Sensitivity Analysis of Single Successive Sub-Basin Elimination Method in Flood Prone Area Derivation to Temporal and Height of Precipitation (Case Study: Zahedan Basin)

One of the useful techniques in flood management planning is identifying flood areas and calculating the flood potential of the basins. However, due to the environmental complexities, this identification has many challenges. Flood prioritization of a basin is the classification of sub-basins of a basin according to the role of each of them in producing the peak discharge of flood produced at the outlet of the basin. There is a wide range of methods for estimating runoff from the basin, such as the use of observational data, experimental and statistical techniques for estimating river discharge, more commonly known as rainfall runoff models. Deriving flood prone areas and identifying flood potential of basins is one of the crucial issues in flood control planning projects. In this way, it will be cleared that which of sub-basins in a whole basin are in priority of implementing flood control scenarios. Prioritization of basin area in term of flood generation is the procedure of dividing and classifying sub-basins based upon their contribution in generating the peak discharge of flood hydrograph. In the other words, the sub-basin with the most contribution in generating the peak discharge, will be categorized in the first flood prone priority.

In recent years, for the flooding status of watersheds, the division of the basin into a number of sub-basins and flood tracking in each sub-basin and then in the main waterway network has been used. With this method, flood-prone and critical sub-basins are identified according to their share in flood production of the entire basin. In the individual sub-basin removal method, first the total discharge flow of the basin is calculated and then in each step, each sub-basin is removed from the simulation process and the peak discharge is calculated again; Then, the flooding index of the sub-basin (F) is calculated and the priority of the said sub-basin in the production of total peak discharge is ranked. Single successive sub-basin elimination method (SSSEM) is one of widely accepted methods in this field. Considering the wide variation of temporal pattern and amount of precipitation, by combining GIS technique and HEC-HMS model, the efficiency of SSSEM method in Lar basin of Zahedan was assessed in this research. After deriving the optimum number of sub-basins, calibration and validation of the simulator model was carried out and flood prone priority method performed using different duration, return period and temporal pattern of precipitation. It should be noted that in previous studies, the sensitivity of the individual removal method in extracting flooding of sub-basins to the height and temporal pattern of precipitation has not been investigated and in this study we have tried to address the sensitivity of this method to the mentioned factors. In this study, different return periods were considered as a criterion for precipitation height and SCS standard time patterns and the dominant regional precipitation pattern were considered as criteria for time pattern change. Meteorological data used in this study include rainfall and hourly runoff; Collected from climatology, rain gauge and hydrometric stations over a period of 22 years (from 1997 to 2020).

According to the obtained results for determining the flooding potential and ranking of sub-basins by repeated method of individual sub-basin removal, it is observed that the individual removal method does not show sensitivity to rainfall time pattern, its continuation rate and return period. At a continuum of 1.5 times the concentration time, the sensitivity of this method to the time pattern of precipitation is observed and with increasing the return period, the anomaly in the ranking according to different patterns increases. There is a sensitivity to changes in the precipitation time pattern and return period at twice the concentration time, but the amount of anomaly is less than before. It should be noted that regardless of the rank of each sub-basin in terms of flood production and its effect on the peak hydrograph discharge, it should be examined what effect the application of flood control scenarios in that basin will have on the damage caused downstream of the city. In other words, in addition to the peak discharge that is the result of this study, other components of the flood, such as its volume, time corresponding to the peak discharge and also the response of the urban drainage system will be effective in the occurrence and severity of destructive flood effects; It may not be the first sub-basin in flood production, but the dynamics of its flood production in interaction with the city's drainage system will impose far more destructive effects, both economically and human losses in the city. Therefore, in order to complete the study and to evaluate the effectiveness of different flood risk scenarios, there is a need for a hydraulic study of rising water levels in different parts of the city, taking into account the sensitivity of each point in terms of economic and population density.