The Anatomy of a Dam Failure

Failed dam on the Green River in Massachusetts shows the river flowing around the spillway and through the retaining wall breach.

Parts of the northeast took a beating from Hurricane Irene in late August, and along with road failures, culvert failures, and significant property damage, we also saw a number of dam failures. The flood peaks were so high, that in some cases it was after the flooding subsided that the actual damage was noticeable.

American Rivers staff have seen lots of failed or breached dams in our work and I wanted to address a common concern that is frequently raised about dam integrity.

Let me start off with a brief explanation of the parts of a “dam.” When we think about dams, most of us tend to think of the spillway structure where the water flows as the entirety of the dam, and we may not notice other important components of the structure. Most dams also include some combination of earthen embankments, retaining walls, gate houses, raceways or sluice gates in addition to the flowing spillway.

When a dam fails, sometimes the spillway concrete breaks or stones come unraveled. But in many cases the dam spillway remains intact, and instead it can be side retaining walls, earthen embankments or raceways that erode and collapse. All of these pieces are part of the structural integrity of the dam and their failure can be just as destructive as the spillway collapsing.

As an example, in the pictures shown here of a dam on the Green River in Massachusetts, the spillway remains intact and dry on the left side of the photo and the river drove its way through the breached embankment and retaining wall. Clearly the dam is no longer functioning even though the spillway itself is in good condition.

This misunderstanding about dams can lead to confusion. In one case, I was at a community meeting where local residents were sure that the century old granite blocks would never fall, and they therefore felt the dam was not a safety issue. Yet, it was not the granite blocks that particularly concerned dam safety inspectors, but rather the 300 feet of leaky 100-year old earthen embankment that connected to the spillway.

So next time you watch water falling over a dam, look around for the embankment, retaining walls and sluice gates and see what kind of condition they are in. In a flood, will the entire structure hold up?

The majority of dams in the Northeast were built decades to centuries ago and many of them, perhaps most, no longer serve the function they were built to provide. Through all of the floods that are repeatedly pounding parts of the country, we continue to believe that the most cost-effective, permanent way for communities to deal with unsafe dams is to remove them.