Investigating sediment transport in karst using hydrograph unmixing, sediment transport modeling and multi-source hysteresis

Abstract

Karst characterizes almost 15% of the worlds terrain, however the mechanics of sediment transport and its prediction in karst river and cave systems remains underdeveloped. Hysteresis analysis has recently been used more to investigate the behaviors of sediments during storm events in surface systems and to some extent in karst systems. Historically, clockwise and counter-clockwise hysteresis typically refer to proximal and distal sourcing for streams. For karst systems, clockwise and counter-clockwise hysteresis has been identified to refer to an saturated and unsaturated aquifer prior to the event.However, most interpretation of hysteresis assumes a single dominant water source, for example runoff, and assumes that baseflow is not contributing to the sediment load. One aspect of sediment hysteresis and its interpretation that has received less attention is the occurrence of several, significant water sources, eroding and delivering sediment to the watershed outlet. It is common for both surface stream systems  and karst subsurface systems to have multiple water sources contributing to the total sediment load. Each of the sources carries their own sediment time distribution, and often lead to complex hysteresis looping behavior after mixing. The primary goals of this work are to (1) study how the complex source water-sediment mixing processes impact hysteresis results and (2) to carry out solutions to the water-sediment mixing processes for karst streams, caves, and springs and show the utility and uncertainty of the method.Several high-resolution sensors have collected data at a karst spring in central Kentucky, USA, for a 2.5 year period. Water unmixing was performed using electrical conductivity as a tracer to separate the groundwater from the surface water and infer sediment sources. Theoretical analyses have shown that not only timing and magnitude of sedigraphs influence the result of the hysteresis loops, but also timing and magnitude of each of the multiple water sources have a strong effect on the resulting hysteresis loop. The groundwater flow shows to have dominant counter-clockwise hysteresis loop, surface water shows to have clockwise loops dominating. Depending on the timing and magnitude of the water sources, the hysteresis loop at the karst spring varies from strictly counterclockwise, to a figure-8 loop, to a complex pattern.