Parking lot grading and drainage (max slope and inlet spacing) 1

[washark]

Site: 1.8ac Proposed parking lot on hill at about 4%-5% for a run of about 200 feet. Asphalt parking with parking bays running with the slope. I can break the site into two drainage areas (1 ac and 0.5ac or thereabout) and collect at the bottom. Location is GA so rain for 10yr is around...say 8.

Question: Should I force a break in the parking lot to collect water half-way down the hill the drive isle? Or restated, is 200 feet overland/shallow channelized flow too far in a parking lot at this slope?

 

[beej67]

I'd be less worried about the length of run to the inlet and more worried about the gutter spread. First question is who's parking in the parking lot. I've done work for industrial clients who didn't mind 50 feet or more of design spread in truck courts at distribution complexes, but that's clearly too much for a commercial shopping center where Grandma parks her Oldsmobile.

Typically I'll start with the municipal regs, see whether they have a requirement for spread in parking lots or just for roads. If it's parking lots, I'll follow it unless the number of inlets becomes overbearing and the client balks, in which case I'll put a half bay of spaces at the bottom of the hill and try to convince the municipality that the spread should "start" at the end of the spaces and only "count" in the drive aisles. They'll buy that sometimes, since the primary point of spread regs is to avoid hydroplaning. Phrase the request carefully.

If there's no reg for spread in parking lots, then I don't provide spread calcs to the municipality at all, but I *will* do a 100 year check of both the pipe system and of the inlets to make sure the HGL stays in the ground and the ponding doesn't top the curb and end up somewhere it's not supposed to, like skipping the detention pond and running onto a neighbor. Some municipalities in Georgia (Cobb, Forsyth, etc) require that you show this process on your plans somewhere, by showing the 100 year HGLs and the 100 year ponding at sag inlets, or showing overland 100 year flow relief on the plans. Don't forget to account for flow bypass of inlets on grade.

Some other areas of the country are even weirder. Durham NC requires you to run a 100 year overland flow analysis assuming every inlet on your site is clogged, and compare that overland flow depth to building FFEs. South Florida requires you to do a 100 year "no flow" analysis, where you dump the 100 year hurricane on your site, assume that the water can't go anywhere but up since every other adjacent site is flooded too, and compare the result to your FFE. The presumptions being A) everyone's evacuated and staying in hotels in Valdosta, and B) they want to check your 100 year site stage vs your neighbors and vs the FIRM, to ensure you're not pushing 100 year flood waters onto your neighbor.

Also, for rational flow for gutter spread make sure you're using an intensity (in/hr) from an IDF curve, not a rainfall depth (in) from an isohyetal map. Appendix A of the Blue Book gives a good table of Is if your municipality doesn't have one they want you to use.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[beej67]

Also when grading your parking lot, don't forget ADA.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Ron]

Just for the record...I hate inverted crowns. In theory they are fine...in practice, most contractors can't do them correctly so 6 months after completion you have a lane crack right where all of your water flows...not good! So I would stay away from the inverted crown idea if you can drain otherwise...and it sounds like you can.

beej67 is correct...consider ADA. Put your ADA spaces on 2% or less slope if you can. As he says, local codes will likely dictate the first inch or so of runoff and retention/detention capacities.

 

[Hoagie]

Since your reach is only 200 feet, I would recommend you keep the grading as simple as possible, and collect at one end with a double catchbasin. The extra basin may, or may not, be needed, but in terms of redundancy, it's the prudent thing to do.

BTW...careful on the ADA grading, it's 2-percent in any direction, not a 2-percent cross-slope.

 

[beej67]

I've seen inverted crowns do some funny stuff if the lot or drive is transverse to a hill. If you're counting on the inverted crown to catch the runoff instead of a curb, and one half of the lot is on more fill than the other half, then you can get differential settlement in the lot over time, which drops one half of the crown and causes the whole mess to sheet flow off site.

The other long term problem with inverted crowns is repaving. Parking lots are often repaved by "gypsy pavers" who don't own their own plant, buy excess batch material from larger companies, and don't pay a lot of attention to cross slopes when they're paving. They'll usually just fill the crown in.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[eea]

All the approaches sound very reasonable and yes watch your ADA. Our DOT usually limits inlet spacing for curbed drainage to 250' maximum, so your slope and spacing sounds good without even without checking Mannings. Also agree to avoid the inverted crown trick when ever possible.

Reading beej67's post almost made me glad I don't work in Florida, then I looked out my window only to see it snowing!

 

[TGLG]

This may or may not be a factor (depending on what the winters are like in GA) but if you get a situation where the weather has been cold for a long time (so that the pavement temperature is below freezing) but the air temperature rises just high enough to get rain instead of snow, and you get a small amount of rain, runoff traveling across the pavement could freeze before it gets to an inlet, particularly if it has to travel a long distance.
So it's best to get water off the pavement as soon as possible.