Zoning of the 25-Year Flood for Use in Determining the Boundaries of the River’s Bed and Boundaries (Til River as Case Study)

One of the management measures that can play an important role in reducing flood damage is preparing a flood zoning map. The preparation, compilation, and having a comprehensive and complete plan in the field of flood controlling and determining the boundaries and bed of rivers is an obligatory and necessary matter that lead to flood management and damage assessment. In this research, for Til River and 2 branches of the tributaries entering this river, in order to determine the bed and the boundary, produce flood zoning map and obtain the quantitative boundary of the river  by using the DLSRS standard, the HEC-RAS hydraulic model and Arc-GIS software with the HEC-GeoRAS connector plugin has been used.

First, the HEC-RAS input file was prepared by using the topographical map of the Til river area from Arc-GIS software and by using the HEC-GeoRAS extension, and after entering the geometric data and data of the processing flow and related calculations in the environment of this software for zoning. The flood took place.to determine the boundaries of the river beds and these waterways, floods with a return period of 25 years are used.Eventually, with the application of HEC-GeoRAS extension and Arc-GIS, flood zoning maps are extracted. Moreover, by using the DSLRS standard, the quantitative boundry of the Til River and tributaries 1 and 2 in the studied area are determined according to the field visits and appropriate engineering advice, and the numbers related to each index and finally the amount of quantitative boundry are calculated.

In accord with the extrcated maps and satellite images and according to the investigations and visits, the width of the hydraulic bed calculated is different from the bed width defined in the law due to many uncertainties. Therefore, at this stage of the studies, expert surveys should be conducted through step-by-step visits to both banks of rivers and waterways along the study area, and eventually, legal bed width should be recognized and marked on the maps.

The results show that integration of HEC_RAS model and GIS geographic system is an efficient and useful tool in determining river floods. According to the investigations, the comparison of existing maps and land uses around the Til River and 2 sub-channels demonstrate that in the event of a flood with a return period of 25 years, it does not cause sever damage to the areas around the river. In accordance with DSLRS standard, qualitative boundary of Til River and sub-channels 1 and 2 in the study area are carried out with field visits and ultimately, the amount of boundry is calculated,As a result, the amount of the quantitative sanctuary of the Tail river in the scope of the study plan was calculated to be 8 meters and also the quantitative sanctuary of Mesil 1 and 2 was calculated to be 4 and 5 meters, respectively.