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... In the longitudinal cross section in Figure 7, the hydraulic heads for the pre-and post-mining phases do not exactly follow Lübbe's concept [11]. Instead, the tilting line derived by the FEFLOW's computations is shifted about 45 m from the center of the pit lake in the direction of the model's outflow border in Scenario 2. The reason is that the 2D flow Equation (5) In addition to the thickness of the saturated zone and the hydraulic head slope, the extensions of drawdown and mounding areas also depend on the local hydraulic conductivity distribution, the shape of the pit lake, its depth and hydraulic connection to any other surface water bodies [7,27,28]. Although the evaluation of each potential effect of a gravel pit lake on groundwater is not the subject of this paper, they can be accurately implemented in a numerical model. ...
... Although the evaluation of each potential effect of a gravel pit lake on groundwater is not the subject of this paper, they can be accurately implemented in a numerical model. Jost et al. (2023) [27] studied the influences of geometrical, hydrodynamical and meteorological factors on the groundwater level and water balances. The potential errors by ignoring these parameters have to be considered during the result analysis of an individual project. ...
... The Dupuit-Forchheimer assumptions are described in Section 2.3. Shifted tilting lines with a parabolic surface of the hydraulic head are also shown inJost et al. (2023) [27]. ...
In Europe, 1132 Mt of sand and gravel were mined in 2019, which causes major changes to the hydrogeological cycle. Such changes may lead to significantly raised or lowered groundwater levels. Therefore, the aggregate sector has to ensure that impacts on existing environmental and water infrastructures are kept to a minimum in the post-mining phase. Such risk assessments are often made by empirical methods, which are based on assumptions that do not meet real aquifer conditions. To investigate this effect, predictions by empirical and numerical methods about hydraulic head changes caused by a pit lake were compared. Wrobel’s equation, which is based on Sichardt’s equation, was used as the empirical method, while a numerical groundwater flow model has been solved by means of the finite-element method in FEFLOW. The empirical method provides significantly smaller ranges of increased/decreased groundwater levels caused by the gravel pit lake as the numerical method. The underestimation of the empirical results was related to the finding that field measurements during pumping tests show a larger extent of groundwater drawdown than calculations with the Sichardt’s equation. Simplifications of the 2D model approach have been evaluated against hydraulic head changes derived from a 3D groundwater model. Our results clearly show that the faster and cheaper empirical method—Wrobel’s equation, which is often preferred over the more expensive and time-consuming numerical method, underestimates the drawdown area. This is especially critical when the assignment of mining permits is based on such computations. Therefore, we recommend using numerical models in the pre-mining phase to accurately compute the extent of a gravel/sand excavation’s impacts on hydraulic head and hence more effective protection of groundwater and other related environmental systems.
