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# Time of concentration calcs for use with the rational equation2

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## Time of concentration calcs for use with the rational equation

(OP)
I am trying to calculate flows into a drainage system we are going to be upgrading. It seems there are a bunch of different ways of computing the travel times for the watersheds. I think the one I like the best is the "watershed lag method" because it seems like it could be the most reproducible. Everything I have read on it though, uses it as a method for calculating travel time for the NRCS hydrograph method. Can this method be used for use with the rational equation as well?

I also like the combo of Kerby-kerpich because it seems to deal with just overland and channel flow (as opposed to sheet flow (and its iterations), shallow concentrated flow, and channnel flow). I did read somewhere however that this method was only good for slopes less than 2%. Does anyone else use this method, or read anything different on this?

Thank you

### RE: Time of concentration calcs for use with the rational equation

I recommend that you check with your governing agency or state standard for methodology. They may or may not accept the method that you are planning on using.

### RE: Time of concentration calcs for use with the rational equation

I personally hate the methods outlined in TR-55 because they are easily gamed by manipulating where you stop "sheet flow" and where you begin "shallow concentrated flow" on your flow path. But those methods are almost universally mandated by the regulatory agencies I deal with.

I feel the most honest way to go for drainage system design is to go with Kirpich, USBR Small Dams, or similar methods which are based on mean basin characteristics instead of flow path summation to a remote point that might not adequately characterize the overall basin, and which presume the engineer has full knowledge of the path in question, which he/she usually doesn't.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

### RE: Time of concentration calcs for use with the rational equation

there are numerous ways to game the rational method, including the Tc calculation, minimum Tc, flow path, estimated slopes, weighted subareas, assumed runoff coefficients etc. unfortunately, we have to use the approved methods from the AHJ and not use the method we like best. So getting the drainage regulations manual and reading it first is required or you will be wasting your time

### RE: Time of concentration calcs for use with the rational equation

I've only ever used the Rational Method for small watersheds... not much roof for fudging... I understand the RM is only intended for small watersheds.

Dik

### RE: Time of concentration calcs for use with the rational equation

small is a relative term. limits vary widely

Texas DOT 200 acres
Iowa DOT 160 acres
Denver UDFCD 90 acres
Australia Bureau of Meteorology 500 hectares (urban) and 25 sq km (rural)
Washington DOT 1000 acres

### RE: Time of concentration calcs for use with the rational equation

cvg…

Years ago I prepared a storm drainage master plan for Kettleman City, a small, rural, unincorporated community hugging the downslope side of Interstate 5 in central California. In my research I found the drainage study prepared by the California DOT (Caltrans) in the 1960s for about a 40-mile stretch of the freeway. I was surprised to find that Caltrans had used the Rational Method and Kirpich for watersheds as large as 5 square miles near Kettleman City and even larger for watersheds further afield. Their study didn't expressly state they had used the Raional Method and Kirpich…I had to "reverse engineer" their method from their data and results.

Fred

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

### RE: Time of concentration calcs for use with the rational equation

seems like a lot of work for questionable results.

I have done similar studies for highway departments but would not have dreamed of using Rational. Linear regression equations for similar watersheds is probably more accurate, much easier and generally conservative.

### RE: Time of concentration calcs for use with the rational equation

cvg: thanks... quite a bit bigger than I've used...

Dik

### RE: Time of concentration calcs for use with the rational equation

(OP)
Can anyone point me to where I can get more information on using the FAA formula for computing Tc? Specifically if it is only for sheet flow? I was thinking it was a general use equation kind of like the kirpich formula, but it produces a lot longer travel time (which i was hoping for) butI can't find anywhere where it says in which cases it can or can't be used

### RE: Time of concentration calcs for use with the rational equation

I assume you mean Federal Aviation Admin (FAA)?

You should be using AC 150/5320-5D. UNIFIED FACILITIES CRITERIA (UFC), SURFACE DRAINAGE DESIGN

Chapter 2, Section 3.2.4 gives the method and it does not limit it to sheet flow. You can find this manual online.

### RE: Time of concentration calcs for use with the rational equation

(OP)
Thank you for your response. I was actually looking for information on the FAA formula developed by the army corps for computing time of concentration. The UFC section 2-3.2.4 uses the velocity method with the 3 different equations plus an iteration in the sheet flow segment. From what I am reading, the FAA equation is an alternative to the velocity equations that is accepted but I can't find anything that says for what cases or situations it is limited to

### RE: Time of concentration calcs for use with the rational equation

not sure what you are reading because the UFC does not reference any formula developed by the USACE

### RE: Time of concentration calcs for use with the rational equation

(OP)
exactly. The UFC isn't what I am looking for. It uses the velocity equations. I am looking for info on the the FAA equation for Tc

### RE: Time of concentration calcs for use with the rational equation

the FAA requires that you use the UFC drainage manual. am I missing something?

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