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TR-55 Time of Concentration assumptions

TR-55 Time of Concentration assumptions

TR-55 Time of Concentration assumptions

Are there any references that give generally acceptable values of time of concentration for urban watersheds that I could use as an assumption? I am developing a floodplain study for ultimate the ultimate land use. My drainage subbasins vary from 80 acres to 400 acres. Since I don't know the ultimate configuration of the storm sewer system I need to assume these values. The land use varies between commercial and 1 unit/acres residential. My initial guess is that travel times will vary between 20 minutes and 40 minutes.

RE: TR-55 Time of Concentration assumptions

You might try using the average slope of a run of pipe or channel and determine the approx. velocity. Assume the shortest routing to be on the consertative side as far as discharge. Divide the length of the run by that value and get the travel time. You will then add that to your overland flow time. Do not forget that some areas may result in rill or shallow channel flow which has a higher velocity than normal overland flow.

Velocity ranges may be in the range of 2-3 feet/sec up to 10 feet/sec or so.

RE: TR-55 Time of Concentration assumptions

For drainage basins of this size ( 80 to 400 acres) FEMA does not accept the Rational Method for calculating flows. If you're not using the Rational Method, you should have little need to know the time of concentration.  For larger basins like these you are probably better off to use either the regional regression approach outlined in the National Flood Frequency program which is free and available on line.  If you are in an urban area you may have urban regression equations available or streamflow data to give you some guidance.  Check with your State Highway Department if you are in the US.  They should lead you to good sources of hydrologic data.

Good luck.

RE: TR-55 Time of Concentration assumptions

Urban drainage systems vary substantially depending on where you are, local standards, how piece-mealed the system is, etc.  My experience in the Seattle area is that sheet flow and unconcentrated flow tend to dominate the time of concentration calculations.  For instance, the Tc for sheet flow on pervious could be 20 m or greater, but the travel time in 1000 ft of pipe is often less than 2 m.

Each basin is different, and you should assemble the available information about the flow paths and existing system to make sure the critical components are considered.  These  components would include (but are not limited to) existing stormwater facilities, drainage channels, very flat pipes, and storm drain trunk lines.  After reviewing this, you can then apply engineering judgement and focus your effort on those areas of the flow path most likely to affect the overall Tc for the watershed.  For example, spend time to understand how ponds are working, if undersized culverts are backing up water in channels, compute velocities in the flattest pipe runs, etc.  Then, you may be able to make simplifying assumptions about some of the other areas and consider/apply PECPESC's advise.  

Good luck


RE: TR-55 Time of Concentration assumptions

While others have offered good advice on how to calculate the time of concentration, I still think you're wasting your time in doing that calculation.  If you are really doing a "flood plain study" which will be reviewed by FEMA or one of their contractors you should visit FEMA's web site.


They have voluminous guidance available for how to do flood plain studies, what computer models they will accept, how to estimate BFEs by approximate methods, etc. etc.

Good luck

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