Reports

Bathymetry and Sediment Accumulation of Triadelphia and Rocky Gorge Reservoirs

Van Ryswick, S., and Sylvia, E.R.

File Reports, Coastal and Estuarine Geology, File Report 15-09

Executive Summary

In response to a request by the Washington Suburban Sanitary Commission (WSSC), State of Maryland, Maryland Geological Survey (MGS) was contracted to study the bathymetry and sedimentation and collect side scan sonar imagery of Triadelphia and Rocky Gorge Reservoirs located in Howard, Montgomery and Prince George’s counties in the State of Maryland. Bathymetric data were collected for the reservoirs, current storage capacities and drawdown curves were determined, and volumes of sediment accumulation since the 2004/2005 MGS surveys for the reservoirs were calculated. The collection, analysis, and presentation of this report were made to be consistent with the most recent bathymetric and sedimentation reports from Loch Raven and Prettyboy Reservoirs (Ortt et al., 2000), Liberty Reservoirs (Ortt et al., 2004), and Triadelphia and Rocky Gorge Reservoirs (Ortt et al., 2007) located within the State of Maryland.

Bathymetric and side scan sonar data for the reservoirs was collected in April 2015 for Triadelphia and Rocky Gorge. This data was collected using differential global positioning service (DGPS) techniques and digital echosounding equipment. Over four hundred thousand discrete soundings were collected and used to generate a current bathymetric model of Triadelphia and Rocky Gorge Reservoirs. Several methods of analysis were used to generate the models. The bathymetric models indicate a current storage capacity of 6.45 billion gallons [24.4 million cubic meters] for Triadelphia reservoir with a surface area of 824 acres [3.33 million square meters] and 5.49 billion gallons [20.8 million cubic meters] for Rocky Gorge reservoir with a surface area of 620 acres [2.51 million square meters]. The side scan imagery shows hardened banks along the shoreline and softer central basins within both reservoirs with the hardened shorelines being broader in Rocky Gorge Reservoir. The side scan data also revealed the pre dam stream bed thalwegs throughout the lower portions of both reservoirs with decreased presence in the upriver portions and coves where historic sediment deposition has been higher. Very little sediment accumulation behind the dams with minimal debris behind Brighton Dam in Triadelphia Reservoir and several large rock clusters with some amount of debris behind the T. Howard Duckett Dam in Rocky Gorge Reservoir were indicated on the side scan imagery.

Downloads and Data

File Report 15-09 (pdf, 2 MB)
File Report 15-09 , Plate 1 (pdf, 24.6 MB)
File Report 15-09 , Plate 2 (pdf, 43.8 MB)
Reservoir Bathymetry Data