Elevated Crosswalk Gutter Drainage 3

[bob3306]

My client would like an elevated crosswalk/speed bump across a street to connect a couple buildings. Since the street is sloped, if I extend the elevated crosswalk like they have asked (from curb to curb), it will block the gutters and pond water on the high side.

I've suggested ramping down and back up at the gutters, but they seem to hate that idea. There are no storm pipes in the vicinity to install a catch basin, that would be too easy.

There's not much drainage so I'm not concerned about capacity, just standing water. I was thinking of using a 4" x 2" rectangular steel pipe. Mounting it along the gutter line, and paving an 1-1/2” directly above it. I only have at best 3-1/2" (the height of a speed bump) to work with.

I'm not sure how that would hold up...and I wonder about end treatments.

Has anyone else ran into this?

 

[ACtrafficengr]

How will you clean out the drain when it gets plugged up with ice or debris? We usually don't allow anything smaller than a 15" diameter pipe in the right-of-way, since smaller pipes often clog.

What are your subsoil conditions? I'd do a perc test, and if soil type allows, design drywells to handle the drainage.

If not, consider horizontal restriction to slow traffic instead, like curb extensions, a pedestrian refuge median, or chicanes. If the road is often used by emergency services, these would be the first option. Ambulance crews hate speed humps because they jostle the patients too much.

If it is not a private road, you'll need a permit from the highway department.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[cvg]

drywells are generally not appropriate for handling stormwater in a roadway. The runoff from the roadway will contaminate the drywell and this may also be in violation of your local agencies groundwater rules. I am in the process of abandoning a drywell in a city street now and the inside of that well is not a pretty sight. Can you divert the runoff into a small retention basin behind the curb?

 

[ACtrafficengr]

Cvg, How is infiltration through a drywell different from infiltration through a retention basin?

Perhaps an engineered stormwater quality treatment device like a Vortech unit before the infiltration system (whatever it is) would be better.

The raised median choke point is starting to sound like a better idea (unless you have to worry about snowplowing).

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[cvg]

drywells around here are drilled, injection wells, often reaching 200 feet deep. They must be registered with the department of water resources. They allow the storm runoff to penetrate directly into the sand or gravel layer directly above an aquifer. A pre-treatment chamber or a retention basin is required to handle the water prior to letting it go down the well.

 

[ACtrafficengr]

Ok, that makes sense! My town is mostly a prehistoric lake bed, with at least 20 ft of fine sand between me and bedrock, so dry wells around here are basically catch basins with holes in the sides, maybe 5 foot deep. It is an MS4 area, with all that entials, but they aren't as complicated as yours.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[bob3306]

Shallow dry-wells are common here, but at $15,000 plus another $5,000 for an interceptor, I don't think the client would like that either...they were thinking of a $2000 speed bump.

I do like the choker or chicane idea - this site is next to a hospital. However, I believe we'd have a similar drainage problem with those - we don’t have the budget to re-grade the street to drain properly. I am going to bring up ACtrafficengr’s point about the ambulance - we may want to rethink our whole approach.

The shallow retention area sounds promising. One side of the street would work fine and there's plenty of room, but there's no room on the other. I'll have to figure out if I can get a pipe to drain to the other side - existing utilities to navigate.

Thanks CVG and ACtrafficengr for your excellent input!

 

[civilman72]

Have you thought about a metal trench grate, placed over your existing curb and gutter? This will allow pedestrian at-grade access between the sidewalk and ped crossing and should maintain flow through the curb and gutter.

 

[gbam]

Check out Neenah Foundry. They have many precast options. The url is to their trench page.

 

[GoldDredger]

I would suggest a prefabricated trench drain spanning the width of the elevated sidewalk at the curb. They can be manufactured with decorative grates.

Or, perhaps design a small concrete rectangular channel, with the exisitng curb on one side and the new wall (curb) on the opposide side. Perhaps 6" to 1' in width. Span it with a cast iron cover. Ideally on hinges so it can be lifted and sprayed out every once in a while.

The idea with either is that you can clean out the area near the gutter. This area is sure to get leaves, coffee cups, cig butts, plastic bags and all mannor of junk in there to eventually block it.

If you enclose it completely as proposed, it might be a hassle to clean that thing out.

 

[bob3306]

I think I'm going with the rectangular channel with a cover. A 6" wide opening and 3" tall both sides at the crossing, tapering from 0" to 3" against the sloping part of the speed bump, full curb height on the opposite side of the taper.

Neenah Foundary has grate and frame that looks almost perfect (R-4990 and R-4999 Series) for this, but it's a little thicker then I'd like at 1-1/2", leaving only 1-1/2" then for the gutter - unless I crank up the height of the speed bump. I asked our local rep if I could get it at 3/4", but he tells me can't be done with them.

Does anyone know of other suppliers?

By the way, Neenah has really good installation instructions, it would be very easy to spec.

Let me know if anyone is interested, I'd be happy to post the detail when I finish it.

Thanks!