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# Retention Basin

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## Retention Basin

(OP)
I need help  to design a retention basing for an area 212,920 sq ft (4.89 acres)with a building of 24,000 sq ft located in the center of the lot. the coeeficient of runoff is 0.95 and i'm using a intensity of 2.85  I can use the Rational Method or other metohd for my volume calculations? Thanks in Avanced.

### RE: Retention Basin

Not enough information here !

Where in the world is this site ?

Who will review your design ?

What are their design criteria ?

Is this to be retention or detention ?

Where did the C= 0.95 and intensity numbers come from ?  Both seem very high.

Good luck

### RE: Retention Basin

Must compare pre-developed to post-developed and you may have to store the difference in volume. If detention, you usually can release it at a max. rate = to the pre-developed rate. If retention, you cannot release it at all. More than likely you want detention.
The I of 2.85 sounds awful low. Here in Dallas, TX the intensity of 100 yr at 15 min. TOC = 7.56. Typically the C for paved/roof = 0.9

### RE: Retention Basin

C value may change with the return interval of the storm.  With lower frequency events, runoff coefficient may decrease.  C=0.95 might be appropriate for a severe storm such as a 100-year flood.  What return interval are you using? What storm duration are you using?

If this is a "detention" basin, you will need to generate an inflow hydrograph.  This is not usually done with rational method.

If a "retention" basin, you need to calculate the total storm rainfall depth and store the entire thing (less losses).

PECPESC is correct, 2.85 inches sounds low.  Here in the Sonorran Desert (Phoenix, Arizona) we would use 100-year, 2-hour rainfall of 2.8 inches and 100-year, 24-hour rainfall of 3.8 inches.

### RE: Retention Basin

I would prefer Rational on the site you described, I can't see unit hydrograph extrapolation for such a small site with so much impervious.  But use what the reviewer wants, as that is the only opinion which will matter.  I agree with most of the others, unless you are in the desert, that intensity is very low for storage volume inflow.  Regardless of methodology, you must weight your coefficients (curve numbers in SCS) first...use .95 for impervious, around .25 for grass and .3 for woods.  Cw for the site will be much lower than 0.95.

Use intensity if you are using the Rational Method and decrease amount for longer Tc.  Here in southeastern PA, intensity ranges from 4.2 in/hr for 2yr to 8.2 in/hr for 100yr storms, at 5 minutes Tc.

If you go with SCS methods (TR20 of TR55), use 24hr rainfall amount, not intensity.  Here, it ranges from 2.5 inches for 2yr to 7 inches for 100yr event.  This amount will adjust itself by using the program algorithms; don't decrease them yourself.

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