EXAMINING SEDIMENTS IN FLUVIOKARST FROM THE INLET TO THE DISCHARGE POINT

Just as flow in karst must be evaluated from the recharge area to the discharge area, understanding sediments in karst requires examination of the inputs and outputs. A sediment budget includes inputs from sediment carried into the aquifer by sinking allogenic streams, soil washdown from the overlying karst surface, plug injection from soil piping failures in sinkholes, and insoluble residuum from dissolution of the bedrock. The net output results from these sources plus or minus sediment in storage within the conduits. In between the inlet and the outlet, the caves can be a mixing chamber for sediments. Evidence for the dynamic nature of sediment storage comes from direct observation by cavers and cave divers, sediment observed at the outlet, and the simple fact that sediment doesn’t fill and clog the caves. The output presents a somewhat blurred picture of sediment present in the system, but nonetheless, sediment transport is a characteristic of karst springs. This sediment discharge is typically fine sediment that is easily transported. Autochthonous sediment (weathered from cave walls) is less common at the discharge point, possibly due to aggressive waters redissolving the carbonates or a patina of sediment that coats the walls and inhibits weathering. The volume of sediment varies and provides evidence of episodic reworking of cave sediments during high flow events. The dynamics of reworking within the cave system are difficult to predict because the flow equations are dependent on the channel size and hydrodynamic conditions which are hard to map in most cases and are subject to change. Thus, despite relationships between flow velocity and particle size, as well as between shear stress and particle size, we are dependent on studying sediment discharge at the outlet to understand how the internal dynamics affect transport of sediment. The most powerful interpretative tool is the spring sediment discharge mapped onto the discharge hydrograph. Such data can be collected for moderate to strong storms. Unfortunately, movement of large particle size sediments (pebble – cobble – boulder size) requires extreme events which are both rare (fortunately) and difficult to observe.