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.
Any input as to the validity of the described method and potential limitations would be appreciated. Thanks.
RE: Sizing Stilling Basin
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
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
RE: Sizing Stilling Basin
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