Tuflow and estry manual Version 3

3.11.4Building the Bed Resistance (Materials)

The bed resistance values for 2D domains are created by using GIS layers containing polygons of different bed resistance. Bed resistance formulation is set to one of Manning’s n, Manning’s M (=1/n) or Chezy using Bed Resistance Values in the .tcf file. The default is Manning’s n. Chezy can be specified as a direct value or by bed ripple heights. Manning’s n is the only supported option. The other formulations were developed for coastal applications and have not been fully tested on the PC version of TUFLOW.

The bed resistance values are spatially located at the cell centres. Values at the cell mid-sides (where the momentum equation is applied) are interpolated from the cell centre values. Future releases may allow direct input of bed resistance values to the cell mid-sides.

The most common approach is to digitise one or more materials layers (2d_mat) and assign Manning’s n values to the materials using Read Materials File and a .tmf file. This approach allows the easy adjustment of Manning’s n values during model calibration.

In creating the base 2d_mat layer, it is good practice to not digitise the most common or primary (most difficult to digitise) material and use the following commands in the .tgc file.

• 
  Use Set Mat to set the most common material to all cells in a 2D domain.
  
• 
  Use Read MI Mat to allocate the remaining material values.
  

The Read MI Mat command may be used as many times thereafter to further modify the materials in parts of a 2D domain.

If using the Chezy formula, a number of commands have been setup to provide backward compatibility. These are Depth/Ripple Height Factor Limit and Recalculate Chezy Interval.