Dam Removal 1

[RedBauer]

We're putting together a set of plans for removing a dam in NY that was built in the 1920's. This is a small dam, approx. 100 ft long and 15 feet high. The stream is approx. 25 feet wide. This may be fairly conservative.

Do we have to do stream bed restoration? We're planning on removing the sediment, however I'm assuming that beneath the sediment is the original stream bed. The affected portion of the stream is 75-100' in length.

Additionally, I believe if we were to place boulders, cobbles, etc. as step or plunge pools, mother nature is just going to redevelop the stream bed during the first flood event.

 

[ACtrafficengr]

At the minimum, I'm sure you'll have to reseed to stabilize.

Is the project big enough to require a contruction activity permit or SWPPP? Perhaps the thing to do would be talk to NYSDEC sooner rather than later, and see what they'll want you to do.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[RedBauer]

SWPPP isn't necessary for this project. Turbidity curtain and silt fence will be required and spec'd on the plans.

 

[francesca]

Any 25' wide stream is a water of the state. You're going to need permits galore. The stream has reached a new equilibrium in the past 90 years and will have to reach a new one again. Yes, you're going to need to help it along. Where is the deposition going to occur downstream? How far upstream will erosion occur? You'd better plan for these things. If you're disturbing more than 1 acre, which you are, by removing the water on top, you need an NPDES permit, which means you need a SWPPP.

 

[cvg]

This would be WUS - Waters of the U.S.

At the minimum you will probably need:

404
NPDES/SWPPP
NEPA documentation
probably a CLOMR which will require ESA compliance
and last but not least
State Dam Safety Department permit to remove a dam

 

[RedBauer]

Looks like we're getting away from the original question. I have the permit requirements taken care of. SWPPP is not required for the site since the disturbed area is less than 1 acre. We plan on stabilizing the stream banks, staging areas, etc. Proposed contours are drawn on the plans.

I'm just curious about the stream bed. Since the dam has been in place, there's a chance the stream bed has changed. Sediment has accumulated behind the dam, and the spillway probably has scoured a portion of the stream bed.

What would you guys do to restore the stream bed? Ideally, I'd like to remove the dam and sediment. I believe mother nature will restore the river as necessary. Additionally, beneath the sediment, I'm assuming the original stream bed still exists.

The total impounded area (basically a small reservoir) is 10,000-15,000 sf. Length of the affected portion of the stream is ~100 ft.

 

[Ryb01]

Redbauer,

The answer you are looking for will take some work to achieve. Fracesca hit the nail on the head when she said that the river has possibly reached an equlibrium state. This condition is not easily achieved in a river/stream and is easily affected by a small change in the flow conditions.

Removing the sediment deposition is a step in the right direction. There are numerous bank stabilization techniques, including native plantings, crib walls, boulder revetments, spurs, etc. Restoring the stream bed could also consider a number alternatives involving pool-riffle sequencing, grade control structures, etc. These types of restorations need to be designed specifically for your site. These type of restoration require careful attention to the stream as a whole and not necesarily to your area of interest.

"I believe mother nature will restore the river as necessary." In short, yes it will.

"The affected portion of the stream is 75-100' in length."

The concern I would have when removing this dam is the negative affects you will definitely create downstream of your site, significantly larger than 75-100' in length. By removing this dam you are adding power to the stream and by adding this power the stream will once again try to re-balance itself to a state of equilibrium. This happens usually happens dowenstream of the site in question as flow depths and velocities change. By removing the dam you will be increasing the velocity, decreasing the flow depth and adding an erosive component to your stabilization considerations. This type of work can be done and has been done in the past but I would stronlgy advise you to consult an expert in fluvial geomorphology to come up with a stabilization and monitoring plan for not only your site but also possibly a few miles (2-10) downstream. This may seem excessive but if private property and or a roadway gets washed out downstream of your site after you've removed the dam..........

Hope this helps.