Prioritization Beheshtabad Sub-basins with regard to Flooding Potential

Flood is a natural hazard that takes place more frequently in recent years. In order to better flood mitigation and control, it's needed to recognize flood production factors and determine the high potential flood areas. Hydrological model is a simplified representation of natural system and the rainfall-runoff model is one of the most frequently used events for flood simulation. HEC-HMS is one of the computer models and for its ability in simulating short time events become very popular. The aim of this research is to investigate spatial prioritization of flooding in Beheshtabad sub-catchments using HEC-HMS software.
Material and Behashtabad Basin located in Chaharmahal-va-Bakhtiary Province, Iran, is 3866 km2 and its mean altitude is 2317 meter above sea level. It is divided into 6 sub basins according to 6 hydrometric stations. Land use categories of study basin extracted using ETM Images for 2009. Using 170 ground control points land use map of Beheshtabad basin was prepared with total accuracy of 99.34 and Kappa Index equals to 0.81. Rangelands covers most of the study area. Soil Hydrological groups and land use data used for mapping sub basin's curve number considering antecedent moisture of past five days as well. The mean curve number of study basin is 72.69. Daily precipitation data of 6 rain gauges in study area used for analyzing maximum 24 hr precipitation in different return periods. Rainfall hyetographs of flood events derived from recording rain gauges data. CN method used for estimating initial loss, SCS method used for runoff hydrograph simulation, and Muskingum method used for flood routing simulation. HEC-HMS model was calibrated using 2 Flood hydrographs and corresponding hyetographs for each sub-basin and validated for 1 flood event.
Rainfall loss in Beheshtabad sub basins varies from 0.13 to 0.19, curve number varies from 69.73 in Kooh-Sookhteh sub-basin to 73.71 in Kharaji sub basin, and lag time varies from 0.99 hr in end Beheshtabad to 5.72 hr in Kharaji sub-basin. Using optimized rainfall loss index derived from calibration stage, HEC-HMS validated for one flood event for each sub basin. Model validation shows very little difference (below 1%) between estimated and recorded data in all sub-basins. Among sub basins, Darkesh-Varkesh has the most and end Beheshtababd has the lowest peak discharge in all return periods. Priorization of sub basins according to their areas show that bigger sub basins havent essentially highest amount of the rate of Qsub/Qtotal. In this comparison Darkesh-Varkesh sub basin with a rate of Areasub/Areatotal of 0.13 has the highest rate of Qsub/Qtotal. Flood routing in streams showed that the rate of participation of sub-catchments in output flood is not proportional to sub-catchment peak discharge. Therefore, in order to eliminate the effect of area in participating sub-catchments, the rate of influencing each unit of sub-catchment area in output flood was calculated as well. The results of prioritization with respect to peak discharge, based on having participation of each sub-catchment in output location of watershed, indicates that Darkesh-Varkesh and Beheshtabad sub-catchments with 29.16 and 2.5 percent, have maximum and minimum of participation in output flood peak discharge of watershed, respectively Results of prioritization based on reducing discharge per unit area show that Beheshabad sub-catchment with having lowest area in comparison to other sub-catchments has highest participation and Tange-Dehno has lowest role and contribution.
In the present study, rainfall-runoff modeling is carried out using HEC-HMS hydrologic model. Results of simulation in 18 events and comparison of simulated and observed hydrographs showed that the model can applied for simulation of rainfall-runoff in study area. Other researches like Kumar and Bhattacharjya (2011) and Hegdus et al (2013) have same results as our findings. Ranking sub basins according peak discharge without flood routing show that Darkesh-Varkesh has the most and end Beheshtabad has the lowest peak discharge. According contribution in total discharge also results are the same. Soleimani et al (2008) and Zehtabian et al (2010) also found the same results. Finally, according to decrease in total Q per unit area, ranking show that end Beheshtabad sub basin despite of having the smallest area, has the highest contribution in total Q per unit area Nasri et al (2011) also concluded that areas are located near the outlet of study basin have the most contribution in flood production. This research show that the Darkesh-Varkesh sub basin need the most attention in selecting management practices especially in optimizing flood control and flood mitigation solutions.