Spatial priority of flood potential areas

Although flood damages normally concentrated along certain reaches of the main watercourse, but the contribution of tributary subwatersheds to the flood effects at downstream should not be overlooked. This research present a methodology, based on the use of mathematical hydrologic models simulating mutual interaction of effective factors, to study the spatial distribution of flood producing areas with in watersheds .> The watershed of interest, was divided into seven subwatersheds which were digitally characterized in a geographic information system (GIS). Subwatersheds flood hydrographs associated with design rainfalls were determind using HMS model and were routed in the stream network to yield the total hydrograph at the outlet. With successively eliminating subwatersheds from the simulation process, in a method titled “Successive Single Subwatershed Elimination method"(SSSE), flood hydrograph at the outlet was determined so that the contribution of each subwatershed in the flood peak at the outlet could be quantified. The routing results show that this contribution is not only a function of subwatershed discharge and size but also a function of other factors (discharge sychoronization at the outlet). Thus, any flood control measure must consider the flood area prioritization in term of contribution at the outlet.> Also, for determine of effective factors, sensitivity analysis on watershed slope, stream slope and CN Was carried out. CN was identified as the most important factor as it can be easily altered in flood control project. Reduction of stream slope by 30% decreases the peak discharge by only 8%, while a l0% reduction in CN causes the peak to drop by 21%.
Keyword: Flood producing potential, priority, sensitivity analysis, subwatershed, HMS model, GIS.