Dam Challenges by Design
This blog is a part of our series this month on River Restoration Challenges.
Dam removal projects come in all shapes and sizes— from the dynamite-using, decades-in-the-making, multi-million dollar, salmon-helping Elwha dam removal in Washington to elbow-grease powered, under $10,000, community-championed, two-in-one, brook-trout-benefitting dam removals on Bear Run in Pennsylvania. Despite the project size, good design must address the impacts of the removal itself, as well as changes in the environment once the dam is no longer there. This blog will outline just a few of the many design challenges associated with dam removal.
Let’s begin with sediment— one of the fastest ways to inflate the cost and complexity of a dam removal. Some dams don’t have much sediment built up behind them like the partially breeched Smitherman’s dam in North Carolina. This dam was removed by simply allowing the modest amount of sediment to move downstream, not unlike it would during a storm event.
Other dams, like the huge Condit dam, have enormous sediment loads built up. The complex (and expensive) design plans included notching the dam at the base to allow sediment to move downstream, and dredging behind the dam to remove large woody debris. Other design questions with the Condit removal: Where should we access the site to dredge? What will be the ecological impact of building a haul road and running thousands of trucks to haul dredge material? Where should we dispose of that material? Where should we allow the dredged material to dry before hauling, or should we haul it wet?
Additionally, some projects may have contaminated sediment trapped behind the dam. In this case, dredging and safe disposal of these sediments is warranted. It is worth noting, however, that just because a dam is small does not mean it is simple to design, and just because a dam is big does not make it necessarily complex.
Another design consideration is the presence of threatened or endangered species. Step one: know what species are present before removing a dam. The Dillsboro dam removal in Franklin, NC, had the challenge of protecting an endangered freshwater mussel, the Appalachian Elktoe. Intensive biological surveying was performed and freshwater mussels in the project area were relocated before demolition and monitored afterwards. Another consideration was timing. For freshwater mussels or any other species, understanding the life history can guide when to perform the work. Typically, demolition is best scheduled around migration or spawning events for both resident and migratory species.
Finally, let’s explore how design can be impacted by what is around the project area, like infrastructure. Heavy equipment must be able to access the site without damaging infrastructure, and sediment movement and water levels should not compromise culverts or bridge abutments. One ongoing dam removal project in NC was delayed after reconnaissance revealed an upstream bridge and a sewer line could potentially be affected by the removal. Additional studies revealed the removal would not negatively impact the infrastructure, but an unforeseen benefit to this delay was locating a leak in the sewer pipe and improving water quality by fixing it.
Every dam removal tells a story and every dam removal has its own design challenges. Good design must consider when to remove a dam, what is in and around the stream, how will the work impact the surrounding area, and how long until the short term impacts are outpaced by the long term benefits. A problematic dam, good design, and innovative, resilient partners will result in a project worth doing.