36" Underground Detentin System

[urbanresource]

All,

I am analyzing a system for use in Colorado. I am using the SCS method to prepare the flood hydrograph, for use in basin routing via HEC-1. The system is basically 3-36" CMP pipes at 200' each. Using HEC-1, with an outlet pipe size of 6", HEC-1 is telling me I need to provide a good amount of CMP for storage. I am only trying to reduce peak flows by about 1.5 cfs, for the 100 year storm. Does anyone think it might be a little unusual that I need 4200cf of storage just to reduce flows by 1.5 cfs? Not sure if there is a quick and dirty way to estimate, but would greatly appreciate any feedback on this. Thanks.

 

[RWF7437]

Not enough information to answer your question. A simple answer is this. The required storage volume is equal to the difference between the inflow hydrograph and the outflow hydrograph. If you plot both hydrographs and calculate the area between them you have the required volume.

I don't use HEC-1 but HydroCAD does this stuff easily and quickly . Have you tried it ?

600 LF of 36 inch pipe does store 4,200 cf of water but without more info we have no way of knowing if that is enough.

A 6-inch diameter orifice under free outlet condition and 2.5 feet of head will discharge 1.56 cfs each. If you have three of them they would discharge about 4.5 cfs.

Are you trying to reduce the peak outflow by 1.5 cfs or did you mean to say "to 1.5 cfs" /

 

[urbanresource]

The proposed condition rational method 100 year peak flow is 4.44 cfs. The existing condition rational method 100 year peak flow is 3.10 cfs. I am trying to reduce flows down to the existing condition. HEC-1 provides the hydrograph for the inflow and outflow, and the corresponding storage volume. 600LF of 36" CMP, and a 6" outlet pipe is showing that the water level would be at about 2.5' in the CMP system, for the 100 year 24 hour duration. I guess my concern is whether it is unusual to require so much storage, just to reduce flows by about 1.5 cfs. But maybe it is what it is, and I'm concerned about nothing.

I have never used HydroCAD. Typically I use HEC-1.

 

[RWF7437]

First, you can't mix the Rational Method and the SCS ( now, NRCS ) methods. The Rational method does NOT produce a hydrograph...only a peak flow. For that reason it is useless for detention basin sizing. Yes, you can imagine a hydrograph using the so-called Modified Rational Method but this is pure fiction.

Also, the Rational Method should NOT be used for drainage basins larger than 20 or 25 acres. So my suggestion is forget the Rational Method except for sizing culverts and drains for very small urban basins.

The two methods will yield vastly different peak flows.

If you want to be consistent use one method, preferably The NRCS method. You don't say what type of storm you're using but let's say you are required to use a Type IA, 24 hour storm which has a 24 hour rainfall in your area of, say, 5 inches ( or whatever it is). Using the NRCS method and reasonable curve numbers for the before and after development conditions you can generate Pre and Post hydrographs. The difference between these two is the required storage. I don't know if HEC=1 will model this but you can do it in any of several available programs and you can even do it in Excel.

Also, remember that whatever you do because it is required by some regulation is only one approximation for one possible storm, of one particular annual exceedence probability. Your solution may satisfy a regulation but it does not take into account all the possible rainfall events that may occur over the life of your project.

From my reading, my recollection is that in some parts of Colorado local agencies have developed calibrated runoff models. Have you looked in to these for your area ?

These would be the best models to use, if available.

good luck

 

[beej67]

RWF has the right of it.

1) Don't mix Rational and SCS methods.
2) The Rational method sucks for everything other than sizing pipes to carry flow. Use SCS.
3) Set your existing/allowable discharge rate with SCS as well. That might change things.


If you're feeling frisky, you can tell exactly how much storage volume you'll need by comparing the existing undeveloped hydrograph to the proposed undetained hydrograph, and looking at the area between the two curves, which turns into a volume when you do the calculus. That's the hypothetical volume you need to hold back to make the peaks match. Old engineer's estimating trick. It never works out to be exact by the time you've got your system designed, but it's close enough as an early check when doing Value Engineering exercises during the planning stages of a project.

 

[msquared48]

I do not know what your rainfall intensity, runoff coefficient ratios, soil type, slopes, time of concentration, lot size or overall head is, but for a 3 to 4 acre site with a 100 year intensity of 3.5 in/hr, 1% slope, class C soil, I think the storage volume seems reasonable.

Mike McCann
MMC Engineering