investigates the hydrological response time of the urban drainage system of Rotterdam and focuses on the effect of imperviousness. This research is based on data which are collected during the first seven months of operation of the monitoring system at 21 combined sewer overflow (CSO) weirs which was implemented in June 2016. As the hydrological response time gives information about the behavior of a sewage system, it’s an important parameter to investigate and the question is how it’s being influenced by parameters such as the imperviousness and to what extent the behavior of the sewer districts in Rotterdam is different from what we would expect from theory. From the results, we see that there is no single hydrological response time for both Time-to-Peak and Peak-to-Peak responses. The response times are highly variable with large standard deviations. There seems to be no clear linear relationship with the imperviousness or the connected surface area for the sewer system of Rotterdam. Furthermore, no significant relationship was found for several rainfall characteristics like intensity, rain event duration and cumulative rain volumes in the previous period with the hydrological response time. Finally, it was shown that the large variability in response times is mainly associated with assumptions on starting time of a rain event, which can produce large Time-to-Peak responses. However, Peak-to-Peak responses found in the research are also significantly larger than the responses found in theory. All in all, the results have shown that the response times are larger than we expected from theory and that variability cannot be explained by variability in rainfall characteristics nor by relations with catchment size or imperviousness for the urban drainage system of Rotterdam. For future research, it is recommended to have a further look into the system responses by doing a signal analysis for individual events in order to understand the high variability in responses. Furthermore, the rain radar, which will be implemented in Rotterdam in the summer of 2017, might help in a better understanding of the influence of local rainfall variability on the response time.Civil Engineering and GeosciencesWater Managemen