14 Level-2 Bankfull Analysis Workflow

🔧 Practical 📐 Theory

Level 2 Workflow

This chapter describes the workflow steps necessary to complete a Level 2 (L2) FluvialGeomorph analysis. The purpose of this level is to calculate bankfull channel dimensions.

Create Bankfull Area

The purpose of this step is to use the final bankfull elevation determined in the previous step to derive a final bankfull_area polygon for each reach.

From the final Estimate Bankfull Report, use the detrended bankfull elevation identified in the last step. In the example above, this value was 104 ft.
Use the 08 - Water Surface Extent tool to extract a bankfull area polygon. This tool creates a new polygon feature class named banks_raw_xxx, where xxx is the detrended elevation specified.
The banks_raw_xxx feature class must be edited to select only the polygons that represent the bankfull area polygon(s).
Open the attribute table for the banks_raw_xxx feature class and use advanced sorting to sort first by gridcode and then by Shape_Area.
Polygons with gridcode = 1 are polygons inundated at the detrended elevation.
Typically, the polygons with the largest area represent the channel.
Begin by selecting gridcode = 1 polygons with the largest area until the entire bankfull area is selected.
Export these selected features to a new feature class named bankfull_area.
Delete the banks_raw_xxx feature class.

Create Banklines

The purpose of this step is to convert the bankfull_area polygon into polylines and edit into a banklines feature class for each reach. The banklines feature class will have two records, one representing the left descending bankline and another the right descending bankline for the stream reach.

Use the ESRI Polygon To Line tool to convert the bankfull_area polygon feature class to a new polyline feature class named banklines.
Add the following fields to the banklines feature class:
- ReachName: Text (50) - The purpose of this field is to store the reach name.
- bank_id: Long Integer - The purpose of this field is to uniquely identify each bank.
- bank: Text (50) - The purpose of this field is to designate which bank is the right descending bank and which is the left descending bank.
Start editing the banklines feature class.
Use the ESRI Explode Multipart Feature tool to explode any multipart line feature in the banklines feature class.
Add the flowline feature class to the map.
Use the ESRI Split Tool to split the line features at the downstream and upstream ends of the flowline feature class.
The features in the banklines feature class should not extend past the end of the flowline feature.
Use the ESRI Split Tool to trim any tributaries from the banklines feature class.
The goal of this step is to have only two features, one representing the left descending bankline and another the right descending bankline. Delete all other line features.
Ensure that the banklines are digitized in the upstream direction (like a flowline feature). Edit the banklines feature class to ensure that each bankline is digitized beginning with the downstream end and digitized upstream.
While editing, select a bankline feature, choose to edit vertices. Ensure that the red endpoint is at the upstream end of each bankline.
Use the “Reverse Direction” command (aka flip) to ensure each bankline is digitized in the upstream direction.
Check that each bankline is digitized in the upstream direction (red endpoint at the upstream end) before going on to the next step.
In the ReachName field enter the reach name.
In the bank field, enter the string right descending or left descending to designate which bank each line represents.
In the bank_id field enter a 1 for the right descending bank and 2 for the left descending bank.

Create Final Centerline

The purpose of this step is to create a stream centerline. The centerline represents the rough midline of the stream between the banklines.

Use the 10 - Centerline tool to create a centerline polyline feature class representing the stream midline at the bankfull water surface elevation.

Calculate Final L2 Cross Section Geometry

The purpose of this stage is to use the bankfull elevation determined in the last stage to calculate the final level 2 cross section dimensions for the base year for each reach.

Calculate Final Cross Section L2 Dimensions

The purpose of this step is to calculate the final L2 dimensions for the regularly spaced and riffle cross section feature classes for each reach. Repeat the following steps for regular spaced cross section feature class, the riffle_floodplain feature class, and the riffle_channel feature class.

Determine the moving window size
Many stream metrics are scale dependent, meaning these metrics are affected by the size of the moving window used in their calculation. To determine the appropriate size of the moving window for this reach, use the following steps:

Many stream metrics are typically calculated using a moving window size equal to two meander wavelengths.
Using the bankfull_area feature class, estimate the typical bankfull width for the reach.
Estimate the length of two meander wavelengths by multiplying the bankfull width estimated in the last step by 10 (e.g., 30ft bankfull width * 10 = 300ft, two meander wavelengths).
Determine how many cross sections two meander wavelengths represent.
For riffle cross section for example, if they are spaced about 300ft apart, then two meander wavelengths would be 1 riffle cross section (i.e., 300ft / 300ft between riffle cross sections).
For regular cross section for example, if they are spaced about 100ft apart, then two meander wavelengths would be 3 regular cross sections (i.e., 300ft / 100ft between regular cross sections).

Calculate Final L2 Dimensions

Delete the initial Level 2 dimensions *_dims_L2 feature classes created earlier in Level 2.
Use the 15b - XS Dimensions, Level 2 tool to calculate final L2 dimensions.
Set the xs_fc parameter to the cross sections feature class.
Set the lead_n parameter to the number of upstream cross sections that you calculated in a previous step depending on the type of cross section (i.e., regular cross section, riffle cross section).
If the elevations in the channel seem noisy, check the use_smoothing parameter and set the loess_span parameter to a value between 0-1.
Confirm that the vert_units of the DEM are in feet.

Confirm the degree of smoothing

Use a chart to verify the choice of the smoothing loess_span parameter in the *_dims_L2 feature class.
Right-click on the *_dims_L2 feature class in the map table of contents and select “Create Chart”, and select “Line”. In the Date or Number dropdown, choose the field POINT_M. In the Aggregation dropdown, choose None. In the Numeric field(s) checklist, check the boxes next to Z and Z_smooth. Click the Apply button to view the chart.
Visually assess the degree of smoothing. The smoothing should be high enough to eliminate LiDAR elevation noise, but not so high as to eliminate meaningful channel elevation change.
If the smoothing is not ideal, re-run the tool and adjust the loess_span parameter.

Add Modeled Water Surface Elevation

The purpose of this optional step is to add an HEC-RAS hydraulic model estimated water surface elevation (if available) to a cross section dimension feature class.

Use the 16 - XS RAS Watersurface tool to add a modeled water surface elevation field to the input cross section feature class.
Use the RAS_model_name parameter to distinguish between multiple RAS model scenarios.

Run Report

The purpose of this stage is to produce the Level 2 report for each reach.

Run the L2 Report

The purpose of this step is to run the L2 report for each reach.

In the Reports toolset, use the Level 2 Report tool to produce the Level 2 Report.
For the stream parameter, use the value of the ReachName field used in the flowline feature class.
For the flowline_fc parameter, enter the flowline feature class for the base year survey.
For the xs_dims_fc parameter, use the *_dims_L2 feature class calculated for the regular cross sections of the base year.
The xs_points_* parameters should be entered with the feature class for the most recent survey first (i.e., the base year) and then the previous surveys in reverse chronological order (e.g., 2016, 2010, 2006).
The survey_name_* parameters are used to label the surveys in maps and graphs.
The feature classes and labels used for the flowline_points_*, xs_points_*, and survey_name_* parameters must be entered in the same order (e.g., 2016, 2010, 2006) in each set of numbered parameters.
For the dem parameter, enter the DEM for the base year survey.
For the banklines_fc parameter, enter the banklines feature class created for the base year survey.
For the features_fc parameter, enter the features feature class for the base year survey.
For the bf_estimate parameter, specify the detrended elevation value that represents the bankfull water surface elevation.
For the regions parameter, select the regions to use for estimating the bankfull water surface elevation.

Perform QA

The purpose of this step is to use the QA Checklist to verify the reports have run correctly and identify any data mistakes that need to be corrected.

Follow the instructions in the QA Checklist Chapter, Level 2 Report section, to verify that the reports have run correctly.
Make the required changes suggested in the QA Checklist and rerun the report.
Repeat these QA iterations until the reports are correct.

☑️ Evaluate ## Determine Next Steps The purpose of this step is to determine what further steps need to be taken.

Review the results of the Level 2 Report and determine if the project goals require proceeding to developing the Level 3 analysis.