Integration of the Flood Risk in Coastal Hinterland Management

Anne Elsner, Stephan Mai, Volker Meyer, Claus Zimmermann

University of Hannover, Franzius-Institute (DE)

Management of coastal hinterland usually is separated from coastal defense planning assuming abso-lute reliability of coastal defenses, i.e. sea dikes or sluices, against storm surges. This assumption is now much under scrutiny due to increasing stress on defense systems, e.g. from accelerated sea level rise or economic restrictions. Risk analysis can integrate coastal defense planning into the manage-ment of the hinterland. Risk analysis comprises the calculation of safety of the coastal defense sys-tems and loss to be expected in case of failure. For this GIS is a valuable tool to analyze the effect from flooding. This is illustrated for the German North Sea coast for the cities of Wilhelmshaven and Bremerhaven.
Basis of calculation of flood damage is the extent of the flood zone. For this dynamic simulations of the flooding process are to be carried out using the numerical models. Results of the simulations, i.e. time series of water depths, are filtered to get the maximum water depth at each location during a storm surge. Maximum water depth is the important parameter to estimate the degree of damage for various property assets within the flooded hinterland. Several numerical simulations are carried out to obtain the influence of location and width of a dike breach on the flooded area and the water volume. Figure 1 shows an example of the influence of the dike breach on the flooded area and the water volume inun-dating. Doubling the width of the dike breach increases the flooded area by a factor of 1,2 and the av-erage flooding depth by a factor of 1,3.

Figure 1 - Effect of dike breachon flooded area and flood water volume (Mai / Zimmermann 2003)

As basis for the analysis of flood damage the land use is obtained from the digital landscape model ATKIS-DLM provided by LGN (Hannover, Germany). The attribution of unit values to different land uses requires two steps. In the first step various asset categories are determined for the affected communities. The assets of each community are to be derived from official statistics e.g. population figures and labour statistics. In the second step asset categories are assigned to land uses (Meyer und Mai, 2003). The outcome are values per unit area of land use. Figure 2 gives an example of the different assets in the hinterland of the city of Wilhelmshaven.

Figure 2 - Example of the Assets in the Hinterland of the City of Wilhelmshaven (Screenshot Loss-Calculator)

For the flood zones obtained from numerical simulations the potential damage, i.e. the total value of all assets, is calculated. Taking into account the grids of maximum water depth in the hinterland the de-gree of damage is derived for each location. This methodology is carried for different damage functions to show the sensitivity of the results. The superposition of damage and potential damage results in a monetary loss.
Analysis is carried out using the geographical information system ArcView 3.1. In this context the two additional tools "Flood-Analyser" and "Loss-Calculator" developed under "Avenue" are used to handle the large data input. The "Flood-Analyser" is used to examine the flooding process with respect to maximum depth of flooding. The next step of the analysis is with "Loss-Calculator", including the dam-age functions, the grid of maximum water depth and the map of land uses with attributed values per unit area. Figure 3 shows a screenshot of this "Flood-Analyser". [Elsner 2002]

Figure 3: Example of the analysis of the land use within the flooded area (Screenshot Flood-Analyser)

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