Time of concentration flow frequency

[Drew08]

Please share your thoughts:

What storm event would you use to determine the velocity, for use in calculating the time of concentration, through a constructed drainage ditch?

For a natural stream, I use bankfull discharge/velocity, which is typically the 1- to 2-year return period event. The assumption is any additional overbank flow would not increase the velocity in the channel.

However, a designed and constructed drainage ditch, in theory, would have a bankfull discharge according to its design, maybe 25- to 100-year event.

 

[beej67]

Doesn't TR55 specify 2 year? I can't remember, don't have it in front of me, but I'm pretty sure they state outright which year to use.

Channel geometry is often ball parked anyway, so I wouldn't sweat the details too much on it. Most of your TC for that method is in sheet flow anyway, which is why I really don't like that method, personally. Too much wiggle room to cheat.

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[TerryScan]

Yes, TR-55 utilizes the 2yr event for Tc calculations. However, I have seen approaches which recompute a Tc for each storm event.

 

[Drew08]

Thank you for your responses, but I'll have to respectively disagree. For sheet flow using Manning's kinematic solution (TR-55 Eq. 3-3), the 2-year rainfall is included in the equation, I suppose with the assumption that sheet flow has the same, insignificant depth whether you’re talking about a 2- or 100-year event.

For channel flow, TR-55 explicitly says, "Average flow velocity is usually determined for bankfull elevation." last sentence page 3-3. Depending on where you are in the world, for a natural stream, bankfull can be anywhere from 0.5- to 10-year return period. For a non-natural designed ditch with 100-year bankfull elevation, this seems over kill.

Oh jees, in researching Beej67's comment, "Most of your Tc for that method is in sheet flow anyway...," I looked closer at the newest version of NEH-630, Ch. 15, and it does suggest that 2-year event is typical, but not exclusive, that’s good enough for me.

I've always been taught, and I thought I read it somewhere, that if the majority of the Tc is sheet flow, you ought to use a different method because that part of the travel time method is the least accurate. In that same topic, I also agree with Beej67 that if channel geometry is just a guess, then use a different method. I use the NEH-630 lag equation in that instance. But if channel geometry is well known, or even part of the design, then why not use travel time method?

 

[beej67]

For small basin hydrology I would much prefer to simply use Kirpich Method and be done with it, since Kirpich was developed for small basins and it makes all your watersheds if not exact, at least consistent. The problem I have in my region, is the reviewers refuse to accept it. They want to see the full breakdown like in TR-55, even when you don't really know the details on what the longest sheet flow path in your basin is, or they won't approve your hydro design. So I give them what they want.

The generally accepted definition of Tc is the time of concentration to the most hydraulically remote point. Well if you're using the sheets from TR-55, and the sheet flow dominates your calculation, then you'd have to know the geometry of every parking lot and every ball field in your basin to find the one with the longest Tc. And even presuming you did, lets pretend somebody forgets to mow it for a year, that could double the Tc of the basin by their rationale, which drastically affects your hydrograph even though the particular Tc location may only be a very small part of your basin. So the whole thing is really somewhat stupid, and the most responsible thing you can do as an engineer is use you best judgment to make the watershed Tc's comparable. Basically, you use judgment to fudge the numbers to make them right. But along come unethical engineers who use judgment to make them wrong in such a way that their design works, and now you've got a problem.

If I were running a municipality, I'd require Kirpich or something similar, and prohibit engineers from using a method with such wiggle room as the one in TR-55. Then again, if I was running a municipality I would also have the sense to realize that a detention pond isn't a watch, and if the calculations are off by a tenth of a percent it probably doesn't need to be torn down and rebuilt. Many reviewers don't agree with that either.

Many municipalities I've worked in actually spell out how much sheet flow you're required to take in your calculation to prevent engineers from gaming the system. Seems to defeat the purpose, to me. Why follow a method at all if you don't trust your method?

Mine is not to question why. Mine is but to Pond Design. (? couldn't think of a better rhyme)


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com