SEDIMENT FLUXES IN KARST AQUIFERS: STORM PULSES, THRESHOLDS, AND FLUID DYNAMICS OF MIXING, STORAGE AND TRANSPORT

Carbonate aquifers with well-developed conduit systems carry a flux of clastic sediment as an intrinsic aspect of the functioning of the aquifer. Sources of clastic sediments include sediments carried by sinking streams, soil wash-down from the epikarst, plug injection by sinkhole piping failures, residual insoluble material from the dissolution of the limestone, and sometimes sediment backflooded from surface streams. The conduit system acts as a mixing chamber where the injected materials are sorted and rearranged. Information on the sediments and their transport processes can be obtained by investigating the source areas, by inspection of cave sediments, and by monitoring clastic sediment discharged from springs as a function of flow conditions.

The engine that drives the sediment transport system is the pattern of storm recharge in the ground water basin drained by the conduit system. Instrumented observations in five karst ground water basins and observations of cave sediments in other conduit systems show that fine-grained clastics move during ordinary storms and can be captured at the springs. Movement of coarser materials requires high-intensity, therefore infrequent, storms so that most of the sediment flux is episodic with long periods of storage interspersed with short periods of movement.

Fluid mechanics provided the basis for calculations of both bedload and suspended load components. However, these calculations could not take into account the discharge-dependent shifts from pipe flow to open channel flow or the effect of blockages due to breakdown and other barriers. Using a commercially available computational fluid dynamics modeling package, Fluent(TM), it was possible to consider the role of channel constrictions, flow depth, and pipe flow/channel flow in controllng the transport of fluid and sediment through karst aquifers. It appears that the morphology of the conduits plays a critical role in controlling sediment transport, particularly during large storm events.