time of concentration
time of concentration
(OP)
when doing a storm water study for a subdivision, how do you know when to stop the length of travel for a water course when calculating the time of concentration. If I am able to collect all runoff at a single pond located in the project. Do I use the travel length from the furthest point in the subdivision i.e. accross the lawn, down the gutter, into a catch basin, through the storm drain, and finally into the detention pond where the final outfall is located? Or is the length just to the first catch basin where it is possable for water to back up and become detained because of the restriction plate/orifice being used at the pond outfall. It makes a big difference in the runoff volume when using shorter travel lengths within the sub catch basins of the subdivision. I would appreciate all coments on how other people calculate the travel length.
Thanks in advance
Thanks in advance





RE: time of concentration
RE: time of concentration
I should also note, as I'm sure you know, the time of concentration is also storm specific.
RE: time of concentration
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: time of concentration
RE: time of concentration
Depending on the size, as long as you are consistant in how you work, you should get a workable model.
RE: time of concentration
If I am able to collect all runoff at a single pond located in the project. Do I use the travel length from the furthest point in the subdivision i.e. accross the lawn, down the gutter, into a catch basin, through the storm drain, and finally into the detention pond where the final outfall is located?
YES, so long as all the runoff is conveyed to the pond. If you have "bypass" areas then look at the route from lawn to where the bypass and pond flows converge.
You will likely find that the initial lawn/gutter flow comprises most of the overall time-of-concentration, and pipe flow is a much smaller portion.
RE: time of concentration
Tc=(2.187Ln/S*0.5)*0.467
n=MANNIG RUN OFF COEFFICIENT THESE VALUES ARE COMMONLY FOUND IN WATER DISTRIBUTION BOOKS
L=MAX. DIST. TRAVELED OF WATER. BUUT THIS DIST. IS LIMITED TO 1198 ft. BECAUSE THE WATER FLOWS FASTER THAN THAN WHAT THE MODEL PREDICTS
s= THE AVERAGE SLOPE OF THE SURFACE TRAVELD BY THE WATER (ft/ft)
TO CALCULATE YOUR RETENTION POND IS THE VOL. IS SMALL ENOUGH TO USE THE RATIONAL METHOD GOOD FOR 54,000 sq.ft.? IF SO YOU MUST HAVE ACCES TO THE LOCAL RAINFALL INTENSITY-TIME-FREQUENCY(RATE OF FALL EX. 1 IN 50 YEARS PROBABILITY) BUT IT HAS NOTHING TO DO WITH "TOC"
"TOC" IS USED TYPICALY TO CALCULATE THE SIZE OF THE STORM PIPES GOING FROM ONE CATCH BASSIN TO ANOTHER IT IS USED TO CALCULATE THE VELOCITY OF THE WATER FLOW IN THE PIPE TO THEN CALC. THE VOL. FLOW AND FINALY TO CALC THE SIZE OF THE STORM WATER TRANSPORT PIPE. ALSO YOU MUST RESPECT THE WATER SPEED IN THE PIPE THE RANG TO ASSUR AUTO CLEANING IS BETWEEN 1.9 AND 10 ft/S
RE: time of concentration