How to make the rational formula match StormCAD results?
How to make the rational formula match StormCAD results?
(OP)
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
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





RE: How to make the rational formula match StormCAD results?
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?
RE: How to make the rational formula match StormCAD results?
RE: How to make the rational formula match StormCAD results?
http://communities.bentley.com/products/hydraulics...
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.
RE: How to make the rational formula match StormCAD results?
A co-worker used 9 inches of rainfall for a 25 year storm instead of 8.95 and had to resubmit the whole package.
RE: How to make the rational formula match StormCAD results?
RE: How to make the rational formula match StormCAD results?
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
RE: How to make the rational formula match StormCAD results?
RE: How to make the rational formula match StormCAD results?
This is not correct. This is simply the correct application of the Rational Method for a storm sewer network.