How to make the rational formula match StormCAD results? 3

[htb723]

I am required to prove my results from StormCAD using hand calculations and my results are off.

First, the drainage area is, 9 but when I look at the properties for the outfall node, it's slightly less, 8.55. I'm not sure why this is.

Using the outfall's given Area in acres, the intensity, and the runoff coefficient, it results in a number less than what I calculated by hand.

Q = CIA

Q = (0.95)(0.991)(8.55) = 8.04

But StormCAD gives a flow of 8.54. Why is it so off?

Thanks

 

[htb723]

The 8.55 came from multiplying the area by the runoff coefficient

9*0.95 = 8.55

So if I update the equation:

Q = (0.95)(0.991)(9) = 8.47 csf

It's still slightly off. What factor's are being taken into consideration here? Is it even possible to prove my hand calcs accurate?

 

[cvg]

you should not be reporting values to 0.01 cfs. round to the neared whole cfs and call them the same

 

[htb723]

After perusing the StormCAD FAQ:

http://communities.bentley.com/prod...ydrology__wiki/5078.general-stormcad-faq.aspx

The unit conversion from acre'in/hr to cfs is taken into consideration. So if I multiply my answer by the conversion factor, 1.008, it gives the exact answer. It's negligible, but now I know why my hand calcs differed from StormCAD.

Thanks for the reply.

 

[jgailla]

Try telling that to a regulatory agency, cvg. I've always carried every decimal point through even though it's wrong in any agency submittal.
A co-worker used 9 inches of rainfall for a 25 year storm instead of 8.95 and had to resubmit the whole package.

 

[cvg]

I have done agency reviews of drainage plans and reports numerous times. I ignore the decimals. it would be so much easier if the designer would do the rounding

 

[beej67]

Every agency I've ever had to provide plans for wanted CFS to the hundredth. Even for watersheds hundreds of acres large. They've also wanted flood elevations to the hundredth. I agree it's dumb, but that's what people want in the age of computers.

When doing StormCAD, please be cognizant that it's doing the "Modified Rational Method" when you're in a storm network. What that means, is that the flows are NOT additive. If you have flow from two pipes into a junction, the flow out of that junction will not be the sum of the two flows in. Instead, StormCAD will throw the flows out from the incoming pipes, re-figure the watershed to the inlet, figure a newer (longer) time of concentration equal to the longest flow path down either pipe, and then use Rational method on that revised watershed. The result is your flow out will be a little less than the sum of your flows in. Provided of course you have your full IDF relationship input into the table.

That's the biggest gotcha I see when people are trying to match StormCAD calculations with hand calculations.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[gbam]

beej67 hits on a key point. Make sure that you hand calc reflect the timing element in the rational method storm sewer computations.

 

[LincolnPE]

When doing StormCAD, please be cognizant that it's doing the "Modified Rational Method" when you're in a storm network. What that means, is that the flows are NOT additive.

This is not correct. This is simply the correct application of the Rational Method for a storm sewer network.