NCED Stream Restoration Toolbox

Channel Planform StatisticsAn ArcMAP Project

By: J. Wesley Lauer

February 14, 2006

The Stream Restoration Toolbox

The Stream Restoration Toolbox consists of current basic research cast into the form of tools that can be used by practitioners. The details of a tool are presented through a PowerPoint presentation, augmented by embedded Excel spreadsheets or other commonly available applications. The toolbox is a vehicle for bringing research findings into practice.

While many tools are being developed by NCED Researchers, the opportunity to contribute a tool to the Toolbox is open to the community. For more information on how to contribute please contact Jeff Marr at marrx003@umn.edu.

Statement of liability and usage

This tool is provided free of charge. Use this tool at your own risk. In offering this tool, the following entities and persons do not accept any responsibility or liability for the tool’s use by third parties:

• The National Center for Earth-surface Dynamics;

• The universities and institutions associated with the National Center for Earth-surface dynamics; and

•The authors of this tool.

Users of this tool assume all responsibility for the tool results and application thereof. The readers of the information provided by the Web site assume all risks from using the information provided herein. None of the above-mentioned entities and persons assume liability or responsibility for damage or injury to persons or property arising from any use of the tool, information, ideas or instruction contained in the information provided to you.

Title PageTitle Page

Tool Title: Channel Planform Statistics ToolboxTool Author: J. Wesley LauerAuthor e-mail: lauerj@seattleu.edu

Version: 2.0Associated files:

1) PlanformStatisticsTools 2.0.ppt2) PlanformTools.esriAddIn

Date: April 2012

• Introduction to the Tool

• Background

• Installation

• Tool 1: Interpolate Centerlines From Two Bank Lines

• Tool 2: Lateral Distance Measurement

• Tool 3: Bank Buffer Boxes

Outline of this Document

The current availability of digital aerial photography allows for the relatively straightforward comparison of historic and recent imagery of river channels. However, making quantitative measurements of planform characteristics such as width, curvature, and channel migration rate, while not difficult, can be time consuming. These tools automate several of the more time consuming aspects of these measurements at discrete points along a single-thread river.

This image of the Bogue Chitto River, Louisiana, shows two channel centerlines developed from bank lines digitized by hand from aerial photography.

IntroductionIntroduction

BackgroundBackgroundThe channel planform statistics toolbox 2.0 is an ESRI ArcGIS 10.x add-in that provides most of the functionality available in the initial ArcGIS 8 and 9 releases of the planform statistics toolbox. The tools perform three primary functions. 1) Interpolate the centerline of two lines (i.e. two banks that have been digitized by hand from an aerial photograph). Width and local radius of curvature at each evenly-spaced point along the centerline are saved in a text file. 2) Estimate the mean lateral normal distances at even increments between two lines developed using tool 1 (i.e. between river channel centerlines at two points in time), and 3) generate a polygon shapefile of boxes adjacent to the channel banks that correspond with a particular centerline point. These boxes are useful if the user wishes to correlate a bank property with one of the observed statistics.

This powerpoint presentation presents rudimentary instructions for installing and using the tools. While the instructions are basic, the tools are fairly intuitive so that the user should be able to develop useful information with only a minimal amount of trial and error.

InstallationInstallation

The Planform Statistics Toolbox for ArcGIS 10 is released as a complied ESRI extension file. To install on a computer that has a valid ArcGIS 10.x installation, simply copy PlanformToolsX.esriAddIn to any folder on the computer and then double click on the file. ArcGIS will install the program automatically.

The tools are the accessed by opening ArcGIS and using the customize menu to add the buttons for the Add-in to any existing toolbar.

The buttons for the tools are as follows:

Centerline Interpolator

Bank Buffer Boxes

Lateral Offset Measurement

Tool 1: Interpolate Centerlines From Two Tool 1: Interpolate Centerlines From Two Bank LinesBank Lines

This tool finds evenly spaced points that are representative of the center of two roughly parallel lines. It then connects these points into a new line. The algorithm used to solve for the points works as follows: The program creates a point a user specified distance from the previous point. It then varies the angle θ until the distance between the closest point on each respective bank line and the new point is nearly equal. (i.e. a = b). This results in a relatively smooth centerline made up of evenly spaced points. The new line is stored in a new shapefile.

Initial

Final

b

a

b

a

Tool 1: Interpolate Centerlines From Two Bank Tool 1: Interpolate Centerlines From Two Bank Lines (cont.)Lines (cont.)

The tool prompts the user to select the two lines between which the centerline is to be interpolated. These lines should be oriented in the same direction. It is recommended that the user represent bank lines with a symbol that includes an arrow oriented in the downstream direction to ensure that this is the case. It then asks for the spacing between the interpolated points. The program seems to run reasonably well when the spacing is on the order of about half a channel width. For smaller spacings,, it is possible for the interpolated centerline to turn back on itself.

The tool is relatively stable in that few geometries cause it to fail. However, it sometimes extends the last segment past the bank lines.

Channel width and other geometric information is stored in a textfile with a name that corresponds to the shapefile used to store centerline information.

To use the tool, select the following icon from the toolbar:

The tool operates using essentially the same algorithm as in the old ArcGIS 8 and 9 releases. However, the output textfile now contains additional the coordinates of the left bank and right bank associated with the given centerline point. Output columns are:

OID: Object IDm: Down-channel distancewidth: Local width (a + b from slide 9)theta: Local angle of downstream segment, θi

dtheta: Local increment in angle, θi-θi-1

r_curve: A first-order estimate for local radius of curvature, Δs/dthetaA value of -99999 is stored for dtheta = 0.

cl_x and cl_y: x and y coordinates of the centerline pointleft_x and left_y: x and y coordinates of the left bank pointright_x and right_y: x and y coordinates of the right bank point

Note that local curvature (dθ/ds) can be estimated using a simple Euler scheme: dθ/ds = (θi – θi-1)/Δs. This is safer than taking the inverse of r_curve because of the way -99999 is stored for points with no curvature. In any case, for most bank geometries, there is significant local scatter in a first-order curvature estimate. It is recommended that higher order schemes be used or curvature be smoothed prior to additional analysis.

Tool 1: New Features in Version 2.0Tool 1: New Features in Version 2.0

θi

θi-1

Δs

Centerline interpolated Centerline interpolated from two bank lines using from two bank lines using Tool 1.Tool 1.

(1952 photograph, (1952 photograph, Pearl River, Louisiana/ Pearl River, Louisiana/ Mississippi.)Mississippi.)

Tool 2: Lateral Distance MeasurementTool 2: Lateral Distance MeasurementThis tool finds the average lateral normal distance between the nodes interpolated using tool 1 and a second line. In the ArcGIS version 8 and 9 tools, the distance was found using a best fit Bezier curve to represent the most likely path of migration for a particular point. In the ArcGIS 10 tool (PlanformTools2.0), the tool no longer uses Bezier curves and instead fits straight line segments to three intermediate centerlines interpolated between the input centerlines. This change significantly decreases computational time. As in the ArcGIS 8 and 9 versions, output is stored in polygon shapefile.

To use this tool, select the following icon from the toolbar: The tool asks for the line to which distances are to be measured, and then for the reference line. It will also prompt the user for a path and name for the new shapefile.

Updates to Tool 2 in Version 2.0Updates to Tool 2 in Version 2.0Basic input for the new version of the tool includes two centerlines, shown in red and dark blue in the adjacent illustration. The tool then splits each of these lines into a series of segments at the mid-point between intersections (red dots). For each set of bounding segments, an intermediate segment is found by averaging the x- and y- coordinates a given fraction into each segment. The process is repeated with each respective bounding segment and the interpolated centerline, resulting in three intermediate centerlines. Because of its simple nature, the algorithm sometimes results in changes in migration direction near the points of intersection of the bounding centerlines. However, migration rates in these areas are usually very small.

Updates to Tool 2 in Version 2.0Updates to Tool 2 in Version 2.0Rather than being represented using Bezier curves (as in the ArcGIS 8 and 9 versions of the toolbox), trajectories are now defined as a set of four straight line segments that connect the older and newer centerline and are approximately normal to each of the intermediate centerlines. The segments are constructed starting at the nodes of one of the input bounding centerlines (shown in dark blue—usually the newer centerline). Each trajectory extends to the nearest point on the first intermediate centerline, then to the nearest points on the second and third intermediate centerline and finally to the nearest point on the other bounding centerline (shown in red)..

Tool 2: Lateral Distance Measurement (cont.)Tool 2: Lateral Distance Measurement (cont.)

Channel Centerline at t

Channel Centerline at t +Δt

For bends that translate primarily downstream without changing form, the trajectory of outward normal migration would change direction, as shown below. In this case, the program has the capability of estimating the short-term outward normal migration rate for the reference line (usually the new centerline position). In the ArcGIS 8 and 9 versions of the program, this was achieved using Bezier Curves. The ArcGIS 10 version achieves this simply by changing mechanism for finding intermediate centerlines, as described on the next pages.

Tool 2: Lateral Distance Measurement (cont.)Tool 2: Lateral Distance Measurement (cont.)To force downstream migration in intermediate centerlines, the program first prompts the user for a shapefile that represents the set of all bend apex trajectories for such bends. The shapefile should be a line shapefile in which, for all bends that translated primarily downstream, straight lines connecting the apex of the older and newer bend has been digitized. This line should be snapped to each of the respective centerlines.

Apex trajectory line. Such lines should be digitized in a separate shapefile before running the distance measurement tool. The lines should be oriented toward the newer centerline, snapped to each respective centerline, and their forward projection should not intersect the newer centerline. If a line is not present at a particular downstream-translating apex, trajectories are computed as described previously.

Channel Centerline at t

Channel Centerline at t +Δt

Updates to Tool 2 in Version 2.0Updates to Tool 2 in Version 2.0The tool represents downstream shifting bends by prompting the user for the trajectory of the bend apex. It then modifies the intermediate centerline interpolation procedure to force the intermediate centerlines to fall on the trajectory. Trajectories are then fit between the two bounding centerlines as described previously, but using the modified intermediate centerlines. The short-term migration rate and direction for the most recent centerline near the bend apex can then be computed based on the length of the newest of the four line segments making up the trajectory of migration (stored in the output polygon shapefile).

Centerline Interpolation on Downstream Translating Bend

Trajectories near Translating Bend

Output from Tool 2 in Version 2.0Output from Tool 2 in Version 2.0The length and sign of each of the four segments making up each migration trajectory are stored in a polygon shapefile. Polygons are centered on the nodes of the centerline the user specifies for storing data (usually the newer centerline). This line serves as the point of origin for trajectories and is shown here in dark blue. A trajectory segment has a positive sign if it extends to the right of the polyline for storing data. (Note that if the user specifies the newer centerline for storing data, then a segment is positive if migration was to the left with respect to the downstream direction.) The down-channel distance for each trajectory on both older and newer centerlines is also stored.

Mig_dist total of Mig_1 through Mig_4i index of apex on centerline used to store datam down-channel distance of trajectory’s origin on centerline used to store dataold_m down-channel distance of trajectory’s end point on other centerlineMig_1 length of 1st segment in migration trajectory (nearest to centerline used to store data)Mig_2 length of 2nd segment in migration trajectoryMig_3 length of 3rd segment in migration trajectoryMig_4 length of 4th segment in migration trajectory (farthest from centerline used to store data)

Output Columns are:

(1952 photograph, (1952 photograph, Pearl River, Louisiana/ Pearl River, Louisiana/ Mississippi.)Mississippi.)

Example usage of Example usage of tools 1 and 2:tools 1 and 2:Centerline interpolated from Centerline interpolated from two bank lines using Tool 1, two bank lines using Tool 1, older photograph.older photograph.

Modern (1998) aerial Modern (1998) aerial photographphotographCenterline interpolated from Centerline interpolated from two bank lines using Tool 1, two bank lines using Tool 1, more recent photograph.more recent photograph.

(1998 photograph, (1998 photograph, Pearl River, Louisiana/ Pearl River, Louisiana/ Mississippi.)Mississippi.)

Results of tool 2: Measured Results of tool 2: Measured lateral migration distances at lateral migration distances at evenly spaced intervals. evenly spaced intervals. (1952 (1952 centerline was used as the “to” centerline was used as the “to” centerline so that measurements centerline so that measurements would be stored at even increments would be stored at even increments along the 1998 centerline.). Note that along the 1998 centerline.). Note that this image was created using the this image was created using the ArcGIS Version 8 tool. Results would ArcGIS Version 8 tool. Results would be similar using the version 10 tool.be similar using the version 10 tool.

This tool finds areas on each bank that correspond with a particular channel centerline point developed using tool 1 (and possibly migration distance computed using tool 2) and saves these as two polygon shapefiles (one each for the left and right banks, respectively). A polygon shapefile representing the union of each set of boxes and the region within the channel corresponding with that set is also saved.

Left Buffer Boxes

Right Buffer Boxes

Union Boxes

Input Bank and Centerline Geometry

Tool 3: Bank Buffer BoxesTool 3: Bank Buffer Boxes

Tool 3: Bank Buffer Boxes (cont.)Tool 3: Bank Buffer Boxes (cont.)The tool creates a sampling corridor of a user-specified width on the upland side of two bank lines. The corridor is then subdivided by projecting evenly spaced lines outward from the centerline (which should be composed of equally long line segments-as is the case for a centerline developed using Tool 1) until they intersect the offset bank lines. In most cases, the subdividing lines are projected normal to the centerline. However, where channel curvature is greater than a threshold value (10° is suggested), the inwardly projected subdividing line is terminated at the point on the outer edge of the sampling corridor nearest to that subdividing line’s origin. This ensures that subdividing lines do not miss the edge of the sampling corridor on the inside of sharp bends. If the above criteria causes two adjacent subdividing lines to intersect inside the outer edge of the sampling corridor, the downstream line is rotated about its origin on the centerline until the intersection occurs at the edge of the sampling corridor.

To use this tool, select the following icon from the toolbar: The user will first be prompted to select the centerline upon which the boxes will be based. The user is then prompted for the buffer thickness and is asked to select two separate sets of bank lines, each of which should be visible in the current view. (The banks should have been digitized in the direction of flow. The first set of bank lines is used only to compute the length of bank within the buffer boxes. The second is used to generate the buffer boxes. The first and second set of lines may be identical, but this is not required. Bank lines must extend past the end points for the centerline.)

The user is next prompted for a maximum expected distance between the channel centerline and the outer edge of the buffer. The program then asks the user for a shapefile name. Union boxes will be stored in this file, while the left and right bank boxes, respectively, will be stored in a file with the same name and the characters “_left” and “_right” appended. Finally, the user is asked for a threshold angle. A value of 10° seems to work well.

Tool 3: Bank Buffer Boxes (cont.)Tool 3: Bank Buffer Boxes (cont.)

Example buffer Example buffer boxes developed boxes developed using Tool 3.using Tool 3.

Bank Line 1 Bank Line 1 (used only for (used only for bank length)bank length)

Bank Line 2 Bank Line 2 (used to create (used to create buffer boxes)buffer boxes)

Lines 1 and 2 Lines 1 and 2 identicalidentical

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This tool is provided free of charge. Use this tool at your own risk. In offering this tool, the following entities and persons do not accept any responsibility or liability for the tool’s use by third parties:

• The National Center for Earth-surface Dynamics;

• The universities and institutions associated with the National Center for Earth-surface dynamics; and

•The authors of this tool.

Users of this tool assume all responsibility for the tool results and application thereof. The readers of the information provided by the web site assume all risks from using the information provided herein. None of the above-mentioned entities and persons assume liability or responsibility for damage or injury to persons or property arising from any use of the tool, information, ideas or instruction contained in the information provided to you.

Disclaimer Notice

Want more information?

For more information on this tool, please contact the author, J. Wesley Lauer, Associate Professor of Civil and Environmental Engineering at Seattle University, lauerj@seattleu.edu, or the National Center for Earth-surface Dynamics.

National Center for Earth-surface Dynamics2 3rd Ave SE,

Minneapolis, MN 55414612.624.4606