Stormwater Conveyance on Steep Slopes

[lindbls]

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.

 

[gbam]

Wow, 180 feet. I think you are on the right track. What is the slope of the bluff, Vertical,near verticl,1:1 or? Since the native soil is loose will your anchor system hold the pipe? Maybe add a concrete outlet structure below grade to help hold things up & to dissipate the energy.


/<- Slope
/ //
___@Grade________/ //<-- Pipe
| | //
Conc Outlet->| |//
|___/

 

[lindbls]

The slope is about 1:1. And yes, anchoring will be a challenge in the existing soil. Any suggestions?

 

[gbam]

Well my drawing didnt come thru very well. Has a Geotech evaluated the soils? Maybe use something at top of slope to anchor - Conc Structure? On a much lower slope I would recommend a concrete spillway. But with the 180 feet I am not sure. How are you going to anchor the pipe into the slope, 10' rails or? Is it possible to use benching every 20 feet +- vertically? If so maybe concrete slabs at each bench to "key" into the slope. I am not sure.

Good Luck!

 

[CarlB]

I've seen something constructed very similar to what you're proposing. Since tree-cutting was not desired, the outfall pipe snakes down the slope to a rip-rap dissipator similar to your description. I believe either 6" or 8" corrugated polyethylene pipe was used, as it was available in rolls and minimized fittings. "Duckbill" anchors were used.

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.

 

[francesca]

I would definitely use a seeded mat at this time of year as it's getting a bit late for grass to grow and be effective. As a long-term measure, shrubs would work better than grass as their root system is deeper. As a minimum, silt fence would likely also be necessary to prevent any (more) soil migrating into the stream.

 

[bltseattle]

Welded HDPE is a common approach to "tightline" stormwater. See FHWA HEC-14 for design of energy dissipators. I've seen both gabion structures and stilling wells used for this application. Gabions do not require heavy equipment to construct or place, as long as you have a way to get rock to the site. You could transport flattened gabion baskets through your limited access, no problem. Stilling wells are easier to construct, using precast manhole components typically 4-ft+ diam (size per HEC-14), but require heavy equipment access. If you follow HEC-14, though, you can come up with other energy dissipation techniques as well; I've designed timber crib structure for this purpose as well.

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!

 

[Prepakt1]

For tight aceess and slope protection you can use fabriform concrete mat.4" thick would suffice for this project. You lay the fabric which will accomodate to the slope.You can pump in concrete from up to 200m if necessary.For anchorage if necessary install helical anchors. That can be done manually by 2 men with small portable equipment.The anchors can be and integrated with the mat. You can also anchor the mat by installing the top part in a trench at the top.

Intrusion Prepakt

http://www.marineconcrete.com

 

[geosavvy]

180 ft, 1:1 slope? Oh my. What type of geologic region is this? I would be scared to even touch the thing for fear of a slope failure... much less divert flow down the thing.

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?

 

[lindbls]

The site is a marine bluff with a residential subdivision on top and a salmon-bearing stream below. The bottom 3/4 of the slope, the stream, and the tidally-influenced wetland across the stream are all part of a US Fish and Wildlife refuge (think federal property, permits, timelines, etc.)

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.

 

[blueoak]

lindbls,

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.