Evaluation of Sediment and Runoff Yields of Asmari and Gachsaran Formations in Ghaleh Gol Watershed, Khorramabad, Lorestan

Erosion is the main cause of wasting water and soil resources and causing natural damage. According to geological characteristics of erosion and sedimentation, it is very important to study the erodibility of the geological formations of the watershed to determine their constructive effects on sediment and runoff. Many factors affect soil erosion and one of the important factors is the erodibility of geological formations. Sediment that moves with water is called suspended sediment load, and the amount of suspended sediment material that passes through a river section in a certain period of time is called suspended load. The suspended sediment load (SSL) of a watershed, which passes through a certain section of the river, depends mainly on the climatic characteristics, the characteristics of the watershed and the capacity of carrying sedimentary materials. The input suspended load is one of the important and influencing factors on the amount of sediment input to reservoirs of dams and lakes. Determining the amount of sediment carried by rivers is important in many aspects. The calculation of suspended load is very important because of various reasons, one of the most important of which is the role of suspended sediment load in the quantitative and qualitative management of surface water resources. Therefore, the distribution and transportation of suspended sediment load (SSL) in rivers have a significant effect on the water resource management, design of hydraulic structures, river morphology, water quality, and aquatic ecosystems.

The present study was carried out to evaluate sedimentation and runoff production of Asmari and Gachsaran Formations in Ghaleh Gol watershed around Khorramabad city, from Lorestan province, Iran, located between 48° 21 '2" and 48° 33 '1", and between 33° 15 '43" and 33° 21 '15" N with an area of 10.76 km2. The studied area has a semi-arid climate with a mean annual rainfall of less than 500 mm.  This study aimed to measure suspended sediment load (SSL) and surface runoff during the November 30th, December 2th and 16th 2020 and also on the 12th of March 2021; for this purpose two square meter plots were used. A tank was installed at the plot outlet to collect sediment and surface runoff. After the rainfall finished, the volume of water and sediment collected in the tank installed at the end of the plot was measured.

The results showed that during the mentioned rainfalls, the average volume of water output from Asmari and Gachsaran Formations were 1.802 and 1.345 liters, respectively; the average output sediment for these two formations were 1.133 and 1.048 g /l, respectively, ant the total output of suspended sediment load (SSL) was 3.083 and 2.227 gr, respectively from Asmari and Gachsaran Formations. Finally, the obtained results suggest that the Asmari formation has the highest erodibility. Also, according to the results, the Asmari formation has the highest flooding level in the study area.

According to the results, considering erodibility and flooding features, Asmari Formation has higher sensitivity compared to Gachsaran Formation in the study area. In fact, the obtained results showed that Asmari formation based on the prioritization of erodibility and flooding has the first rank, which must be prioritized for conducting management operations in the Ghaleh Gol watershed.