PLT - Savage River Dam, A Wild Ride: Developing 86 Miles of Inundation Mapping and Emergency Action Planning for 10 Municipalities/Counties Downstream of Savage River Dam.

Savage River Dam is one of the tallest rockfill earth dams in the US with a 180-foot hydraulic height and impounding an approximately 6.3-billion-gallon reservoir. The dam is owned by the Upper Potomac River Commission (UPRC) and operated by the UPRC and the US Army Corp of Engineers. The Savage River Reservoir is a major water source for a prized native trout stream and the Potomac River which serves three major utilities in the greater DC area. This presentation will focus on the recent breach failure inundation analysis and emergency preparedness activities including an Emergency Action Plan (EAP) update, interagency coordination, and tabletop exercise for Savage River Dam.

Past modeling processes involved orographic Probable Maximum Precipitation (PMP) determination, HEC-1 hydrologic modeling for dam runoff determination and spillway peak flow calibration, and Probable Maximum Flood (PMF) breach analysis using DAMBREAK. New hydrologic and hydraulic models were developed in HEC-HMS and two dimensional (2D) HEC-RAS software, respectively. The hydrologic model was developed using Geographical Information Systems (GIS) watershed boundary and infiltration analysis as well as hydrologic model calibration. The 2D hydraulic model totaled approximately 250,000 cells and included 30 bridge crossing structures along 86 miles of stream, 55 miles on North Branch Potomac River and 27 miles along the Potomac River. Five breach methodologies were applied to determine final overtopping and sunny day failure breach sizes. The breach failure inundation area was estimated to impact 4,600 structures in at least 25 different municipalities along with 13 roadway and 17 railroad crossings.

The EAP was updated to include 38 pages of the resulting inundation extent and to meet to current Maryland Department of the Environment (MDE) Dam Safety guidelines. Updating the EAP involved coordination between UPRC and MDE to define appropriate notification flowcharts for different emergency levels. Notifications for a Level 3 (Imminent Failure) involves two states, 5 counties and 10 downstream municipalities/counties. Guided by the Federal Emergency Management Agency (FEMA) as part of the Maryland/West Virginia Collaborative Technical Assistance Program, an EAP tabletop exercise was held for all responsible parties to demonstrate emergency response procedures following a simulated failure scenario.