A 3D description of river channels using 2D NHD reaches

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A 3D description of river channels using 2D NHD
reaches

• Venkatesh M. Merwade and David R. Maidment

Center for Research in Water Resources,
University of Texas at Austin
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Overview

• Motivation
• Objective
• Data collection and representation
• Conceptual Model
• GIS framework
• Results and Conclusions

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Motivation Instream flow studies in Texas
Extrapolation of channel bathymetry at regional
scale
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Objective

• Based on the knowledge gained from a detailed
  dataset collected for a reach of river, develop a
  framework for describing the 3D river channel
  form at regional scale.

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In other words
Now, we have blue lines
We need the ability to produce a 3D description
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Data collection
GPS Antenna
Depth Sounder
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Bathymetry of the Brazos River
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Data representation
Profile lines and cross-sections
Points
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3D CrossSections and ProfileLines
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Conceptual Model
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The GIS framework

• Get the shape from the Blue lines
• Using the shape, locate the thalweg
• Using thalweg location, create cross-sections
• Network of cross-sections and profile lines

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Site1 and Site2 on Brazos River
_at_ 5 miles
_at_ 30 miles
The data for Site 1 and Site 2 are available as
(x,y,z) points.
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Normalizing the data
For any point P(ni,zi), the normalized
coordinates are nnew (ni nL)/w znew (Z
zi)/d
For nL -15, nR 35, d 5, Z10 P (10, 7.5)
becomes Pnew(0.5, 0.5)
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Normalized Data
Original cross-section
Modified cross-section
Depth and width going from zero to unity makes
life easier without changing the shape of the
original cross-section
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Shape characterization through radius of curvature
r1
r3
c1
c2
r2
c3

• If radius of curvature is small, the thalweg is
  close to the bank.
• If the channel meanders to left, the center of
  curvature is to the right hand side of the
  centerline and vice versa.
• When the center of curvature is to the right, the
  radius of curvature is considered positive and
  vice versa

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Locating thalweg using the shape
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Thalweg and cross-section

• Cross-section should have an analytical form to
  relate it to the thalweg
• Many probability density functions (pdf) have
  shapes similar to the cross-section
• Beta pdf was found feasible
• its domain is from zero to one
• it has only two parameters (a,b)

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Cross-sections as beta pdfs
beta c/s (beta1 beta2) k
a15, b12, a23, b23, factor 0.5
a12, b12, a23, b27, factor 0.6
Create beta cross-sections for different thalweg
locations
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The final framework

• Start with a blue line (s), locate the thalweg
  (t) using the relationship, t f(s).
• Using t, describe cross-sections (c) using the
  relationship, c(a,b) f(t).
• The resulting cross-sections have a unit width
  and unit depth.
• Rescale the normalized cross-sections using width
  and depth (hydraulic geometry)

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Results
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Lower Brazos in Texas
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Lower Guadalupe in Texas

• Model parameters are changed and are related to
  sinuosity

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Sulphur River in Texas
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Regional Application (300 miles along Brazos
River!)
To study how frequently the oxbow lakes are
replenished (1D hydraulic modeling)
Study area
Hydraulic Geometry DOQs
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Summary

• The GIS framework provides a mean surface for the
  channel bed.
• Analytical model is useful for describing channel
  bathymetry at regional scale.
• Caution! while using the framework.
• Additional features need to be added (pools and
  riffles).
• The framework should be verified in other areas.

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Courtesy Texas Water Development Board
Venkatesh Merwade vmmerwade_at_mail.utexas.edu http/
/webspace.utexas.edu/vmm2675/www/