Debris fence in channel

[sdcivilguy]

I am designing a debris fence to be constructed across a channel at the downstream end of a rock energy dissipator. The fence is to be constructed with chain link fabric on heavy steel pipe posts. The fence will be 3 feet high to catch the first flush debris during a 5-yr to 10-yr storms and would be overtopped for storms greater than that. Assuming the fence becomes totally blocked with debris, what should be the basis for determining the hydrodynamic force on the fence for sizing the posts and their spacing. I have considered the totally blocked fence as a sharp crested weir but have not found any information on calculating the pressure on the surface (fence) below the weir crest. I am thinking of using the hydrostatic pressure due to the total depth of water flowing over the fence less the hydrostatic pressure at the top of the fence.

 

[RWF7437]

You can model this in HEC-RAS. That will give you some reasonable estimates of the hydrodynamic forces acting in the "fence". The add a factor of safety ( factor of ignorance).

good luck

 

[JedClampett]

If the fence is totally blocked the largest forces will be created by the static pressure on it. Conservatively (ignoring the pressure on the back side) it will be the triangular distribution of pressure starting at zero at the weir crest going to a maximum at the base.
The dynamic forces for most channel flow situations are negligible. For instance, if the flow was five ft./sec. (which is really cooking), the dynamic force is about 6 additional inches of water.

 

[RWF7437]

If velocity v = 5 ft/sec then,
v^2/2g = 0.388 feet of water = 4.66 inches of water
additional pressure of 4.66 in. of water =0.17 psi = 24 lb/sf

If v= 10 ft/sec,
additional pressure = 96.7 lb/sf

Whether or not this is negligible you mat determine for yourself.

good luck

 

[cvg]

why not just make the fence "break away" during large events? use a hinge at the base of the supports and insert a shear pin. Seriously, unless the "debris" is nothing but plastic and paper, the fence will probably break away anyway when the first big log comes along, regardless of the size of the posts.

 

[swearingen]

I have to disagree with JedClampett's assertion that the force is zero at the top of the wall. With water flowing over it, there is a pressure of at least the depth of the overtopping water if you still want to discount dynamic forces.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[sam74]

Is the rock energy dissipation for a pipe? If so check out this link

http://www.kristar.com/media/brochures/ntspecs.pdf

 

[JedClampett]

swearingen, you're right. I used the wrong terminology when I said the weir crest. I should of said water level.

 

[sdcivilguy]

Thanks everyone for your responses.

Swearingen and JedClampett: I think you're both agreeing that the hydrostatic force will be the trapezoidal pressure diagram equal to the height of the fence but based on the total depth of water. See the attached calculation.

Sam74: my dissipator is at the end of a concrete trapezoidal channel.