Generating a Bankfull Buffer Polygon for Confinement

The Bankfull Buffer Polygon is used in the Confinement Tool for determining where the Active Stream Channel is likely abutting a Valley Margin. These locations are called "Confining Margins".

##Before You Start:

• Download the modified FHC tool.
• Make sure you have Spatial Analysit Installed and Enabled.
• I tend to use .tif files as outputs for Raster Datasets in this process, but any Raster format should work.

Prepare the Inputs

###Stream Network

Make sure the Line Network is:

• Projected
• Z and M Values Disabled
• Singlepart
• Segmented? > The Input parameter says Segmented, but I'm not sure if this is required. If it is, then we need information on what segmentation should be used.

Flow Accumulation Raster in SqKM

The Bankfull Polygon Tool requires a Flow Accumulation Grid in SqKM. Use the following steps to generate one.

1. Create a folder to store working datasets, as well as a scratch folder for temporary files.

2. Obtain a DEM clipped to the watershed.

   We typically use a 10m DEM.

3. Project the DEM using a Bilinear (or cubic?) Resampling Technique using ArcToolbox/Data Management Tools/Projections and Transformations/Raster/Project Raster. > Do not use Nearest as the Resampling Technique. This can introduce artifacts into the Projected DEM.

4. Fill the Projected DEM using ArcToolbox/Spatial Analyst/Hydrology/Fill.

5. Create a Flow Direction Raster from the Filled DEM using ArcToolbox/Spatial Analyst/Hydrology/Flow Direction. > You do not need to save a Drop Raster.

6. Create a Raw Flow Accumulation Raster from the Flow Direction Raster using ArcToolbox/Spatial Analyst/Hydrology/Flow Accumulation

   • Output type should be FLOAT

7. Create a Flow Accumulation Cells Raster by entering the following formula into ArcToolbox/Spatial Analyst/Map Algebra/Raster Calculator:

   • "<Name of Raw Flow Accumulation Raster>" + 1
   • The Raw Flow Accumulation Raster does not consider each cell itself, only the number of cells that flow into each cell (i.e. cells with no accumulation a value of zero). This step will add one cell to each cell in flow Acc Raw to account for each cell, which we will need in order to calculate area in the next step.
8. Finally, Create the SQKM Flow Accumulation Raster
   1. Determine the Cell Size of the Flow Accumulation Cells Raster in meters
   2. Calculate: Cell Area km^2 = (cell size m * (1km/1000m))^2
   3. Use the following formula in Raster Calculator:
      • "Flow Accumulation Cells Raster" * cell area (use the cell area in km^2)

Precip Shapefile

1. Download the Precip Shapefile per USU instructions.
2. Clip this to the area of the watershed.

##Running the Modified Bankfull Channel Tool

Remember, you are trying to create a Buffered Bankfull polygon that estimates the potential edges of the active margins of the stream channel. You are not trying to create a model of bankfull based on stream area. This approach is used to account for uncertainty in:

• DEM (10m Cell Resolution)
• Digitization of the NHD Network relative to the DEM.
• Overestimation of Valley bottom widths
• Estimation of Bankfull width values.

###Input Recommendations:

• Minimum Width: Use a large enough size to cover the typical minimum width of headwater valley bottoms, but not too large to mask potential flood plain pockets in these locations. In many watersheds, 20-25m is an ok value to use.

• Percent Buffer: Use 100 percent to effectively double the calculated bankfull width. This places the Active channel (bankfull) edges at up to a bankfull width away from the stream network in each direction.

Assessing the Bankfull Buffer Quality

1. Add the Valley Bottom Polygon to the map. Optionally, you may add a Basemap Image to the map to compare the stream visually.

2. Determination of Confinement is straightforward. Any location where the bankfull Polygon falls outside of the Valley Bottom polygon defines a confining margin. Scan through the map and assess:

   • Wide Valleys are likely to be unconfined to partially unconfined. The buffer should likely 'bump' into the valley bottom as in snakes down the valley. You want to capture these confining margins!
   • Most of the headwaters should be considered "confined" and therefore the bankfull buffer should cover most of the valley bottom in these locations (use the Minimum Width value to adjust this). While it is important to cover the Valley Bottoms in these areas, avoid using too large of a value so you do not miss flood plain pockets in these locations.