SECTION 4: RISK ASSESSMENT
non-residential buildings, agricultural land, or low volume roads.
• Class B (Intermediate Hazard) includes dams located where failure may damage highways or secondary railroads, cause interruption of use or service of public utilities, cause minor damage to isolated homes, or cause minor damage to commercial and industrial buildings. Damage to these structures will be considered minor only when they are located in backwater areas not subjected to the direct path of the breach flood wave; and they will experience no more than 1.5 feet of flood rise due to breaching above the lowest ground elevation adjacent to the outside foundation walls or no more than 1.5 feet of flood rise due to breaching above the lowest floor elevation of the structure. • Class C (High Hazard) includes dams located where failure will likely cause loss of life or serious damage to homes, industrial and commercial buildings, important public utilities, primary highways, or major railroads. Table 4.16 – Dam Hazard Classifications Hazard Classification Description Quantitative Guidelines
Interruption of road service, low volume roads
Less than 25 vehicles per day
Low
Economic damage
Less than $30,000
Damage to highways, interruption of service
25 to less than 250 vehicles per day
Economic damage
$30,000 to less than $200,000
Intermediate
Probable loss of 1 or more human lives
Loss of human life*
Economic damage
More than $200,000
High
*Probable loss of human life due to breached roadway or bridge on or below the dam
250 or more vehicles per day
Source: NCDEQ
The most recent failure of a high hazard dam occurred in 1996 with the failure of the Lake Raleigh dam following Hurricane Fran. Based on classification criteria, a high hazard dam failure could cause death and/or injury as well as severe property damage and economic impacts within the affected area. Therefore, though the affected area would be negligible in size relative to the entire planning area, the potential impact of a high hazard dam failure is critical.
Impact: 3 – Critical
Spatial Extent: 1 – Negligible HISTORICAL OCCURRENCES
The following table details historical occurrences of dam failure reported in Wake County. No additional failures are known to have occurred in the County since 1996. Table 4.17 – Dam failures in Wake County from 1996 to 2023 Dam Name Location Class at Time of Failure Year of Failure Cause of Failure Cedar Hills* Wake County Intermediate* mid 1970s Heavy rain Coachman’s Trail Lower Wake County High late 1970s Heavy rain Beaman’s Lake Wake County Intermediate late 1980s Heavy rain Bass Lake* Holly Springs Low* 1996 Hurricane Fran
Wake County, NC Multi-Jurisdictional Hazard Mitigation Plan
WSP June 2024 Page 94
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