Sizing and Grading a Detention Basin
2

Question

Sizing and Grading a Detention Basin

racv2 (Civil/Environmental)

(OP)

12 Sep 04 13:38

After calculating peak outflow discharges for existing and developed conditions using the TR-55 Tabular Method and figuring out the Storage Volume, I need to desing a basin that will hold 80,000 c.u.ft of runoff.
I would like to know if there is any good literature (online or books) I can read that will guide me through the process of sizing and grading (designing) detention basins.
I have no clue of where to start? How do I sketch/draw it in my drawing? How do I set the elevation of the bottom of the basin? and how do I pick an appropiate depth?
The only information that I am thinking I can use to start is an existing pipe with an elevation of 265 feet but from there I don't know what else to do.
Thank you for any help.

gbam (Civil/Environmental) · Answer 1 · 2004-09-13 14:27:23

First, are you planning to detain the R-O or retain it?  If you need to retain the volume then it is simple: depth time area.  If you are to detain the flow to match existing condition outflow then I will describe a simple way.

Take the inflow hydrograph for developed conditions.  Graphically draw a straight line from q=0cfs to your desired outflow value.  The volume above this line it the amount you need to store.  Take that volume and increase it by 10% or so to get a starting point for your basin volume.  Based on your site layout start outlining the bottom of your basin then calculate the volume to natural ground.  Inroads and or Eaglepoint software will make this easy.  Recalculate your stage storage and discharge for this basin and route the inflow hydrograph thru it to see if your basin will work.

Remember that this is a starting point.  You will need to tweak it here and there to get your desired results.  Goodluck and have fun. Oh yeah,  I typically use a 0.5% slope for the bottom of the basin to the outlet.  However I do not use slopes less than 0.3%.  That is so there will not be standing water for mosquito abatement.

Maury (Civil/Environmental) · Answer 2 · 2004-09-13 14:37:26

There is a Federal Highway Administration publication that can help you. Hydraulic Engineering Circular No. 22, "Urban Drainage Design Manual". It's a free download.
(Publication No. FHWA-NHI-01-021)

racv2 (Civil/Environmental) · Answer 3 · 2004-09-14 09:00:05

Thank you gbam and Maury for your help.

Gbam: I am planning to detain to match existing conditions outflow.

LHA (Civil/Environmental) · Answer 4 · 2004-09-14 10:26:57

Before grading basin, read local ordinance.  It will dictate parameters...how flat the bottom can be, how steep side slopes, what the spillway must pass w/ what freeboard.

Then, read your state's E&S guidelines.  If you can make this basin work as both an E&S and Stormwater basin with little modification, your client will save big money.  You almost always can, if you plan ahead.

But first, check the ownership, capacity and condition of that "existing pipe" to ensure it can handle your routed outflow as a point source, plus basin bypass.  This will be a different hydrograph than existing conditions.  If the pipe is substandard, the entity which owns it will probably want it upgraded.

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Sizing and Grading a Detention Basin
2