Tenth Anniversary of the Lake Needwood Dam Incident

摘要

Located North of Rockville, Maryland, Lake Needwood Dam stands 65 feet high and has a drainage area of 12.8 square miles. Constructed in 1965 using PL-566 authority, the Soil Conservation Service (now Natural Resources Conservation Service) assisted with the planning, design and construction of the dam. The dam provides flood protection, sediment control, and recreational use. With a breach inundation area encompassing hundreds of homes and businesses, the Natural Resources Conservation Service and the Maryland Department of the Environment - Dam Safety Division classify the dam as high hazard potential. The Maryland-National Capital Park and Planning Commission owns and operates the dam. Normal dam operations provide for a half-full stage in the reservoir above the dam. This allows for recreation on Lake Needwood, while maintaining storage capacity for flood control. During heavy rains in June of 2006, the lake level rapidly increased 23 feet. On June 27, dam operators discovered uncontrolled seepage on the downstream slope of the dam and the dam owners initiated the emergency action plan for the dam. At 1:00 AM on June 28, the Montgomery County Emergency Operations Center issued an evacuation order to at-risk residents below the dam. The county completed the evacuation by 8:00 AM the following morning, with 2,400 people evacuated from downstream communities. The evacuation lasted two days. During the evacuation, county managers, Natural Resources Conservation Service personnel, and Maryland Department of the Environment - Dam Safety Division personnel assessed and monitored the seepage. They examined the fractured bedrock below the dam and installed observation wells to determine potential causes of the seepage. After two days, the reservoir stage receded and pressures decreased in the observation wells. Officials then allowed residents to return to their homes. Afterwards, officials instituted continuous monitoring of the dam until completion of a rehabilitation of the dam two years later. The incident at Lake Needwood exemplifies the usefulness of an emergency action plan. The dam owner, designer, regulatory agencies, law enforcement, and emergency action officials worked well together. Despite the leakage, the dam served to attenuate floods downstream. The sponsors continue to operate and maintain the rehabilitated dam and maintain an updated emergency action plan.