Strom Drain 3

[queque]

I just reviewed a three pipe storm drain Hydraulic Grade Line calculation. How can the peak flow from the combined
flows of the two inlets be routed through a detention basin and through a final outlet pipe.
A SKETCH IS SHOWN BELOW -

outlet pipe from detention basin
x

detention basin here x


inflow from storm drain to detention basin here x
cfs is known from rational equation calculations

 

[cvg]

you need to route the inflow hydrograph through the basin using a stage - storage - outflow relationship to get the outflow from the basin. This can be done with various programs including HEC-1 and others. However, by using the rational method, you do not have a hydrograph. you need to calculate your runoff with a rainfall - runoff model which produces a hydrograph, not by using the rational method.

 

[queque]

Three different parking lots were analyzed using the Rational Equation to produce peaks. Each parking lot peak flow was used to size the three connected storm drains. The final storm drain pipe includes the three flows and the composite flow finally reaches the retention basin. Since no routing wsa used to simulate peak flow attenuation, a conservative (large)peak flow reaches the retention basin.
Thus, using HEC-1 to simulate the peak may be a approach to the start of reservoir routing, given a stage storage relation. Is this realistic?

 

[sam74]

check out the responses to the most recently updated thread in the storm/flood engineering forum. The recommended methods for pipe sizing and detention/retention modeling are discussed there.

 

[bltseattle]

In addition to CVG and SAM74's response, here is another opinion.

If detention is required, then the applicable stormwater regulations should specify the acceptable routing methods, and the engineer should have followed them.

If detention is required by code, but no computation standards for how to size the pond are provided, then what criteria are you using to review the engineers calculations?

It seems inappropriate to determine the criteria for the engineering analyses after receiving the submittal instead of providing clear criteria/direction before-hand. It should be somewhat cut and dry whether the engineering analysis fulfilled the stormwater code requirements for the methods or not. If the engineer is trying to use some un-standard method then go ahead and ask them to resubmit. If you are in over-your-head maybe your state DOT can be of assistance, in most areas DOT have there own stormwater criteria to prevent flooding and control downstream risks.