Transcript Slide 1

NCED Stream Restoration Toolbox
Channel Planform Statistics
An 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 [email protected].
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 Page
Tool Title: Channel Planform Statistics Toolbox
Tool Author: J. Wesley Lauer
Author e-mail: [email protected]
Version: 1.0
Associated files:
1) PlanformStatisticsTools.ppt
2) planform_statistics_tools_v91.mxd
Date: February 2006
Outline of this Document
• Introduction to the Tool
• Background
• Installation
• Tool 1: Interpolate Centerlines From Two Bank Lines
• Tool 2: Lateral Distance Measurement
• Tool 3: Bank Buffer Boxes
Introduction
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.
Background
The channel planform statistics tools are a set of visual basic programs
written for ArcMAP 9.1. The tools have been installed in an ArcMAP map file
named planform_statistics_tools_v91.mxd. 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) Measure 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 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.
Installation
The tools have already been installed in the ArcMAP map
planform_statistics_tools_v91.mxd. Technically, the code is associated with
UItoolcontrol1, UItoolcontrol2, and UItoolcontrol3 in “This Document” of the
project. However, these three UI tools have already been associated with a
toolbar in the migration.mxd map, so no installation should be required to
access the tools, aside from opening the file planform_statistics_tools_v91.mxd
in ArcMAP.
Tool 1: Interpolate Centerlines From Two
Bank 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.
a
a
b
q
q
Initial
b
Final
Tool 1: Interpolate Centerlines From Two Bank
Lines (cont.)
To use the tool, select the following icon from the toolbar:
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 then asks for the spacing between the interpolated points. The
program seems to run reasonably well when the spacing is about half a
channel width or less.
The tool is relatively stable in that few geometries cause it to fail. However,
it sometimes extends the last segment past the bank lines.
Since channel width is measured as part of the interpolation process,
channel width at each interpolated point is saved as a separate text file with
the same name as the shapefile used to store centerline information.
Centerline interpolated
from two bank lines using
Tool 1.
(1952 photograph,
Pearl River, Louisiana/
Mississippi.)
Tool 2: Lateral Distance Measurement
This tool finds the average lateral normal distance between the nodes
interpolated using tool 1 and a second line, using a best fit Bezier curve that is
assumed to represent the most likely path of migration for a particular point. It
stores the length of these curves in a new 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.
It is possible that the program will blunder under certain geometries,
particularly when the line to which measurements are made ends before the
reference line. To make it obvious when this occurs, the program creates
screen graphics representing each migration trajectory. If a trajectory looks
unreasonably long, the geometry of the centerlines should be changed or that
measurement should be discarded.
Tool 2: Lateral Distance Measurement (cont.)
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 computing the short-term outward
normal migration rate for the reference line (usually the new centerline position)
by fitting the Bezier curve between a user-defined bend apex trajectory rather
than the historic centerline.
Channel
Centerline
at t
D
l
di
Channel
Centerline
at t +Δt
ci 
di
t '
, where t ' 
l
t
D
The program reports the distance doutput=ci*Δt rather than the rate. Simply dividing
doutput by Δt results in a reasonable approximation of the local migration rate.
Tool 2: Lateral Distance Measurement (cont.)
The program automatically prompts the user for a shapefile that represents the
set of all bend apex trajectories. 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.
Channel
Centerline
at t
Channel
Centerline
at t +Δt
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, the
program simply fits Bezier Curves between
the two centerlines.
An example of the
An
exampleprocedure
of the results of the
correction
downstream translation function
of Tool 2.
The procedure ensures that
the method does not predict
outward migration at downstream
translating bend apices.
Example usage of
tools 1 and 2:
Centerline interpolated from
two bank lines using Tool 1,
older photograph.
(1952 photograph,
Pearl River, Louisiana/
Mississippi.)
Modern
(1998)
aerial from
Centerline
interpolated
photograph
two bank lines using Tool 1,
more recent photograph.
(1998 photograph,
Pearl River, Louisiana/
Mississippi.)
Results of tool 2: Measured
lateral migration distances
at evenly spaced intervals.
(1952 centerline was used as
the “to” centerline so that
measurements would be stored
at even increments along the
1998 centerline.)
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.
Input Bank and
Centerline Geometry
Right Buffer Boxes
Left Buffer Boxes
Union Boxes
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.
Tool 3: Bank Buffer Boxes (cont.)
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.
Example buffer
boxes developed
using Tool 3.
Bank Line 2
(used to create
buffer boxes)
Bank Line 1
(used only for
bank length)
Lines 1 and 2
identical
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information on this Web site is offered on an “as is” basis without warranty. The readers of the
information assume all risks from using the information provided herein.
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.
Want more information?
For more information on this tool or the NCED Stream
Restoration Toolbox please contact the author of this tool,
J. Wesley Lauer, or the NCED Stream Restoration Project
Manager, Jeff Marr at [email protected]
National Center for Earth-surface Dynamics
2 3rd Ave SE,
Minneapolis, MN 55414
612.624.4606