Sidewalk drainage between buildings

kansasguyFebruary 20, 2014

Hi all,

Sorry in advance for the long post, but I'm hoping to tap into the expertise here. I am handling some maintenance for the townhome community I live in. We have a drainage problem between two of our buildings.

The buildings are about 40 feet apart, with a sidewalk running between them that residents use to get from the parking lot down to their units. The landscape from each building is slightly sloped down toward the sidewalk, and the general slope is downward from the parking lot toward the far end of the sidewalk. The total sidewalk run is about 200 feet from the parking lot down to the last units, with another 50 feet or so past that before the landscape really drops off toward the street. The problem is that the slope is not consistent and there are several areas along the sidewalk where ponding is an issue. In the winter this is particularly bad as we have sheets of ice covering the sidewalk.

The total square footage of roof being drained into this area is around 4200, coming from 4 downspouts on one building, and 3 on the other.

I've discussed the issue with the board of directors and am tasked with presenting a couple of possible solutions to address the issue before heavy spring rains start up. Right now I've got two ideas I'm working with and I'd like to present them here for some feedback.

Option 1 is to remove the existing sidewalk in its entirety, and replace it with a slightly wider sidewalk that integrates a channel drain (something like this: on each side, to catch runoff from each of the buildings. In my opinion, this would offer the cleanest look, but would also be the more expensive of the two options, and would have the greatest inconvenience to residents, who would be without a sidewalk for at least a few days. We would most probably hire this out as we do not have laborers with concrete experience.

Option 2 is to install a french drain on each side of the sidewalk. I'm thinking along the lines of 4" perforated PVC, buried in a 10" wide x 12" deep trench lined with landscaping fabric, and filled in with a 1.5" gravel of some kind. I think the best option for optimum drainage would be to top the trench off with decorative 1 to 1.5" stone, such as Colorado river rock, in a sort of irregular "wavy" pattern on each side of the walk. We have laborers that can do most of the work for this task, making it less expensive. It is also less inconvenient for residents.

I'm under no illusion that in a torrential downpour either option will completely handle all of the runoff we're dealing with. Our goal is to handle water from the more frequent small showers, and to drain the water that normally would be left standing.

Do these sound like feasible options? Which sounds like the better solution? Am I missing a more obvious solution?

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If in the ponding areas there is a natural drainage to one side or the other, ( ideally all to the same side).

Option 3; install a collector box beside the walk at the ponding areas connected with 2" PVC and yes there is a specific drain hub which prevents "other" occupancy.

    Bookmark   February 20, 2014 at 7:52PM
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Here is a picture - it's taken from back by the parking lot, looking down between the buildings. The dark spots on the sidewalk are where the water ponds - they're mud left over from where yesterday's rain pooled up. Unfortunately it happens on both sides of the sidewalk. Just beyond the end of the walk, down by the giant oak tree, there is a very large ponding area, and about 30 feet beyond that, the landscape drops off toward the street.

    Bookmark   February 21, 2014 at 1:28AM
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Of the two options,french drain is the more practical because those surface drains will be a maintainance nightmare keeping them from becoming clogged. Perhaps an open concrete gutter could be poured alongside the walk. If you go that route,install a redwood or asphalt expansion between new and old concrete. Would overlaying the sidewalk with brick or flagstone prevent water ponding on walk? .

    Bookmark   February 21, 2014 at 2:46AM
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Where the pond is substantially balanced a collector can be placed on both sides and interconnecting.

And yes there can be a substantial cost of annualy removing the covers, scooping the acccumulated debris into a single 5gal. bucket and replacing the box grate.

Possibly as much as as 1 and 1/2 man hrs.

As opposed to the cost of the french drain.

Something to keep in mind, you are acting on behalf of the community.

    Bookmark   February 21, 2014 at 10:05AM
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For the collector drains in the spots that pond - would we need to dig a trench and lay gravel/pipe for those to discharge on the slope down to the street? I can see that being a far more cost-effective option.

    Bookmark   February 24, 2014 at 11:50AM
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Yes, with the exception of the gravel.
Thankfully, you have a natural grade to follow.

Also, you'll need to be prudent in the removal and replacement of the sod.
Where the collectors are placed on both sides, an inexpensive sidewalk drill connected to PVC pipe does a good job.
You had also mentioned "mud". If the source of this alluvium is from other than natural occurring dust, it should also be a concern.

This post was edited by snoonyb on Tue, Feb 25, 14 at 5:21

    Bookmark   February 24, 2014 at 12:18PM
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"would we need to dig a trench and lay gravel/pipe for those to discharge on the slope down to the street?"

Yup,them gates need drain lines and when them lines clog,there is no "scooping" them clear. The solution is called Hydro-Jetting,ask a plumber what that costs and while you are asking,ask if cheap,thin pipe will resist damage by the jetter. If you are digging ditches anyhow to bury pipe,add gravel and use preforated pipe for french drains. The french drains should run to daylight. Have you noticed all the grass clippings,sticks and mud in the ponding areas after heavy rain? If that gets in the lines it will cause blockage more often than you want to deal with.

    Bookmark   February 24, 2014 at 10:04PM
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It's why "maintenance," is called that.

These collector boxes are available with screens to mitigate those issues.

    Bookmark   February 25, 2014 at 5:27AM
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Or MAYBE install shallow swales on each side of main sidewalk.

1) Use a string level to make sure you have a positive grade from one end of main walk to the other, and can outlet water at low end with no problems.

2) Grade ground to slope sides gently to bottom of swale, especially making sure side next to main sidewalk slopes gently away from sidewalk for two to three feet.

3) For the short walks coming from the units perpendicular to the main walk, you'll need to bore under them and install 3 small diameter PVC pipes in series under each. These pipes should be about 10' long.

4) Slope ground away from perpendicular walks about 3 to 4 on each side to inlet and outlets of the pipes you just installed (to keep folks from stepping into a drop off if they happened to step off the walk. Also looks better).

5) Add gravel to discharge end of pipes to prevent erosion. Could also add larger rock to inlet ends to help prevent clogging.

6) Grass the swales, or create rain gardens with appropriate plants for function, beauty and to treat runoff.

This should stop ponding in low areas unless there's an area of the walk itself that's depressed. If so, that section should be removed and relaid.

NOTE: in large storms swales may fill and top walkways temporarily, depending on number and diameter of pipes under perpendicular walk, width and depth of longitudinal swales, etc.

    Bookmark   November 19, 2014 at 4:30PM
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Are those dark spots on the sidewalk the only places you get ponds? Or is that the center spot of a much larger pond?

Bcs if it's only those small places, maybe the problem is that your sidewalk is not level, and is not elevated or graded properly. Maybe you only need to re-pour the sidewalk. Or just those sections.

But I don't really know anything. Since no one else has suggested this, maybe it's only in my brain.

    Bookmark   November 21, 2014 at 6:09PM
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