Urban Flash Floods

Water-management issue

The Hagen case study focuses on local heavy rainfall events and how the negative impacts on the city's urban areas can be mitigated. For planning purposes the municipality so far uses data of observed rainfall events and added a flat 10% increase in heavy rainfall precipitation due to climate change based on extrapolations from the observation data.
For investigating which areas of the city would be flooded in the event of e.g. a 100-year return period event, runoff simulations it would be good to make use of the latest climate projections. On this basis simulations could be carried out that more accurately calculate the flood depths and flow velocities of stormwater in Hagen. The results could then be used as a basis for developing adaptation strategies at neighbourhood and city level by integrating other information e.g. on land use, population distribution etc.
The more accurate rainfall and run-off simulation data are of particular interest to Hagen’s Urban Development department and Civil Protection department (see below).

Decision support to client

Urban development (city-wide level)
In which urban areas should the development of new settlement areas (residential, commercial, industrial, sensitive uses) or infrastructural facilities (electricity supply, transport infrastructure) be avoided or only developed in a climate-adapted manner?
Civil Protection (local level)
Which local areas (e.g. road intersections, individual buildings, infrastructure) are expected to be affected in the event of a disaster? Which roads are flooded at what depth? Are there risks to life and health due to the depth of flooding and flow velocities?

Temporal and spatial scale

Urban development (city-wide level)

  • Time scale: the next 10 to 25 years (planning horizon for the new integrated urban development plan)
  • Spatial scale: entire jurisdiction of the City of Hagen (about 160 km2)

Civil Protection and emergency response (local level)

  • Time scale: immediately
  • Spatial scale: very local scale (e.g. road sections, individual buildings, infrastructure)

Climate Indicators

Pan-European and local indicators

  • Data and forecasts on heavy rainfall
    • 20-yearly events up to 100-yearly events
    • Regarding 1 hour to 1 day of heavy rainfall
    • Small-sized / urban scale data (mostly raster data)
  • Other local indicators (non-climate indicators)
    • Elevation models
    • Land-use data (e.g. surface sealing, buildings, infrastructures, soil conditions)
    • Population and age distribution data

Case study workflow

A – Urban Development

Precipitation Data For the case study of Hagen, values for very heavy rainfall events (that will increase in the context of climate change) are relevant. The data provide modeled rain quantities for a specific annuality of an event (e.g. 100-year event) for different time periods (e.g. one hour). In Germany, KOSTRA data from the ‘Deutschen Wetterdienst’ (German Meteorological Service) usually provide these data at raster cell level.

Local Disaggregation of Data The rainfall data are adjusted using stationary measurements with regard to local conditions (e.g. topography) in order to achieve more realistic values. This results in more precise, smaller-scale grid cells of the rainfall quantities than in the original data.

Hydrological Simulation With the ArcGIS extension FloodArea, a runoff simulation will be used to determine spatial areas of the city of Hagen that are flooded during heavy rainfall or where high flow velocities are projected to occur. As input data for the simulation, digital terrain models, current land use and existing buildings are used in addition to data on rainfall quantities.

Flood Extent and Depth Map As a result of the simulation, the calculated areas of the city are determined, which are flooded during a certain rain event and in which time. Furthermore, the flood depths are determined.

Future Settlement Planning Oriented Flood Map On the basis of the flood extent and depth map, further maps are generated that take into account future settlement development with regard to the potential risk of flooding. These maps serve as an aid in which areas of the city the future development of residential or commercial areas should be avoided with regard to the risk of flooding or in which areas special adaptation strategies should be taken into account in planning.

Vulnerability Map The vulnerability mapping shows existing areas of the city which are sensitive to flooding. On the one hand, these can be area-based uses such as residential areas or industrial estates, or on the other hand, social or traffic infrastructures such as kindergartens, retirement homes and subway stations. The mapping is independent of the modelled flooding results.

Flood Risk Map The Flood Risk Map compares the results of the Flood Extent and Depth Map with the Vulnerability Map and thus shows areas that are projected to be at risk in the event of heavy rainfall.

Adaption Plan The adaptation plan identifies vulnerable areas where specific adaptation strategies can be used to minimize the risk of flooding.

Integrated Urban Development Concept Within the framework of an Integrated Urban Development Concept, both the existing areas and future settlement areas are identified, where the topic of heavy rainfall should be given special attention and adaptation measures should be considered.

B – Civil Protection and Emergency Response

Precipitation Data For the case study of Hagen, values for very heavy rainfall events (that will increase in the context of climate change) are relevant. The data provide modeled rain quantities for a specific annuality of an event (e.g. 100-year event) for different time periods (e.g. one hour). In Germany, KOSTRA data from the ‘Deutschen Wetterdienst’ (German Meteorological Service) usually provide these data at raster cell level.

Local Disaggregation of Data The rainfall data are adjusted using stationary measurements with regard to local conditions (e.g. topography) in order to achieve more realistic values. This results in more precise, smaller-scale grid cells of the rainfall quantities than in the original data.

Hydrological Simulation With the ArcGIS extension FloodArea, a runoff simulation will be used to determine spatial areas of the city of Hagen that are flooded during heavy rainfall or where high flow velocities are projected to occur. As input data for the simulation, digital terrain models, current land use and existing buildings are used in addition to data on rainfall quantities.

Flood Extent and Depth Map As a result of the simulation, the calculated areas of the city are determined, which are flooded during a certain rain event and in which time. Furthermore, the flood depths are determined.

Flood Depth and Flow Velocity Map A second result of the simulation is a map of the flow paths in the context of heavy rainfall events. Where does the water run off very quickly and with what depth, so that potentially life-threatening situations would occur?           

Vulnerability Map The vulnerability map shows existing areas of the city which are sensitive to flooding. This applies in particular to infrastructural facilities such as roads and supply and disposal facilities. In addition, social institutions such as kindergartens or retirement homes are very sensitive areas. Mapping population and age distribution is also used for identifying vulnerable areas.

Flood Risk Map The Flood Risk Map compares the results of the Flood Extent and Depth Map with the Vulnerability Map and thus shows areas that would actually be at risk in the event of heavy rainfall. This shows where emergency and civil protection services would be expected to have to act in the event of a heavy rainfall event.

Emergency Risk Map The Emergency Risk Map compares the Flood Depth and Flow Velocity Map with the Vulnerability Map. Where are densely populated areas that are quickly and deeply flooded by rainwater so that there is a risk to life and limb? In addition, roads are identified which, due to their flow velocity and flood depth, can no longer be driven on with emergency and disaster control vehicles.

Emergency Response Plan The results of the Flood Risk Map and the Emergency Risk Map can be used to provide concrete support for disaster management in the planning of operations and thus enable the operations manager to create targeted Emergency Response Plans.

 

Importance and Relevance of Adaptation

The City of Hagen so far uses data of observed heavy rainfall events and on this basis extrapolated that climate change would lead to a 10% increase in heavy rainfall precipitation. Different climate change projections have not been taken into account so far. This method is used for e.g. planning the stormwater drainage system. The urban planning/urban development department has so far not been using flash flood – not to mention projected flash flood risks (on the basis of climate change projections) for their long-term plans of future urban development. A new Integrated Urban Development Concept is currently in development, with comprehensive citizen and stakeholder involvement. The new inputs provided by AQUACLEW (as described above) would be highly appreciated in order to integrate the issues of future flash floods into this plan.
The same is true for the civil protection department, which is currently in the process of developing emergency response plans for flash flood events. Results of AQUACLEW (as described above) would support these efforts and provide the basis for more accurate and detailed emergency plans and management.

Scientific Publications

One article each in a scientific journal is planned for a) the urban planning application and b) the civil protection application.

Consultancy or Knowledge Purveyors

Institute of Spatial Planning, Faculty of Spatial Planning, TU Dortmund University (TUDO); Germany

Clients

City of Hagen

Contact

Johannes Lückenkötter, Jörg Peter Schmitt, Institute of Spatial Planning, Faculty of Spatial Planning, TU Dortmund University