关键词: GIS, 洪水淹没, 种子蔓延

Abstract: In this paper, a model of flood area display and damage evaluation is discussed, which adopts seed spread algorithm to calculate and visualize flood area based on a DEM data set and under the condition of a given water level of flood. The raster image containing flood area is overlaid with raster data converted from thematic vector maps such as land use map and related to other socio economic data, and then quantitative evaluation of flood damage is achieved. There are two different cases, the so called “non source flood” and “source flood”, for determining flood area based on DEM and under the given water level condition. Non source flood corresponds to the case with well distributed and large area rainfall where all the low lying land may have a flood disaster, and in this case all the points with elevations below the given water level should be included in the flood area. The “source flood” is the case of flood (e.g. from bank burst) flushing through the surrounding lands that needs to consider “circulating” condition. Because flood may be obstructed by ring structures or high lands, thus it can only cover the place where it flows and reaches. One of the computer processing methods suitable for source flood is the seed spread algorithm. The principle of this method is to select a representative pixel as a seed and to examine its contiguous pixels outward along 4 or 8 directions. The pixels, contiguous to it and meeting the given conditions, become seeds, and then contiguous pixels to the new seeds will be examined in the same way. This process repeats until an aggregation of the pixels which meet the given conditions on a certain plane are obtained. Adopting seed spread algorithm for source flood area calculation is just to achieve an aggregation of pixels, which satisfies the connectivity requirement and the given conditions of water level. The initial position of seed pixel should usually be selected on the characteristic points, e.g. adjacent to a reservoir dam or stream boundary. The precision of flood area calculation is mainly determined by spatial resolution of the DEM, the area on the ground is represented by each pixel; whereas the accuracy of flood area calculation, to a great extent, is dependent on the precision of the DEM’s pixel value or elevation. Once the area below the flood water level has been calculated, the flood damage evaluation can be calculated by overlaying the flood range with thematic maps and by relating to other socio economic data. All thematic maps were converted to raster data because algorithm of the overlay analysis implemented in raster format is relatively simple and computing is fast. In order to improve accuracy of the flood damage evaluation it is necessary to accurately register and georefer all raster layers before overlay analysis. The calculation results of flood damage evaluation are generally represented by tabulation, which is a kind of classified statistics of flood damage data, e.g. a table representing total area of every land use type in different administrative divisions. The results of flood area calculation and damage evaluation can also be visually and dynamically provided in the forms of graph, image, 3D or virtual reality. The model has been successfully applied in the “Zhejiang Provincial Water Conservancy Management System” developed for a national project.

Key words: GIS, flood, seed spread