CONDUIT HYDRAULICS IN KARST AQUIFERS: CONCEPTS AND PROBLEMS

The characteristics of the conduit component of karst aquifer permeability can sometimes be determined by direct surveying of cave streams and sub-water table conduits, by tracer studies, and by interpretation of spring hydrographs and chemographs. To interpret hydrographs and chemographs there must be some understanding of the hydraulic behavior of the conduit system. The intent of this paper is to examine the hydraulic consequences of various features in the conduit system known to exist from direct exploration. As a beginning, conduit flow is treated as pipe flow described by the Darcy-Weisbach equation. A single segment of conduit must be treated as a rough pipe with constrictions and blockages. Attempts to measure the wall roughness in terms of the Darcy-Weisbach friction factor have yielded values differing by several orders of magnitude. Constrictions can be described by the Bernouli equation but blockages can produce entirely new flow regimes. There are alternating transitions from pipe flow to open channel flow with resulting shifts in flow regime. Converging conduits (branchworks) are treated by mass balance and flow regime. Anastomotic and network renticulated conduit systems result in distributed flow, lower velocities, and a higher critical threshold for the onset of karstic transport regimes. A distinction must be made beween low locked-in gradients and higher evolving gradients. For all of the hydraulic responses, pulsed recharge from storm inputs is a useful probe of the system.