Maximum depth of sheet drainage for parking lot

[RHOeng]

Local codes not withstanding, what is everyone's consensus as to the maximum desirable depth of sheet drainage in a parking lot before putting in an area inlet or island with curb inlet to intercept it? The lot in question is a 500 car high school parking lot, about 5 acres. The existing site is almost square, a 2 percent gradient diagonally, and will need very little grading. The school wants to put in no islands or depressed area inlets and allow it to sheet drain to one corner and dump into a detention pond. Being a government body, they are not required to adhere to local ordinances, expecially for onsite drainage. I think it is too much water for the kids to have to splash through for that long a distance. Has anyone seen design references to this I could show to the school?

 

[ccor]

I've never really found any good written guidelines for parking lot grading and drainage, I would be interested in seeing some. I always limit the product of velocity times depth to be three or less for safety (occasionally I will allow it up to four in a remote area). But I also limit the total depth to less than half a foot for the minor storm and one-foot for the major event. In areas with many freeze-thaw cycles and snow storage I don't like to let water surface drain more than 100-150 feet. Intercepting the surface runoff may also increase pavement life.

 

[jheidt2543]

Here's my two cents:

A 2% parking lot grade is fine for either concrete or asphalt paving. If it is raining, people expect to get wet.

The real question in my mind is, is there enough capacity in the detention pond to prevent back up of the water onto the parking area during the design storm? If there is, then I don't see a problem, if there isn't then the depth of water on the parking lot begins to deepen as the water backsup.

Just make sure there is enough capacity in the pond to accept the design storm without backup onto the pavement.

 

[ccor]

It's not uncommon to provide "parking lot detention" for the major storm, where parking lots are allowed to be under standing water up to less than 1-foot.

 

[RHOeng]

OK. Maybe I wasn't specific enough. The question wasn't about the adequacy of a 2% gradient. That was put in there in case you wanted to calculate the flow depth for various storms or to come up with an inlet spacing. The question wasn't about a detention pond on the parking lot or even backing up onto the parking lot. In the last 30 years I've done dozens of those. The outflow dumps into a pond 200 feet away and 20' lower in elevation. The question is still, "What do you feel the depth for sheet flow across a parking lot should be"? If it was your kids would you want them to walk 500' through 1/4", 1/2",...etc. How wet do you want them to get going or coming from school?

 

[cvg]

how about 6 inches or less for flowing water, 12 inches for ponding.

 

[petros77]

Will you be able to establish a ridge in the middle and convey the runoff by gutters on the sides? That way you can shorten the flow path and consequently the depth, it also prolong the pavement lifetime.

 

[bltseattle]

3/4 to 1" would provide a very user-friendly condition. You don't want people to have to sit there all day with wet socks...

 

[RHOeng]

Thanks guys and/or ladies. I had assumed 1/2" to 3/4" for the 50 year and I think I'll go with that. It'll be a little deeper for the 100 year, which we seem to get here (Oklahoma) frequently when there's hurricanes in the gulf.

 

[TDAA]

Have you considered permeable pavement to allow the water to run through? This may give you even better perfomance, depending on soil types.

 

[RHOeng]

Permeable pavement doesn't work too good in Oklahoma. Most of our soil is highly plastic clay, PI in 35-70 range, and has very little permeability. If you use that type pavement then you have the expense of an underdrain system to collect the water after it penetrates the pavement. Also, we are on a hill and undercutting rock in places to put in a cushion before paving. Thanks for the suggestion.

 

[saruman26]

The more shallow the runoff is, the better.

Have you considered using a slot-drain system to collect and convey the water? Duraslot drains manufactured by ADS (

http://www.ads-pipe.com)

are ideally suited to long, consistent grades like you are describing and can be installed for H20 loading.

 

[RHOeng]

Thanks saruman26 for the idea. Slotted drain works well. I used it for the first time in the late 1970s at a hospital, have used a couple hundred feet at this school site and am using a small amount for the next school building. I've also proposed 10,000 feet for a future highway project. But, in this case, the school won't part with the money required to extend it across the parking lot.

 

[blueoak]

I think you are going to have problems with the length of the life of the paving. You will never have a perfectly flat surface, especially with high plastic soils. The water will concentrate and asphalt isn't the best at holding up with those rivulets. My old street was a mess after a neighbor couldn't control his sprinklers. I think concrete collection gutters will lengthen the life. Also you will have puddles that will have to evaporate unless you concentrate it. I think people have an expectation that if you groom a surface flat it will stay that way. You might let the school board consider how often they want to pave the lot rather than control drainage now.

As for kids, i don't think it is an issue unless you have puddle areas which they normally love. On a perfectly graded smooth surface I can't see you getting any of the depths you are worried about.

 

[RHOeng]

Thanks blueoak for the comment. Flat surfaces and plastic soils aren't a problem when you deal with them on a daily basis. Just like asphalt airport ramps which are very flat aren't a problem with plastic soils when you take care of the expansiveness. I've done some large asphalt parking lots that are flatter than 2% with very few structural problems. I went out last week and looked at one 12,000 space lot I did at a race track that is almost 20 years old and it is in really good shape for its age. Shows some oxidation but almost no structural problems at all.