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# TR-55 Calculation for Time of Concentration

## TR-55 Calculation for Time of Concentration

(OP)
Could someone tell me how to handle a time of concentration in TR-55 to take into account a vertical wall, specifically in the case of water flowing into a quarry.  In addition, could someone tell me where to measure your wetted perimeter and area in channelized flow.  Specifically in a case where the typical ditch has 3:1 side slopes extending up for 4 feet; however, the stream flowing through the channel flows within a 1' deep x 2' wide rectangular cut in the base of the channel.  Is the wetted perimeter assuming full flow conditions at the top of the 4 feet or is my area only the 1'x 2' rectangular channel in the bottom of the channel.  Is there a good document that clearly describes how to measure channel flow geometrics?  Any help would be greatly appreciated.

### RE: TR-55 Calculation for Time of Concentration

There are many texts written about and usually entitled "Open Channel Flow".  My favorite is written by Vin T. Chow.  Using Manning's equation to analyze open channel flow, the wetted perimeter is litterally that perimeter of the the irregular cross sections that get wet with a given flow, slope, and n-value.  Since my calcs are typically "quick and dirty" I derive the wetted perimeter by trial and error.  I draw the irregular cross section in AutoCAD, draw an arbitrary water surface, calculate the resulting flow with Mannings and compare it to the design flow.

### RE: TR-55 Calculation for Time of Concentration

You should also have a copy of TR-55 itself which has plenty of worksheets for estimating Tc.

Area and wetted perimeter are straightforward.  The area of flow is what you want to calculate and the entire perimeter that is wet.  Often easiest to just draw the cross section to scale and measure it.

King's Handbook of Hyadraulics is another "classic" reference.

### RE: TR-55 Calculation for Time of Concentration

with a vertical wall I have just used 33 f.p.s. (gravity) scince there are no limiting factors like in a downspout.

### RE: TR-55 Calculation for Time of Concentration

Water flowing down a vertical wall under the influence of gravity does NOT travel at 33 feet per second.  It accelerates at a maximum of 32.2 feet per second per second.

For TR-55 it would be conservative to assume a travel time of zero down the wall. Unless the wall is hundreds of feet high the travel time must be very short; on the order of a few seconds one would think.

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