Floodwaters from Hurricane Matthew washed away a portion of Mirror Lake Drive, which runs along the crest of its namesake dam, and a portion of the dam along with it. Damages encompassed the downstream and upstream slopes of the embankment, several utilities, and parts of the spillway outlet system. The city of Fayetteville hired Schnabel to assess the dam’s condition, design repairs, and investigate upgrade options.

Following the October 2016 hurricane, as part of the FEMA 406 hazard mitigation process, we evaluated the dam and spillway, designed repairs to the dam and road, and developed options to upgrade the facility to meet state dam safety requirements. We based our design on a variety of factors, including an engineering analysis that revealed the dam overtops for storms greater than 10 year/6 hours. We also considered changes in upstream and downstream flood levels, constructability, and cost.

We presented three alternatives: a new concrete riser with larger culverts under the road; a combination of embankment overtopping protection and a new riser and culvert system; and eliminating culverts with an open-channel overflow spillway and a bridge over the spillway to maintain road access. The selected alternative – designed for a 25-year flood and responsive to the dam’s hydraulic conditions – was the combined embankment overtopping protection and new spillway system.

Our subconsultant, Dewberry Engineers, Inc., designed repairs to Mirror Lake Drive. The improvements elevated the road to match the existing spillway’s low point and location, with guardrails on both sides.