Requirements of Dam Declassification

[blong93water]

Hello, I'm currently working on a project where our goal is to decommission a dam on a site that is going to be mostly abandoned in the near future. My question is, how do you figure out what the minimum criteria is for dam decommissioning?
1. Dam height is defined as the vertical distance between the downstream toe of the dam and the minimum elevation of the dam crest. Can that minimum elevation be located at the emergency spillway invert? How long would your "emergency spillway" have to be before it is considered the lowest point of the dam crest?
2. Dam storage is defined as the maximum amount of water stored at the minimum elevation of the dam crest. Again, can this be considered the emergency spillway invert? Or, can you run a design storm event (say, the 100-year, 24-hour storm) and determine what the water storage volume would be for that storm event?

Let me know what you think!

 

[psmart]

These are really policy issues that would have to be answered by the applicable agency. (Don't know where you're located, but that's the situation in NH)

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[coloeng]

In some states (US), a full breach of the dam would be required. If you are decommissioning and abandoning the dam you are implying that there will be no future maintenance of the dam in the future. With no future maintenance, potential deterioration of the dam could adversely affect downstream activities, thus the need for a full breach.

 

[cvg]

1. Dam height is defined as the vertical distance between the downstream toe of the dam and the minimum elevation of the dam crest. Can that minimum elevation be located at the emergency spillway invert? YES

How long would your "emergency spillway" have to be before it is considered the lowest point of the dam crest? IT IS NOT CONSIDERED PART OF THE DAM CREST

2. Dam storage is defined as the maximum amount of water stored at the minimum elevation of the dam crest. Again, can this be considered the emergency spillway invert?

“Storage capacity” means the maximum volume of water, sediment, or debris that can be impounded in the reservoir with no discharge of water, including the situation where an uncontrolled outlet becomes plugged. The storage capacity is reached when the water level is at the crest of the emergency spillway, or at the top of permanently mounted emergency spillway gates in the closed position. Storage capacity excludes dead storage below the natural ground surface. “Surcharge storage” means the additional water storage volume between the emergency spillway crest or closed gates, and the top of the dam.

Or, can you run a design storm event (say, the 100-year, 24-hour storm) and determine what the water storage volume would be for that storm event?

 

[blong93water]

Thanks all! Great input from everybody, that helps us out a lot.

 

[GeoEnvGuy]

So you want to decommission a dam, which means you want to remove the structure so it no longer impounds or diverts water. So you are removing the reservoir and constructing a drainage channel through the dam.

 

[cvg]

geoguy
it might still divert water. flood control dams are sometimes breached and turned into levees / dikes which function to divert water

 

[blong93water]

Our goal is to reduce the storage capacity below 50 ac-ft to remove the classification and requirements related to being an official "dam". The impoundment will still be used to detain water, but the storage capacity will be significantly less.

 

[GeoEnvGuy]

below 50 ac-ft to remove the classification

This is an arbitrary criteria, you want to get this from the regulator that they will accept this as official declassification.


Ontario Canada definition from the government
Dam: For the purpose of the administration of the LRIA, a dam is defined as a structure that is constructed which holds back water in a river, lake, pond, or stream to raise the water level, create a reservoir to control flooding or divert the flow of water.

With the above definition a drainage swale in my backyard is a dam. This type of regulation also prohibits anything being truly declassified from dam status, which is the governments right as they do not wish to have any liability even if the closed out condition exceeds the adjacent natural lands risk of failure.

 

[cvg]

I believe 49 of 50 states in the US use the 50 acre / 6 feet rule to define state regulated dams. Alabama is the sole holdout and does not have a State Dam Safety Program. there are numerous smaller, unregulated dams that do not meet the 50/6 requirement

 

[coloeng]

In Colorado a non-jurisdictional dam is one that retains 100 acre-ft or less, is 20 surface acres or less and has a jurisdictional height of 10 feet or less.