Impact of the Digital Elevation Model in Pluvial Flooding : A Study on Pluvial Flood Modelling with Temporary Flood Barriers in Karlstad, Sweden

Sammanfattning

Urban pluvial flooding is an increasing challenge due to urbanization, climate change, and the increased frequency of intense rainfall events. Hydraulic flood modelling is commonly used to assess flood risk, but the accuracy of model outputs depends strongly on the quality and resolution of input data, particularly the digital elevation model (DEM). In urban areas, where small-scale terrain features influence the water flow, the DEM spatial resolution may affect this representation as well as affecting the flood modelling results. The aim of the study was to investigate how DEM spatial resolution influences urban pluvial flood modelling results and the simulated efficiency of temporary flood barriers. A two-dimensional hydraulic model was created in HEC-RAS for the urban area of Sundsta and Norrstrand in Karlstad, Sweden. Flood simulations were performed using DEMs with spatial resolutions of 1 m, 5 m, 10 m, 15 m, and 20 m, with a synthetic rainfall based on a historical 100-year rainfall event with a duration of 60 minutes. Two temporary flood barrier scenarios were analyzed: protective barriers intended to reduce flooding within a designated area, and a blocking barrier intended to intercept and redirect surface water flow. The results showed that DEM spatial resolution significantly influenced simulated flood extent and the water depth, indicating the importance of DEM resolution in pluvial flood modelling. The efficiency of the temporary flood barriers was also found to be dependent on the DEM resolution. The protective barriers reduced flooding within the intended area to a varying degree depending on the DEM resolution. This suggests that the efficiency of protective flood barriers is sensitive to the DEM resolution. The blocking barriers proved to be more sensitive to DEM resolution, as a higher resolution intercepted the flowing water and reduced flooding downstream of the barrier. When using coarser DEM resolutions, the water flow paths were less concentrated, and the barrier efficiency decreased as a result. The study confirms the impact of DEM resolution in urban pluvial floods. It also suggests temporary flood barriers’ sensitivity to DEM resolution. It highlights that, in complex urban environments, terrain resolution is an important aspect to consider in flood risk management.