Terrain, Infrastructure and Flood Processes on 26/27th September 2024

by Helen Steen

The terrain in Buncombe County North Carolina is a mixture of rolling hills and steep wooded slopes, interspersed with valleys some of them dammed. Prolonged heavy rainfall over the 26 and 27th of September led to run-off over steep wooded slopes, filling creeks, rivers, reservoirs and lakes. Catastrophic flooding caused complete and partial destruction of infrastructure close to creeks and rivers, and major damages in the overflowed areas away from channels. Primary flood processes - flash floods initiated on steep hillsides and from ponds, lakes and dams, while secondary flood processes occurred when debris ladened flash floods entered flooded main water courses.

Asheville’s main water supply reservoir the North Fork Reservoir (NFR) discharged when an auxiliary spillway gate activated on 27th of September. The NFR Improvement Plan, completed in 2021, involved the construction of a mechanical automated Fusegate overflow system. The manufacturer of the Fusegate describes a dilemma inherent to the system: “Failing too early would result in frequent replacement of the Fusegates and unnecessary lowering of the reservoir. Failing too late or not failing at all could result in overtopping of the earthen embankments, and ultimately lead to a breach of the dam.”

Activation or tipping of Fusegate units occurs when the water level rises above full pool until pre-set inflow rates to the units are reached. The system consists of eight units in series designed to discharge stepwise one after another, until the water level recedes, so downstream areas are protected from a flash flood. Why only one Fusegate unit activated can be understood by way of the previously mentioned dilemma inherent to the system, however described in terms of pre-set inflow rates; too high inflow rate risks a flash flood while too low inflow rate risks a dam breach.

On the 27th of September, by the time unit No. 1 tipped over, when the pre-set inflow rate was reached, water in the reservoir had risen to a critical level. The inflow rate to the other seven units was not reached so they remained upright. A lower pre-set inflow rate to unit No. 1 was necessary for it to have activated earlier followed at intervals by units No. 2. then No. 3 and so on until the water level fell to full pool. The resulting outflow volume would have been equal to the single unit discharge but having a lower flow rate over a longer time period, avoiding flash flood conditions downstream in North Fork Creek.

Damage to terrain and infrastructure is a function of type of flood and flood type is a function of geomorphology and infrastructure. Three types of inland flood; fluvial, pluvial and flash flood have been identified. A fluvial or river flood is when a river overflows its banks and floods surrounding land but fluvial floods do not exert powerful forces. A pluvial flood or surface water flood can become a flash flood when an extreme rainfall event creates a high velocity torrent over steep terrain or when there is a sudden release of water from an upstream dam. Flash floods exert powerful forces therefore can be very dangerous and destructive under certain circumstances.

Damage to infrastructure and terrain downstream of lakes and reservoirs such as North Fork Reservoir, Bee Tree Reservoir, Flat Top Mountain Lake and Tahoma Reservoir (McDowell Co.) can be explained by flash floods due to releases, overflows or breaches of dams, in combination with pluvial flash floods due to runoff over steep terrain. Damage to infrastructure in main water courses was due to secondary flood processes, which occurred when fluvial flooding conveyed flash flood debris flows. Secondary flood processes such as block/break-away effects occurred when debris flows became held back by obstructions, such as building, bridges or natural narrows blocking the flow until forces overcame the blockage that then broke away with increased velocity. By this means secondary flood effects became compounded as more and more debris accumulated with repeated block/break-aways adding to destructive forces.

Seventy one dams are listed on the US National Inventory of Dams (USNID) in Buncombe County 47 of which are classified high hazard potential ie. risk to life. The few dams in existence before the Great Flood of July 1916 were washed away, so all 71 on the register were either reconstructed or constructed after 1916. Flooding on the 26/27th September 2024 caused the Swannanoa River to crest at 26.1 feet, which was more than 5 feet above the old record of 20.7 feet in 1916. High rate of rainfall over an extended period up to the 27th of September led to reservoirs and lakes failing to hold very high run-off volumes generated within their catchments. Flash floods caused by their discharges were not only destructive within downstream creek systems but caused secondary flood effects raising the hazard level in main water course due to debris flows.

Operation of the emergency Fusegate system at NFR on 27th of September prevented a breach of the dam, however the pre-set inflow activation rate for unit No.1 was too high resulting in a flash flood down North Fork Creek. In the aftermath the creek was unrecognisable according to one observer. Downstream from the confluence of the creek with the main Swannanoa River there is a bend in the river. A debris ladened flash flood originating from the North Fork Reservoir entered the flooded main river and failed to take the bend sweeping away or destroying most of the homes in Beacon Village situated on the bend. According to local news reports only 11 homes were left habitable.

The consequences of concurrent flood processes over the 26th and 27th of September 2024 were catastrophic; taking lives, destroying infrastructure, altering communities and left behind a changed landscape strewn with debris.