The Navy Ships Parts Control Center near Mechanicsburg, Pa., occupies an area of 824 acres, of which 358 are covered by impervious surfaces. Most of the impervious area is drained by stormwater systems that discharge to an open channel that extends about 7,900 feet from its headwaters to its confluence with Trindle Spring Run. The channel drains an area of 992 acres, of which 435 are covered by impervious surfaces. The entire area of the Center including the stormwater-drainage channel is situated in karst terrain. Parts of the drainage channel contain large sinkholes and most of the storm runoff that enters the channel drains to the sinkholes.
From 1992 to 1994, the U.S. Geological Survey, in cooperation with the Department of the Navy, conducted a detailed study of water and sediment flows in the stormwater-drainage channel. The purpose of this study was to quantify the discharge of stormwater and suspended sediment to the ground-water system, by way of sinkholes, and to Trindle Spring Run. From October 1, 1992, to September 30, 1993, the data-collection period for the study, discharge and suspended-sediment concentrations were measured at three sites along the drainage channel. During the period, water inflow to the channel totaled 679 acre-feet and outflow to Trindle Spring Run totaled 131 acre-feet. Water loss to sinkholes in the drainage channel totaled 548 acre-feet or 81 percent of inflow. Total sediment inflow to the drainage channel was 97 tons, outflow to Trindle Spring Run was 22 tons, sediment loss to sinkholes was 63 tons, and the residual 12 tons of sediment was deposited in the channel.
The effect of filling the sinkholes on flooding was estimated through use of a step-backwater model. The model was used to simulate undampened water-surface elevations that would result from the maximum instantaneous discharge recorded during October 1992-September 1993. The model is constrained by uncertainty in the values of the channel-roughness parameter. Analysis of the model results indicates that during high flows, inflow to sinkholes results in a moderate reduction in discharge and water-surface elevations in the drainage channel. This analysis shows that filling the sinkholes will result in increased frequency and magnitude of flooding in downstream parts of the drainage channel and increased discharge of storm runoff and suspended sediment to Trindle Spring Run.