Rational Vs NRCS for ponding depth. 1

[jjv2006]

I have a problem in determining the ponding from a 10 acre developed site with the following parameters.
Rational: A = 10.00 acres; Tc= 15 minutes; c = 0.76;

SCS: A = 10.00 CN = 85; Tc= 15 minutes; Storm Duration = 24 hours; Time Interval – 1 minute; Rainfall: Type – III;


I am trying to find out the ponding within an existing basin six feet deep with these two different inflow hydrographs. The depth of ponding when the outlet is clogged is much lesser for the rational hydrograph than for the SCS hydrograph although for the same time period of 48 hours (total iterations for the routing procedure).

It is true that the rational method is more accurate for determining peak runoff but when you route the 24 hour rainfall hydrograph, the depth of ponding is almost 10 times for that of the rational hydrograph.
Anyone with expertise in this area, please post ur comment.



Thanks.

 

[RWF7437]

You cannot use the Rational Method alone to determine pond volume. Q=CIA provides only the peak flow; not a hydrograph.

It is NOT true that the Rational Method gives you an accurate measure of anything.

The choice of a 24 hour storm using the SCS method will yield one of many possible hydrographs. That is not necessarily the most critical storm duration. What rainfall pattern are you using Type I, II, III IA or a custom "storm"?

What exactly is it you are trying to do ?

 

[DMcGrath]

You are missing (or forgot to supply) a critical piece of information: the design storm frequency (2-yr, 10-yr, 100-yr, etc.) I agree the two methods will yield drastically different results in peak runoff rates, but you can only figure a volume from the NRCS method.

 

[jjv2006]

The rainfall distribution is Type III. The storm of concern is the 25 year storm.
Is the 6 hour rainfall distribution more reasonable or the 24 hour rainfall distribution to determine the pond volume.

All I really want to model is the amount of ponding that will occur when the outlet is totally blocked for the 25 year storm...

 

[RWF7437]

"All I really want to model is the amount of ponding that will occur when the outlet is totally blocked for the 25 year storm..."

If that's all you really want to do then calculate the hydrograph for the storm of interest and take the area under the hydrograph. ( because the outlet is blocked the outflow will be zero cfs). That area equals the runoff volume for the storm you've chosen to model and is the volume that must be stored. Using the stage/discharge/ depth rating curve for the pond will tell you the depth in the pond.

 

[bltseattle]

"All I really want to model is the amount of ponding that will occur when the outlet is totally blocked for the 25 year storm..."

For this specific question, you do not need to generate a hydrograph. You can use the NRCS equations that predict runoff depth from a CN. All you need is a curve number and precipitation depth; it should be a 10 minute exercise once you have the CN and precip. I recommend the 24-hour precipitation for your scenario. You do not need the storm type (eg I, II, III etc) to use the runoff depth equation.

See TR-55, Chapter 2 for all of the necessary equations.

ftp://ftp.wcc.nrcs.usda.gov/downloads/hydrology_hydraulics/tr55/

 

[cvg]

If you are looking to be conservative, do both the 6 and 24 hour storms and pick the one with the greatest volume of runoff

 

[blueoak]

The type III is a pretty gentle curve so it sounds like your 24 hr will definitely have more rain. The only NRCS hyetograph for a 6 hr is the the type B (please let me know if there is another).

As to which storm you have to model, what are you trying to do? Will it take crews 24 hrs to get out there? Is the outlet accessible with the basin full? The 25 yr storm is a very high frequency event.

I would check your routing procedure. With a 15 minute Tc your flow should be pretty minimal 15 minutes after it stops raining.

 

[MEM1]

Example:

Rainfall 6 inches
NRCS CN 90
Drainage Area 50 acres
Runoff/Acre 4.85
Total Runoff 20.1911589 Acre Feet
Total Runoff 6,579,309 Gallons

Let me know if you have any questions

Scott

 

[jjv2006]

Thanks for the input. I did calculate using the runoff equation.

Thanks again

 

[kedpe]

Would the direct runoff calculation apply in this case?
One of my architects regularly asks me how much detention would be required on a site. This is before I provide a proposal to design the project. He gives me a conceptual site plan; Would it be conservative to calculate the direct runoff and muliply by the acreage to give him a detention volume?

 

[cvg]

kedpe
If you are looking for the amount of "retention" then the answer is yes, direct runoff will yield a total volume to be retained. If "detention" as you state, then you need to generate a hydrograph and then use that to determine the amount of volume required in your detention basin to achieve the desired peak outflow.

 

[bltseattle]

kedpe
Yes, that would be conservative so long as you apply all the same safety factors as you would in further design. In many jurisdictions you would take the difference in the proposed and existing/predeveloped site runoff volumes (not the entire proposed runoff volume) to be a conservative detention volume estimate, understanding that once you perform pond routing computations the volume will decrease. Don't forget to apply agency-required safety factors, if any, to your estimate!

 

[env21tech]

Step 1- assume you will be detaining during the IDF 60-minute duration. The average intensity associated with longer durations is usually a drizzle.
Step 2 - rainfall depth for this duration = 60 min x average intensity. This is what you would need to detain if you had no outflow pipe. This is all the rainfall that the raincloud can generate during 60 minutes.
Step 3 - assume that all of this rainfall is going to get to the detention basin one way or another because the leading edge of the storm has saturated the ground and filled in the birdbaths. There is no need for a runoff hydrograph.
Your problem is another illustration of why focusing on the ground (concentration time) instead of the sky (IDF data) can lead to strange results.