Vermont dam weathers Hurricane Irene

Hurricane Irene barreled up the East Coast of the U.S. this weekend, bringing high winds and heavy rain to a broad swath of the continent.  About 5 million electricity customers lost power at some point during the storm's progress, with service still off for many consumers.  Hurricanes and other storm events place added stresses on our electric infrastructure, resulting in not only power outages but possible dam failure.

In rain-lashed Vermont, utility Green Mountain Power worked to prevent the failure of the Marshfield Dam near the town of Cabot.  Located on the Winooski River less than 20 miles upstream from Vermont's capital Montpelier, the earthen Marshfield Dam creates a 400-acre impoundment known as Molly's Falls Pond.  Green Mountain Power uses the water impounded by the dam to run a 5-megawatt hydroelectric project.


Irene dumped a lot of water on Vermont.  As a result, the water behind the Marshfield Dam rose within 10 feet of the dam's crest on Sunday night -- far too close for comfort.  When the impoundment behind any dam gets too full, the dam is at increased risk of breaching; if a dam gets overtopped -- particularly an earthen dam -- this risk of dam failure is even greater.  Dam failure at Marshfield could have serious impacts for the people and property downstream in the Winooski River Valley, from Montpelier further down.  As a result, Green Mountain Power's emergency plan calls for a large controlled release of water through the dam once the water gets within 6.5 feet of the dam's crest.  As a precaution, the utility asked the state emergency management agency to evacuate about 300 households in Marshfield, Plainfield, East Montpelier and Montpelier.  Dam safety was at risk.

In the end, the water stopped rising, and then returned to a lower level.  (Check out the USGS's streamflow data for the Winooski River near Montpelier.)  Although Vermont suffered major damage from Hurricane Irene's remnants, the Marshfield Dam survived this storm.