Time of Concentration

[CivilEngMatt]

I'm working on a project that has an existing pond on the property. I delineated the drainage basin contributing stormwater runoff to the existing pond. My question is: How should I determine the flow path for time of concentration calculations. I'm using the LAG method. I have two options both begin at the most hydraulic point in the basin. Option-1 is to end the flow path when runoff first enters the pond. Option-2 is to end the flow path at the pond outlet. The difference in flow path distance is 450 feet. I'm not sure what option best describes the time of concentration.

 

[TerryScan]

Terminate the flowpath where it enters the pond. Then route the hydrograph through the pond.

 

[psmart]

What Terry said. Since the Tc is a runoff parameter, it should describe the runoff and flow only to the edge of the pond. From that point, the pond routing effects take over. Including the pond in the Tc calculation will make a double allowance for the pond.

Note: If you are not doing a separate pond routing calculation, it's possible to include the pond as part of the Tc, perhaps using the wave velocity. However, it's more accurate to use a separate hydrograph routing to simulate the pond's behavior.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Ryanemma]

Every time I did a tc calc to the outlet of a basin, the reviewer would tell me to stop it at the edge of the basin.

I agree with terry

 

[fattdad]

Just recall if the flowpath you determine is greater than 200 ft, you should use 200 ft and make up the difference considering channel flow to the edge of the pond - the presumption being after 200 ft the flow will find a channel.

At least that's what I recall. . .

f-d

¡papá gordo ain’t no madre flaca!