National Land Cover Data (NLCD)

Step 1: Curve Number Generator method

• Load the Curve Number Generator from the Processing toolbox.

• Set the Extent to Watershed Layer.

• Set the name and path.

• Uncheck the other items.

• Click Run

Step 2: Download the USGS data

• Connect to the NLCD Land Cover data.

• Repair the data Source

• Select the layer and click OK.

• Right click the Land Cover layer and click Export.

• Fill the form and click OK.

• Move the Land Cover layers to the Manning’s group.

• Copy the Style from the NLCD Layer.

• Paste the Style to the Downloaded Layer.

• Note the difference between the Curve Number Generator 2021 and the Land Cover NLCD Server 2024 data.

Step 3: Review look-up tables

Land cover lookup table. The full table is in the Excel Spreadsheet.

• Modify these table per local Flood Control Specifications.

• Manning’s roughness for shallow overland flow developed for FLO-2D. (source: FCDMC)

Step 4: Run NLCD Land Cover Roughness processor

• Open the Processing Toolbox.

• Open the Existing Processor Model.

• Double click the Reclassify module.

• This table may be modified to reflect local Manning’s n value standards.

Note

Every raster value must be explicitly assigned a corresponding Manning’s n value. If one value is omitted, the processor may fail during roughness surface generation.

Hint

Processor models are script-based. Editing the table in a plain text editor is often faster than using the graphical interface.

• Run the processor model to reclassify the land cover data as roughness data.

Step 5: Interpolate Manning’s n to the Grid

• Run the Roughness Raster processor.

• Move the Land Cover and Roughness layers to the Manning’s n group.