Investigating the Effect of Land Use Changes on the Distribution of Dam Reservoir Sediment (Case Study: Lasak Dam) Using the HEC-RAS Model

In the last few decades, due to the growth of industries, drastic changes have occurred in the climate of the planet, and its average temperature has increased significantly. Land use changes and climate have had a great impact on discharge and sediment production in watershed. The increase in the production of sediments has many harmful environmental and constructional effects; among these effects we can mention the reduction of the useful depth of the dam and consequently the reduction of the life of the dam. The purpose of this study is to investigate the process of sedimentation along the Mubarakabad River (Emamzadeh Ebrahim watershed) and the effect of time on the increase in the sedimentation depth of the Lasak dam reservoir located in Guilan province using the HEC-RAS model in different states for the time range of 1997 to 2071.

Mubarakabad River is one of the main branches of Pasikhan River, which is the most important river supplying water to Anzali wetland. This watershed has various uses, including forest, degraded pasture, medium pasture and good quality pasture, residential, paddy field, etc. To carry out this study, land use changes in the years 1997, 2007, 2020, 2040 and 2071 were used. In order to measure the runoff, sediment and flow rate in the coming years (2022 and 2071) with the help of the LARS-WG statistical model in two scenarios (RCP 4.5 and RCP 8.5) and SWAT tool was investigated. To implement the HEC-RAS model, three models of surface reduction were used: Borland and Miller's method, Moody's surface reduction method and surface increase method. The input variables of the model are loaded in three sections: topographic map, discharge information, discharge-sediment, and sediment grading. The model was calibrated using discharge and sediment data during the study period. In this study, changes in sedimentation along the Mubarakabad River from upstream to the construction site of Lasak Dam were investigated. Also, the sedimentation depth of the dam reservoir was investigated in different years using the HEC-RAS model.

The simulation results of this study showed that with passing of time, the percentage of residential areas will increase significantly and the area of pastures located in the southern part of the basin will decrease. Currently, in this basin, the total area of 1.87 km2 has been allocated to the residential sector, which includes several villages. While in 2071, this amount will reach 21.45 km2. Also, pastures with dense coverage in this basin in 1997 were equal to 99.65 km2, and in 2071 this amount will decrease to 4.82 km2. The results of this study showed that from the source to the construction site of the dam, sediment deposition has increased due to the reduction of the slope, and the largest amount of sediments have accumulated in the reservoir of the dam. It was also observed that with passing time, the depth of sediments behind the dam reservoir has increased significantly, which reduces the efficiency of the dam. The results of this study showed that there is an increasing trend in the sedimentation depth of the reservoir, so that its maximum value was obtained in 2071, equivalent to 39.1 meters from the height of the dam intake. The results of this study show that in the years 2071 and 2040, 2.02 and 1.92 million tons of sediment will settle in the Lasak dam reservoir, respectively.

According to the results obtained from the HEC-RAS model simulation in the Emamzadeh Ebrahim watershed, it can be seen that if detailed and executive planning is not done in this area, land use change will occur severely. This change of uses causes the increase of soil erosion and production of sediment in the watershed, in other words, this change of uses can be considered as an alarm for the destruction of Anzali Wetland. In general, due to the conditions of the watershed and its high erosion upstream, the life of the Lasak dam will not be long, and its construction will reduce the water rights of the Anzali wetland and lead this international wetland to complete destruction at a faster rate.