2.0

Table Of Contents
Chapter 5 Working with Hydrology Commands
558
4 In the Shallow box, enter the shallow flow. You can either enter this value
directly, or click Select to display the Shallow Flow Time Calculator dialog box.
For more information, see Specifying the Shallow Flow in this chapter.
5 In the Channel box, enter the channel flow. You can either enter this value
directly, or click Select to display the Channel Flow Time Calculator dialog box.
For more information, see Specifying the Channel Flow in this chapter.
NOTE
Normally only one length for each flow type is used, but you can enter up
to two lengths for each type in the Time of Concentration calculator. This
allows for differences in various flow types, such as different slopes.
Autodesk Civil Design calculates the sum of the sheet, shallow, and channel
flow times and displays it as the Total Tc at the bottom of the Time of
Concentration Calculator dialog box.
6 Click Save to save the current values as the new defaults for the Time of
Concentration Calculator. This stores the calculated data so that when you
access the Time of Concentration Calculator from a runoff calculation dialog
box, you can load the defaults you saved by clicking Default.
The Default button is available only when the Time of Concentration
Calculator is activated from another calculator (in which case defaults are not
automatically loaded).
7 You can also output or clear the information from the dialog box by using the
following guidelines:
If you want to} Then click}
output the total time of
concentration to a text file
Output.
clear all the boxes in the dialog box Clear.
8 Click OK.
Specifying the Sheet Flow
Sheet flow is water sheeting over a plane surface, such as an even amount of
water flowing over a parking lot. It is the first component of Tc and starts at the
hydraulically most distant watershed point. Sheet flow normally occurs at a
depth of 0.1 ft. or less, and the length of sheet flow rarely exceeds a few
hundred feet. The maximum length of sheet flow is 300 ft.
To calculate sheet flow, you need the following information: a two-year rainfall
amount; length of flow; average slope along flow path; and ground roughness
over which the water is sheeting (measured in Mannings n factor).
The Sheet Flow Time Calculator dialog box is available from the following
dialog boxes:
n
Time of Concentration Calculator
n
Time of Travel Calculator