How Will a Dam Removal Impact Your Community?

By Janeen McDermott, PE, Senior Water Resources Engineer

While every dam removal is unique, each project shares similarities. A dam removal can require years of preparation for the specific circumstances of the surrounding area, characteristics of the impoundment and sediments, and special consideration of the dam age, materials, and condition. So, what can you expect if a dam is being removed in your community?

Dam removals can be complicated. The entire process takes years of planning, investigation, analysis and engineering, and years of river restoration post dam removal. The actual deconstruction of the dam is a relatively short component of the whole process, but reshaping a natural river channel where there once was an artificial lake can take a year or more of active construction.

Many steps must be completed before dam deconstruction. The entire affected area must be carefully assessed beforehand. These investigations include topographic and bathymetric surveys (to map the depths and shapes of underwater land), sediment sampling to characterize sediment quality and type, sediment probing to determine sediment quantity, habitat assessments for threatened and endangered species, land surveys to determine ownership of the bottom lands, and geotechnical and structural data for engineers to assess existing dam conditions.

Removing a Dam is a Slow, Careful Incremental Process

There are many misconceptions about the actual deconstruction, or demolition, of dams. First, dam removal is a slow, incremental, process. Water must be released very slowly, typically dropping only six inches per day, at most. To people downstream in the river, the additional flow is largely negligible, but may look like a higher flow typically seen following a heavy rainstorm.

Another common misconception is that the dam is suddenly blown up or knocked down. That is never the case in an urban setting and is rarely the case anywhere for medium or large dams. The dam is removed piece by piece from top to bottom over weeks or months, often after much of the water has been drained from the impoundment, the artificial lake, or pond above the dam

Construction Work Sometimes Requires Special Equipment and Procedures

Once the impoundment has been fully dewatered and the water level above the dam approximately matches the water level downstream, any remaining concrete structures of the dam can be removed. The major difference, of course, is that the work is being done in a river, which requires additional planning and safety precautions. Some contractors have access to amphibious excavators which are excavators that can float and move on the water. These are specialty pieces of equipment that can help the contractor access hard to reach locations within the flowing river.

Sediment Management is Often a Constant Focus

Before, during, and for some time after the dam removal, sediment will likely need to be managed. The sediment quality and quantity are key factors determining the extent of sediment management required. The goal of managing sediment is to minimize the transport of sediment downstream. Rivers normally move a lot of sediment, but a dam causes sediment to build up unnaturally. When removing the dam, we want to avoid surges of sediment moving downstream.

We also want to prevent erosion along the banks of the river. The water level will drop, and in places, the river will move faster than it once did. Faster moving water can scour banks of sediment. In cases where sediment management has been determined to be required or important, the dewatering and dam removal process needs to be intricately tied with the project’s sediment management plan. Sediment moves as the water level upstream of the dam is lowered. The contractor will be able to manage a large portion of the moving sediment by slowing the dewatering rate and actively managing sediment within the impoundment.

Cloudy Water During Removal is Common

Even following best practices, visitors to the river during and immediately after a dam removal should expect to see turbid, cloudy, black or dark brown water. We expect this, because the finest particles of sediment – the silts and fines— are very difficult to capture. Most of the sediment, the sand and larger grain size material, will be captured through active and passive management within the project area, but the finest particles will result in noticeable cloudy water through the project area and downstream. This will be most noticeable during active dewatering and construction work within the river. Once the dam is removed and a new river channel is established, the turbidity will decrease and return to existing conditions with minimal long-term effect on wildlife and habitat.

Restoring a Free-Flowing River and Shorelines

After the water level starts dropping in the impoundment and a restored river channel is established, restoration work will begin. Sensitive areas, like existing infrastructure and wetlands, and public spaces will be a priority, with the goal of cultivating native grasses and plants before invasive species can populate the area. Some sections of the shoreline, like those on the outside bends of the river channel, will likely need to be fortified with tree root wads or boulders or a combination of the two. Later, native trees and shrubs may be planted to help hold the restored banks in place and improve habitat in the former impoundment.

After one or two growing seasons, the impoundment area begins to look quite green. After three or four, it can look natural and restored. After nine or ten years, it will be hard to tell there was ever a dam on the river.

Dam Removal Can be A Positive Solution

Many dams have surpassed their intended design life or no longer serve their intended purpose and pose a risk to downstream communities and environmental habitat.   There are many factors that go into dam removal, and every project has unique challenges ranging from environmental impacts, funding, public perception, and more. Engaging a team of experienced engineers, scientists, and restoration practitioners early in the planning phase can save time and money while achieving the desired vision for the project. Removal of an aging dam or one that no longer serves its intended purpose can be a positive solution for the owner, community, and environment.

Contact me if you have questions about dam removal and the unique challenges you may be facing.