Rhodes Pond Dam Repair, Dunn NC
NC Wildlife Resources Commission
The Rhodes Pond Dam is located on the Black River in north Cumberland County, approximately six miles south of Dunn, North Carolina. The pond and dam, which have existed in some form since the year 1770, provide about 120 acres of surface water for residents and visitors’ recreational use. During Tropical Storm Andrea (June 2013), Rhodes Pond Dam suffered damage due to overtopping, and the dam was reclassified as High Hazard by NC Dam Safety. McGill designed and permitted repairs and overtopping protection for the dam to withstand a 100-year flow event, as required by NC Dam Safety. During Hurricane Matthew in October 2016, while construction of the repairs was ongoing, the dam was overtopped, resulting in breaches on both sides of the spillway, as well as soil loss and void space development under the foundation of the small spillway and around its endwalls. NC Dam Safety officials directed McGill and Wildlife Resources Commission (WRC) that the spillway must be redesigned to accommodate flows from a more extreme rainfall event – up to 1/2 probable maximum precipitation (PMP).
Two main challenges were predominant to repair the dam:
1. The dam is located just upstream of the US 301 bridge over Black Creek, approximately 0.5 miles upstream of the I-95 bridge over Black Creek, and 2 miles upstream of the confluence of Black River and South River. As such, the dam is subject to substantial backwater effect from this succession of conveyance contractions and major confluence.
2. The flat area surrounding the dam and the limited hydraulic head of 2.3 ft between normal water level and overtopping require a massive structure to convey extreme flood events.
McGill prepared an incremental hazard evaluation (IHE) of the dam to determine a reasonable storm design flood since the entire area surrounding the dam floods during a 1/2 PMP. The IHE concluded that the appropriate design storm for the spillway is the 500-year event. McGill performed the hydraulic modeling and preparation of design drawings, technical specifications, cost estimate, and permitting for the installation of a (185′ x 80′) 6-cycle labyrinth weir with an 8-degree wall angle and half-round weir crest to replace the existing spillway and accommodate the 500-year event. Design began in May 2019 and NC Dam Safety permit application with design report, plans, technical specifications, emergency action plan, and operations and maintenance plan were submitted in August 2019.
The complete redesign will minimize overtopping concerns. As added value to the project, McGill incorporated design elements to improve the habitat for area wildlife and provided accommodation for the installation of a future pier and parking area to bring more outdoor recreation opportunities. The project results in indirect economic benefits through an adjacent event and celebration venue.
“I think you’ve done a great job of designing this to provide a range of flows that will passively mimic natural conditions in the Black River, while maintaining water levels in Rhodes Pond.”