GIS and River Channels

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GIS and River Channels

• By Venkatesh Merwade
• Center for Research in Water Resources,
• University of Texas, Austin

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Instream flow studies

• How do we quantify the impact of changing the
  naturalized flow of a river on species habitat?
• How do we set the minimum reservoir releases that
  would satisfy the instream flow requirement?

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Objective

• Objective
• To model species habitat as a function of flow
  conditions and help decision making
• Instream Flow
• Flow necessary to maintain habitat in natural
  channel.

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Methodology

• Species habitat are dependent on channel
  hydrodynamics hydrodynamic modeling
• Criteria to classify species depending on the
  conditions in the river channel biological
  studies
• Combine hydrodynamics and biological studies to
  make decisions ArcGIS

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Process Flowchart
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Data Requirement

• Hydrodynamic Modeling
• Bathymetry Data (to define the channel bed)
• Substrate Materials (to find the roughness)
• Boundary Conditions (for hydrodynamic model)
• Calibration Data (to check the model)
• Biological Studies
• Fish Sampling (for classification of different
  species)
• Velocity and depth at sampling points

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Study Area (Guadalupe river near Seguin, TX)
1/2 meter Digital Ortho Photography
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Depth Sounder (Echo Sounder)
The electronic depth sounder operates in a
similar way to radar It sends out an electronic
pulse which echoes back from the bed. The echo is
timed electronically and transposed into a
reading of the depth of water.
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Acoustic Doppler Current Profiler
Provides full profiles of water current speed and
direction in the ocean, rivers, and lakes. Also
used for discharge, scour and river bed
topography.
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Global Positioning System (GPS)
Tells you where you are on the earth!
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Final Setup
GPS Antenna
Computer and power setup
Depth Sounder
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Channel Movie!
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Channel Movie
A boat is moving along a River and bathymetry is
recorded as set of points with (x,y,z)
attributes.
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Final Data View
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2D Hydrodynamic Model

• SMS (Surface Water Modeling System)
• RMA2 Interface
• Input Data
• Bathymetry Data
• Substrate Materials
• Boundary Conditions
• Calibration Data

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SMS mesh
Finite element mesh and bathymetric data
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SMS Results
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Biological Studies (TAMU)

• Meso Habitat and Micro Habitat
• Use Vadas Orth (1998) criterion for Meso
  Habitats
• Electrofishing or seining to collect fish samples
  for Micro Habitat analysis
• Sample at several flow rates and seasons
• Measure Velocity and depth at seining points
• Statistical analysis to get a table for Micro
  Habitats classification.

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• Mesohabitat Criteria V, D, V/D, FR
• (Vadas Orth, 1998)

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Micro Habitat Table
Species 50 MinD 50 MaxD 50 MinV 50 MaxV
Group 1 1.5 2.7 1.5 2.9
Group 2 0.9 1.7 0.9 2.3
Group 3 0.5 1.2 0.6 2
Group 4 0.6 1.2 1.6 2.3
Group 5 1.8 4.6 0.3 1.6
Group 6 4.3 6.5 0.5 0.9
Group 7 1.5 3.3 0.1 1.2
Group 8 1.1 10 0.01 0.9
Group 9 0.5 2.0 0.4 1.6
Group 10 0.3 1.5 0.01 0.8
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Habitat Modeling using ArcGIS
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(No Transcript)
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Results
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GIS database for river channels
Measurement points
Surface
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Measure in ArcGIS
A PolylineMZ can store m and z at each vertex
along with x and y coordinates.
64.0056
0
112.3213
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Defining a Thalweg
Input
Output
Step 1
Step 3
Step 4
Step 2
User defines an arbitrary centerline over the
measurement points
Thalweg tool creates a surface using the
measurement points
Normals are drawn at each vertex of the
centerline to locate deepest points
All the deepest points replace the vertices of
the old centerline
Final result is a 3D polyline defining the thalweg
Densify the initial centerline to get more points
Old vertices
New vertices
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(s,n,z) coordinate system
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(x,y,z)?? (s,n,z)
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Surface in transformed coordiantes
Straightened river
Profile line and cross-sections
Sinuous river
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FishNet comparison
Hydraulic FishNet
Regular FishNet
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Profile Lines and Cross Sections in 3D
Birds eye view!
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Courtesy Texas Water Development Board

• Priority segments are 100s of miles long
• Representative reaches (study areas) are only a
  few (lt5) miles long
• Can we develop a channel description for the
  segments using the data for representative
  reaches??
• Useful not only for instream flows but also for
  other hydrologic studies

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Analytical framework for river channels
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Some thoughts on blue lines

• Blue lines on the hydrography map are pretty, but
  it would be nice if we know more about our river
  channels than just their location and shape
• If we have the three-dimensional form of river
  channels then we can use it for preliminary
  studies and save lots of

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What do we know about river channels?
Meandering shape
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Methodology

• We can get shape from the Blue lines
• Using the shape we can locate the thalweg
• Using the location of the thalweg, create
  cross-sections

3D form is not a problem, what about the
dimensions?? They are different everywhere..
Work in a normalized domain where everything is
Unity (one). We can re-scale the results using
additional information..
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Site1 and Site2 on Brazos River
The data (bathymetry) for both sites is 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

• If radius of curvature is small, the thalweg is
  close to the bank and as it increases the thalweg
  moves towards the center of the channel.
• 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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Channel shape and thalweg
0
0.5
1.0
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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 is found feasible
• its domain is from zero to one
• it has only two parameters (a,b)

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beta probability density function
Beta pdf looks good, but..
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Combination of two beta pdf
beta c/s (beta1 beta2) factor
a15, b12, a23, b23, factor 0.5
a12, b12, a23, b27, factor 0.6
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Thalweg location and beta
Thalweg 0.20
Thalweg 0.40
a13.75, b15, a2 1.75, b2 1.75, f0.25
a12.25, b17.5, a2 2.25, b2 2.25, f0.225
a16, b13, a2 2, b2 2, f0.24
Thalweg 0.70
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The final framework

• If we start with a blue line, we can locate the
  thalweg using the relationship, t f(s).
• Using t, we can find the shape of cross-section
  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
  (obtained from aerial photographs) and depth
  (hydraulic geometry)

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Results
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Results (2)
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Venkatesh Merwade Email vmmerwade_at_mail.utexas.edu
http//civilu.ce.utexas.edu/stu/merwadvm/