Chain-Link Fence Across Spillway 1

[gjeppesen]

Hi,

I inherited a project that requires something I've never had to do before. One of the agencies is requiring us to fence in a 10 acre wet pond because of steep sideslopes (not my design). This is fine, except there is a 75' wide spillway that discharges into this pond. The spillway is made of soil cement and is flat.

Design parameters:

100 year stage above spillway is 4' and velocity is less than 5 fps. We are wanting to use a chain-link fence. I really don't want to put anything across this spillway but it's the only way to "close off" the pond from the public.

Any ideas?

 

[DMcGrath]

I don't suppose running away is an option...I can think of several scenarios in which this fence is bad. I would seek some legal advice along with engineering advice.

1. Fence is installed and then "removed" by flood waters. Who pays to replace fence and clean debris out of pond? Contractor/engineer/fence company/locality?

2. Sturdy fence is installed, but is clogged with debris during a storm event, surround areas flooded, residents sue the locality/owner.

3. No fence installed, some member of the public goes for a swim (despite the otherwise fenced area) and drowns. Family sues locality/owner.

Fences in general define boundaries. If someone wants to get in, they will. One of my clients was successfully sued because someone willingly climbed over a fence into an obviously dangerous area and was killed. The main argument was the fence was "insufficient" to prevent entry.

 

[sam74]

Maybe you could fence across the spillway with a separate fence (separate fence posts than the rest of the fence so the two are not tied together)whose abnormally long fenceposts reside in metal tubing buried in the ground with a slightly larger diameter than the fence posts. Put flotation device at the ground level and if water rises your fence and fence posts rise with the water. Sounds like it could work?

 

[msquared48]

What about a fence that would open swing mechanically when the water level was sensed, say 3" or so above the top of the spillway, and close when the water receeded. Could intermediate post(s) be added to cut down on the size of the fence gates needed? This would allow the installation of a solid fence, although it still coould be chain link. Just thinking here. Might depend too much on electricity to operate unless there was a backup system for power outages. Starting to add up $$$$.

Mike McCann
McCann Engineering

 

[cvg]

breakaway fence is the typical solution to your problem. fence is installed on hinges with a shear pin. shear pin breaks and fence folds down flat when water pressure rises. O&M crew then goes in after the storm and replaces the pin. very labor intensive

second option is to put the fence either upstream or downstream of the spillway weir control section so that water elevation does not rise and blockage does not impede flow through the spillway. This will require the top of the fence to be below the maximum water surface elevation. fence is likely damaged during design flood and has to be repaired. also expensive

preferred option in my opinion is no fence and signs that say danger - stay out of spillway.

 

[civilperson]

Fence support from 20' high pivot points such as space truss with cables down to six foot of chainlink fabric with weights attached at the bottom. The fence swings out to allow flows under it during spillway use. "Fences are for cows, signs are for people", so put up some written notices.

 

[cvg]

civilperson
I like your idea, but wonder what some bored redneck, a 12-pack of beer and a 4x4 will be able to do with the space truss on Saturday night... I guess about the same as with the breakaway fence...

 

[eea]

I believe the USDA or Corps has a detail of a safety cable and sign strung across the spillway. Fencing along the sides of the spillway instead of across as Sam74 suggested would be my choice as well. The cable is a last grasp measure before going over.

DMcGrath is right to point out the liability. Present the options to the client, let them decide and document it.

 

[newnickname]

A ten acre wet pond is more like a small lake, therefore, I would consider it more beneficial to bench it.

Have you run a quick cost estimate on how expensive it is for 2650 LF of fence? I'm thinking the materials alone will run about $20,000. (with labor $30,000-40,000)

I'd check with the client and see what they would rather do. Redesign might cost about the same, but the final product will be much nicer.

 

[TerryScan]

Interesting point, newnickname. In my state, benching is required on any stormwater pond with a permanent pool.

The authorities have repeatedly rejected substitution of fencing. Their argument is: that kids will somehow manage to find a way into the facility. When kids do find a way in, and get into trouble, the fence is a severe obsticle to rescue efforts.