Minimum slope to drain a parking lot.

[okconstruction]

I am constructing 2 buildings with a driveway and parking lot inbetween each building.

I am in Moore Oklahoma.

My buildings are 300' by 60'. My concrete drive and parking spaces are 70' between buildings.

I have a gabled roof on each building I am draining 30' of run off per building or a total of 60' of roof run off in a 70' parking lot. ie 130' total run off.

I am taking the water to the street. So at the back of the parking lot in the center I am planning on sitting my valley 8" below the floor level of each building.

My question is what is the absolute minimum that I can set the flow line grade?

Concrete people say 1 foot per 100 (1%). So at the front of my building my "V" in the driveway would be 3 feet 8 inches making the parking slope pretty steep.

I am sure there is math to figure this out. Everyone says if you don't have this 1% slope the water will not drain. If I put in a .5% slope I would have 26 inches of fall at the front. I don't see how a driving wind can hold back a wall of water 26 inches tall to flood buildings.

Any help is greatly appreciated.

Thanks

 

[TehMightyEngineer]

I'd say you can get away with 0.5% slope but keep in mind that your tolerance is very small at that slope. You have to be sure that you don't go below 0.5% anywhere or you'll have the water pooling there.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[cvg]

the minimum is 100-year water level must be below the finished floor. you need to do some engineering analysis to make sure that works. otherwise you can set your driveway to anything you want assuming the building department will approve it. you will need a grading and drainage permit so that is part of the process. normally I like to see 2% minimum for both cross slope and longitudinal.

 

[Ron]

okconstruction....why are you asking such a question here? The civil engineer who did the site drainage plan should be consulted. If you don't have a Civil Engineer on the project, you should have.

There might be local codes that dictate minimum slopes. Further,assuming you are in the US, you have ADA requirements that must be met as well. Not as simple as you might assume....additionally, if you obtained a permit based on a particular civil drainage plan, changing that might require a permit revision.

 

[msquared48]

Why don't you set the minimum finish grade at 1% to the catch basins, and let the flow line exist from the catch basins to your below grade outlet. To will then not have such a deep vee.

Is this a concrete slab? Usually parking lots are in Asphalt. In either case, I have always maintained 1% for the finished grade and as little as 0.5% for the piping depending on the minimums allowed.

Ron: I think he is in Moore (or less) Oklahoma. Kinda flat there???

Mike McCann, PE, SE (WA)

 

[Ron]

OKconstruction and Mike.....
Long day of deposition....still cranky!

Yes, Mike's advice is good; however, the project civil engineer should have shown this on your plans. It isn't whether a wind will hold back that amount of water for a backup, but how fast the water gets to its exit point and the ability of the exit point to handle that amount of water quickly.

A parking lot will drain at 0.5 percent slope. If you are putting in an asphalt parking lot, not a big deal. If concrete, this slower movement of water can affect the joints in the concrete, so make sure they are sealed properly.

 

[okconstruction]

Ron: "why are you asking such a question here?" Because I thought I could get a better answer than 1% that is just the way it has to be. And your ADA statement is on my mind also.

Yes it is a concrete drive.

There is no drainage retention on the lot. We drain to the street and at the bottom of the drive 3/4 mile away there is a huge pond that serves as retention for the sub division. So the drainage flows directly out of the parking lot down the approach and to the gutter curb.

CVG: What is the 100 year flood level statement. I do not know what the level is. I am more or less at the top of a slope, no one drains on me, I am probably 50 feet above the overflow of the pond down the street. I don't see how I could possibly flood unless I pour a concrete drive just inches below the floor grade. I see parking lots, Walmart, Lowes ect much larger than mine with almost no slope to their drainage ares.

Ron: "Further,assuming you are in the US, you have ADA requirement." Yes I am in the US, and ADA is a pain in the rear and that is one reason why the question is asked. If a ADA parking spot is located at the front, you will need a elevator to lift the car to the spot if it is poured on a 1% slope.

My plan is to put the ADA spot about halfway down the buildings.

 

[cvg]

ask the building department for grading and drainage regulations and start there. you should do that before starting the design, not afterwords