Create → Channel Geometry Grids → With Area Grid

This dialog allows you to create grids for several key channel properties by parameterizing them in terms of a contributing area grid, denoted as A. The channel cross-section is modeled as a trapezoid, with bottom width, w, bank angle, a, and a Manning roughness parameter, n. The cross-section is triangular when w = 0, and is rectangular when θ = 0.

The curve-fitting formulas shown have been selected for their ability to fit widely observed trends in hydraulic variables.

Enter measured values into the bottom table and click on the button labeled "Update c, b and p". The parameters c, b and p will be updated in the top table to give the best fit to the measured values. Note that the rows in the bottom table are labeled Overland, Transition and Channel. The Overland row represents the hillslopes and the endmember case where A=0. The Transition row represents an approximate threshold area where hillslopes end and channelized flow begins. The Channel row represents values for a well-defined channel in the model basin that has a large contributing area.

The bed width grid is special in the sense that for hillslope pixels it is used to model overland flow. The value for hillslope pixels is therefore determined by the grid spacing and you should leave the width in the Overland row set to zero. Technically, there should be no channels below the threshold area.

A Manning's N value of 0.3 is often used for hillslopes (A=0), while for channels, N is typically between 0.01 and 0.05, with 0.03 being a typical middle-range value. N varies with the bed material and may be modeled by Strickler's formula:

N = D501/6 / 21.2,

where D50 is median grain size in meters.

The value of p for bed width should be close to 0.5, and the value of p for Manning's N should be negative. The value of p for bank angle can be set to 0 to indicate that bank angles do not change with increasing area. Bank angles should never be set to 90 degrees and should be set to 0 on hillslopes.