Time of concentration calcs for use with the rational equation
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
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
RE: Time of concentration calcs for use with the rational equation
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
RE: Time of concentration calcs for use with the rational equation
Dik
RE: Time of concentration calcs for use with the rational equation
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
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
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"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
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
Dik
RE: Time of concentration calcs for use with the rational equation
RE: Time of concentration calcs for use with the rational equation
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.
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RE: Time of concentration calcs for use with the rational equation
RE: Time of concentration calcs for use with the rational equation
RE: Time of concentration calcs for use with the rational equation
RE: Time of concentration calcs for use with the rational equation