Assistant Director of Water Resources City of Asheville
Abstract Description: On September 27, 2024, Hurricane Helene roared through the Blue Ridge Mountains of North Carolina leaving a wake of destruction from record-setting rainfall. The rainfall forecast in the Asheville, NC area on the afternoon of September 25 jumped significantly from 6-8-inches estimated the previous day, to 15 inches with the possibility of more in the Swannanoa Valley, leading to the recommended evacuation of people within the 500-year floodplain along the Swannanoa River downstream of North Fork Dam. Many tributaries that contribute to the overall drainage into the Swannanoa River and ultimately into the French Broad River were overrun by out-of-bank flows well in excess of 20 feet high. The destruction was one of the worst ever recorded for the metro-Asheville area, including the historic flood of 1916.
North Fork Dam impounds the Burnette Reservoir which provides 70% of Asheville’s water supply. About 17% of the Swannanoa River drainage basin flows into the reservoir. Dating back to 2014, concerns were identified related to the spillway capacity at North Fork Dam. After three-years of alternatives evaluations, a Fusegate™ (HydroPlus) system was selected as the appropriate solution to address this concern, with NCDEQ’s Dam Safety Program’s approval. The solution has a smaller footprint compared to other alternatives and provided the necessary capacity to pass the design flood. Construction began in 2017 and was completed in 2020. In addition to the Fusegate auxiliary spillway construction, further remediation design efforts included seismic stability upgrades to the embankment, installation of a four-foot-high parapet on the main dam crest, modifications to the primary spillway to reduce reliance on gate operations, and primary chute reinforced concrete overlay to address hydraulic jacking potential.
On September 27, 2024, after 16.4 inches in two days, the water level in the reservoir was lapping at the toe of the newly constructed parapet on the main dam crest when the first cycle of the Fusegate system tipped. Water levels soon began to lower after this estimated 1000+ year storm event passed. The dam was safe. The operation of the Fusegate system as intended prevented the potential for a dam failure and significantly more downstream flooding than what occurred.
This presentation will discuss the design of the Fusegate system, the significant storm that occurred, and the operation of the system and other components of the dam during this unprecedented flood.
Learning Objectives:
Example of a Fusegate Spillway System in action.
Benefits of upgrading a spillway system.
Dam owner's preparations and response during an extreme rainfall event.
