Surface Runoff on Green Urban Areas : A study on driving forces behind surface runoff generaiton

Sammanfattning: Recent flooding events, such as the one in Germany 2021 have caused irreversible damages to infrastructure and lives. The aftermath of events like these have underlined the importance of an accurate risk management by predicting them in urban areas. This is done by hydrological modelling in which topography and presence of hardened surface often determine the magnitude of the flood. However, the contributory part from green areas to a flooding event is often simplified by a runoff coefficient not taking all of the soil hydraulic properties into account.  This thesis aims to study the surface runoff generation from three different soil types from different green urban areas in Uppsala, Sweden. Simulations have been conducted using the modelling tool Hydrus 1-D and two different types of rainfall-runoff simulations were investigated. Firstly, a 25-year historic rain series was simulated to investigate what soil moisture content was present in the soils before large rain events and what the main driving force for surface runoff generation was. The second part of the simulations was conducted using five type hyetographs depicting large rain events in Sweden. The aim of these simulations was to further investigate main driving forces for surface runoff generation and to examine if the temporal distribution of a rain event effect the surface runoff generation.  The results showed that the soil moisture content present in the soils before large rain events were very close to the field water holding capacity and that the main driving force for surface runoff generation on the studied sites was peak precipitation intensity, but also that the mean precipitation intensity and soil moisture had a significant effect within a 95 % confidence interval. The hyetograph simulations showed that the temporal distribution of a rain event effects the surface runoff generation on the studied soil, with the events whose peak came later generally generated more surface runoff.