More than 40 people showed up at Horizons Day Camp on a hot Saturday afternoon to hear Roy Schiff, a water resource scientist and engineer, speak about dams in Vermont generally, and about our Lake Fairlee dam in particular.
Roy first directed us to the recently published Report Card for Vermont’s Infrastructure, which contains a summary evaluation of the approximately 1150 dams in the State. LINK (see pp. 11-15) He put our dam in the context of other Vermont dams, explaining that it is unremarkable and somewhat typical in its age, history, size, and condition. He spoke a little about the permitting process for a dam like ours, any work on which requires approval by both the State and the Army Corps of Engineers.
We had given Roy an opportunity to visit our dam, both from the water in a kayak, and then from downstream on foot. He was able to explain how it was constructed. The older portion is masonry (stone), with a newer concrete top. It is sitting on ledge, and its ends run into the earthen banks of the watercourse. What is unusual about our dam is the structure sitting in the middle.
Because our dam is classified as “low hazard,” the spillway is required only to carry the amount of water expected from a 100-year flood. Since the watershed that drains into the lake is about 20 square miles, the spillway capacity required would be about 1230 cubic feet per second. This is almost certainly more than the present configuration, so it is likely that any repair or replacement of the dam would require a significant redesign. There is no working low level outlet, which is required to manually lower the lake level for repairs.
The process of dam repair/replacement:
- Survey first – takes a week
- Analysis of alternatives – location, tye of structure, dimensioning to flow, spillways, abutments, hydraulics analysis, modeling how water surfaces will change.
- Preliminary design – after one alternative has been chosen
- Permitting – could be up to six months.
- Certification (by the State)
When the audience began asking questions he answered candidly, with the disclaimer that these opinions were not based on a proper engineering evaluation of the dam. He said
- There is no way to know when the dam will fail, or how badly. It is in need of repair, however.
- If the dam fails, the water level in the lake will only go down 5 or 6 feet. Nonetheless this could create significant issues for abutting landowners.
- Dam construction is generally limited to the warmer months. We will be challenged to get it done without impacting the prime recreation season.
- The lake will need to be drawn down for any dam work, unless (expensive?) “cofferdam” is employed.
- Installing a “low head” hydroelectric turbine is probably not an economical option in our situation. If the dam were ten feet higher . . . also permitting requirements are more stringent.
- When pressed, Roy opined that repairs on our dam might cost in the neighborhood of $100,000. (NOTE: see below)
The meeting lasted about an hour. We expressed our gratitude for his help, and promised to keep in touch.
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Two days later Roy sent along the following thoughts:
I took a few more minutes and looked into dimensions and ideas we discussed to refine my off-the-cuff cost estimate. I actually think that a more realistic ballpark for such a project is $300,000. I think that my frugal side got the best of me when pressed in front of the crowd. At this point costs could easily go 50% higher or lower, I just do not know without more information.
My thoughts after the talk lead me to recommend to you a two or three-step project. Step 1 is a dam repair or rebuild feasibility study that will get you an alternatives analysis, initial regulatory feedback, and a ballpark cost opinion. This study is important to allow you to resolve the unique setting with the camp on the structure and put some reality into the discussion to see what the fundraising requirement is. It will also hopefully land on a preferred alternative to move forward from. Step 2 would be design and permitting. Step 3 would be construction.
Also it was suggested that some readers might not be familiar with the idea of a structure sitting in the middle of a dam. HERE is our first post about the dam from August of 2010 that should help.