Thursday, December 17, 2009

All Dams Provide Flood Control

"Maintain 'em or drain 'em" a motto adopted by the National Park Service Dam Safety Program, aptly expresses the dam owner's dilemma. Faced with the choice of repairing or upgrading a dam, or the less expensive option of draining a lake, many owners choose the latter course; however, where dams provide drinking water or flood control, dam removal may not be a viable alternative. The loss of a reservoir of any size often has negative economic and social impacts on local communities that have depended upon the impoundments created by dams for water supply, recreation and flood control.

The function of flood control has become increasingly important in recent years, as more and more development has occurred in historic floodplain areas protected by dams. As these dams deteriorate, they should be fixed or replaced, as dam removal leaves property in downstream floodplains highly vulnerable to flooding.

The Ben Smith Dam is in excellent shape. It is properly maintained and is not at risk of failing.

Most of the Town of Maynard is down stream of the Ben Smith Dam. And therefore will be at greater risk of flooding if this dam is removed.

All dams even ones that have been converted to fixed weir, provide flood control. It is a natural consequence of restricting the river. During most times the restriction is obvious in a static sense. The impoundment is visible as the pond behind the dam. In a dynamic situation where more water is added from rain, snow melt, and other run-off, the dam serves a second purpose, to attenuate the flow. The Dam acts as a restriction and allows less flow downstream than would have occurred without it. The additional flow is distributed over the large surface area of the impoundment causing only a minor change in the water level above the dam. This water is then retained in the impoundment and released more slowly, over time, than without the dam. However if this flow were allowed past the dam, because it was not there, then the level of the river below the dam would rise much much higher as its surface area is much much smaller than that of the impoundment. Homes and Property above the dam need not worry as the new flood plain will start 7 feet lower than it is right now.

But those below the dam need to be concerned as the river level below the dam will fluctuate more than it does now.

1 comment:

  1. Maybe we shouldn't be constructing homes and other such facilities in artificially protected, flood-prone areas. Just because that's the way something is doesn't mean its right.

    Dam removal does need to be assessed on an individual basis, but saying all dam removal in flood control use scenarios is bad is an invalid statement.