Time of Concentration and Rainfall Intensity - Rational Method
Time of Concentration and Rainfall Intensity - Rational Method
(OP)
I am reviewing some calculations for peak discharge using the rational method from a 4 acre area. A statement in the calculations is that due to the steep slope of the drainage area (8.8%), which is not explained, the time of concentration can be neglected. I cannot find any backing for this statement. The calculation goes on to use a value for the rainfall intensity of 6 inches/hour and references the 24 hr/25 year storm event (Madison County, NE). I do not know if this is connected to the first statement or not. We do see this all the time where the rainfall intensity in the rational equation is mistaken for the 24 hr/25 year storm event from the maps incldued in TR-55.
Any suggestions would be great.
Any suggestions would be great.





RE: Time of Concentration and Rainfall Intensity - Rational Method
They could ignore the sheet flow component of the Tc, leading to a shorter Tc. I've seen people claim a full 300 ft of sheet flow in woods... like you've ever seen a beautifully flat woodland with no tree roots, fallen limbs, etc. to help concentrate the flow. I typically assume <50 ft of sheet flow in woods and on a 10% slope would probably assume 0 ft.
I would toss the calculations back at them until they provide some graphics showing the Tc path, which length is assumed sheet/shallow concentrated/channel flow and also the NOAA rainfall intensity charts with the assumed intensity circled.
RE: Time of Concentration and Rainfall Intensity - Rational Method
RE: Time of Concentration and Rainfall Intensity - Rational Method
RE: Time of Concentration and Rainfall Intensity - Rational Method
The Intensity associated to 25-year 5 min duration storm is usually [i]overly[]/i] conservative and might result in over-design.
Whether if it's acceptable or not really depends on the interest of the calculation.
is it : 4 ac x 6 inches/hour = ... 24 cfs.
Then it's rather irrational method...
RE: Time of Concentration and Rainfall Intensity - Rational Method
Really? 6 inches per hour over 24 hours is a 144 inch storm. Which is nonsense. The appropriate units for I in the rational method are inches per hour, not inches. They probably saw that there were 6 inches (not in/hr) in a 24 hour storm for their region, and then decided to use 6 in/hr as their I by just arbitrarily changing the units. Then since they had magicked up their I without actually calculating a Tc, they probably said "it can be neglected" because they didn't know what else to say.
I agree with some of the above posters. Tc calculation is often so arbitrary anyway that for smaller basins it's conservative to just go with a minimum, usually 5 or 10 minutes, and pick their intensity off an IDF curve or from a published table. If they do this, a reviewer shouldn't have a problem with it, although I must admit I've been raked over the coals by picky reviewers for assuming a minimum Tc without providing a backup calculation. One job I had to provide 20 Tc worksheets each showing a Tc of 3 minutes or less, to justify to the reviewer my choice of picking 5 for each basin.
Tell them to pick a Tc and pull their I off an IDF curve. If they don't know what you're talking about, then go through all their work with a fine toothed comb. I'm sure plenty of engineers here on Eng-Tips would love to do so for you on a contract basis, just to have some fun stories to tell their friends.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Time of Concentration and Rainfall Intensity - Rational Method
For example, I just went to the NOAA website and randomly picked a location in Arizona. The 25-year, 10 minute precip intensity was 6.19 inches/hour for 25-year storm and was over 8 inches/hour for a 5 minute duration. This is not unusual as most storms in Arizona are short duration, high intensity thunderstorms. The corresponding 25-year, 24-hour storm precip "depth" was about 3.3 inches. precipitation intensity and rainfall depths do not have a direct relationship.
As far as assuming anything else, why are we wasting time? Reviewer should just ask for re-submittal of the calculations with the appropriate backup material.
RE: Time of Concentration and Rainfall Intensity - Rational Method
"Just throw them in an Appendix and circle what you used, it is only paper" is what I was taught.
Ask them to verify their statement that the Tc can be neglected using a reference from the regulatory stormwater manual.