Evaluation of Flood Hazard Potential and Investigation of Damage Caused by it in Talar Drainage Watershed

      Flood is defined as an unconventional increase in river discharge. Complete protection from flood hazards is rather impossible. For those living next to floods, implementing new policies is necessary regarding land use management and development of residential areas along rivers to reduce the effects of destruction. One of the basic steps to reduce the harmful effects of floods is to identify flood-prone areas and grade these at risk. For this purpose, one of the solutions against floods is to prepare for a zone. By using zoning maps in watersheds, flood preparedness can be created. The purpose of this study is to perform the zoning of flood-prone areas in Talar drainage watershed and to calculate the damage in areas eroded and destroyed by floods in 20 meters of river, which due to the geographical location and weather conditions in recent years, this region has been faced with various floods and in this regard, the importance of risk zoning and management and planning of the region intensifies.

In order to conduct this research, input data including digital geological file, digital vegetation file, digital soil file, digital elevation model (DEM) with a spatial resolution of 10 meters, satellite image, and flood event statistics and information were used on 19 July 2015. HEC-1 model in WMS  was used to estimate the hydrograph of SCS unit in Talar basin and Kasilian sub-basin.To determine the flow and boundary conditions, the peak flow number for the flood of 19 July 2015 is 163 cubic meters per second. In order to zone the flood, software WMS and HEC-RAS models have been used.  Since the upstream and downstream slope of the river have a significant impact on the flooding process, the upstream slope is 0.02, and the downstream slope is 0.01. Water elevation points were imported from HEC-RAS program in WMS, and cross section profile plots were prepared at the location of sections located in rural and urban areas, and finally, water depth maps were prepared and compared for the event on 19 July 2015. The Kruger Method was used to determine the maximum instantaneous discharge for 50-year and 100-year return periods. In order to calculate the damage in flooded and eroded areas, a 20-meter area of the river has been considered. Then, the monetary values in each square meter of the covered uses have been estimated according to experts, and finally, the flood damage has been calculated. Duncan-Tukey and ANOVA tests were used to assess the hazard.

    In the SCS unit hydrograph, the peak time is 390 minutes for the Kasilian basin and 555 minutes for the Talar basin. The height of the current water level and the height of the water level at the critical level in some cross-sectional profile plots have coincided, which indicates the dangerous situation in these areas. The largest area of ​​flood zones in the flood of 19 July 2015  in Shirgah, Zirab, and Pol Sefid cities at a depth of  0.6  meters, Do Ab villages at a depth of  0.3  meters, Lerd and Rudbar villages at a depth of  1.2 meters, Darzikola, Vazmela and Sangdeh villages at a depth of  0.6  meters. The highest area of ​​flood zones in the 50-year return period is in Pol Sefid and Shirgah cities at a depth of  0.3  meters with an area of ​​181 hectares, and the 100-year return period in Shirgah with an area of ​​292 hectares at a depth of  0.3  meters. Also, the largest area of flood zones in the 50-year and 100-year return period in rural areas is located in the villages of Valik Ben Sang Deh, Darzikola, and Vazmela, with an area of 276.9 and 188.6, respectively, at a depth of  0.3  meters. The results of statistical tests in the risk assessment section showed that in total, two uses of rangelands with an average water depth of   0.57 meters have the lowest average water depth and urban residential areas with an average water depth of  0.84 meters have the highest average water depth.

According to the flood zoning on 19 July 2015, it was determined that the highest areas of water depth zones of  0.3 meters,  0.6 meters,  0.9 meters,  1.2 meters, and  1.5  meters are located in Pol Sefid city. The zoning of the 50-year and 100-year flood return period also indicates an increase in the area of ​​water depths of 0.3 meters,  0.6 meters, 0.9  meters, 1.2 meters, and  1.5 meters compared to the flood of 19 July 2015. Considering the area of ​​flood depth zones in the flood of 19 July 2015 and the area of ​​land uses covered by flood risk, it was determined that the most damage is related to the Zirab city in residential use with a damage of  266,482,744,183 Rials. Then the city of Pol Sefid with agricultural use with damage of 96,979,434,983 Rials and the city of Shirgah with garden land use with damage of 78,544,366,182 Rials. Finally, flood risk assessment with Duncan, Tukey, and ANOVA tests showed that residential land use has the highest average depth of  0.84 m, and Lerd rural area with an average depth of  1 m has the highest flood potential.