The role of flood routing in determination & separaration of flood-generated regions in watershed

Mohammad Khosroshahi [1] Bahram Saghafian[2]

[1] -Research Institute of Forests and Rangelands. P.O.Box 13185-116,Tehran, I. R. Iran.
Email: khosro@rifr-ac.ir

[2]- Soil Conservation and Watershed Management Res.Cen.P.O.Box 13445-1136,Tehran, Iran.
Email : saghafian@scwmrc.com

Abstract

Most areas in the country are subject to frequent flood damages in term of increasing loss of lives and properties. Considering the direct and indirect consequences of flood in economic terms, high priority must be given to flood studies. In this regard, areas with high flood potential must be identified. Since climatic factors may not be altered, flood prevention measures should be sought on the ground. Thus, watershed areas with high runoff potential are to be determined. Existing methods have considered the watershed as a lumped unit, focusing on regional or approximate analysis. In limited reported cases, the hydrologic response has been assumed linear and no hydrograph syncronization and attenuation due to stream routing have been taken into account.
This article presents a methodology, based on the use of mathematical hydrologic models and the interaction of effective factors, to study the spatial distribution of flood potential areas in watersheds. The flood potential index can be determined in the context of hydrologic response units.
The watershed of interest, Damavand, was divided into seven subwatersheds which were digitally characterized in a geographic information system(GIS). Subwatersheds flood hydrographs associated with various design rainfalls were determind using HMS model and were subsequently routed in the stream network to yield the total hydrograph at the outlet. With suceessively eliminating subwatersheds from the simulation process, in a method titled “successive single subwatershed elimination method", flood hydrograph at the outlet was determined so that the contribution of each subwatershed in the flood peak at the outlet could be quantified. Then, all subwatersheds were ranked with respect to the order of contribution to the outlet flood peak. The routing results show that this contribution is not only a function of subwatershed discharge and size but also a function of other factors. Another words, nonlinear response is obvios. Thus, any flood control measure must consider the flood spatial prioritization in term of contribution at the outlet.
The methods outlined in this article is believed to enable the flood potential zonation. Execution of the methods is strongly recommended in flood studies.
Keyword: Flood potential zonation, Flood control, priority, Spatial flood generation, HMS model, GIS, Damavand basin.