Goldilocks and the Three EAPs: Working with EAP Templates and Inundation Mapping Guidelines to Ensure They are "Just Right" for your Dam

ABSTRACT ONLY - The City of Hagerstown (the City) owns and operates three high-hazard dams as part of the water supply system for Washington County, Maryland. Edgemont Dam is a 55-foot-high, earth-filled embankment that impounds a 258 ac-ft reservoir at normal pool. The Edgemont Reservoir feeds the W. M. Breichner Water Treatment Plant (WTP) 1.75 miles southeast. The Smithsburg Dam, located on the WTP property, is a 16-foot, off-stream, 700-foot-long ring earth-filled dike that can store a maximum of 75.4 ac-ft. Currently, the main water supply source for the City is the R. C. Willson intake and WTP located along the Potomac River. In 1955, a 2,300-foot-long, 30-foot-high levee was designed and constructed around the WTP to protect the facility from significant flooding events on the Potomac River.
In 2020, the City requested that Hazen and Sawyer (Hazen) update their existing Emergency Action Plans (EAPs) for the Edgemont and Smithsburg Dams in accordance with current Maryland Department of the Environment's Dam Safety (MDE) regulations. Since the EAPs were initially created, MDE released both a new EAP template and guidance for completing breach analyses for dams in Maryland. In addition to the two updates, the City requested that a new EAP document be prepared for the R.C. Willson WTP Levee, recently reclassified to a state-regulated, high-hazard structure.
Work involved developing two new 2-Dimensional HEC-RAS hydraulic models and simulating breaches through the Edgemont and Smithsburg Dams. ArcGIS Pro tools and existing FEMA Flood Insurance Rap Maps were leveraged to analyze Potomac River flooding conditions around the R. C. Willson WTP Levee. Inundation extents, whose distances downstream ranged from several hundred feet at the R.C. Willson WTP Levee to approximately 1 mile at Smithsburg Dam and over 20 miles at Edgemont Dam, were mapped for inclusion in the EAPs. Coordination with the City to populate the site-specific information and determine emergency level triggers in MDE's 2021 EAP template was also necessary to complete the work.
This project highlighted the importance of using engineering judgment, coordinating with stakeholders, and working with regulators when developing inundation maps and populating EAP templates to ensure they provide maximum benefit to all stakeholders for managing and responding to a potential emergency. This presentation will address the assumptions for the breach analyses and inundation mapping approaches as well as the customization of MDE Dam Safety's EAP template necessary to address the site-specific conditions and concerns at each of the City's dams.