hydrology-Time of concentration for large watersheds 2

[arj00]

I have used Kirpich formula for calculating the time of concentration for small watersheds,however for large watersheds it is giving conservative results"shorter duration " compared to other methods as the Kinematic wave equation which is developed by the University of Maryland & is recommended by Federal Highway Administration for overland flow.
my question can we use the wave equation to calculate the time of concentration for large watersheds(10sq miles or larger)

 

[sam74]

For large watersheds I have in the past used USGS or similar type regression equations to estimate a peak flow for varied design storms. They require no time of concentration, generally just a drainage area. Our DOT uses their own as does TVA. If you require a hydrograph try several different Tc methods until you get SCS peak flows similar to what the regression equations gave you. If this doesn't help you might try posting this in the Storm/Flood engineering forum.

 

[LHA]

I agree w/ sam. For anything over a sq mile or two, use a statistical model (if you are in PA, USA, PSU-4 is universally accepted, including PennDOT). Creating hydrographs will lead to excessive design flows, and, therefore, overdesign.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[ashhafPE]

Regression equations in our area (South Texas) resulted in unrealistic Q's. We have been monitoring bridge structures with different frequencies. Reg. Equ.'s indicate W.S. elevations higher than bridge elevations at 25 and 100 yr frequencies, yet we have never seen those bridges go under water.

 

[francesca]

Been watching those bridges for 25 to 100 years?

 

[RWF7437]

As far as I know, Regression Equations are widely used for such things as flood plain studies, bridge design, channel design, etc. They are not "accurate" in the sense that they typically have errors of plus or minus 30%, or more. Still, they are the best available estimates for larger watersheds. The only thing better would be a nearby stream gage with a long period of record.

Unless one has a long record of flows at a bridge site, it is probably not wise to trust anecdotal evidence. Just knownig that a bridge has not been overtopped in recent years wouldn't tell you very much unless you can relate flows, water surface elevations and probability of recuurance over a significant period of time.

good luck