Volume III, Number 4
eMDE is a monthly publication of the Maryland Department of the Environment. It covers articles on current environmental issues and events in the state.
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Dams are designed and constructed to contain water and reduce flow to residential and sensitive areas during times of flooding. The evacuation of the Lake Needwood Dam area during a week of heavy rains in June 26, 2006, brought to light many of the challenges dam owners face over the long life cycle of high hazard dams in urban settings. When flooding caused uncontrolled seepage and elevated water levels outside the dam’s normal capacity, the owner, Maryland-National Capital Parks and Planning Commission (MNCPPC), sent park managers to monitor it.
Lake Needwood Dam is a flood control structure that captures floodwaters within the upper Rock Creek. The control structure is designed to discharge floodwater at a rate that stays within the stream banks or channel below the dam. The water runs through a 42-inch principal spillway through the dam over a 10-day period to reduce downstream flow.
Lake Needwood Dam was built in 1964-1965, and has a 65-foot high earthen embankment constructed on a fractured rock foundation. June 2006 heavy rains caused a 23-foot rise of the water level in Lake Needwood resulting in uncontrolled seepage and elevated water levels from the downstream slope. The dam developed a leak and was on the verge of failure. Within a matter of hours, 2,400 people were evacuated from the downstream communities, including part of the City of Rockville.
“Maryland Department of the Environment (MDE) dam safety engineers were deployed at 11:30 p.m. and arrived at 2 a.m. to assess the structure. Emergency evacuation had already started at the county level and by 8 a.m., the at-risk residents had been evacuated from the inundation area,” said Hal Van Aller, Water Resources Engineer, MDE’s Dam Safety Division.
It starts when national weather service issues a flood warning. Actions are taken by the landowners and local jurisdictions to evacuate downstream residents. With a prepared emergency action plan, and flood maps that showed areas to be flooded should the dam fail, the multi-agency effort went into action. A bucket brigade brought in sand bags, and heavy equipment showed up with stones and abatement materials.
Luckily, this was one of the first dams in Maryland that had flood maps prepared back in early 1980,” said Van Aller. “It was good that we had the plan, but the contact information was somewhat outdated. The Montgomery County Emergency Operations Center was able to overcome these challenges by using newer mapping technology, such as GIS systems. This county was one of the first areas to recognize GIS systems as a useful tool to develop updated evacuation plans. It is critical for situations like a potential dam failure that we have accurate information.”
The instruments showed that the water pressures continued to increase, even after the storm had passed and the lake began to recede. MDE engineers and county managers examined the bedrock below the dam and determined that it had open joints and fractures. They used geologic records and historic construction drawings and set up observation wells in the dam to provide water level monitoring data.
It took two days for us to be comfortable with the water pressure rates,” said Van Aller. “After 48 hours of intense evaluation while repairs were being made, the residents were allowed to move back in. We were past the emergency, and decidedly out of imminent danger, but not all clear for evacuees to return. We provide recommendations to the emergency operation center, then it is their call,” said Van Aller. “Since it takes 10 days to recede to normal level, and we had the 4th of July holiday coming up - a decision had to be made as to whether we would allow residents to move safely back into their homes, understanding that we were not 100 percent sure that the dam would not fail.” There was a possibility of having to re-evacuate in short notice.
MDE issued an immediate directive to the county outlining procedures to follow, requiring 24-hour monitoring, retaining an engineer to investigate the problem, and requiring recommendations for permanent repairs be submitted within 30 days. Repairs underway included curtain grouting of the bedrock together with a new drainage/filter blanket. A berm is being constructed on the downstream slope this summer. The berm is the last phase and is expected to be completed this fall. The grout curtain will significantly reduce the potential for water from the reservoir to flow through the rock under the dam. The downstream drain/filter will control any water from the reservoir that may flow past the grout curtain or water from the abutments that might exit the downstream slope of the dam.
By mobilizing response teams and experts on a multi-agency level, MDE and MNCPPC addressed public safety issues to ensure public confidence in the long-term integrity of the facility. As dam owner, MNCPPC was required by MDE to develop and implement a remedial measures program to address the seepage and update the Emergency Action Plan (EAP). “To the county’s credit, they placed priority on this project to protect public safety, based on our recommendations,” said Van Aller.
The Lake Needwood Dam experience brings to light many of the challenges dam owners face over the long life cycle of high hazard dams in urban settings. There are over 400 dams in Maryland, ranging in height from six to 296 feet. MDE provides dam safety recommendations to agencies that own or operate dams but their primary function is not dam safety. The Dam Safety Program ensures all dams in Maryland are designed, constructed, operated and maintained safely to prevent dam failures and the consequences of failure. MDE inspects dams based on their "hazard class;" issues permits for construction, repairs or for modifying dam structures; and works with dam owners and emergency management professionals to develop and exercise an 'Emergency Action Plan' to be used in the event of dam failure.
Dams can provide many benefits for Maryland's citizens including water supply, flood control, hydroelectric power, fishing and recreation. However, dams can also be a great threat to the safety and well being of downstream property and people if they are not properly constructed or maintained. The state has been assuring the safety of dams since 1934 through a permit and inspection program. Click here for more information about MDE’s Dam Safety Program.
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