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Chapter 5 Working with Hydrology Commands
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4 In the Channel Slope box, type the slope of the channel.
You can also click Select to calculate the slope by selecting contours on the
drawing or by typing D for DTM and have the elevation interpolated from a
DTM surface.
NOTE
If you don't know the wetted perimeter value or the channel slope value,
but you do know other channel values, then you can click Channel to
display the Advanced Channel Calculator dialog box. You can use this
calculator to calculate channel parameters for this flow segment, and then
you can type these values into the Channel Flow Time Calculator dialog
box.
5 In the Mannings n box, type the Mannings n roughness coefficient for the
channel. You can also click Select and select a value from the Channel Flow -
Mannings n dialog box.
6 In the Flow Length box, type the channel flow length. You can also click Select
to specify the length by selecting either an object or points in the drawing.
When you specify all five values in the dialog box, the following calculations
are displayed:
n
Hydraulic Radius: The calculated hydraulic radius of the channel.
n
Velocity: The computed velocity of the channel.
n
Channel Flow Time: The computed channel flow time. When you click OK
to return to the Time of Concentration calculator, this value is inserted into
the Channel box.
TIP
To display the channel flow data in a text editor, click Output. From here,
you can print the data to a file or printer.
7 Click OK.
Calculating the Watershed Time of Travel
Travel time (Tt) is the time it takes runoff to travel (already flowing in shallow
or channel flow) from one watershed subarea through another subarea as the
runoff flows toward the composite watershed outflow point.
Tt can be a combination of two types of flow: Shallow Concentrated and
Channel. Both flow types use different equations and require different values.
For most cases, the travel time is composed of only channel flow segments with
no shallow concentrated flow segments. For more information, see Specifying
the Shallow Flow and Specifying the Channel Flow in this chapter.
The TR-55 travel time method has the following limitations:
n
The maximum Tt allowed in the Tabular Hydrograph method is 3 hrs.
n
A watershed with only one subarea does not have a Tt. Travel Time is
computed only if the runoff from an upstream subarea passes through a
subarea before it reaches the composite watershed outflow point.
n
It is sometimes difficult to determine exactly where shallow concentrated
flow ends and channel flow begins. Use your best judgment and/or consult
your local Soil Conservation Services office.