Numerical modeling of the influence of karst-conduit structure on variation of spring flow

The spring hydrograph, such as variation of flow, can be used to extract information on the structure of karst conduits, while the complexity of these conduits makes such study not very successful. To solve this problem, this work employs a numerical model of the known karst conduit structure to reveal the relationship between the spring hydrograph and the conduit structure, which will help address this issue for real cases. This conduit flow model is built with MODFLOW-CFP to study the karst aquifer response in a storm period. The basic features of the karst aquifer of this simplified model are taken from a deeply-cut valley of the Liuchong river in Guizhou Province. This karst aquifer consists of a single conduit and a fracture network which is treated as an equivalent continuous porous medium. The valley is regarded as the river boundary of the karst water system, and the water level of the river is higher than the location of the spring outlet, so the conduit can be saturated with water. In the modeling spring hydrograph, the peak flow value is chosen as the dependent variable in the analysis of the karst conduit, and 4 parameters of the conduit structure and the recharge ratio of sinkhole are studied, respectively. The results show that the pipe diameter, wall permeability and the concentrated recharge of the sinkhole are characterized by positive correlation with the spring flow peak, of which the wall permeability has the greatest effect on the flow peak. When pipe bending is smaller, the spring flow becomes bigger, while the growth of the flow peak is limited by the tortuous flow path. The water flow status experiences two abrupt changes at the two critical Reynolds number of conduit-wall roughness, leading to twice of sudden variations of flow peak.