TR-20 Lag Method - proper use
TR-20 Lag Method - proper use
(OP)
How do I apply the required hydrologic length in a Lag Method time of concentration computation for a roadway surface having a closed drainage system? Example: Drainage area A is a 300 ft. long x 12 ft. wide, paved, curbed area sloping to a catch basin. The cross slope is 2% and the longitudinal slope 1%. Is he hydrologic length just centerline to gutter line (20 l.f.±) or does it include the 300 ft. of gutter flow? The resulting times are drastically different - < 1 min. or 7 min.± depending on the length used. Know of any on-line articles on this?
Thanks in advance.
Thanks in advance.





RE: TR-20 Lag Method - proper use
It would help to know more about the drainage area and what qovernmental regulations apply.
RE: TR-20 Lag Method - proper use
Keep in mind, that TR-20 was developed for rural applications and was never intended nor calibrated for urban modeling. Another suggestion is to use the methods in TR-55. Although method was developed for small watersheds, it too was calibrated on rural land uses. I think your best bet outside of the Rational Method is to calculate the tc without lag. If you are measure the tc within the curb/catch basin, there is no lag.
Let us know how you tackled your problem
RE: TR-20 Lag Method - proper use