Hazard analysis of runoff

Runoff resulting from heavy rain over urban areas differs significantly from runoff caused by river flooding (delimitation can be sometimes ambiguous). Due to widespread anthropogenic sealing in urban areas precipitation triggers surface runoff almost immediately. What we aim at with this subtask is to analyse the impact of different structures of urban areas and public space as well as topography on runoff behaviour in detail.

Runoff in a city

Since a sewer is normally designed for a return period of 5 to 30 years, rare heavy rainfall events trigger runoff on streets and pathways, where instead of once natural streams and channels now only heavily regulated little streams exist. In addition, surface runoff from nearby natural catchments can also flow into an urban area. Concentrations of runoff and development of flow paths depend on topography and housing estate structures. Great importance is attached to natural and anthropogenic hollows and sinks (e.g. underground parking), where temporary water pools may develop. Runoff situations might tighten when hydraulic structures congest e.g. as a consequence of an extreme hail event. Technical structures of buildings such as eaves gutters or water supply tanks are not designed for heavy rain events. Thus, heavy rain is able to rapidly overload such structures and cause extreme damage on affected buildings and infrastructure.

Research focus

• Runoff development as a result of heavy rain over an urban area
• Special hazard areas in urban areas (industrial land use, infrastructure)
• Heavy rain impacts on buildings and building services
• Runoff drainage in urban areas depending on topography, structures of urban areas and public space as well as housing estate
• Impacts of housing estate related changes on the natural topography and flow paths
• Behaviour and capacity limits of structures for rain water drainage and secondary flow paths (streets, paths)
• Identifying of hot spots within a floodwater drainage system in urban areas
• Runoff prediction through hotspot observation

Approach and results

Flash floods usually exceed the capacity of urban sewers. Runoff takes place in open space, streets and paths, following downward slope. The development of this precipitation-runoff-process will be analysed by means of case studies and related derived various housing estate structure types. Based upon detailed digital elevation models (laserscan surface models) a high resolution unsteady 2D-runoff-simulation “in-between-buildings” and with regard to various open space structures will be performed by using HYDRO-AS_2D. Expected results will be spatially detailed and time-dependent depths of flow, flow directions and flow velocities. At the same time runoff drainage in the sewer as well as runoff losses (backwater in basements) will be considered and evaluated. The analysis also takes into account imponderabilities in case scenarios such as runoff bottlenecks, sedimentation, clogging of intakes etc. As results are expected knowledge about a “typical” runoff situation and its course as well as identification of significant hotspots relevant for urban planning. Based on this, municipalities may conclude on necessary research and contract sewer problems to experts. Existing results from previous research projects such as RiSurSim will be taken into account.