Sedimentation and Its Challenge for Sustainability of  Hydraulic Structures : A Case Study of Mosul Dam Pumping Station

Abstract: A successful management and operation of water resources projects are essential to maintain their functions. Dams and reservoirs are one of the largest worldwide infrastructures. They serve one or more functions; reliable store and release of water for different purposes, hydropower generation and   flood and draught controls. Sedimentation is one of the serious problems that affects the reservoir`s efficiency; it leads to reduction in storage capacity and reliability for water supply. Furthermore, deposition of sediment near and inside the intakes and hydropower plants cause a negative effect on plant efficiency and corrosion of turbines and pump`s impeller.    Generally, degradations of the watersheds, surface runoff and river flow are the main sources and transporters of the reservoir’s sediment.    Sediment management techniques are the most economical and efficient approach for sustainability of reservoirs and attached structures. Pumping rate, operation schedule, sub watersheds sediment control and earth dike is the reasonable alternatives that were applied in this study for sediment control and sustain water intakes.  In Mosul Dam reservoir, the pumping station is considered as a case study, the station is suffering from sediment accumulation in front and inside the intake. The work includes application of Soil and Water Assessment Tool (SWAT) models to estimate the runoff and sediment load delivered by sub watersheds surrounding the studied area, and a sediment rating curve was considered to assess the sediment load carried by the main river (Tigris River). The Hydrological Engineering Centre’s River Analysis System ( HEC-RAS) model   as a one dimensional model (1-D) was applied for sediment routing, and as a two dimensional model (2-D) for flow analysis. This aims to estimate the sediment load deposited in the studied reservoir and   evaluated the effects of pumping rate and flow depth on flow velocity distribution, flow stream power and sediment transport. As this study focuses on the sedimentation problem on the area around the intake’s structure and due to compound flow regime and sediment transport near the intakes and withdraws outlets, a three dimensional (3-D) model is considered more suitable than a 1-D or a 2-D model. The Sediment Simulation in Intakes with Multiblock option (SSIIM) model was considered also in this study; a proper control code for studied case was developed. This model depends on Computational Fluid Dynamics (CDF) techniques as a numerical method to solve fluid motion problems.The applied models were   calibrated and validated based on measured data of previous studies. The considered statistical criteria indicate that the models’ performances were reasonable for both flow and sediment assessments.  The results of all applied strategies show an improvement with a different percent in the amount of sediment deposited in front and inside of the intake, in comparison with the current situation. The optimal improvement was obtained by adding a control earth dike upstream the station. It is considered the most efficient and practical strategy that can be applied for sustainability of the   pumping station efficiency and lifespan with fewer dredging requirements.