STORMWATER INFLOWS AND SENSITIVE SPECIES: NEW FRONTIERS OF KARST HYDROGEOLOGY

In the eastern area of the United States, karst drainage networks have been long used as integral components of storm-drain systems, to avoid directly discharging storm flow into channels. Karst aquifers in this area of the United States are generally large, ranging from ten to several hundreds of square miles. In central California, smaller, high-relief karst aquifers -- several hundreds of acres to a few square miles in outcrop area -- play a key role supplying potable water to the growing number of residents in the Santa Cruz area, as well as sustaining streams which support listed species, including coho salmon (Oncorhynchus kisutch ) and steelhead (Oncorhynchus mykiss), species that benefit from low peak runoff and high baseflow. New techniques and approaches are needed for these small edaphic systems.

Utilizing simple analytical solutions developed for reservoir management, we have developed possibly new field techniques to facilitate the calculation of sinkhole infiltration rates and predict the rate of sinkhole inflow which would result in storm spills to newly evolving channel corridors. The technique was tested with two sinkholes during two different rainfall events in December 2003 and February 2004. These two small-recurrence, low-intensity rainfall events resulted in only 1.5 to 2 inches of total rainfall over 10 to 14 hours; one sinkhole spilled and the other sinkhole came within 8 inches of spilling. Geomorphic field evidence suggests that these sinkholes likely spilled only during significant rainfall events in the past, on the order of one to several centuries over the past several thousand years. Results indicate that the two tested sinkholes presently have similar maximum infiltration rates (5 to10 cfs), despite very dissimilar morphologies. The apparent dramatic increase in the frequency of sinkhole spills – and possible increase in sediment transport -- during ‘typical’ winter storms highlights the need to focus our attention towards understanding the hydrologic dynamics of sinkholes. With an increased understanding, preservation of cavern and subterranean habitat and downstream channel corridors can be planned in parallel with campus growth. We believe our capacity-analysis technique and the broader habitat based approach can be widely applied in karst environments.