Stormwater Conveyance on Steep Slopes
Stormwater Conveyance on Steep Slopes
(OP)
What's the best way to get water to the bottom of a bluff w/out eroding the whole slope?
I work for a county public works department and am dealing with a stormwater outfall on a steep slope. Currently, a perforated 12" pipe carries stormwater runoff from a catch basin in a county road to the top of a steep slope. Runoff flows from the pipe (elev=180 feet), down the slope, and ends at a salmon-bearing stream below (elev=0 feet). The slope includes alternating layers of sands, gravel, clean pea gravel, with a few dense aquitards interspersed.
The property owner notified us recently of significant erosion at the top of the slope that has reached within about 40 feet of a house.
We're proposing replacing the perforated pipe with welded HDPE, installing the HDPE on the surface down the slope (anchored), and ending in two upturned HDPE elbows within basically a big pile of light loose rip rap for energy (splash)dissipation.
Access is tight between existing homes (about 8 feet between a house and a iron fence set in concrete).
Any suggestions? Any other pipe ideas?
How about erosion protection for the area that's already eroded? There are now scour channels 5 to 15 feet deep. Again, the native soil can be very loose.
I work for a county public works department and am dealing with a stormwater outfall on a steep slope. Currently, a perforated 12" pipe carries stormwater runoff from a catch basin in a county road to the top of a steep slope. Runoff flows from the pipe (elev=180 feet), down the slope, and ends at a salmon-bearing stream below (elev=0 feet). The slope includes alternating layers of sands, gravel, clean pea gravel, with a few dense aquitards interspersed.
The property owner notified us recently of significant erosion at the top of the slope that has reached within about 40 feet of a house.
We're proposing replacing the perforated pipe with welded HDPE, installing the HDPE on the surface down the slope (anchored), and ending in two upturned HDPE elbows within basically a big pile of light loose rip rap for energy (splash)dissipation.
Access is tight between existing homes (about 8 feet between a house and a iron fence set in concrete).
Any suggestions? Any other pipe ideas?
How about erosion protection for the area that's already eroded? There are now scour channels 5 to 15 feet deep. Again, the native soil can be very loose.





RE: Stormwater Conveyance on Steep Slopes
/<- Slope
/ //
___@Grade________/ //<-- Pipe
| | //
Conc Outlet->| |//
|___/
RE: Stormwater Conveyance on Steep Slopes
RE: Stormwater Conveyance on Steep Slopes
Good Luck!
RE: Stormwater Conveyance on Steep Slopes
Regarding eroded gullies I would say just fill them (sand or gravel), compact, topsoil & seed, maybe surface erosion protection such as jute mesh. Should not need greater erosion protection if there will be no more concentrated runoff.
RE: Stormwater Conveyance on Steep Slopes
RE: Stormwater Conveyance on Steep Slopes
Consult a geotech about anchoring, and restoration of gully. For anchoring the steep part, you may be able to use a Manta Ray or duckbill type device attached to a collar around the pipe.
HDPE pipe design must accomodate expansion/contraction of the HDPE. I had a tightline project where the change in length was about 3 feet from summer to winter; slip connections consisting of one pipe sleeved inside a slightly larger pipe are typically used.
This would be a nice little project for your local engineering consultant - maybe you should get someone experienced to crank this design out for you!
RE: Stormwater Conveyance on Steep Slopes
Intrusion Prepakt
www.marineconcrete.com
RE: Stormwater Conveyance on Steep Slopes
However, if it is a "natural" slope then its probably more stable than I am envisioning.
I don't have any suggestions for dissipating your water energy. However, whatever you do keep in mind that you may not want to mess with the toe of the slope (ie excavating or washing out the material at the toe).
Really, its bad in general to dump water down a slope. If you get a lot of infiltration the internal stability of the slope is weakned. If its a fill slope, it would be especially prone to slope failures if you inundate it (though I can't imagine anyone being allowed to build a 180', 1:1 fill slope).
I'm assuming there's no option of directing flow around the slope?
RE: Stormwater Conveyance on Steep Slopes
Like all marine bluffs, it will continue to erode. All we can really do is try to minimize the impact the our conveyance has on the erosion...we don't expect to stop it forever. THe pipe in question conveys runoff from the low point in the county road to the edge of the bluff. It was installed when the neighborhood was built in the 1970's. Short of a pump station to move the water elsewhere, there's no other option we've seen.
Based on advice of our geotech consultant, we're going to fill the upper part of the "gulley" with rock, being careful not to cross (in theory)the property line into federal property and leave the existing pipe in place for this winter. In the meantime, we're going to submit for permits for a permanent fix. At this time, I envision the permanent fix being fill the gulley with rock, hard-pipe the water down the bluff, dissipate the energy at the bottom.
RE: Stormwater Conveyance on Steep Slopes
Sounds good, one other thing you may want to check to is that when the pipe is at capacity, does the overflow spread out or cut somewhere sensitive. I have seen where a pipe got plugged and the new low spot had started a cut into someones yard, a good trash protection may suffice if the county maintains it.
As for permitting, call it a treatment aeration system to compensate for BODs from residential runoff.