FEMA Issues Final Report: Independent Evaluation of Recent Flooding in New Hampshire

By Jim Gallagher

The Federal Emergency Management Agency has issued its final report on the Independent Evaluation of Recent Flooding in New Hampshire. The evaluation was performed to determine the specific causes of floods of May 2006 and April 2007 in New Hampshire and provide recommendations for improving water management procedures and dam operations to reduce the impacts from future flooding. The evaluation was requested by New Hampshire Governor Lynch following the floods that devastated Southern New Hampshire in 2007.

Rainfall and Runoff

According to New Hampshire Department of Environmental Services (DES) Commissioner Tom Burack, “This report will provide DES with invaluable information as we continue to improve our emergency preparedness and management processes. DES has already begun to act upon many of the recommendations put forward in the report."

The evaluation found that the causes of the flooding in May 2006 and April 2007 were different for the two events. The May event was unusual because of the sheer volume of rainfall, which ranged from 6 inches inland to over 14 inches along the seacoast over a two-day period. The region normally receives only about 3.5 inches of rainfall in an average spring month. The April 2007 event was extraordinary because of the combination of heavy rainfall, which ranged from 4 to 8 inches across south central and southeastern New Hampshire, and rapidly melting snow.

The runoff produced during these events overwhelmed the region’s rivers and streams, and inundated the region’s floodplains. The evaluation found that the high runoff also lessened the effect of operations performed at dams in the region. All but the largest lakes in the upper reaches of the rivers filled rapidly and passed all inflows downstream.

Mitigate Future Damage

The evaluation recommended several actions to mitigate future flood damages, including: improved floodplain management; improved flood forecasting; and a watershed approach to flood operations. These recommendations are based on the following findings:

Flood events as large as, and larger than, the May 2006 and April 2007 floods are likely to happen in the future. Communities and the State should plan accordingly. Many of the floodplains adjacent to the rivers and streams in the region are still relatively undeveloped. Building in these floodplains will subject the structures to flood risk and will increase the flood elevations and flow rates elsewhere, and should be discouraged. Sound floodplain management, based on accurate information about the floodplains, is critical to reducing the effects of future floods. Flood forecasting, while not always sufficiently accurate, should be used as a tool to help decision makers take appropriate actions during flood events. Storing waters in the region’s lakes, ponds and reservoirs, and coordinated dam operations help reduce flooding. However, storage opportunities in south central and southeastern New Hampshire are highly limited, and the effect of improved dam operations will be relatively minor. Implementing flood management recommendations can reduce local flooding, but cannot prevent widespread flooding from events like the May 2006 and the April 2007 events.

A complete copy of the report, including appendices, is available for downloading in PDF format.

Jim Gallagher is Chief Engineer of the New Hampshire Department of Environmental Services’ Dam Bureau.