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Time of Concentration and Rainfall Intensity - Rational Method

Time of Concentration and Rainfall Intensity - Rational Method

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.

RE: Time of Concentration and Rainfall Intensity - Rational Method

Well, no, the Tc can't be neglected, and for a 4 acre watershed they can't use a 24 hr rainfall intensity. However, 6 in/hr doesn't sound like a 24 hr storm rainfall intensity.

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

My guess is that the statement is just bad word smithing.  I don't believe he is saying that the Tc is being ignored, but due to the relatively steep slope and small watershed, the engineer is saying that a they assumed a minimum Tc(ie 10min) and no calculation for Tc is included.  The 6 inches/hour reference could be a mistake since the 6 inches may have been taken from a 24 hr/25yr isohyetal map, which would have the units 6 inches/day in that case.  Without reviewing the entire calc it's hard to say, and I have no feel for the magnitude of storms in your area. Sounds like for both issues a simple clarification is warranted, but I would hesitate to assume that the calculations are erroneous based on the information given.

RE: Time of Concentration and Rainfall Intensity - Rational Method

I agree with ccampttu on the presumption of minimum Tc.  You may ask the engineer to clarify their description.  The engineer should have included the IDF graph or table on which you can confirm the appropriate intensity and Tc.

RE: Time of Concentration and Rainfall Intensity - Rational Method

However, I wouldn't calculate a Tc for a 4 acres basin and would probably use 5 min or 10 min.

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

The biggest telling clue is this:


6 inches/hour and references the 24 hr/25 year storm event

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 -

RE: Time of Concentration and Rainfall Intensity - Rational Method

assuming for a minute that the Tc is 5 or 10 minutes, an "intensity" of 6 in/hr is very possible.

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

They should have provided the data table, graph, map or a reference where they got the information for the calcs. Same with the formulas they used to calculate the flows.

"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.  


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