Investigation of Flood Risk in Ungauged Watersheds using Geomorphic Features (A Case Study: Vazrood Watershed)

Due to the lack of discharge stations and rainfall in the Vazroud basin, it is not possible to have a relatively accurate hydrological behavior of the region. Therefore, by addressing the geomorphic features, this problem can be overcome to reduce the damage caused by floods in the future. In this study, in order to determine the flood risk of flood-prone areas, two geomorphic factors including topographic wetness index and stream power index were used.

Due to the lack of sufficient data related to the discharge of Vazroud watershed, the characteristics of historical flood risk were determined in this area. In the first step, the SCS method was used to simulate the peak discharge. After determining the quantitative characteristics of flood risk (peak discharge) in the next step, the qualitative risk of flood was predicted. In order to determine the qualitative risk of floods, a combination of two factors, the strength of the river and the level of floodplains, was used as the damage that could be caused. The amount of flood-prone regions with different return periods was determined by topographic moisture index threshold method.

The results of this method showed that with increasing the flood return period, the amount of flood-prone lands increases and the amount of topographic moisture index should decrease with increasing the flood return period. The results of flood risk maps with different return periods showed that as the flood return period becomes larger, the areas affected by flood risk in the hazard classes generally increase. The results of total lands that will be at risk of floods with return periods of 2, 10, 25, 50 and 100 years will reach 2.2, 3, 4.2 and 9.8% in the whole basin, respectively. Also, Vaz Tangeh, Nojmeh and Vaz olia villages with 20.6, 16.4 and 13.1% of flood-affected lands were determined as the most vulnerable rural areas in Vazrood watershed, respectively.

Flood risk prediction depends on the characteristics of the catchment and the hydrological and geomorphic factors of the region. So that in areas that have incomplete and low statistics as one of the non-structural flood management tools and complementary to structural methods, it plays an important role in flood control and reduction of potential damages.