Void Evolution in Soluble Rocks Beneath Dams Under Limited Flow Condition

In this study, the numerical model CAVE (Carbonate Aquifer Void Evolution) was used to simulate the development of solution conduits beneath a weir that is constructed on river sediments overlying gypsum bedrock. The weir site is located at Hessigheim on the Neckar River in the State of Baden-Württemberg, Germany. After flowing through the State of Baden-Württemberg over a length of 367 km, the Neckar flows into the Rhine River near Mannheim and Heidelberg. At this site, a non-soluble layer overlying soluble rock limits the maximum leakage from the reservoir. The model was used to identify the key parameters controlling void evolution under flow conditions controlled by soluble and non-soluble rocks. In all, three scenarios were simulated. Sensitivity analyses indicate that the breakthrough time was influenced by permeability of the stratigraphic units, initial conduit diameter, conduit length and the number of nodes coupled to the gypsum layer. Leakage rates of the order of 10^–7 m³ s^–1 (about 9 L day ^–1) were observed after breakthrough for the standard scenario. Thus, under limited flow conditions, if the hydraulic conductivity of the intervening layer is sufficiently low, leakage rates may not pose a serious threat to the dam's ability to store water. However, the development of conduits having diameters of the order of tens of centimeters or more may pose a threat to the long-term physical stability of the dam.