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# Tt for Open Channel

## Tt for Open Channel

(OP)
TR-55 states that "Average flow velocity is usually determined for bank full elevation."

Would this be considered the most conservative approach to calculate a sub-basin's Tc when using the rational method & IDF curves to determine a peak flow rate from a basin for a culvert design?

Or would it make sense to assume a Tc to get a flow rate from the sub-basin then use that flow rate to determine the channel depth and then in turn find the avg. velocity and Tt of the channel segment (the channel geometry is known).  After finding this Tt based on the assumed Tc & flow rate use this Tt to determine a new Tc & flow rate and repeat the process until the numbers converge?

### RE: Tt for Open Channel

I would be surprised if there is much difference.  I like to divide channel length by 5 fps.  Are they diferent by more than 10%?

### RE: Tt for Open Channel

Why are you mixing TR-55 with the Rational Method ?  If your basin is large  you should probably not use the Rational Method.  Although FEMA allows the Rational Method for basins as large as 640 acres most Engineers I know seldom use it for anything larger that 20 acres.

In TR-55 you can use the kinematic wave method or any of several other methods described in the literature to calcualte Tc.

### RE: Tt for Open Channel

(OP)
Basins are small 2-4 acres, using the rational method to estimate the peak flow rates.  Using the procedure described in Chapter 3 of the TR-55 literature to calculate Tt & Tc only.

Just curious how others solve for travel time (Tt) in open channels seems to be a few choices:

1.  Assume the channel is at a "bank full elevation"

2.  Assume a velocity  (would the assumed velocity change based on the channel slope?)

3.  Or an iterative solution based on a initial assumption of flow depth.

### RE: Tt for Open Channel

It would be valid, I think to use actual flow.  Over a long enough channel, it would make a difference.

Given a trap. grass swale of b=2, D=1, z=3 and slope =.020, capacity is 11.8 cfs.  Full flow yields a Manning's of .063 and v = 2.37.

Same swale with 1 cfs yields a Manning's of .134 and v = 0.70.

I don't like Rational on anything over a couple of acres, and really don't like it for hydrographs at all.

Signature under construction, sorry about the mess - Steve

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