Bass Lake Dam Emergency Action Plan

Client: Town of Holly Springs, NC

Bass Lake Dam Emergency Action Plan

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Services provided in this project:

 

The North Carolina Dam Safety Program required that the emergency action plan (EAP) for the dam at Bass Lake, a recreational park in the Town of Holly Springs, include a hydraulic analysis that accounted for the possibility that a breach at Bass Lake may also result in a breach of the dam at Sunset Lake, located roughly one mile downstream. JEWELL developed a hydraulic model in HEC-RAS, built upon the cross-section geometry from preliminary FEMA models for Basal Creek and Middle Creek, with updates to account for the new bridge under construction by the North Carolina Department of Transportation just downstream of Bass Lake. The preliminary FEMA hydrologic model was used as the basis for developing an inflow hydrograph at Bass Lake during a 1/3 Probable Maximum Precipitation event, recommended by FEMA as the flood magnitude to be used in design of high-hazard dams of this size, as well as emergency planning for a breach event.

Based on this analysis, JEWELL concluded that the projected flood level at Sunset Lake could be high enough to also result in a breach of that dam, and developed a dam breach analysis for Sunset based on the projected inflows from the Bass Lake breach in a 1/3 PMP event. The results of the unsteady flow model in HEC-RAS were used to delineate the downstream inundation area. Based on input from the Town staff and research of appropriate contacts and services, JEWELL developed and submitted to DENR the online EAP, inclusive of inundation mapping.