SECTION 4: RISK ASSESSMENT
Table 4.14 – 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
Intermediate
Economic damage
$30,000 to less than $200,000
Loss of human life*
Probable loss of 1 or more human lives
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 the historical occurrences of dam failure reported in the 2015 Wake County Hazard Mitigation Plan. No additional failures are known to have occurred in the County since 2015.
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
Lake Raleigh
Raleigh
High
1996
Hurricane Fran
Penny Hill Lake
Zebulon
Low
1996
Hurricane Fran
Silver Lake*
Raleigh
Intermediate*
1996
Hurricane Fran
Yates Mill Pond Hurricane Fran *These dams are now classified as high hazard due to downstream development and/or increased downstream traffic. Source: 2015 Wake County Multi-Jurisdictional Hazard Mitigation Plan Wake County Intermediate 1996
Probability of Future Occurrence Given the significant presence of high hazard dams in Wake County, failure of a dam is possible. Dam failure has not occurred in the county since 1996, however historical events alone do not provide an adequate estimate of potential future occurrence. With heavy rain events becoming more frequent and intense, conditions conducive to dam failure may occur more frequently as well. Probability: 2 – Possible
Wake County Multi-Jurisdictional Hazard Mitigation Plan 2019
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