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# 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?

### RE: TR-20 Lag Method - proper use

This would not be an appropriate use of the TR-20 method. The drainage area you describe would be to small for this method. It might be better to look at the Rational Method if the example is the type of drainage you need to calculate.
It would help to know more about the drainage area and what qovernmental regulations apply.

### RE: TR-20 Lag Method - proper use

For the application you are looking at, the Rational Method is an excellent choice. If you are forced to use TR-20 for this type of application, although your drainage area is too small, get a copy of the National Engieering Handbook. It will give you great examples on to calculate the time of concentration. I believe that manual is now available on the NRCS web site.

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

The Rational Method is the Rational Solution here. However, if you are required to use it by local agency, they should have criteria establishing its use. For example, in Miami-Dade County, Florida, the local agency provides recomendations for estimating tc for local conditions as well as established a minumum tc of 10 minutes. It is rare when tc is greater than 10 minutes, therefore tc is almost always 10 minutes. However, we also have a requirement that we must capture the 1st inch of runoff. Therefore, t = tc + t1". Check with local regulators, they shouls provide you with their guidlines (It is common prcatice to request guidelines since different agencies have different guidelines and standards). Good Luck.

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