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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?

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gis and river channels

GIS and River Channels

By Venkatesh Merwade

Center for Research in Water Resources,

University of Texas, Austin

instream flow studies
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
  • 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
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

Depth & velocity

Species groups

Habitat

Hydrodynamic

Habitat

SMS/RMA2

Data Collection and some statistics

Model

Model

Descriptions

ArcGIS

Instream

Flow

DecisionMaking

Process Flowchart
data requirement
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
Study Area (Guadalupe river near Seguin, TX)

1/2 meter Digital Ortho Photography

depth sounder echo sounder
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
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
Global Positioning System (GPS)

Tells you where you are on the earth!

final setup
Final Setup

GPS Antenna

Computer and power setup

Depth Sounder

channel movie13
Channel Movie

A boat is moving along a River and bathymetry is recorded as set of points with (x,y,z) attributes.

2d hydrodynamic model

Surface Water Modeling System

(Environmental Modeling Systems, Inc.)

RMA2

(US Army Corps of Engineers)

2D Hydrodynamic Model
  • SMS (Surface Water Modeling System)
    • RMA2 Interface
  • Input Data
    • Bathymetry Data
    • Substrate Materials
    • Boundary Conditions
    • Calibration Data
sms mesh
SMS mesh

Finite element mesh and bathymetric data

biological studies tamu
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.
slide19

Deep Pool

Run

Depth [feet]

Medium Pool

Shallow

Pool

Fast Riffle

Slow Riffle

Mesohabitat Criteria: V, D, V/D, FR

(Vadas & Orth, 1998)

gis database for river channels

Thalweg/Centerline

Cross-sections

ProfileLines

Fishnet

GIS database for river channels

Measurement points

Surface

measure in arcgis
Measure in ArcGIS

A PolylineMZ can store m and z at each vertex along with x and y coordinates.

64.0056

0

112.3213

defining a thalweg
Defining a Thalweg

Input

Output

Step 2

Step 1

Step 3

Step 4

User defines an arbitrary centerline over the measurement points

Thalweg tool creates a surface using the measurement points

Densify the initial centerline to get more 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

Old vertices

New vertices

s n z coordinate system

P

s1

Centerlin

e

s2

n1

(s = 0, n = 0)

n2

Bankline

s

P(n1, s1)

Q(n2, s2)

Q

(s,n,z) coordinate system
x y z s n z

n

+

o

n

-

y

n

s

n

+

x

s

o

(x,y,z)

n

-

s

(s,n,z)

(x,y,z) (s,n,z)
surface in transformed coordiantes
Surface in transformed coordiantes

Straightened river

Profile line and cross-sections

Sinuous river

fishnet comparison
FishNet comparison

Hydraulic FishNet

Regular FishNet

slide32

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
some thoughts on blue lines
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

Thalweg location

Cross-section form

C

C

C

C

B

B

B

B

A

A

A

A

What do we know about river channels?

Meandering shape

methodology36
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

Site1 and Site2 on Brazos River

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

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, Z=10

P (10, 7.5) becomesPnew(0.5, 0.5)

normalized data
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

shape characterization through radius of curvature

r1

r3

r2

Shape characterization through radius of curvature
  • Ifradius 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

Y = 0.087*log(x) – 0.32

Channel shape and thalweg

0

0.5

1.0

thalweg and cross section
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)
b eta probability density function

a < b

a = b

a > b

real cross-sections (in red) are different especially at the tails

beta probability density function

Beta pdf looks good, but…..

combination of two beta pdf
Combination of two beta pdf

beta c/s = (beta1 + beta2) * factor

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 b eta
Thalweg location and beta

Thalweg = 0.20

Thalweg = 0.40

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

a1=6, b1=3,

a2 =2, b2 =2, f=0.24

Thalweg = 0.70

the final framework
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)
slide49

Venkatesh Merwade

Email: vmmerwade@mail.utexas.edu

http://civilu.ce.utexas.edu/stu/merwadvm/