Hur avrinningskoefficienten varierar med återkomsttid - En jämförelse mellan fyra olika områden i Sverige

Sammanfattning: As climate change progresses, heavy rainfall in Sweden is becoming more frequent and intense, coinciding with an increase in impermeable surfaces in urban areas. This raises the risk of flooding as drainage networks may exceed their capacity due to increased runoff. Understanding the amount of water flowing into drainage networks is crucial for assessing and managing flood risks, as well as appropriately designing and expanding these networks. Hydraulic models can simulate the flow from precipitation into the drainage networks. The purpose of this study was to investigate how the runoff coefficient, which is the parameter describing runoff in the models, varies depending on the return period of the rainfall loading the model. To examine this, pre-existing hydraulic drainage network models from SVOA and NSVA were used for four different areas. One area was in Sköndal, Stockholm, and three areas were locations in Åstorp municipality in Skåne. Flow and precipitation measurements were taken in the summer of 2023 at each location. Each area was subjected to 4 to 5 selected rain events with varying return periods. The models were calibrated by loading them with the selected rains, after which parameters were adjusted to make the calculated and measured flow at the points match. The modified parameters included the reduction factor (symbolizing the runoff coefficient), time of concentration, and time-area curve. The results showed that, on average, rains with shorter return periods had lower reduction factors compared to larger rains when analyzing individual locations. However, this trend did not hold when comparing all locations, likely due to multiple factors influencing runoff beyond just the return period of the rain. Factors such as initial soil saturation, rainfall intensity, soil type and the slope of the area also affect runoff. The results also indicated that the size of impervious surfaces affects runoff, but other factors play a role as well. In conclusion, isolating a single parameter is challenging, and additional studies with more rain events would be needed to obtain a better understanding of the results.