Investigation on Sand Extraction Capacity and its Effect on Morphological Changes of River (Case study of Shara River)

Background and ObjectivesSurface runoff and water flow in rivers cause soil erosion and carry sediment materials. Knowledge of how erosion and the ability to carry sediment in rivers and waterways should be considered in every engineering plan. One of the most important factors in disrupting the hydraulic balance of the river is the indiscriminate removal of sediments from the river bed. Today, the materials obtained from rivers are very vital in human life, especially in civil and industrial activities, and are of great importance. Indiscriminate and out-of-capacity harvesting of sand causes changes in the morphology of rivers, which results in the river's reaction to establish a new balance. The study of past research indicates that sand harvesting has caused different changes in the hydraulic pattern of rivers, changes in morphology, and changes in the sediment balance of rivers. The extraction of sand from the country's river beds has increased in recent years. Due to the complexity of hydraulic and sediment problems in rivers, it is not possible to solve the equations analytically, hence numerical methods are usually used. Various studies have been conducted using the HEC-RAS model to investigate the flow of hydraulics in the river. Various researchers in the country have also used this model to investigate sediment transfer in rivers and reservoirs of dams.MethodologyIn this research, a part of the Khara River located in Khandab City, which is subject to excessive sand harvesting, was investigated. In this study, HEC-RAS 5.3 model was used to simulate sediment flow. For Joshirvan hydrometric station, discharge was calculated with a return period of 2, 5, 25, 50, and 100 years and used for hydraulic simulation of the river. To use the HEC-RAS model for hydraulic simulation of flow and sediment, river information including geometric data, hydraulic data, and sediment data was used. In the geometric section, the general plan of the river along with cross sections was introduced to the model. Using the topographical map of the river course in two time periods before sand and sand harvesting (2015) and after sand and sand harvesting (2017), a digital elevation model of the Kurkhane river course with a scale of 1:2000 was prepared for both periods. Then, a TIN was made from the prepared digital elevation model, and 200 sections were extracted in the HEC-GeoRAS add-on package in the Arc Map software along the length of 3.5 km of the river. After preparing cross-sections and measuring the distances of the left and right banks of the main channel of the river, the relevant information was connected to the HEC-RAS model in the form of a GIS file to introduce the geometry of the river. Finally, the changes made in the geometry and morphology of the river were compared by examining the parameters before and after harvesting the materials and field investigations in the periods when this operation was carried out.FindingsBy comparing the longitudinal profile of the bed in the post-harvest conditions compared to the pre-harvest conditions, it was found that in the first half of the studied interval from the upstream side (section 1-98) a sedimentation phenomenon occurred with a shallow depth, while in the middle half of the studied interval study (section 100-150) there has been an uplift of the river bottom. The results showed that the amount of erosion and sedimentation in the first 1.5 km and the last 1 km is almost low, and the materials in this area are not very removable. The best area for collecting materials in this interval from 1.5 to 2.5 km is almost in the middle of the interval. According to the changes in the river bed, the maximum change is about 1.2 meters and the average changes are about 0.5 meters along the river path.ConclusionResults showed that the rate of erosion and sedimentation in the first 1.5 km and the last 1 km almost decreased in the sedimentation state and the materials in this area are not very harvestable. The best area for harvesting materials in this range is from 1.5 to 2.5 km approximately in the middle of the range. Due to the slope and cross-sectional shape, the flow velocity has decreased and sedimentation with an average depth of 1.2 meters has been done. Calculations showed that the amount of sediment that can be removed from the river in this period is about 200 tons per year.