Grading for sheet flow of stormwater

[MCD917]

For a proposed parking lot, the engineer we are working with drew up a site plan which has sections of the parking lot graded at what I believe is a 1% slope (6" over 30'). We aren't in disagreement, but we question whether or not this is sufficient. I'm aware that this is acceptable, but afraid that no one will be able to pull asphalt level enough to keep the grade at 1%. Should we be concerned, or should the water be able to flow consistently accross the asphalt without 'puddling?' I'm not an engineer, nor do I have a solid background in engineering. I'm just seeking advice for a development project. Thank you in advance.

 

[cvg]

I have always assumed 2% minimum for parking lots to avoid bird baths. Others will likely recommend even steeper...

 

[Hoagie]

Min. 2% for AC pavement; 1% for smooth PCC pavement.

 

[cvg]

note that 6" over 30'is not 1% - it is 1.7% which is probably acceptable

 

[TerryScan]

I typically use 1% min.-2% desired-4% desired max. This is close to the spec by some big box store clients.

I am in a coastal pain where the land is quite flat. One cannot run steeper slopes very far. Either one is getting too low for drainage or creating too much of a fill condition. Contracters here don't seem to have a problem paving 1%.

 

[MCD917]

Thank you for the advice. Fortunately, we were able to get the engineer to increase the slope a bit. Our flatest section is now at 2.6% and our steepest is at 4%. Our parking lot only extends 80' beyond our building.

 

[Dezigner]

2% minimum on asphalt and 0.5% on concrete... Asphalt at less than 2% will have a tendency to have birdbaths and ponds that will eventually result in sub-grade failure... In Southern California any way...

 

[LHA]

But in the US, ADA requires 2% max on Handicapped Accessable spaces. If 2% is the min which will drain, how do you drain HC spaces?

I've spec'd bituminous at 1% many times. I've never had an RFI from the contractor regarding the slope, so there must be no problem laying it at that.

On one warehouse we did, the owner actually spec'd 0.5% at the docks. Don't know why, but we graded to .005. I can say, though, that that project (now complete) does have very small low spots with standing water. Not enough to pose a hazard, just a little ugly.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[Dezigner]

Actually, ADA allows for a handicap ramp of 8.33% with a max cross fall of 2%.

 

[woodleigh]

1.0% is fine, it's not too flat, just make sure base rock is compacted tight, and it's grade is checked. If the contractor tells you that they can't pave it without birdbaths, find a different paving contractor.

 

[saruman26]

I have never experienced significant problems with 1% slope on asphalt surfaces.

Just to note - the 8.33% grade for ADA is only for curb cut ramps or building access ramps, and then only for a maximum distance of 12-16 feet before a landing is required. Across the HC parking spaces, 2% slope is the maximum.

 

[cvg]

I would assume that not the entire parking lot is required to be ADA compliant, but only the areas marked for handicap parking and the adjacent ramps and sidewalks. As indicated above, the concrete flatwork for the ramps and sidewalks can be constructed to much higher tolerances than pavement and 2% or flatter should not be a problem to construct without low spots.

 

[Hoagie]

ADA NOTE....8.33% max up to a max rise of 0.50'.....if over 0.5' rise, then handrails apply.

 

[dln001]

I live near the coast in Virginia and we generally use 1% as our minimum slope for asphalt. The land here is so flat that I often find myself struggling to meet this requirement.

 

[bcallagan]

Sorry to get in so late: 1% could be sufficient depending on the grading concept...if it's a large plane not broken up very much and the contractor has a good way to control the equipment (laser level, etc), I go down to 1% , but not below.
I almost never go over 4% in commercial shopping centers, because that's the slope that shopping carts start running away with groceries, kids in child seats, and crash into car doors (liability).

 

[bperry211]

To add to bcallagan's liabilities, slipping on ice in the winter is another concern (aka liability) for slopes in excess of 4 or 5%. Many local ordinances in central NJ will permit slopes as low as 0.5% to 0.75% on pavement (bituminous or concrete). This is not that big of an issue with ponding etc. From a stormwater management standpoint, this actually helps slow the flow. If water does pond a little, so what, you've just increased your initial abstraction. We've actually designed some areas of overflow parking to purposely hold water for emergency storage.