This will compute channel depth, flow and velocity based on channel parameters such as side slopes, base dimensions and Manning's n value. It handles triangular, trapezoidal , rectangular and irregular channels. Entry of a depth leads to calculation of flow and velocity. Entry of one of the other items (flow or velocity) will lead to calculation of the remaining items. In addition to functioning as a channel calculator, the program will output a typical section or detail of the channel as well as a report of the channel output. The routine also works in metric units. It applies to non-erodible channels, primarily. The user can select the Report button to output a report of the channel input and output values as well as a standard detail shown below for the above example.
When the routine is selected, the dialog box shown below appears. Select, for example, a trapezoidal channel, equal sides, with side slopes of 3 (for 3:1) and a base dimension of 16. Enter the Manning's n value and channel slope as shown. Note that 0.1, in English units, means 0.1 foot drop per 100 foot of length. Then at the lower right, plug a value of 4.5 for the depth. This will calculate a flow of 862 CFS and a velocity of 6.5 fps, as shown below. If the channel is divided into two types of materials (paved lower portions and vegetated upper portion), you can specify a second Manning's n for the upper banks, as shown in the dialog.
To use the routine as a calculator, enter the known
value in the lower right area of the dialog (flow, depth, or
velocity), then press enter while still in the entered item. The
other two items are then calculated. Note that the routine will
default to the last values used during the current Carlson work
session, and will capture the flow values calculated in Water
Runoff under the Watershed Pull-down. When entering the Manning's n
value, a table of n values can be brought up and an appropriate
Manning's selected. Among the output options is "Draw Channel
Detail", which will draw and annotate as shown below. Since
Open Channels are often used as emergency spillways, the Write
Stage/Discharge File will output an .STG file for use in the
command Locate Structure under Watershed, for pond design and storm
routing and hydrograph calculations.Use Drawing Setup to activate Metric or English outputs. If English is configured, the formula v=(1.486/n)(R^2/3)(s^1/2) is used, where n is the Manning's value, R is the water cross section divided by the wetted perimeter and s is the slope ratio. If Metric is configured, the formula becomes v=(1.0/n)(R^2/3)(s^1/2) and outputs are in meters. To test metric, set to metric in Drawing Setup. Then for a rectangular, concrete open channel of 12.0 meters width, slope of 0.28%, Manning's n of 0.013 and depth of 2.5 meters, you should compute a velocity of 5.94 m/s and Q (flow) of 178 cubic meters/second.
Channel Design (Manning's Eq) dialog: Fill in values.
Pulldown Menu Location: Structure in Hydrology