How do I Size ditch for Parking Area Flow
How do I Size ditch for Parking Area Flow
(OP)
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!!





RE: How do I Size ditch for Parking Area Flow
RE: How do I Size ditch for Parking Area Flow
RE: How do I Size ditch for Parking Area Flow
v = (20.3282)*(S^1/2) for paved surface
S= 0.074 ft/ft
thanks
RE: How do I Size ditch for Parking Area Flow
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.
RE: How do I Size ditch for Parking Area Flow
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).
RE: How do I Size ditch for Parking Area Flow
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RE: How do I Size ditch for Parking Area Flow
RE: How do I Size ditch for Parking Area Flow
slope of the ditch is 2.33%.
RE: How do I Size ditch for Parking Area Flow
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.
RE: How do I Size ditch for Parking Area Flow
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: How do I Size ditch for Parking Area Flow
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.
RE: How do I Size ditch for Parking Area Flow
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: How do I Size ditch for Parking Area Flow
did you mean velocity (feet per second)? 5 cfs could be a minimal velocity in a large channel.
RE: How do I Size ditch for Parking Area Flow
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.
RE: How do I Size ditch for Parking Area Flow
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