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WEIR DETAIL??
4

WEIR DETAIL??

WEIR DETAIL??

(OP)
Does anyone know where I can find a concrete overflow structure (weir) detail for a retention pond?  The local municipality is looking for something more permanent than a "grass"-type weir.  My weir opening length is about 50' wide and top of bank is about 1.2' above bottom of weir elevation (ponding depth about 9").  Any help would be appreciated.

RE: WEIR DETAIL??

We regularly make weirs out of fabriform concrete mattress
or fabric bags which are relatively inexpensive to install.You can use a variety of different materials and designs. You really have to decide what you need or what the city requires.

RE: WEIR DETAIL??

(OP)
the city is looking for a weir constructed of concrete.  i have a detail that shows a 6" wide, 42" deep weir.....but the total length says it should be 3 times the length of the weir opening (the detail is typically used for smaller sized weirs i.e. 5' opening).  A 45' weir opening x 3 = 135' long.   the 45' on each side of the opening sounds like overkill....is that typical?

RE: WEIR DETAIL??

set the length as needed to pass your flow with sufficient freeboard.  Additional length is only necessary if you are accounting for debris clogging or some other safety factor

RE: WEIR DETAIL??

2
Is this like a vertical weir "wall" or a concrete lined spillway crest?  Here are a couple of ideas...

For a vertical weir, which is what is sounds like your detail is for, I would have a structural engineer look at it if possible, especially for the length you mention - a 45-ft long weir wall would need a foundation design and hydrostatic forces accounted for.  

For a concrete lined spillway crest on a pond berm, i.e. a broad-crested overflow weir, consider if you can adapt a concrete slab detail like a driveway approach (in terms of thickness and reinforcing bar spacing/diam.), but add a vertical cutoff lip or alternative barrier to prevent any piping through the bedding material.

RE: WEIR DETAIL??

with only 9 inches depth on your weir, I don't think a structural analysis would be essential.  It sounds like your maximum flow is under 100 cfs.  However, the concrete slab idea is a good one and cutoff at the top and the bottom would be standard practice.  Basically, you will be just replacing the grass with a concrete slab...

RE: WEIR DETAIL??

Is the weir an emergency spillway for the retention basin with an orifice for primary discharge?  If the weir in question is an emergency spillway..the designer should check with the agency approving the basin design for the storm event that must pass over the weir.  With the installation of a concrete weir and not a vegetated spillway, I assume a high velocity is proposed over the weir.  I suggest a point of anaylsis downstream of the weir should be analyzed for scouring velocity.  The detail should include soil erosion control and slope stabilization measures.

Just a thought I figured that I would mention.

RE: WEIR DETAIL??

Check out labyrinth weirs - think of folding the weir's width (in plan view) in order to get the same effective weir length without the real estate.  They're really slick!  Just be sure the downstream channel can handle the flow -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.  See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

RE: WEIR DETAIL??

(OP)
thanks for the help.....bltseattle, what do you mean by a vertical cutoff lip?

the local municipality is requesting a concrete lined overflow weir (broad-crested overflow weir) in lieu of the vertical-type weir.  Has anyone had experience in a design of this type (i.e. slab thickness, rebar spacing, etc.)

thanks

RE: WEIR DETAIL??

as suggested before, it seems that flow depth would be quite low over your spillway, therefor the structure will be lightly loaded.  Unless you plan to drive on it, the rebar is for temperature / crack control.  Suggest welded wire mesh 6 x 6 x W1.4 x W1.4 and 6 inch thick slab.  See MAG spillway detail 550 which you can modify.  

Download at the following website (2004 english uniform standard Details for Public Works Construction):

http://www.mag.maricopa.gov/publications.cms

RE: WEIR DETAIL??

As mentioned before and noted above by cvg maybe try to use a standard pavement slab type detail if the flow depth is shallow.  I do have to caveat that I have not needed  this type design in any locations where frost heave is a factor, so you may have to consider that element.

By vertical cutoff lip I mean a downward vertical extension (say 18-24 inches) integrated with the concrete-pour to prevent water from seeping/piping beneath concrete slabs used to construct a spillway.  Actively design to prevent hydraulic piping through the bedding material, if any, beneath the concrete...

RE: WEIR DETAIL??

We designed a concrete lined weir for the above-referenced project.  The client was wondering if we could do anything to the concrete surface to discourage skateboarders from using the weir (and ultimately falling into the pond).

The weirs are relatively large (could be ideal for kids and skateboards).  I've seen surfaces that look "bubbled" with concrete but this may change the weir coefficent and as a result increase the weir dimensions (and add more concrete).  Any ideas/details/suggestions (i.e. weir speed bump???)?

Thanks

RE: WEIR DETAIL??

6 to 8 inch rock placed in the wet concrete on the downslope will provide energy dissipation, prevent skateboarders and also looks nice.

RE: WEIR DETAIL??

If you think the skateboarding is a problem, you could integrate 2-3" diameter metal discs into the edges of the weir (disc "on edge" with 1/4 embedded and 3/4 protruding)at 5-10 ft intervals, that is what I see done with planters & retaining walls here in town...  But if the weir is accessible what's to keep non-skaters from falling into the pond?

My personal opinion (I am not a lawyer, owner should consider legal ramifications):  Sign it or fence it

Soap-box speech:
If the pond is a risk, fence it off for everyone, otherwise dont' worry about the boarders.  Put up a sign about risks if that is the concern, something you should do anyways if the facility is unfenced.  Skateboarding is not a crime (at least shouldn't be; there are much worse things kids can get into.... a really creative design for a mixed use stormwater facility could incorporate some cool skating features for when the facility is dry).  Why single out skaters, what about people walking their dogs, or mountain bikers...... if skaters in the neighborhood are also hell-raisers there are already likely to be laws/ordinances to control nuisance behavior/graffitti/vandalism/etc.


Please let us know what you decide.

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