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This article addresses the reliability and safety of an earth dam in the case of a change in the reservoir water level. The water level must often be reduced to remove water or as a response to an emergency situation in the process of operation of a hydraulic structure. Lower water levels change seepage conditions, such as the surface of depression, values and directions of seepage gradients, seepage rates, and volumetric hydrodynamic loading. Practical hydraulic engineering shows that these changes can have a number of negative consequences. Higher seepage gradients can lead to seepage-triggered deformations in the vicinity of the upstream slope of a structure. Hydrodynamic loads, arising during drawdown, reduce the stability of an upstream slope of a dam and cause its failure. Potential consequences of a drawdown can be evaluated by solving the problem of drawdown seepage for the dam body and base. A numerical solution to this problem is based on the finite element method applied using the PLAXIS 2D software package. Results thus obtained are compared with those obtained using the finite element method in the locally variational formulation. A numerical experiment was conducted to analyze factors affecting the value of the maximum seepage gradient and stability of the earth dam slope. Recommendations were formulated to limit the drawdown parameters and to ensure the safe operation of a structure. [ABSTRACT FROM AUTHOR]

Changes in the water level in a reservoir lead to changes of filtration stream parameters in the soil structure. The position of the depression curve, value and direction of filtration gradients and speeds changes near the slope of the structure. Operation practice of waterworks in such conditions shows that emergencies are possible. Changes in filtration stream parameters can cause filtration deformations of soils and loss of slope stability of a soil dam. The article considers unsteady filtration in a homogeneous soil dam for five options of composing soils having various filtration coefficients. The reservoir drawdown rate is considered as an affecting factor. Using the numeric finite-element method and PLAXIS 2D software suite, filtration stream parameters were found for the considered options. Maximum filtration gradients near the upstream slope of the soil dam were found for various options of dam soil permeability and drawdown rate. As the reservoir draws down, directions and filtration gradients and rates change near the slope surface. Maximum gradients occur at various stages of drawdown depending on its speed and reservoir soil permeability. Maximum gradients are quite high, which shows the need for anti-filtration measures.