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Geoinformatics-8-2009
article finite difference raster. spatial interpolations of these data including the determination of flood extents result, in general, in an enhanced mapping of flood characteristics. coupled one- and two-dimensional hydraulic models (1d/2d) allow for optimizing modeling and postprocessing efforts according to the hydraulic regime to be modeled. here, flow velocities and water levels are available at nodes of the finite element mesh and at cross sections in the one-dimensional case resulting in procedures for interpolating water levels and flood extents similar to the ones described earlier. step 2: intersection of water levels with digital terrain model data the water level and dtm data are intersected on the basis of tins in order to determine flow depths and flood extents. using tins in this operation is to be preferred against employing a raster-based analysis although the computation is more complex and the overall execution time longer. tins allow for a comprehensive inclusion of breaklines and other information relevant for hydraulic analysis in this process. in particular, connections or disconnections of possible inundated areas with the main stream, due to dikes and flood protection walls for example, can be automatically taken into account in this analysis by using tins. in order to effectively handle these time consuming tin intersections in batch mode, a special arcgis geoprocessing tool has been developed and added to the toolbox. alternatively, these calculations can be carried out on grid basis. figure 5. schematic diagram of a stage-damage curve for a residential house. purpose, but these are generally not free of charge. some examples include wspwin from bjornsen consulting engineers, sobek river and sobek rural from deltares, hydro_as-2d from nujic engineering firm, mike 11 and mike 21 from danish hydraulic institute, telemac from sogreah or hec-ras from us corps of engineers (also available free of charge). hydraulic models determine water levels and flow velocities at computational model nodes, e.g. at cross sections in one-dimensional hydraulic models. these data need to be reprocessed taking other data, such as dtms, land use, distribution of people and assets, into account. the overall objective of these works is to generate meaningful and geographically validated information and to depict this information in flood hazard and flood risk maps. step 3: determination of inundation depths after having intersected the water level and dtm data tins, the resulting tin is converted to a grid with a user-selectable resolution. this dataset might contain blurs which directly result from the intersection of water level and dtm data tins. these maps show flood areas and inundation depths which sometimes appear to be non-reliable due to uncertainties in dtms as well as in the hydraulic modeling results. these effects can be compensated for in step 4 of the workflow. flood hazard mapping the gis toolbox has been designed and implemented in order to automate the required processing for generating flood hazard maps to the utmost extent. these tools enable experts to fully control each step in the process chain and correct data according to his/her hydraulic and technical expertise. esri s arcgis, version 9.3 provides the framework for operating the tools; the tools are contained in a separate arcgis toolbox and are implemented in phython, vb and c#. the workflow consists of six work steps. figure 3 depicts this workflow including examples of corresponding geoprocessing inputs and results. step 4: analysis of the inundation areas the aforementioned blurs appear as very small areas within a close neighborhood on the map which are either dry or inundated; the latter possessing a small variance of inundation depths. small inundated areas with no connection to the main stream will be eliminated from the data set; small islands within inundated areas are redefined as being inundated. larger dry areas within inundated surroundings are classified as possible dry island areas which need to be verified by an expert review. possible inundated areas due to subsurface flows in the vicinity of the main inundated areas are also automatically detected. these areas are subject to verification by an expert review as well. in order to eliminate these effects geostatistical functions may be employed by the user for classifying inundation depths according to e.g. area size and water depth as well as connectivity of these areas to the main stream. the parameterization of these features is preconfigured in the system but can be altered easily by means of a graphical user interface by the expert. step 1: interpolation of water levels from modeling results import filters for a variety of hydraulic modeling systems are available, e.g. wspwin, sobek, mike 11, hydro_as-2d. new import filters for hydraulic modeling results can be easily implemented and added to the system. due to the variety of input data different methodologies are required for generating triangular irregular networks (tin) or raster datasets (grid). these methods have been integrated into the toolbox. in general, a complete coverage of water levels for the flood plain is required in order to determine the full flood extent by intersecting the water level tin coverage with the tin derived from the dtm. one-dimensional hydraulic models (1d) reduce real world three-dimensional hydraulic processes to one dimension on the basis of characteristic cross sections. flow velocities and water levels are determined for these cross sections by the modeling systems. therefore, these water levels need to be spatially interpolated between consecutive cross sections. the flood extents also need to be generated from these data by utilizing dtm data. two-dimensional (2d) hydraulic models determine water levels and flow velocities on the basis of a two-dimensional mesh of finite elements or a step 5: correction of the inundation areas after having analyzed and verified the identified features with the toolbox in step 4, the toolbox now reclassifies all of the identified features according to the results of the expert review. the result comprises semi-automatically derived inundation areas on grid basis which may be subject to further manual processing in step 6. step 6: manual editing of inundation areas due to modeling restrictions and missing information on culverts, which are not part of the dtm, the inundated areas might be extended or clipped by manually defined polygons in order to take care of these local conditions. december 2009 22
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