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Sand and gravel are essential materials for developing purposes in infrastructures and many various purposes. Iran, is a developing country and numerous infrastructure projects all around the country are under construction. This issue, demonstrates the growing demand for sand and gravel harvesting. Irregular and non-technical harvesting of sand and gravel from rivers, plays an important role in unwanted morphological and environmental side-effects. Physical modeling and numerical simulation are two main techniques to investigate this phenomenon. Considering the high cost of constructing physical models, application of numerical tools for simulation of hydrodynamics and sedimentation has made a significant help for understanding the related phenomenon including the effects of sand and gravel removal in different rivers. In this study, the accuracy of the MIKE21 as a two-dimensional numerical tool, in simulation of sand harvesting hole displacement was investigated by comparison with laboratory data. For this purpose, nine experiments with different dimensions of excavation holes were designed in a 10 m long and 0.7 m wide laboratory flume with uniform sand bed materials. (D₅₀=0.71mm). Two types of triangular and trapezoidal excavation holes were tested. Four important point plus depth and area of the excavated hole were considered as base points of comparison between simulated and experimental results. The flow depth was constant during all experiments (12 cm) and clear water condition was considered (v/vc=0.95). Acceptable agreement between numerical and experimental results was observed. However, the accuracy of the model was more in larger holes whereas the maximum error in predicting the migrated hole geometry in trapezoidal holes was about a half of triangular ones. After verifying the numerical model in laboratory, a specific reach in Helleh river was considered as a case study. Initially one-dimensional model of the river was simulated with HEC-RAS. 25 years return period flow hydrograph was introduced as the upstream boundary condition. Normal flow depth at Helleh Lagoon and time series of the water surface elevation changes of the Persian Gulf were introduced as two downstream boundary conditions. The boundary conditions of the selected reach for   two-dimensional modeling were extracted from one-dimensional simulation. After setting up the two-dimensional model, the effect of sand and gravel mining on a bridge in the reach was investigated in two different scenarios. The distance of the sand mining hole to the bridge was selected as 1000 m and 100 m respectively in two scenarios. It should be noted the simulation was conducted only for a 25 years return period within 16 days. More severe floods can leave more significant effects on the river and in-line structures. The results indicated that for a flood event with a return period of 25 years which was considered for simulation, sand and gravel mining had changed the hydraulic parameters and bed profile significantly, so that the flow depth at the vicinity of the excavation hole was raised up to 77% in second scenario. The flow velocity was reduced up to 75% in the first scenario and the bed profile was decreased up to 1.27m at the foundation of the bridge in the second scenario. Initial signs of river meandering were emerged in the second scenario where the flow was deviated to the mining hole.
 

Keywords: Sand and gravel mining, Numerical simulation, Sediment transport, Mike21 model

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