| dc.description.abstract | As a result of climate change, the intensity and frequency of extreme precipitation events in the Netherlands increases. The impact of these precipitation events can be problematic, especially in urban areas. In these areas storm water can barely infiltrate into the soil and needs to be drained by the sewer systems. Due to the intensity of the precipitation events, the capacity of the sewer systems is insufficient and water accumulates on the surface, known as pluvial flooding. Since increasing the sewer capacity is expensive and time consuming, and a large share of urban areas is private property, it might be efficient to focus on adaptation on a private scale. Therefore, the Dutch water authority Hoogheemraadschap Hollands Noorderkwartier (HHNK) aims to motivate citizens in problem areas to take measures in order to reduce the risk of pluvial flooding. By temporarily storing the water on private properties, the pressure on the sewer system can be reduced, which might reduce the risk of pluvial flooding and possible damages. One potential tool HHNK considers to use is the so-called 'Waterlabel'. This label indicates whether a property buffers the storm water well (A) or that it discharges it directly to the sewer system or surrounding area (G).
The aim of this study was determine to what extent improvement of the rainwater retention capacity on private properties can contribute to the reduction of pluvial flooding in urban areas of the management area of HHNK. Firstly, the calculation method of the Waterlabel was evaluated based on expert judgement and scientific literature, to determine to what extent the label provides a decent indication of the rainwater retention capacity. It became clear that several improvements can be made to increase the representativeness of the label, such as inclusion of soil type and groundwater level. However, a balance should be maintained between a realistic representation of the water retention of a property and the purpose of informing people in order to improve water awareness. Therefore, the calculation of the Waterlabel on the website should be kept as simple as possible, in order for people to understand the principles. Secondly, to test the statement of the Waterlabel (2017) that a better Waterlabel (e.g. A or B) results in less pluvial flooding, a correlation between these two variables was determined. It appeared that there is a very weak negative correlation, meaning that when the Waterlabel increases the risk of pluvial flooding decreases, which is in line with the statement.
By combining the spatial pattern of bad Waterlabels (G) with that of a high pluvial flood risk, the locations where adaptation is most needed were identified. To determine which private adaptation measures are most effective, three neighbourhoods were selected and six adaptation measures were evaluated by executing simulations in the program RainTools. This tool simulates the effect of different measures on a property by using a water balance approach. The results of these simulations showed that lowering the garden, implementing of an infiltration crate and pervious pavement were the most effective measures.
The reliability of the obtained results might be influenced by the methods and data used. For the definition of pluvial flood risk, a map from the 'Klimaatatlas' was used that visualized the water depth after an extreme precipitation event. However, infiltration of water into the soil and drainage by the sewer system was not taken into account in the composition of this map.
Therefore, it is recommended to improve the water depth map, as well as the calculation method of the Waterlabel, before starting communication of HHNK with regard to private adaptation. Additionally, it appears that private adaptation can reduce the risk of pluvial flooding, however further research is needed to determine to what extent. Finally, it is recommended to exchange knowledge and experience between different water authorities and other organisations, to learn from each other and cooperate where possible. | |
