Dam Wall

[Awsim]

Hi.

An acquaintance of mine owns a farm with a through-flowing river. He would like to increase the wall height of an artificial retention dam wall by approximately 800mm(See attachment). The 800mm increase will cause the top of the wall to be flush with the higher, inclined water retaining wall to the right of the weir.

1. Is the 0.8m increase in height(Add. 7.85kn/m2 at the base) significant enough to reconsider the design of the existing dam wall, or can concrete just be poured on top?
2. Can anyone provide a source that would enlighten me as to how to go about the design? and the concrete works?

Thanks in advance.

Jaco

 

[csd72]

tis is a question that would be better asked in the civils forum.

 

[Awsim]

Hi CSD72

I have posted this thread in the civils forum, but figured that if point 1.(civils) in my post does not apply, point 2 which is a structural issue, would be appropriate for this forum.

 

[DaveAtkins]

If the height of the weir does not change, the depth of water behind the dam does not change (or am I missing something?).

Assuming I am correct, the overturning moment on the footing below the dam won't increase. However, the vertical load will increase, so I think the footing should be checked for the new loads.

DaveAtkins

 

[csd72]

That wall is probably lower for a reason, you do not want the water overflowing in a region that is not protected against erosion.

 

[Awsim]

Hi Dave

Thanks. The weir will also be lifted, hence lifting the water level. The aim is to widen the retention pond/dam(Increase water volume). The additional concrete, like noted by yourself, will require me to reanalyze forces on the footing below the dam wall. I can only assume that the additional weight of the water on the footing will also come into play.

Seeing that a design is not available, or the fact that I cannot visibly asses the geometry/properties of the footing; is there some way I can arrive at a conclusion about the current properties of the footing by considering the geometry & design of the dam wall? The water retaining properties of the wall must have some relationship with that of the footing as they work together in ensuring stability.

csd72 - Thanks. But erosion is not an issue.

 

[msquared48]

Dave:

Any permanent height change to the spillway will affect the stability of the dam structure by increasing the ultimate pool height during spill.

As this is a gravity retaining structure, the analysis should not be too complicated if the dimensions are known.

I just wonder why stoplogs could not be used in lieu of more concrete. Could be simpler with only a .8 meter, or less than a foot increase in the pool height. These can be easily pulled if a problem is encountered.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[letrab]

Hi Jaco,

Raising the dam wall will affect the stability of the wall. It is a basic gravity structure and the analysis is straightforward. The only tricky bit is the asumption on cohesion between the rock foundation and the concrete. From the piccie it seems as if the back slope is quite steep and you may have to rely on cohesion.

The USACE has great publications that you can download, look at the one for gravity dams and you will find all the info you need to calculate the stability.

I would be concerned about erosion, the overall flow depth will decrease but the water on the sides will spill on the banks and it may not be competent rock
There is also not visible energy dissipating structures and downstream erosion is likely

 

[cvg]

looks to me like you are raising the entire dam. It's not a large raise, but engineering analysis is definitely recommended. Is there an emergency spillway somewhere not shown in this photo, or does all water flow over the dam? How tall is the dam and how much water does it impound? In the US and in other countries also, dams meeting a minimum size requirement are required to go through a permitting process with the state. This process generally requires a thorough engineering analysis including hydrology, spillway and outlet hydraulics, geotechnical - foundation, settlement, stability and seepage analysis, geologic mapping, structural analysis and finally an emergency action plan. First, I would do a little checking and make sure you do not fall in the permit required category. If you do, and even if you dont, it would be a good idea to ask for advice from the dam safety agency. They are usually glad to help and can answer all your questions.

 

[DaveAtkins]

I didn't know the weir is also being raised. All bets are off.

The structural issues are minor here. The big issue is dealing with your state's Department of Natural Resources. They may not even allow this, even if this is on private property, because it affects everything downstream.

DaveAtkins

 

[paddingtongreen]

Is this a back door job? if yes, stay away from it, if not these questions need answers:

Who owns the Right of Way on the river?
Are code authorities involved?
Is a permit required?

Michael.
Timing has a lot to do with the outcome of a rain dance.