Estimation of damages caused by storm tides in the city of Hamburg K.-H. Pöschke DHI-WASY GmbH Waltersdorfer Str. 105, Berlin, Germany

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1 Estimation of damages caused by storm tides in the city of Hamburg K.-H. Pöschke DHI-WASY GmbH Waltersdorfer Str. 105, Berlin, Germany 1 Introduction The city of Hamburg is potentially endangered by storm tides. The city is about 50 km away from the Elbe river mouth, but high tides and bad wind conditions can press enormous masses of water into the Elbe river. Figure 1: Location of Hamburg During the last storm tide catastrophe in 1962 more than 300 inhabitants lost their lives and more than 6000 houses were damaged. A tide of NN m caused this enormous disaster. Figure 2: Situation during the 1962flood Meanwhile the flood protection facilities have been reconstructed and today the dikes are much higher than in the sixties. These facilities could already prevent eight higher tides than these of Every inhabitant in these residential areas knows, no technical system can provide 100% safety. Dike breaks, dike overflow and defective flood protection facilities can still cause flood disasters. Several areas of the Hamburg city have to be protected during every high tide. That means, the inhabitants in these areas must trust every 12 hours in the technical facilities. When it comes to a flood event, then the rescue service needs detailed information for effective handling. In the middle of the 90s the Hamburg Senate required a flood simulation system for the development of disaster scenarios and rescue planning. It should also be able to calculate real scenarios during a flood event in a reasonable time. The fast rising water level during a storm tide requires very fast responses from the rescue service, which otherwise require detailed information about the present situation and the estimated damage.

2 Figure 3: Storm tide in a not protected city area In 1998 the Senate of Hamburg ordered a software solution for damage estimation in the endangered areas from WASY GmbH. The system HWSIM was designed as a planning tool for the estimation of flood events and for developing evacuation guidance. The first system was developed on the basis of ESRI ArcView 3.2 GIS. Since then the system has been extended by the area of the Hamburg port (2000) and redesigned for the latest ArcGIS technology (2007/8). The short lead time for storm tides requires a fast flood model combined with a geographical information system. HWSIM can simulate dike overflow, dike breaks and damaged or obstructed flood protection facilities like tide gates. HWSIM gives flood information for different time steps. A mouse click on the map provides the water level and the flood curve for the specific place. It is possible to define different observation points that automatically generate alarms when predefined conditions appear. The solution is now used for developing catastrophe scenarios, staff training and in operative disaster control during storm tides.

3 2 Hydrological Basics 2.1 Flood Diagram The flood diagram for the reference tide scale Sankt Pauli is the dynamic input for the calculation. The values and the shape of the flood diagram depend on the wind conditions at sea side, waves and the outflow of the Elbe River. There are different kinds of storm tides: Normal storm tide Late bloomer is mostly not seen during the low tide phase at the costal site Collapsed storm tide breaks down during the tide Figure 4: Flood diagram for the reference tide scale Figure 5: Calculated diagrams for the specific city tide scales The shape of the flood diagram also depends on the water flow into the polders. For creating a flood diagram the time and the water level at the low tide and the time and the estimated water level at the high tide must be defined. The present outflow of the Elbe is part of the calculation. Separate water levels for three additional times within the tide period can be defined. Depending on the flood diagram of the reference tide scale additional flood diagrams for the other tide scales in the city area are calculated. 2.2 Digital Elevation Model (DEM) The DEM is the static part of model input and has to be created for the whole observation area. Many tools have been developed for data transformation and for creating a unique elevation model from the different data sets. Converting LIDAR XYZ-file in Shape / Grid / Mosaic Converting ESRI ASCII Grid in Grid / Mosaic TIN-creation / TIN converting in Grid Converting CAD data (DWG) to Shape File Grid replacement Clip Grid with Polygon Shape File Creation of Grid Mosaics Converting Grid to Polygon Shape File Converting Grid to Shape-file / ASCII file (XYZ)

4 Calculation of the polder statistics / polder area & -volume at Zj Calculation of the polder edge statistics / edge Z 2.3 Water level calculation The calculation area is divided into so-called polders. A polder is an area which is separated by edges with specific shapes. Figure 6: Calculation procedures For every polder a function for the volume depending on the water level is created. For every edge a function for the flow rate depending on the water level is defined. The calculation of the water distribution in a polder on the river side depends on dike overflow, wave overflow, dike break and defective flood protection facilities. After that the distribution between the certain polders is calculated. The water level and the flooded areas are stored for every time step.

5 3 HWSIM the software The software is now used by the Hamburg city and the port authority. The client version bases on ESRI s ArcGIS technology and does not need any ESRI extension (Spatial Analyst, 3D Analyst) for the flood estimation. The user friendly graphical interface allows an easy handling and provides all required information for every requested time step. Figure 7: HWSIM GUI The calculation starts with defining the flood diagram for the reference tide scale. After that, expected dike breaks or defective flood protection facilities can be defined. Observation points can be set and the system will generate an alarm call when the water level reaches the critical value. Figure 8: definition Dike break Figure 9: Definition of defective flood protection facilities Figure 10: Definition of observation points

6 At last the value for the time steps and the kind of simulation must be agreed. This can be done time step by time step, continuously until a critical water level or until the end of the tide. At the end of the simulation, the results can be shown in diagrams, tables and maps. Figure11: Flood map with dike break

7 4 Preview HWSIM Integration into GeoFES At the moment only flooded areas and water level diagrams for specific points can be displayed. Messages can be automatically generate depending on water levels of observation points. It is planned to integrate HWSIM features into GeoFES the decision support and rescue management system of DHI-WASY. The flooded areas will be used as areas of analyses. A lot of additional information like number of inhabitants, stored dangerous material, schools, and kindergartens can then be used to find the right decision. The integrated management tool for human and technical resources helps the rescue service to optimize the handling of the rescue forces. Figure 12: Rescue management of human & technical resources That means HWSIM can then be used for the management of flood disasters including resources control and disaster documentation. At last the rescue event will be documented in tactical maps and the complete handling will be recorded for later analysis. The update to the latest ESRI GIS technology provides new features and a much easier handling for the users. The very effective tool set allows an easy creation of a digital elevation model, even when no unique dataset is available for the analysis area. The fast calculation routine qualifies the software for operational use during real disasters in other cities, too.