Detention Basin Design

[GoldDredger]

I have a project where a municipality requires that a detention basin be able to store the 100-year, 2 hour rainfall event for the site without exceeding the pre-developed discharge rates.

The area in question is relatively small, under 2 acres of parking. I'm not sure if I should be using the modified rational formula using various intensities vs time to determine peak storage (using different C values for pre development allowed discharge vs post development peak flow, storing the difference)

Or if I should simply take the intensity given for a 2 hr storm where volume V= (Acreage)(C)(intensity 2.5 in/hr)(time of storm 2hrs) and get an acre-ft of post and pre development, and store the difference.

Any thoughts?

 

[Ron]

Use TR-55 from the USDA. If you only use the rational formula, you will likely underestimate the storage capacity on a small site.

 

[NeffariasBredd]

If it's available, Bentley's PondPack software is very intuitive and has a number of different models to use. Definitely do not do the second method you suggested. Almost every municipality requires that you model the runoff using SCS Unit Hydrograph, the Rational Method, or some other method. What Municipality are you in? Most towns that I've worked with have some sort of Stormwater Design Manual that lays out the method that they require be used in your calculations. If they don't, I'd recommend finding the nearest city that does and going off of theirs.

If you can't find a manual just use SCS Unit Hydrograph as it's generally a safe bet.

 

[gbam]

Golddredger - there are procedures to follow for designing a detention basin. I recommend that you check with the agency to see what hydrologic method they require, don't assume. If they don't have a requirement, check with the County then State. You may also ask what modeling program they accept. Most agencies, at least in my area, have design criteria spelling out your constraints and methods to use.

 

[beej67]

I don't have my copy in front of me (am on vacation) but I believe TR-55 is limited to 24 hour storm distributions, not 2 hour. But truthfully your reviewer probably doesn't know the difference.

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

http://www.campbellcivil.com

 

[cvg]

Or if I should simply take the intensity given for a 2 hr storm where volume V= (Acreage)(C)(intensity 2.5 in/hr)(time of storm 2hrs)

if i understood your description of the drainage regulation correctly, than use the total rainfall depth, not intensity. calculate the volume for the developed condition. now calculate the basin size required to hold that amount such that the overflow through the spillway or outlet pipe is no more than the pre-development runoff.