Crested Weir Wall Design

[rlewistx]

I believe I have the terminology correct in the title. I'm designing a concrete wall V-notch weir. It is about 15 feet tall at the middle and 300 feet overall length. My questions has to do with the concrete wall forces.
1. In the vicinity of the weir, will I have added forces on the wall due to the moving water through the weir?
2. If the water over-tops the wall by 1.0 feet, will I have added forces on the wall due to the moving water over top of the wall?

If I have added forces, how are they calculated? I know the flow rates of the water through the weir and over the wall.

Thanks for your assistance.

Rich

 

[cvg]

you will have a small amount of shear stress on the wall caused by flowing water. same with overtopping. in addition with overtopping, your lateral hydrostatic load will be slightly higher. google shear stress to find the appropriate equations for estimating the boundary / bed shear stress

 

[rlewistx]

Thanks for this information. As I understand the bed shear stress, this would be the shear stress on the surface parallel to the flow of the water. Is that correct?

What I want to know is if the moving water is also putting a force on the back side (upstream) of the wall due to moving water in the vicinity? Is there a significant horizontal force against the wall due to the moving water above the wall, that needs to be included in my design, or is it just the hydro static water head pushing on the wall?

Rich

 

[cvg]

the force is hydrostatic

 

[TerryScan]

I am not experienced in a structure of this size, but wonder whether there may be additional forces to consider:
1. Forces due to debris strikes during storm events (logs etc.)
2. Strikes of ice flows (depends on location).

 

[rlewistx]

Thanks for the responses.

I can see this as being primarily static pressure as this is a settlement pond and the water rises until it tops the wall, they spills over. The weir is also always active. Regarding ice, this is in Texas and ice is not a problem in this area. It is a good thought though.

My next question would be when does it change from a pond type condition to more of a flowing stream? I'm not saying this will be a flowing stream, but in general, I see a flowing stream as having momentum behind it and a wall would be a barrier and need to hold back the flowing pressure. Just a curiosity question.

Rich

 

[cvg]

you will have a zone behind the wall which is an ineffective flow area. the velocity will be lower than the center of the channel where the water is deeper. there will be little momentum in this ineffective zone. so assuming hydrostatic conditions with the depth of water 1 foot over the top of the wall is a good approximation.

unless you have a trash rack to prevent it, trash including logs could impact the wall. also, is there a chance that the weir could get partially blocked with debris, causing the water level to rise?