Template:Chezy Friction

Chezy Friction

The Chezy Friction tool is used to populate the spatial attributes for the Chezy friction coefficient at the sea floor. A new dataset is created from NLCD, C-CAP, or other land use rasters.

The tool will extract values from the raster object specified by the tool. For each node in the geometry, the "area of influence" is computed for the node. The area of influence is a polygon that represents the area of the geometry around a node. This is computed as the area encompassing the node and half the distance to each of his neighboring nodes. All of the raster values within the area of influence are extracted from the visible raster objects. A composite Chezy friction value is computed taking a weighted average of all the raster values. If the area of the extracted values is less then the area of influence around the node (because the process passed the extent of the raster objects or encountered null values), the default value is used to compute the Chezy friction using the area not represented in the values.

The Chezy Friction tool dialog contains the following options:

• Input landuse raster – This is a required input parameter and should cover the entire target grid. Specify which raster in the project to use when determining the Chezy friction.
• Landuse raster type – This is a required parameter. Specify what type of landuse raster to use.
  • "NLCD" – Sets the landuse raster type to National Land Cover Dataset (NLCD). A mapping table file for NLCD can be found here.
  • "C-CAP" – Sets the landuse raster type to Coastal Change Analysis Program (C-CAP). A mapping table file for C-CAP can be found here.
  • "Other" – Sets the landuse raster type to be set by the user. This adds an option to the dialog.
    • Landuse to Chezy friction mapping table – The Select File... button will allow a table file to be selected. The entire file name will be displayed in the text box to its right.
• Target grid – This is a required input parameter. Specify which grid/mesh the canopy coefficient dataset will be created for.
• Default Chezy friction option – Set the default value to use in computing the Chezy friction using the area not represented in the values. This can be set to "Constant" to use a constant value or set "Dataset" to select a dataset to use.
  • Default Chezy friction value – Enter the a value to use as a constant default value.
  • Default Chezy friction dataset – Select a dataset to use as a default value.
• Subset mask dataset – This optional option allows using a dataset as a subset mask.
• Output Chezy friction dataset – Enter the name for the new Chezy friction dataset.

If the landuse type is chosen as NLCD or C-CAP, the following default values are used in the calculation. If there are different landuse raster types, or wishing to use values that differ from the defaults, specify the raster type as Custom and provide in CSV file with the desired values.