Basin outflow recharacterization

[proletariat]

I'm designing a basin within very limited space. To the east is a 3:1 slope leading down to wetlands and then a stream. To the west are lots (I'm an evil land developer consultant so I have to preserve as many lots as possible).

I'm trying to come up with a way to discharge the basin outflow and recharacterize it into sheet flow in as little lateral space as possible. This means that I don't want to take up the 21' that a 3' deep stilling trench/level spreader would take at 3:1 w/ 3' bottom. I was thinking about using a manifold of sorts. Water would flow into a main HDPE pipe from the outflow structure. This pipe would have an upward facing tee every 20' or so with a grate in it. The water would flow up and out of the tees. Sheet flow would be acheived by placing the grate elevations about .5' below a berm to equalize the flow. I could do in 3 feet what would otherwise take 21'.

- How would I calculate for such a thing? Weir equation with each tee circumference added to the overall length of the weir? Just make the cross sectional area of the tee openings much much larger than that of the outflow pipe?

- Is there a way to make it self cleaning?

 

[francesca]

All you're really doing is splitting the flow. If it takes 21' to spread the flow to a non-erosive velocity, it takes 21', whether you split it into seven sections of 3' or use one 21' level spreader. (Except that it's easier to construct 3' level than it is to construct 21' level.)

Consider a flow of 9 cfs and you've calculated (say) that a non-erosive velocity of 3 fps requires a sheet flow depth of 0.03 ft or less, then the width of level spreader required is (9 cfs / 0.03 ft / 3 fps) = 100 ft of level spreader.

No matter how many flow splits you have, you're still going to come out at your flow rate = length of level * depth of flow * non-erosive velocity.

Another thing to consider is that basins with 3:1 slopes are hideous beasts, and not things that people like to have in their back yards. (This weekend I drove through a subdivision I designed with a 3-throated sinkhole that I desingned at 3:1 slopes to conserve space and it's hideous.) Many people don't look at the final plat and see that in a big storm they'll have standing water in their back yards, so a flatter, bigger basin often results in more valuable lots.

21 ft is very short for a level spreader! The one I looked at putting in was for a 36" pipe discharging around 100 cfs and it came out at over 1000' would've been required. Fortunately(?) the subdivision snuck in before the sheet flow-requiring legislation applied.

 

[proletariat]

Thanks for the input. I actually meant the stilling basin would be about 21' deep (horizontal), 3' deep (vertical), and up to 330' long with 3:1 slopes and a 3' wide bottom.

I did find something to help me save space. It seems that the Corps of Engineers has some Energy Dissipator publications to aid in the design of stilling wells. It's basically a discharge pipe that shoots into the bottom of a vertical pipe that is some ratio greater in diameter than the discharge pipe. Once this energy is dissipated by the stilling well, it can surcharge out into a 0.5'- 1' trench (as opposed to 3' deep) and spill out in sheet flow.

I don't know how to account for the losses in the stilling well. About the best I can do is model the basin discharge pipe with a tailwater condition equal to the head provided by the proposed weir.

My goal here is to NOT have a channelized flow to the stream to avoid a Pennsylvania DEP and Corps of Engineers Joint permit.

 

[francesca]

Energy dissipation is one thing, but you're still going to have to do the flow rate = flow depth * velocity * width calculation. There simply isn't any getting around it.

The energy dissipation is if, for instance, the inflow pipe to your level spreader is at a steep grade and the water is more likely to overtop the level spreader than to spread out correctly.

I looked into level spreaders a bit, and I believe it's South Carolina (or North Carolina) that has some good guidance on how to design them. There are some creative designs, like PVC pipes that you lay just below the surface and they fill up and then spill out on the surface through perforations, but your flow rate is still your dictating force.

 

[sam74]

How about some free software from FHWA for help in designing energy dissipation devices. Try this link to download the software

http://www.fhwa.dot.gov/engineering/hydraulics/software/softwaredetail.cfm

I had it installed about a week ago but have not had a chance to use it yet. It automates HEC 14 which I have used to model stilling basins by hand before. It models several different energy dissipation alternatives some of which I would never try to design by hand.

 

[queque]

Which software product are you referring to?

 

[proletariat]

I downloaded HY8 Energy, but haven't had a chance to use it yet.