by Peter Corrigan
Dams and dam safety are not topics that jump to the mind when thinking about the National Weather Service (NWS). However, the NWS plays a critical role in the unlikely event of a significant problem occurring at a dam.
There are literally hundreds of dams within the Blacksburg NWS Hydrologic Service Area (HSA). Our office maintains a database with information on 665 different dams in our 40-county area of North Carolina, Virginia and West Virginia. This database contains a variety of information on the dam including location, downstream hazard, physical characteristics, and data regarding dambreak scenarios.
There are a variety of ways in which dams are classified such as area of impoundment, height of the dam, or type of construction material. For purposes of dam safety however, dams are classified according to the potential impact that a failure of the dam would have and there are three categories:
• High Hazard - dams which upon failure would cause probable loss of life or excessive economic loss
• Significant Hazard - dams which upon failure could cause possible loss of life or appreciable economic loss
• Low Hazard - dams which upon failure would not likely lead to loss of life or significant economic loss
In the Blacksburg HSA there are 138 High, 147 Significant, and 380 Low hazard dams. These dams range from massive concrete hydroelectric facilities such as Claytor dam on the New River, to small earthen dams used for a variety of local purposes. Overall, dam maintenance, safety and inspections are the responsibility of the dam owner. This may be a federal agency such as the U.S. Army Corps of Engineers (Gathright Dam) or a power company such as American Electric Power (AEP) (Smith Mountain Lake Dam). These larger dams tend to receive frequent detailed inspections and testing of emergency procedures. However, the great majority of dams are privately owned and many receive no routine inspections due to the perceived low hazards downstream. Regulation of most dams is performed by the state, North Carolina, Virginia, or West Virginia in our area.
The NWS responsibility with respect to dam safety occurs when there is some indication that a dam may fail. Usually a dam operator would initiate a phone call to the NWS as part of an Emergency Action Plan (EAP). From an operational standpoint there are essentially three options the office can take when there is a dam failure potential. 1) Issue a Flash Flood Warning; 2) Issue a Flash Flood Watch; 3) Take no action. The Flash Flood Warning is issued if there is a credible report that the dam has in fact failed or is about to fail. This Warning will get immediate and maximum dissemination via media outlets, the internet, NOAA Weather Radio and other systems such as cell phones or pagers. If the report is that there is a potential failure, the dam is leaking for example, then the NWS will issue the Flash Flood Watch. The Watch of course has a lower urgency than a Warning but will still be disseminated via normal channels of communication.
Little River Hydroelectric Dam near Radford, VA
Fortunately, dam failures are rare and even rarer when it comes to large, high hazard dams. The last significant dam failure in our HSA occurred in June, 1995 when the Timberlake Dam in Campbell County failed after 6 to 10 inches of rain fell on June 22-23. The subsequent dam failure and uncontrolled release of water caused 2 deaths and substantial damage.
The NWS regularly participates in dam failure tabletop exercises with important dam operators, such as AEP and Dominion Power. These exercises bring together regulators, dam operators, emergency management agencies and others for mock-failure scenarios that test communications and response plans. Most large dams are routinely inspected and the odd of a failure extremely low. It is the small, privately owned dams that are the more likely to fail, however these are usually not the High Hazard dams that would cause serious danger. The aging of the dam inventory is one of the biggest concerns as we move into the 21 st century. A number of factors, including age, construction deficiencies, inadequate maintenance, and seismic or weather events, contribute to the likelihood of dam failure. For example, some failures are the direct result of flows larger than the dams were built to withstand. With the exception of seismic or weather events however, age is a leading indicator of dam failure. In particular, the structural integrity and operational effectiveness of dams may deteriorate with age and some older dams do not comply with current dam safety standards established in the 1970s. Overall, nearly half of dams in the U.S. are older than 50 years, the design life of many dams.