Transcript GIS and River Channels By Venkatesh Merwade University of Texas, Austin

GIS and River Channels
By Venkatesh Merwade
Center for Research in Water Resources,
University of Texas, Austin
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?
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.
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
Process Flowchart
Criterion
Hydrodynamic
Model
Depth &
velocity
Habitat
Species
groups
Model
SMS/RMA2
ArcGIS
Instream Flow
Decision Making
Habitat
Descriptions
Data Collection and
some statistics
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
Study Area (Guadalupe river near Seguin, TX)
1/2 meter Digital Ortho Photography
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.
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.
Global Positioning System (GPS)
Tells you where you are on the earth!
Final Setup
Computer and power
setup
GPS
Antenna
Depth
Sounder
Channel Movie!
Channel Movie
A boat is moving along a River and bathymetry is recorded as set
of points with (x,y,z) attributes.
Final Data View
2D Hydrodynamic Model
• SMS (Surface Water Modeling System)
– RMA2 Interface
• Input
DataWater Modeling System
Surface

Bathymetry
Data
(Environmental
Modeling Systems, Inc.)

Substrate Materials

Boundary
RMA2 Conditions

(US Army
Calibration
Data Corps of Engineers)
SMS mesh
Finite element mesh and bathymetric data
SMS Results
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.
Deep Pool
Run
Medium Pool
Shallow
Pool
Fast Riffle
Slow Riffle
Mesohabitat Criteria: V, D, V/D, FR
(Vadas & Orth, 1998)
Micro Habitat Table
Species
Group 1
50% MinD 50% MaxD 50% MinV 50% MaxV
1.5
2.7
1.5
2.9
Group 2
Group 3
Group 4
0.9
0.5
0.6
1.7
1.2
1.2
0.9
0.6
1.6
2.3
2
2.3
Group 5
Group 6
Group 7
Group 8
1.8
4.3
1.5
1.1
4.6
6.5
3.3
10
0.3
0.5
0.1
0.01
1.6
0.9
1.2
0.9
Group 9
Group 10
0.5
0.3
2.0
1.5
0.4
0.01
1.6
0.8
Habitat Modeling using ArcGIS
Results
GIS database for river channels
Measurement points
Thalweg/Centerline
Cross-sections
Fishnet
ProfileLines
Surface
Measure in ArcGIS
A PolylineMZ can store
m and z at each vertex
along with x and y
coordinates.
0
64.0056
112.3213
Defining a Thalweg
Input
User defines
an arbitrary
centerline
over the
measurement
points
Step 1
Thalweg tool
creates a
surface using
the
measurement
points
Step 2
Densify the
initial
centerline to
get more
points
Old vertices
New vertices
Step 3
Normals are
drawn at each
vertex of the
centerline to
locate deepest
points
Step 4
All the
deepest
points
replace the
vertices of
the old
centerline
Output
Final
result is a
3D
polyline
defining
the
thalweg
(s,n,z) coordinate system
s1
s2
(s = 0, n = 0)
Centerline
P
n1
n2
Q
P(n1, s1)
Q(n2, s2)
Banklines
(x,y,z) (s,n,z)
+n
o
-n
y
n
s
+n
x
(x,y,z)
s
o
-n
s
(s,n,z)
Surface in transformed
coordiantes
Straightened river
Profile line and cross-sections
Sinuous river
FishNet comparison
Regular FishNet
Hydraulic FishNet
Profile Lines and Cross Sections in 3D
Bird’s eye view!
Courtesy: Texas Water Development Board
•
Priority segments are
100s of miles long
•
Representative
reaches (study areas)
are only a few (<5)
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
Analytical framework for river
channels
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 $$$
What do we know about river
channels?
C
C
B
A
Meandering
shape
B
A
Thalweg
location
C
C
B
B
A
A
Cross-section
form
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..
Site1 and Site2 on Brazos River
@ 5 miles
@ 30 miles
The data (bathymetry) for both sites is available as (x,y,z) points.
Normalizing the data
nL
nR
0
-
+
Z
P(ni, zi)
d
Zd
w = nL + nR
For nL = -15, nR = 35, d = 5, Z=10
P (10, 7.5) becomes Pnew(0.5, 0.5)
For any point P(ni,zi), the
normalized coordinates
are:
nnew = (ni – nL)/w
znew = (Z – zi)/d
Normalized Data
Original cross-section
Modified cross-section
13
0
Normalized width
0.5
0.75
0
11
Normalized depth
Elevation (m)
12
0.25
10
9
8
Bathymetry Points
7
6
-75
-50
-25
n-Coordinate (m)
0
0.25
0.5
Bathymetry Points
0.75
25
1
Depth and width going from zero to unity makes life easier
without changing the shape of the original cross-section
1
Shape characterization through radius
of curvature
r1
r3
r2
• 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
Channel shape and thalweg
Y = 0.076*log(x) + 1.21
Thalweg location
1.00
0.75
2
R = 0.8238
0.50
R2 = 0.8717
0.25
0.00
-15000
-10000
-5000
0
5000
10000
15000
Radius of Curvature (m)
Y = 0.087*log(x) – 0.32
0
0.5 1.0
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)
beta probability density function
(a  b ) a 1
x (1  x ) b 1
(a ) ( b )
a  0, b  0
f ( x |a, b ) 
0  x  1,
0.75
1.00
0.25
x
0.50
a<b
Beta pdf looks good, but…..
0.75
1.00
0.00
0.25
x
0.50
0.75
f(x)
0.25
0.00
f(x)
f(x)
0.00
x
0.50
ab
ab
real cross-sections (in red) are different especially at the tails
1.00
Combination of two beta pdf
beta c/s = (beta1 + beta2) * factor
0.25
0.75
1.00
Beta c/s
Beta1
0.00
beta c/s
beta c/s
0.00
x
0.50
0.25
x
0.50
0.75
1.00
Beta c/s
Beta1
Beta2
Beta2
a1=5, b1=2, a2=3, b2=3, factor = 0.5
a1=2, b1=2, a2=3, b2=7, factor = 0.6
Thalweg location and beta
Thalweg = 0.20
Thalweg = 0.40
n
0.00
0.20
0.40
n
0.60
0.80
1.00
0.00
0.25
0.25
Beta
0
Beta
0
0.5
0.75
0.75
Beta
0.60
0.80
1.00
Beta
1
a1=3.75, b1=5, a2 =1.75, b2 =1.75, f=0.25
a1=2.25, b1=7.5, a2 =2.25, b2 =2.25, f=0.225
0.00
0.20
0.40
n
0.60
0.80
0
0.25
Beta
a2 =2, b2 =2, f=0.24
0.40
0.5
1
a1=6, b1=3,
0.20
0.5
0.75
Beta
1
Thalweg = 0.70
1.00
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)
Results
Results (2)
Venkatesh Merwade
Email: [email protected]
http://civilu.ce.utexas.edu/stu/merwadvm/