Maryland Department of Natural Resources

Reports

Bathymetric survey and sedimentation analysis of Loch Raven and Prettyboy Reservoirs


2000, Ortt, R.A., Jr., Kerhin, R.T., Wells, D.V., and Cornwell, J.

File Reports, Coastal and Estuarine Geology, File Report 1999-04


Abstract

As part of a larger Gunpowder Watershed study, Maryland Geological Survey (MGS) was contracted to study the bathymetry and sedimentation of the Prettyboy and Loch Raven reservoirs. In cooperation with the United States Geological Survey and the University of Maryland, bathymetric data were collected for both reservoirs; sedimentation rates for Loch Raven Reservoir were calculated; and, the chemical and textural characteristic of the bottom sediments of Loch Raven Reservoir were documented.

Bathymetric data for the reservoirs were collected in the Fall of 1998 and in the early Summer of 1998 for Loch Raven and Prettyboy reservoirs, respectively. This data was collected using differential global positioning service (DGPS) techniques and digital echosounding equipment. Over thirty-two thousand discrete soundings were collected and used to generate a current bathymetric model of Loch Raven Reservoir. Over forty-eight thousand discrete soundings were collected and used to generate a bathymetric model of Prettyboy Reservoir. The bathymetric models indicate a current storage capacity of 19.1 billion gallons [72.3 million cubic meters] and 18.4 billion gallons [69.7 million cubic meters] in Loch Raven and Prettyboy respectively.

Sediment accumulation rates within Loch Raven Reservoir were calculated using historical comparisons, volumetric comparisons, radiometric dating techniques, and visual identification of pre-reservoir surfaces using gravity cores and sub-bottom seismic-reflection records. All of these methods concluded that the sediment accretion rate within Loch Raven reservoir is between one and two centimeters per year. The annual percentage loss rate of capacity from this sediment accumulation is 0.13 percent or 26.8 million gallons.

Using only historical comparisons for Prettyboy reservoir computations, the annual percentage loss rate of capacity is estimated to be 0.12 percent or 23.1 million gallons.

These losses are significantly below the national average of 0.27 percent for other reservoirs (Morris, 1998).

The Loch Raven sedimentation rates may be misleading for future planning. The upper portion of the reservoir and the stream channels leading into the reservoir, that was not part of this study area, have reached a sediment balance and will no longer retain additional sediment. Consequently, sediment is transported increasingly further downstream into the reservoir resulting in an increasing rate of sedimentation.

Sediment in Loch Raven was analyzed for cadmium, chromium, copper, iron, manganese, nickel, lead, zinc, nitrogen, total carbon, organic carbon, reactive-organic carbon, sulfur, and phosphorous. Slightly elevated levels of zinc and lead were observed in the sediments collected from Long Quarter Branch of Loch Raven Reservoir. The chemistry showed no other significant findings.

The bathymetric and sediment data collected for this study establishes a solid baseline for future calculations of sediment accumulation rates and sediment chemistry changes.