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# Sizing Stilling Basin

## Sizing Stilling Basin

(OP)
I've been asked to review a plan that involves the construcution of 3 apartment buildings and related parking in Connecticut.  The total impervious area is approximately 5.5 acres and the 25-yr flow was calculated to be about 33 cfs.  I don't have any issues with the flow calculation, however a stilling basin is proposed and has been sized in a way with which I'm not too familiar.  The method seems reasonable, albeit crude.  The designer used the time of concentration and went to the state rainfall intensity chart and obtained a corresponding intensity.  With the intensity he/she went into the rainfall frequency-duration-intensity chart and determined a storm duration.  Finally, the duration was converted to seconds and then multiplied with the flow (cfs) to obtain a volume.  This volume was then used to size the basin.  Seems fairly reasonable to me, but have not encountered this design approach before - usually have hydrograph and the computer figure the area under the curve, etc..

Any input as to the validity of the described method and potential limitations would be appreciated.  Thanks.

### RE: Sizing Stilling Basin

I would ask them to submit a more appropriate calculation, this is not correct!

The rainfall intensity at the time of concentration is not equivalent to the total rainfall depth.  You need the total rainfall depth for the storm, the area and the runoff coefficient.  Multiply these to get the total runoff volume, then add a safety factor.

For instance:

Area = 8.2 acres
Rainfall depth = 2.6 inches
Tc = .46 hours
Intensity = 4.06 inches/hour
C = .62

100-year, 2-hour storm runoff = 1.1 ac-ft

### RE: Sizing Stilling Basin

I'm assuming that the purpose for this basin is storm water detention.  CVG is exactly right that this calculation isn't correct.  It uses all the right charts and terminology but mixes them together without understanding what they mean.

The first step in a review is to know the applicable ordinance and determine if the plan is in compliance.   Every storm water detention ordinance I've seen defines the type of calculation that is required.  If you have no ordiance to go to, you should take exception to this as not conforming to good engineering practice.

### RE: Sizing Stilling Basin

(OP)
cvg and Maury... thanks for the input, after doing some digging I realize why this design method is in fact incorrect.  After reviewing the TR-55 manual (Chapters 2 & 4) I found that I could reasonably estimate the storage volume needed to have post development flow equal pre development.  As it turns out the proposed basin has approximately 1/4 the required volume.  Thanks again.

### RE: Sizing Stilling Basin

I think you are all talking about a 'detention' basin, in which case flowrate or quantity is used for designed as previously explained.  Maybe it is a matter of regional semantics...here in PA a 'stilling' basin is a drop structure at the end of a pipe which slows the water down.  It looks like a big riprap apron with a depression in the middle.  It is sized based on pipe size, capacity and expected velocity; its dimensions have nothing to do with flowrate or quantity.

Separately, a 'sediment' basin is designed arbitrarily for a given contributory acreage, with no hydrology applied.  It is used to remove sediment, but not to slow or depeak inflow.

All three BMPs are valid if designed as required.  ndpw, I would verify whether the design is for a detention, sediment or stilling basin.

### RE: Sizing Stilling Basin

(OP)
Thanks lha... you are correct in noting the difference.  This basin is labeled as a stilling basin (and it will slow the flow) however the designer is attempting to also pass it as a mechanism for detaining the 25-yr storm, which it will not.

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