HURRICANE-RELATED PRECIPITATION EVENTS REVEAL THRESHOLD BEHAVIOR IN WATER LEVEL AND SEDIMENT CONCENTRATION AT ARCH SPRING, PA, USA

In September 2004, three major hurricanes, Frances, Ivan, and Jean, traveled up the east coast from Florida bringing large amounts of rain to Central Pennsylvania. During these storms, 15-minute time series of stage, conductivity, and temperature collected at Arch Spring, Blair County, show that flow at the spring is regulated by the capacity of the upstream cave system, with a larger rain event producing a similar maximum flow to a preceding smaller event. Ivan was a much more devastating storm to the area, but the net stage increase at Arch Spring was greater during the earlier Frances, a 74 cm increase versus a 54 cm increase. Sharper up and down swings in conductivity and temperature occur during the first event of the three, and the system's response to a new event is affected by preceding storms.

In addition to data logging, an automatic stormwater sampler at Arch Spring collected water samples at two hour intervals from September 9 to 10, 2004, capturing flow during Hurricane Frances. The samples' suspended sediment concentrations show another threshold in the karst system. Concurrent with the sharp rise in water level that accompanied Frances, maximum sediment concentration (933 mg/L) exceeds previously recorded concentrations by almost an order of magnitude. The highest suspended solids concentrations are coincident with water of high specific conductance reaching the spring. This indicates the highest sediment concentrations occur not when the storm water reaches the spring, but rather when older water is being flushed out of the conduit system. This sediment deposited in the conduit system is flushed only when adequate flows occur in the system, indicating that sediment transport in karst is controlled by thresholds and is a strongly non-linear process.