Chapter 8 Level 3 Workflow

This chapter describes the workflow steps necessary to complete a Level 3 (L3) FluvialGeomorph analysis. The purpose of this level is to extract planform dimensions.

8.1 Define Valley Line

The purpose of this stage is to define the valley trend line for the base year for each reach.

8.1.1 Determine Final Floodplain Extent

The purpose of this step is to use the detrended bankfull elevation identified in L2 to delineate the active floodplain.

  • From the final Estimate Bankfull Report created in Level 2, use the detrended bankfull elevation identified. In the example from Level 2, this value was 104 ft.

  • Determine the bankfull depth: The detrending process used in FluvialGeomorph expresses stream elevations using a base index value of 100 ft. Therefore, a detrended bankfull elevation estimated to be 104 ft. means that at bankfull discharge, the water surface elevation is 4 ft. above the detrended stream base index value of 100 ft. for this reach:
    104 ft detrended bankfull elevation - 100 ft detrended stream base index = 4 ft bankfull depth

  • Determine flood prone height: Flood prone has been determined through empirical studies to be approximately two times the bankfull depth. For example, using the bankfull depth calculated in the last step, calculate the flood prone depth using the following formula:
    2 * 4 ft bankfull depth = 8 ft flood prone height

  • Therefore, the flood prone detrended elevation would be 108 ft:
    100 ft detrended stream base index + 8 ft flood prone height = 108 ft detrended flood prone height

  • With the detrended flood prone elevation for your reach determined in the last step, use the 08 - Water Surface Extent tool to extract a flood prone area polygon. This tool creates a new polygon feature class named banks_raw_xxx, where xxx is the detrended elevation selected.

  • This feature class must be edited to select the flood prone 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 selecting gridcode = 1 polygons with the largest area until the entire flood prone area is selected.

  • Export these selected features to a new feature class named flood_prone.

  • Delete the banks_raw_xxx feature class created in this section.

8.1.2 Develop Candidate Valleylines

The purpose of this step is to create a set of candidate valleyline feature classes through iterative smoothing of the flowline for each reach.

  • The following steps are performed in the site geodatabase for the base year. This is done to ensure that a valleyline feature class is created smoothly through all of the reaches in a site.
  • In the site geodatabase for the base year, use the ESRI Smooth Line tool to smooth the site flowline feature class.
  • Choose the PAEK smoothing algorithm.
  • For the first candidate valleyline, use a Smoothing Tolarance value of 200 meters. In this case you would name the output feature class valleyline_200
  • Repeat the previous step using the Smoothing Tolerance values such as 400, 800, and 1000.
  • Depending on the detail of the flowline feature class geometry, you may need to select a different range or set of step values of the Smoothing Tolerance parameter values to evaluate.
  • The goal is to create a wide degree of flowline smoothing in the candidate valleyline feature classes to select from in the next step.

8.1.3 Choose Final Valleyline

The purpose of this step is to choose from the candidate valleyline feature classes the one that best captures the overall valley trend line.

  • The goal of this step is to select from the candidate valleyline feature classes the one that is the most smoothed, yet still fits primarily within the flood_prone polygon extent.
  • Add the flood_prone polygon and all of the candidate valleyline feature classes to the current map.
  • Identify the valleyline feature class that has the highest smoothing tolerance value that also is mostly contained within the extent of the flood_prone polygon.
  • In the site geodatabase of the base year, rename the selected valley line to valleyline.
  • Delete the unused candidate valleyline feature classes from the site geodatabase of the base year.
  • Copy the valleyline feature class you just created from the site geodatabase to each of the base year reach geodatabases.

8.2 Define Meander Loops

The purpose of this stage is to define meander loops and bends for the base year for each reach.

8.2.1 Define Loop Points

The purpose of this step is to create a new feature class named loop_points and use it to identify the start and end stream meander loops and bends.

  • Create a new point feature class named loop_points. Enable z and m values. The feature class should contain the following fields:
    • ReachName: Text (50) - The purpose of this field is to store the reach name.
    • loop: Long Integer - The purpose of this field is to store the loop unique identifier for the point.
    • bend: Long Integer - The purpose of the field is to store the bend identifier for the point.
    • position: Text (10) - The purpose of this field is to store position identifier for the point. This field must have one three values: “start,” “end,” or “apex.”

Create Loop Points

  • loop_points features are used to define the start and end location of loop bends and the location of a loop’s apex.
  • loop_points features are always placed along (i.e., snapped to) a bankline feature.
  • Begin numbering loops starting at the downstream end of the reach and increment the loop integer values moving upstream.
  • In FluvialGeomorph, the valleyline is used segment the flowline into loops.
  • The beginning and end of loops are delineated where the valleyline definitively crosses the flowline.
  • If the flowline crosses the valleyline and then crosses back over in a short distance, this is not considered a definitive crossing. A definitive crossing is one where the flowline approaches the valleyline, crosses it, and then continues to move away from the valleyline for a significant longitudinal distance.
  • Loops must alternate from one bank to the opposite bank. For example, if loop 1 is delineated along the right descending bank, then loop 2 is delineated along the left descending bank.
  • Loops are composed of one or more bends (bends nest inside of loops). For example, loop 1 can have bends 1, 2, and 3 (i.e., loop 1, bend 1; loop 1, bend 2; loop 1, bend 3).
  • Bend numbering restarts within each loop. The first bend value within a loop always begins with the value 1.

Multiple Reaches:

  • For a site with multiple reaches, loops must be uniquely numbered across all reaches. The loop field values of loop_points should not repeat within the reaches of a site.
  • The downstream-most loop in the site should be numbered starting with the loop field value of 1 and increase moving upstream.
  • Set the loop field value for each upstream reach to the upstream-most value (i.e., the highest loop value of the downstream reach’s loop_points feature class) of the downstream reach. For example, set the value of the field loop of the first Reach-2 loop_points feature to 8 if the maximum value of Reach-1’s loop_points feature class loop field is 7.

Check Loop Points:

  • Use the Check Loop Points tool in the Check toolset to verify that loops and bends are defined correctly.
  • Review the messages produced by this tool to identify and correct problems with loop and bend delineation.

8.2.2 Derive Bankline Points

The purpose of this step is to convert the banklines to a set of bankline_points and assign elevations and loop and bend locations to these features.

  • Use the 14b - Bankline Points tool to assign loops, bends, elevations and valley line positions to the bankline_points.
  • When the tool completes, check that this tools runs correctly by running the Check Bankline Points tool in the Check toolset.
  • The check tool ensures that the loop, bend, position, valley_POINT_X, valley_POINT_Y, and valley_POINT_M fields are populated. Only bankline_points features within loops and bends will have values in the loop, bend, and position fields.
  • All records should have values in the valley_POINT_X, valley_POINT_Y, and valley_POINT_M fields.
  • Additionally, apply the following definition query to the bankline_points feature class: “loop IS NOT NULL.” Only bankline_points features between loop_points features should be visible.

8.2.3 Assign Cross Section Loops

The purpose of this step is to assign loop and bend identifiers to regularly spaced and riffle cross section feature classes for each reach.

8.3 Calculate L3 Cross Section Geometry

The purpose of this stage is to calculate the Level 3 dimensions for each reach.

8.3.1 Calculate Cross Section L3 Dimensions

The purpose of this step is to calculate the L3 dimensions for the regularly spaced and riffle cross section feature classes for each reach.

  • If any *_dims_L3 feature classes exist in the reach geodatabase (created during a previous run), delete these feature classes.
  • Use the 15c - XS Planform, Level 3 tool to calculate the L3 dimensions.
  • Verify in the attribute table that the L3 dimensions have been calculated.

8.3.2 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.

8.4 Run Report

The purpose of this stage is to run the Level 3 report for each reach.

8.4.1 Run the L3 Report

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

  • In the Reports toolset, use the Level 3 Report tool to produce the Level 3 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_L3 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.

8.4.2 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 3 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.

8.5 Determine Next Steps

The purpose of this step is to determine what further steps need to be taken.

  • Review the results of the Level 3 Report and determine if the project goals require further analysis.