Evaluation of bottom outlet effect on the behavior of entered turbidity current into the Dez Dam reservoir

Sediment deposition issues for reservoirs are important because the severe deposition could excessively decrease the reservoir lifecycle. Extreme storm events usually can carry a massive amount of sediment into reservoirs, and deposition will happen unless the incoming material can pass through sluice gates. When it comes with high concentration, the density current flow is prone to be generated, and the bottom outlets are the most effective sluice gate to release the sediment. Venting of turbidity currents is known as an efficient measure to prevent reservoir sedimentation and is applied in many reservoirs globally. It has several economic and ecological advantages compared with other evacuation methods. Dez Reservoir is one of the many reservoirs in Iran which suffer from sedimentation due to a lack of proper low-level outlet. Currently, the major issue which threatens the Dez Dam is the continual accumulation of silt in the reservoir near the dam endangering the operation of the entire hydraulic scheme. Mean annual sediment deposition in the reservoir is about 15 million tons. The elevation of sediment deposition has now reached to nearly 14 meters under the intakes of power stations (Research center of power Ministry, 2003). In this study, density current releasing through a proposed low-level outlet is considered as a proper solution to minimize the consequences of sedimentation.

The aim of the study is to numerically assess the proposed low-level outlet. A high resolution 3-D numerical model is employed based on Navier-Stokes equations is employed to simulate the evolution of turbid density current in Dez Reservoir. Frist in order to calibrate the simulation model, Several models including various roughness height in combination with RNG and LES turbulence models were employed. Then, the flow depth and velocity results were compared with field measurements (KWPA 2007). In the next step, the proposed low-level outlet was added to the model and its impact on the behavior of turbidity current was investigated by using three turbidity current concentration and also three low-level outlet opening.

The results indicated that using large eddy simulation turbulence model (LES) presents a higher accuracy than the simulation by comparing with the field measurements, so that the maximum error related to the calculation of the depth and velocity of the turbidity current is estimated to be 14.67 and 13.82%, respectively. In the next step, the proposed low-level outlet was added to the model and its impact on the behavior of turbidity current was investigated. Based on the simulated results the amount of sediments evacuated from the dam reservoir is directly related to the input concentration and the opening percentage of the low-level outlet. In such a way that by averaging the results for different inflow turbidity current concentrations, venting efficiency at the openings of 25%, 50% and 100% was calculated to be 15.4%, 20.8% and 25.1% respectively. In the next section of study, velocity profiles in the upstream sections of the low-level outlet were used in order to evaluate the water entrainment coefficient (Ew). It was found that as the opening low-level outlet increase from 25% to 50% and from 50% to 100%, the water entrainment coefficient increase 20% and 16% in avarage respectively.

In a general point of view, The numerical results show good accuracy in simulating the evolution of the density current in the reservoir. Based on the simulated results, as the turbid density current approaches the low-level, water entrainment coefficient increases in avarage. Moreover water entrainment and also venting efficiency has direct relationship with low-level outlet opening and inflow density current concentration.