TR-55 Time of Concentration assumptions
TR-55 Time of Concentration assumptions
(OP)
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
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
Good luck.
RE: TR-55 Time of Concentration assumptions
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
BT
RE: TR-55 Time of Concentration assumptions
http://www.fema.gov
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