HILLSIDE DRAINAGE HELP
2

Question

HILLSIDE DRAINAGE HELP

rasheng (Civil/Environmental)

(OP)

3 Nov 10 02:32

Hi all,
We have a old hillside house that's being standing for a 100 years, and we decided to remodel it by taking the roof off and building a basement etc. Then the city starts asking us to have grading/drainage plans prepared, of which we hired someone to prepare. The house sits about 10' below the street on a downslope lot, and further down is another property. There is unfortunately no public or private improvements in between the properties to divert drainage to the street (such as a swale)
The civil consultant we hired designed a concentrated drainage system that pipes everything from rain gutters, sheet flow, basement drain tiles to a small energy dispersion wall down slope. The city rejects such a design, and now we have problem. We've asked around the city and different engineers, and there seems to be a few solutions. I am here to see if there are anything better?
1. sump pump all the drainage to the street which is sort of ridiculous for us to do provided the house has been draining like this for a 100 years. That's a lot of water to pump, like a whole 30% of the entire lot.
2. create a dispersal wall across the width of the entire lot. We are not sure how this is done, does anyone have any idea what the city is talking about?
3. ask the lower neighbor's permission to run a drain pipe across their side yard all the way to the street
4. dig a 16' deep dry well and send all the water under bedrock per soils engineer recommendation
5. we thought about infiltration systems downslope but the city does not accept those either.
Any comments would be greatly appreciated, thanks so much

EngWade (Civil/Environmental) · Answer 1 · 2010-11-03 09:35:05

It is difficult to say without more specific information, such as city codes and state regulations (if applicable). It does seem to me that you should be able to be grandfathered in one way or another (provided you aren't increasing the site's impervious area or altering the hydrology). Have you tried sitting down at the same table with your consultant and the city engineer and hatch it out? More often than not, doing just that will net you more gain than anything else.

Otherwise, I'd first ask the neighbor if you could build some cross lot conveyance to discharge to the lower street. You might be able to sell it to the owner that this could be in their best interest as well by reducing potential runoff from entering their home.

rasheng (Civil/Environmental) · Answer 2 · 2010-11-03 12:03:24

Hi EngWade,
Thanks so much for the comment. I will try to talk to the neighbor (happens to be a 8 unit apartment), so I hope the owner is actually around town.
The civil consultant does not seem very cooperative, so that's why we are asking everyone else. They are just telling us to install sump pump and get it over with...
We'll see if we can have a conference with the Los Angeles city plan checker...

cvg (Civil/Environmental) · Answer 3 · 2010-11-03 17:44:40

i would question the need for grading and drainage plans if you had not planned to do any grading or drainage... However, considering that poor drainage on your property could end up flooding your planned new basement or flooding your neighbors down the hill, then it does seem prudent. Regarding the "dispersal wall", why don't you ask the city what they mean?

rasheng (Civil/Environmental) · Answer 4 · 2010-11-03 18:00:15

Thanks for the comment. We are indeed doing some grading because the existing basement has a 6'-6" head clearance, so we are digging down another 2~3' to give us adequate head room. Estimated about 50 c.y. of grading though, very little work. There is an existing flat, concrete area behind the house, which gets to dig down 2~3' as well to line up with the basement floor. We also proposed a deck over this flat area, but it should not affect the drainage that much since the deck is permeable to rainwater.
When I asked them what is a dispersal wall, the guy does not seem to know exactly what he is talking about...but something that breaks down the water velocity into much slower velocity, like a stretch of small pebble rocks which dissipates the discharge energy. Has anyone seen something like this?

cvg (Civil/Environmental) · Answer 5 · 2010-11-03 18:14:07

a line of riprap along the edge of your property will do little or nothing for your drainage issues. It will however increase your liability if downstream neighbor gets flooded. If you do anything at all, then make sure it is effective. A swale or storm drain with a posative outfall to the street is recommended. A dry well could help with small storms, but will probably not work for larger ones.

bimr (Civil/Environmental) · Answer 6 · 2010-11-03 18:35:53

Some other approaches consist of a soak-away pit, infiltration trench, grassed swale, etc.

This design guide for stormwater may give you some ideas:

http://www.ene.gov.on.ca/envision/gp/4329e_4.htm

The local stormwater utility should be able to provide some guidance as to what type of designs are used locally.

EngWade (Civil/Environmental) · Answer 7 · 2010-11-04 10:56:46

The concern that always comes up with putting infiltration practices upstream of other homes is what happens with the increase in infiltrated water? It doesn't disappear. Will it have enough time to reach the groundwater table before it reaches the neighbor downstream? Are there hardpans in the soil profile that will cause a perched table, and develop additional runoff on the neighbor's footing drains (hoping they even have them). Also, you must have a positive overflow mechanism anyway, and that puts you right back where you were when you started.

I'd still reccommend a sit down with all the parties and come to an agreement. After that, I'd ask the neighbor for permission to install a conveyance line through his property, and try to sell him/her on the idea (IF the city absolutely forces you to do something).

msquared48 (Structural) · Answer 8 · 2010-11-04 11:31:45

Unfortunately, in this economy, municipalities are looking for any way to up the revenue, and invoking the percentage of value guideline for renovations to see if a permit is required, which is questionable here, is an option for them.

I hate to say it, but if push comes to shove, ie, they insist on being unreasonable, you might want to engage the services of a local attorney who has dealt with similar situations in the past.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

csd72 (Structural) · Answer 9 · 2010-11-12 11:15:49

I have come across this exact situation before and the solution we came up with was as follows:

Prevent as much runoff as possible coming down the hill in the first place.

Pipe as much of the front of the building as possible to the street.

Add the rest into a soakage pit.

At the moment most of this runnoff is soaking into the ground anyway.

beej67 (Civil/Environmental) · Answer 10 · 2010-11-14 22:20:40

What state/municipality is this in?
How steep is the slope?
What sorts of soils and land cover are you dealing with at the property line?
Is there any existing evidence of scour?
Do you intend to increase the amount of impervious coverage on the lot from what exists there?

I'd be a little sketched out proposing an infiltration system on the side of a hill like that, unless you've got a geotech that's firmly on the hook saying it's okay.  In the interests of avoiding future litigation, I'd explore the idea of piping it through the downhill property *first* before looking at other options.

By "dispersal wall" they probably mean a level spreader, which is sort of like a no slope ditch at-grade that follows the land contours and has a downslope wall that overtops simultaneously, so the water runs off in sheet flow.  There's different design specs depending on what state you're in.  North Carolina happens to have a pretty detailed design standard for their level spreaders, here:

http://portal.ncdenr.org/web/wq/ws/su/bmp-ch8

It's more explanatory and comprehensive than the Georgia one.

If you're in C or D soils (maybe even B) I'd avoid infiltration systems unless they're pretty far away from your building.  You mentioned the house was 30% of the lot, so I've got visions of McMansions sliding down muddy hills when the soil beneath their newly-monkeyed-with foundation loses its bearing capacity.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

BigInch (Petroleum) · Answer 11 · 2010-11-19 06:02:49

Reality check.

Seems like the pump would be the way to go, provided that the city is agreeable. Why take on the liability issues of screwing up the neighborhood's entire natural slope drainage system. Its apparently been working quite well for the most part of the last 100 years. I'd go for minimal footprint and be done with it. Wouldn't be surprized if it wasn't cheaper too. It's not like the pump is going to be running 24/7 or something.

//virtualpipeline.spaces.live.com

ClarkBart (Civil/Environmental) · Answer 12 · 2010-11-20 08:40:48

HI:

I know that I am a little late in the discussion, but, like many of the people responding have mentioned, there is a lot more to know about your particular situation in order to provide you with a definitive solution.  But, there are a lot of potential solutions for this type of situation.  Green infrastructure techniques should be considered whenever conditions allow.

Green Infrastructure techniques include infiltration/runoff reduction measures like rain gardens, rain barrels for stormwater re-use, green roofs, pervious pavement, etc.  I'm sure that Los Angeles has some design standards for this as part of their Stormwater/combined sewer or water efficient landscaping efforts.

The hillside situation could potentially be a problem toward implementing these techniques but not an insurmountable barrier.  Rainwater is a resource that needs to be managed for its beneficial uses and for its destructive potential.  Find a balance for your yard and your yard will be a more pleasant place to be in and the stormwater will be less of a burden for the city.

Good luck.  Let us know what solution all parties finally settled on.

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HILLSIDE DRAINAGE HELP
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