A Numerical Study of the Movement Pattern of Sand and Gravel Mining using 1D/2D Models

Several construction projects such as dam construction, road construction and building, may increased the sand and gravel materials demand in the country. However, cost savings, convenience and high quality of fluvial materials, will lead to excessive use of the sediment capacity of river in construction projects. Non normative utilization of the fluvial materials destroy the lands overlooking the river, aquatic habitats, birds, and animals of the region, and interrupt the natural balance of the rivers, as well. In many cases, the economic value of the damage, which has been done, is much more than the interest earned by the sale of the materials. If we removed sand and gravel materials from good sites, under appropriate technical principles and proper management, not only the negative consequences are minimized, also the performance and stability of the river are increased.
In case the exploitation of rivers exceeds the legal limit, the river restores its balance in response to changes. Hence, the river could be considered as a living organism and must be treated totally carefully to reduce side effects. In other words, even small changes in the structure of river, will lead to a broader range of changes. Therefore, it is necessary to predict how river responds to changes before any action is taken (Hedayatipur et al., 2010). In this research, HEC-RAS one-dimensional and CCHE2D two-dimensional models have been used to evaluate and to compare the existing sediment transport functions. Furthermore, some scenarios have been designed to sand mining at beds and banks of Sarbaz river in Sistan and Balouchestan province and its effects on hydraulic and sediment characteristics of river flow have been investigated using HEC-RAS model. The main reason to choose Sarbaz river had been illegal and successive mining from the river in recent years. Improper and excessive instream mining activities during a seven-year time period (between 2002 and 2009) has caused more than 4 meters degradation on Sarbaz riverbed, riverbanks failure, destruction of agricultural lands near Rask Village and consequent problems. Riverbank excavation has caused water diversion to this part, and riverbank erosion. This poses hazards to bridges and structures on the river.
Sand and gravel mining in the studied area of the Sarbaz river has caused disruption of the flow pattern and as a result of the concentration of the river flow, especially in case of flood to the riverbanks. The concentration of flow at the toe of the riverbank caused their erosion and followed by bank failure. Consequently it caused destruction of gardens, agricultural lands and moving to residential buildings located on the riverbank. Considering that the hydrograph with a 2-year return period is usually used for dominant discharge and the hydrograph with a return period of 25 years is used in floodway and flood fringe studies, these two have been used as the main hydrographs. To investigate the effect of various sand and gravel mining patterns on the hydraulic properties of the Sarbaz River, nine hypothetical scenarios, including removal of materials from the bed and the riverbank, were simulated in a 10-kilometer interval by a one-dimensional model, HEC-RAS. The scenarios implemented were: 1- Sand and gravel mining from the bed of the river at depths of 1, 2 and 3 meters,
2- Sand and gravel mining from the banks of the river with a width of 20, 40 and 80 meters,
3- Sand and gravel mining with a depth of 1 m from the bed and a width of 20 m at the bank, a depth of 2 m from the bed and a width of 40 m at the bank and a depth of 3 m from the bed and a width of 80 meters at the bank.
According to the above scenarios, the model was implemented for floods with return periods of 2, 25 and 50 years.
To study how sand and gravel pits moving in Sarbaz river, and to compare the results of field measurements with those of simulation by one-dimensional HEC-RAS and two-dimensional CCHE2D models, an artificial pit with 32 m length, 20 m width and 3 m depth was dug in a reach of Sarbaz river. Dimensions of the pit was surveyed using surveying instruments (theodolite camera and levelling staff) at different times. Measurements done over a period of 31 days in May 2014. The root mean square error (RMSE) has been used to evaluate each function in the selected models. In calibration step, the longitudinal profile obtained from each sediment transport function was compared with observational data, and the function that showed the least difference (using the RMSE function) was considered as the appropriate sediment transport function.
The results of this study showed that the effect of degradation pattern in the upstream part of the mining site is more than that of river widening, e.g. the effect of 80 meters river widening is even less than the effect of 1 meter deepening. The maximum bed-level changes due to degradation was also 3 meters. The results of this study showed that sand and gravel removal, reduced the water level upstream of the excavation pits. In addition, due to the reduced groundwater level in the area, the results of previous studies (WCC, 1980; Majnonian, 1999; Femmer, 2003; IWRM, 2005) on the groundwater level decrease due to the decrease in the permeability of the river bed in gravel pits are confirmed. Besides reducing groundwater resources, it causes severe damage to agricultural lands. Moreover, based on the values obtained from the RMSE, two-dimensional CCHE2D model give more reliable results than HEC-RAS one-dimensional model in terms of flow characteristics, grain-size of bed material, sediment transport functions, and other parameters under the same conditions.
The results of this study showed that the influence of degradation pattern in the upstream part of the sand and gravel mining, is more than the effect of river widening and if the material was removed from the river bed at high distances, the river bed level will be changed more severe. Finally, numerical simulation results show that in the same context of flow characteristics, grain-size of bed material, sediment transport functions and other parameters, the 2D model will obtain more reliable results.