Minsi Lake Dam

Located on the East Branch of Martins Creek in Upper Mount Bethel Township, PA, Minsi Lake Dam has a maximum height of 26.5 feet and a length of 3,300 feet. This earthfill dam is categorized as a Hazard Category 1 (high), Size Class B structure, with a spillway design flood (SDF) of Probable Maximum Flood (PMF). Schnabel supported the development of a rehabilitation design for the dam.

The Schnabel team performed an alternatives analysis, with a hydrologic and hydraulic (H&H) analysis of the dam and spillway. Various spillway and embankment configurations were evaluated. The goal: to pass the SDF safely without overtopping the dam, while maintaining consistency with floodplain mapping and stormwater management plans. An assessment of the embankment related to seepage control and slope stability was also performed.

The alternatives analysis showed that installing a labyrinth weir and raising the top of the dam two feet provided a cost effective and viable design. During the design stage, the labyrinth was further refined.

The final design of the labyrinth consisted of two half-cycles for the low stage capable of passing the 100-year flood, while the remaining four cycles were set to safely pass the PMF (SDF) without overtopping the raised embankment. Downstream of the labyrinth, the stepped chute and stilling basin were sized for storms up to the 0.5 PMF, consistent with guidance provided by the U.S. Army Corps of Engineers’ EM1110-2-1603, Hydraulic Design of Spillways.

Schnabel also performed a geotechnical investigation, toe drain and conduit inspection, and provided contract administration and quality assurance during the rehabilitation of the dam.