Urban Flooding in Gothenburg - A MIKE 21 Study

Abstract: Climate change has affected the extreme weather throughout the world. Studies have shown an increase of the extreme precipitation in Gothenburg, Sweden. At the same time, space limitations and cost considerations prevent the development of an optimized drainage system. Designing a stormwater system for an extreme rainfall is impossible due the high cost. In addition, the lack of space limits the availability of infiltration areas. However, the cost associated with floods in the cities is very high due to high property and infrastructure value and the high economical activity. In this paper, a two dimensional (2-D) hydrodynamic simulation model using MIKE 21 was developed as a tool to simulate stormwater related flooding in the central part of Gothenburg. This was followed by development of flood risk maps. The model was developed by using topographical data and fine time resolution precipitation data. This was then used to simulate 2-D overland flow in the catchment to develop, implement and verify the model approach in urban areas. The topography data is first processed in ArcGIS so that it can be further used in MIKE 21.Then, a bathymetry for the area is created in MIKE 21 considering boundary layers and other parameters including land value. For the Manning number, two cases were considered: in the first case, a constant value of the Manning number was assumed over the entire computational grid and in the second case, a land use map was used with different Manning numbers for buildings, roads, parks and water. The model simulates unsteady flooding and drying processes over the urban areas. The developed two-dimensional model shows that the parts of the study area including the canals, the streets and the space in between the residential buildings would be flooded if the drainage system is blocked due to extreme precipitation. The model results provide spatial flood risk information such as water depth and flow velocity during flooding and can be used to evaluate the effectiveness of flood protection practices in central Gothenburg. Flood maps with maximum water level have been generated and presented in the study. The findings from this model can be used for development of a comprehensive urban flood management plan of Gothenburg. The overall objective of the output from model is to provide an integrated planning and management tool to allow cost effective management for urban drainage systems.