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Heavy rain volumes in Maryland starting on June 25, yielded 10 to 15 inches in less than 12 hours in Dorchester County, and more rainfall continued through June 28. To illustrate how much rain this was, we’ll take a common parameter used in dam design, known as the 100-year storm, or the storm that has a one in one hundred chance of occurring in any given year. For Dorchester County the 100-year rainfall over a 24-hour period is 7.8 inches. The June 25 precipitation level corresponds to a 500 to 1,000-year storm based on the National Weather Service latest rainfall charts.
Even with these dramatic rainfall totals, dams in Maryland performed well. Dams are classified depending on the damage that may be caused if they fail. The greater the hazard, the better the design necessary to hold water.
Dam Hazard Classifications in Maryland current numbers in each
category:
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Classification |
Number of Dams in MD |
Potential Hazard |
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High Hazard Dams |
67
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Probable loss of life, major increases in existing flood levels at houses, buildings, major interstates and state roads with more than 6 lives in jeopardy.
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Significant Hazard Dams |
84
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Possible loss of life, significant increased flood risks to roads and buildings with no more than 2 houses or 6 lives in jeopardy.
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Low Hazard Dams |
228
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Unlikely loss of life, minor increases to existing flood levels at road and buildings.
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During the June storms, the only failures in Maryland were to five low hazard dams, all located on the Eastern Shore. When storms exceed the 100-year storm, which is extremely rare, some low hazard dam failure can be expected. The cost of repairs from these failures tends to be less than the increased cost of improving the design and construction required of greater hazard dams.
Emergency Action Plans Implemented
Emergency Action Plans (EAP) are required by the Maryland Department of the Environment (MDE) for high hazard dams and significant hazard dams in order to construct or repair dams in this state. These plans are used to alert the public in the event of high water levels or other potential concerns. There were three occasions where these plans were implemented as a precaution for public safety. After conditions returned to acceptable levels the actions were cancelled, and residents were allowed to return to their homes
Dam that Broad Creek!
The EAP for the Broad Creek high hazard dam in Harford County was activated when the depth of water over the concrete spillway reached four feet. The local fire department evacuated properties downstream of the dam the morning of June 26.
Also on June 26, operators of the Howard Duckett (a.k.a. Rocky Gorge Dam) high hazard dam in Prince George’s County opened its gates to release water. The owner, the Washington Suburban Sanitary Commission, activated its EAP by notifying the County and the City of Laurel. In turn, the City coordinated with the County and the Maryland Emergency Management Agency to activate a flood warning evacuation within Laurel.
Finally, on June 27, conditions at both Lake Frank and Lake Needwood in Rockville, Montgomery County made it necessary to activate their EAPs. Lake Needwood was experiencing major seepage on the downstream embankment. Approximately 2,200 individuals were evacuated from their homes. A remedial action is underway to repair the dam damage for future public safety. The two lakes are owned by Maryland National Capital Park and Planning.
Maximizing Public Safety
MDE’s 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 staff continues to work with dam owners to improve safety across the State. Efforts include developing and updating EAPs to be used in the event of dam failure, repairing and upgrading structures, inspecting and monitoring existing dams, and the detailed review and issuance of permits for new dams. MDE requires that the EAPs be tested and updated each year by the dam owners and emergency management officials to provide emergency preparation, minimize risk, and maximize public safety.
Click here to learn more about MDE’s Dam Safety Program.
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