Increasing spillway capacity at Bee Tree Dam

Bee Tree Dam is located on Bee Tree Creek, a tributary of the Swannanoa River, in Buncombe County, North Carolina. The dam is owned by the Regional Water Authority of Asheville-Buncombe and Henderson and is a part of the water supply system for the City of Asheville, Buncombe and Henderson Counties in North Carolina. The embankment dam, constructed in 1925, is a semi-hydraulic fill structure with a height of approximately 177 feet. The original spillway was located on the rock abutment at the right end of the embankment dam. The spillway structure consisted of an ungated concrete ogee weir section, approximately 100 feet in length that converges into a concrete-lined chute outlet channel 30 feet in width. The 125-foot high intake tower is a concrete structure located in the reservoir. The dam is classified as a Very Large, high-hazard dam under the criteria of the North Carolina 1967 Dam Safety Law. The original spillway was significantly undersized, passing approximately the 1/3 PMP storm event without overtopping. The required design storm for this dam was determined during the 1980's by the NC State Dam Safety Office to be the ¾ PMP storm event.
The dam was repaired, including total spillway replacement, in 2001-2002. To accommodate the required design storm, the spillway was replaced by a new spillway that is 180 feet wide at the ogee weir, with sidewalls that converge to a 60-foot wide chute section. Model studies were conducted at the St. Anthony Falls Hydraulics Laboratory to confirm the required chute width for alternative combinations of weir widths and sidewall convergence angles. In addition, the top of dam was raised with a 7-foot high reinforced concrete parapet along the top of the existing dam. The new spillway ogee weir is divided into two levels (a 100-foot wide low level and an 80-foot wide high level) so that the 100-year storm discharge will not be increased in the new spillway, as the downstream valley is heavily populated. This project is being submitted for consideration for the 2003 National Rehabilitation Project of the Year. 11 pp., 8 figures, 7 references.