Flow and stability of Al-Wand earth dam during rapid drawdown of water in reservoir

The rapid drawdown is known as one of the most dangerous conditions for the upstream slope. When the countervailing upstream water pressure is lowered, it results in distress to the upstream slope. Soils within the dam body remain saturated, and flow commences towards the upstream slope. Seepage in addition to hydrodynamic pressures generates downward forces that act on the upstream slope. These forces act adversely to the stability and generate a critical condition in the upstream slope. In this study, the finite element method is used to study seepage through the body of an earthfill dam. For this purpose, the software Geostudio 2007 is used through its subprograms SEEP/W and SLOPE/W. The water levels on the upstream and downstream sides, the properties of materials and boundary conditions of the dam were input variables and the water flux, exist gradient, and pore water pressure were the target outputs. Al-Wand dam in Iraq has been chosen for analysis for the original period of emptying the dam reservoir and emergency periods. It was concluded that when the reservoir is rapidly emptied and water is drawn down, pore water pressures in the dam body are decreased in two ways: the first way is through slow dissipation of pore water pressure due to drainage, and there is an immediate elastic effect due to the removal of the total or partial water head. During rapid draw down, the pore water pressure at all points within the dam body decreases linearly which indicates that steady state flow takes place. Some points in the dam downstream may be affected by negative pore water pressure during the period of water draw down which indicates that the water level becomes below these points.