time of concentration 3

[sinusoidal]

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

 

[LHA]

Use the longest flowpath for both pre and post, in your case it is the first scenario...I am confused, though; how does the detention volume (in your second scenario) in your pond back the whole way up to the first inlet in the pipe system? If that is happening, make sure you are not flooding inlets in larger storms, or at least that you have provided an alternate emergency flowpath away from structures and utilities.

 

[jheidt2543]

The time of concentration is a point specific concept. You have a time of concentration at each inlet for its own little drainage area. As you travel down through the drainage system to the the pond, there is a sumation of times of concentration untill you get to the pond outlet. By definition, the time of concentration at a point is the longest time path from that point back upstream. So, all the upstream points affect the downstream points, but like lha says, I don't see how the reverse can happen without a back up in the system.

I should also note, as I'm sure you know, the time of concentration is also storm specific.

 

[Focht3]

Please describe your site a bit better - that might help us. Where is it located? Design storm? Site grade, cover, etc.? Other special circumstances?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[cvg]

If you are doing a retention basin design, the volume is not directly related to the time of concentration or to the peak flow rate. Typically, it is based on the storm precipitation depth x area x runoff coefficient (V = DAC)

 

[0verflow]

I saw one reference to time of concentration in which the point you are discussing is called "the point of interest."
Depending on the size, as long as you are consistant in how you work, you should get a workable model.

 

[bltseattle]

You said:

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.

 

[genieconseil]

AS CVG POSTED DEC 19TH 2003, THE TIME OF CONCENTRATION(TOC) IS BY DEF. THE TIME FOR A DROP OF RAIN THAT FALLS ON THE GROUND TO REACH THE POINT OF CAPTATION (CATCH BASSIN), NATURALLY THE "TOC" IS DIRECTLY PROPORTIONAL TO THE MANNING ROUGHNESS COEFFICIENT RELATIVE TO DIFFERENT TYPES OF SURFACES SUCH AS PERMEABLE AND IMPERMEABLE AND INDERECTLY TO THE SLOPE OF THE SUFACE BEING ANALYSED, THIS IS ACCORDING TO THE KERBY (EQUATION) MODEL, THIS MODE IS USEFUL IN EVALUATING THE TIME NESSECARY FOR A THIN FILM OF WATER TO RUN OFF ON SURFACES OF DIFFERENT TYPES (OTHER MODELS EXIST SUCH AS KIRPICH, SCHAAKE, FEDERAL AVIATION AGENCY)

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

 

[treg]

In the work I've done, Time of Concentration would be calculated for each subwatershed. In your example, it would be across the lawn to the gutter. Travel time would be calculated to the catch basin. Calculations for outlet works downstream of the catch basin would be calculated based on the peak discharge of the catch basin.