How do I Size ditch for Parking Area Flow

[ChipperB]

I am working with a large (1.7 ac) asphalt parking area that drains to an existing ditch located parallel to the parking area's down stream edge (see attached sketch). This ditch also receives flow from 1.0 ac of undeveloped land. I think the existing ditch is undersized because it appears to be supplemented with an earthen berm and in one area it appears the storm water has breached the berm. I am looking for information on how to determine the cross sectional area of a ditch that can accept this flow. Also, how can I determine if this flow will erode the ditch and if it has enough velocity to jump the ditch and scour the berm. Using the Rational Formula I determined the Q for the parking area is 21.5 cfs and the Q from the undeveloped land that enters the ditch is 1.6 cfs. Thanks for your support and happy Thanksgiving!!

 

[gbam]

ChipperB - Is the existing ditch on the down gradient side of the lot? If so the runoff will enter uniformly alogn the edge of the parking lot. You can evaluate the size of the ditch two ways. First is conservative, size entire ditch for the entire 22-cfs. Second is to prorate the discharge linearly along the parking lot reducing the swale as the discharge entering it decreases. You did not mention which drainage area controls. If you combine the tow onsite and offsite areas and use the longest Tc is the flow rate greater or less than the onsite? Your scenario is not uncommon, I would look at 2 or 3 reaches along the edge of lot and size accordingly. How did you determine your velocity for the sheetflow on the lot?

 

[gbam]

I forgot to address you main question, use Manning's Eqn to size the ditch.

 

[ChipperB]

I determined the velocity using the following equation:

v = (20.3282)*(S^1/2) for paved surface

S= 0.074 ft/ft

thanks

 

[ChipperB]

gbam,
forgot to tell you if I combine the offsite and onsite drainage areas and use the longest Tc the flow rate is lower than the onsite flow rate.

 

[cvg]

looks like your pavement slope is less than 7.4%

use the highest flow rate for design

Mannings equation will only give you the normal depth. you need to verify that you will actually have normal depth in your part of the ditch (ie, no backwater).

 

[gbam]

ChipperB - You can should for backwater as cvg suggests but if your slope of the swale is steeper than the critical slope, normal depth will work. What is the slope of your swale? FHWA's hydraulic Engineering Circular #15 provides step by step analysis and design for roadside ditches/channels using flexible linings to mitigate erosion. It can be downloaded for free from:

http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm

 

[cvg]

slope of the ditch is 1 factor, culverts, changes in dimension or roughness may also cause backwater

 

[ChipperB]

gbam,
slope of the ditch is 2.33%.

 

[gbam]

Really quick evaluation of a 4' bottom 3:1 SS swale gives .8' depth with Fr=1.0 Manning = .035 (+/- 6" riprap), Vel=4.5 fps. You may or may not have room for that configuration.

Results will change if there is backwater. If not, the FHWA manual I mentioned before will aid in designing a stable channel. The extend to which the backwater impacts your ditch will depend on the normal flow regime of the ditch.

 

[beej67]

Use the rational method to estimate the flow. Don't forget to include the offsite (undeveloped) area as well. Design the ditch by approximating it as a trapezoidal channel and using Manning's equation.



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

http://www.campbellcivil.com

 

[jgailla]

Is 21.5 cfs a typo? That is more than twice what I wold expect, even from a big storm.
In Florida, we would use a five or ten year storm, which is about 5" to 6.5".
Even with a ten minute time of concentration and a C factor of 1.0, that would only yield (1.7*1.0*6.5)~=11 cfs.
That should only be a couple of feet deep, depending on slope.

 

[msquared48]

4 to 5 cfs will cause scour of the sides. Limit it to 1.5 cfs.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jgailla]

msquared,
did you mean velocity (feet per second)? 5 cfs could be a minimal velocity in a large channel.

 

[ChipperB]

All,
thanks for your help. To jgailla, the the total acreage that drains to the site is 2.7 ac. 1.7ac parking area and 1.0 ac. undeveloped, but rocky, not much vegetation, and sloped. I designed for the 25 year storm. I developed a composite "c" of 0.97.

My Q=cIA is: Q=(0.97)(8.20)(2.7ac.)-21.5 cfs

again thanks to everyone for helping.

 

[beej67]

Your C seems too high, but it's conservative, so that's okay. I would expect around 0.9ish, maybe lower. What C are you using for the rocks? Is it like a big sheer rock face or is there dirt?

What's your time of concentration? Are you using Kirpich method, or something more "impressionable" ..?


jgailia and chipper - I'm sure msquared meant ft/s

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

http://www.campbellcivil.com