Fluvial Geomorphic Analyses of the
Llano River and Sandy Creek Basins, Central Texas, Using Geographic Information
Systems (GIS) and Arc Hydro Tools
Franklin T. Heitmuller
CE 394K
River channel adjustment has long been a fundamental
topic of fluvial geomorphology. Two primary dimensions of this topic concern
controls on channel pattern (planform geometry) and channel shape
(cross-sectional geometry). Discharge and sediment are commonly referenced as
the controls on channel adjustment. The most common index of discharge (m^{3}/s)
is bankfull discharge (Q_{bf}),
which is frequently related to the scale of size of channel features. Indices
of sediment commonly include bedload (tons/day), bed material size (mm), or
bank material (% silt-clay), which are related to the shape of a river channel.
Much of our knowledge on the topic of channel adjustment derives from studies
in humid mid-latitude settings. Recent studies, however, have shown that specific
indices controlling channel pattern vary regionally. This is particularly
important when considering spatial variability in channel adjustment along
transition zones in hydrology and lithology, such as are represented for
drainage systems within
A study of the mutual adjustment of channel pattern and
channel shape of streams draining the
Figure 1.
Map of study area.
Figure 2.
Annual peak streamflow (m^{3}/s) for the
Channel geometry
can be described in three planes of adjustment: (1) planform (referred to as
pattern), (2) cross-section (referred to as shape), and (3) longitudinal
(referred to as the profile). Common indices used to describe channel pattern
include curvature (radius of curvature (m)/channel width (m)), meander
wavelength (m), sinuosity (P), and
degree of channel division for a given reach (Figure 3). Channel shape is most
commonly determined by the ratio of bankfull width (m) to depth (m), the
presence of bars or islands, and the symmetry of these components. The channel
profile is a plot of the bed elevation along a reach of interest. Three planes
of geometric adjustment commonly have been independently examined (e.g., Leopold
and Maddock 1953; Leopold and Wolman 1957; Schumm 1963;
Figure 3.
Classification of channel pattern. Source: Church (1992), modified from Schumm
(1985).
Figure 4.
Model for mutual adjustment of channel slope, shape, and pattern of meandering
rivers. Source: Rosgen (1994).
An adequate understanding of mutual adjustment of channel
geometry in the
PURPOSE AND SCOPE
The purpose of this report is to present a variety of procedures
and techniques used to store, organize, and analyze fluvial geomorphic data
associated with the
Much of the work completed for the project was performed
in ESRI ArcGIS 9.1 using Arc Hydro Tools. Maidment (2002) provides a synthesis
of concepts, techniques, and guidelines associated with Arc Hydro Tools.
Hydrography and elevation data used in the project were downloaded from the U.S.
Geological Survey’s National Hydrography Dataset (NHD) website (http://nhd.usgs.gov) and Seamless Data
Distribution System (http://seamless.usgs.gov).
No field data collection efforts were necessary for completion of the project.
HYDROGRAPHY AND ELEVATION DATA
Hydrography data were downloaded from the U.S. Geological
Survey National Hydrography Dataset (NHD) website (http://nhd.usgs.gov). High resolution
(1:24,000) data were selected, and four subbasins were needed to cover the
Figure 5.
High resolution (1:24,000) National Hydrography Dataset (NHD) subbasins for
Elevation data were downloaded from the U.S. Geological
Survey Seamless Data Distribution System website (http://seamless.usgs.gov). 30- meter and
10-meter digital elevation models (DEMs) (Figure 6) were downloaded to
encompass the
Figure 6.
10-meter DEM mosaic spanning the
ARC HYDRO TOOLS
Arc Hydro Tools (Maidment 2002) in ESRI ArcMap 9.1 were
used to generate hydrologic spatial datasets for the study area. The datasets
generated by Arc Hydro Tools are useful for systematic delineation of river and
stream networks and watersheds from DEMs. Unique identification numbers,
including HydroID and DrainID, are given to stream reaches and watersheds.
These are useful in associating hydrologic data from the reach scale to the
watershed scale.
Procedures implemented using Arc Hydro Tools for the
Llano River and Sandy Creek basins were: (1) fill sinks in the raw 30- and
10-meter DEM mosaics, (2) calculate flow direction grids, (3) calculate flow
accumulation grids, (4) calculate stream definition grids, using a threshold of
100,000 cells (10 square kilometers) for the 10-meter DEM, (5) calculate stream
segmentation grids, (6) calculate catchment grids, (7) process drainage lines,
(8) process adjoint catchments, (9) process drainage points, (10) delineate
watersheds at the outlets of the Llano River, the James River, Beaver Creek,
and Sandy Creek using batch point processing, (11) generate HydroEdge and
HydroJunction datasets using Hydro Network Generation.
The drainage divides downloaded from the NHD and
delineated from the 30- and 10-meter DEMs using Arc Hydro Tools differed from
one another (Figure 7). The NHD subbasin drainage divide is a generalized,
hand-digitized line. The drainage divides processed for the 30- and 10-meter
DEMs tend to follow one another, but can substantially deviate from one another
in particular localities. In one instance, a small area of the 10-meter
delineated watershed of the
Figure 7.
Drainage divides from the NHD and processed from 30- and 10-meter DEMs using
Arc Hydro Tools. The northeastern edge of the
10-meter
DEM 30-meter
DEM
Figure 8.
Missing portion of the
LONGITUDINAL PROFILES
Longitudinal profiles are plots of channel bed elevation
with distance downstream. Typical longitudinal profiles of a river or stream
are characterized by a concave-up shape with a steep slope in the upper
headwaters that gradually decreases with distance downstream. Plots of
elevation and distance have been observed to have a power, logarithmic, or
exponential form (Leopold and Langbein 1962; Snow and Slingerland 1987; Morris
and Williams 1997). Profiles are very useful in fluvial geomorphic analyses. Slopes
are visible and can be determined for the overall system or for particular
reaches. Channel slope is a necessary parameter in computation of bed shear
stress (N/m^{2}) and stream power (W/m^{2}). Longitudinal
profiles are also useful in detecting geologic controls on channel slope,
including resistant bedrock outcrops. If available, a time series of
longitudinal profiles can be used to track the upstream rate of channel
degradation, possibly induced by anthropogenic channel straightening or
sediment depletion/extraction.
Beaver Creek, a major tributary to the
Figure 9.
ESRI ArcMap 9.1 interface and the XTools Pro toolbar. The arc for the
Figure 10.
The ESRI ArcMap 9.1 Measure Tool showing the distance is not equal to the
user-designated 100 meters between two ordered strings of points created using
the XTools Pro tools. The distance between points is 100 meters within each
ordered string.
Figure 11.
Manual notes taken to keep track of distance downstream at each point along
Beaver Creek. The point with ObjectID 381 is the furthest point upstream. All
points between 381 and 390 are 100 meters apart. A new ordered string begins
with ObjectID 375, the distance between the two ordered strings being 71.82
meters.
Figure 12.
Beaver Creek generally displays a concave-up longitudinal profile, but slope
remains remarkably constant between 8 and 60 kilometers downstream. Slight
increases in elevation downstream are attributed to both the proximity of the
stream channel to a steep valley wall and the resolution of the DEM.
Figure 13.
The point highlighted in blue along Beaver Creek represents a location where
elevation increases in a downstream direction. This is explained by the
proximity of the channel to the steep valley wall and the resolution of the
DEM.
POTENTIAL FOR STREAM POWER MODELING
Stream power (W/m^{2}) is commonly used to assess
sediment transport (Bagnold 1977; Carson and Griffiths 1987) and channel
geometry (Ferguson 1987; van den Berg 1995) in river systems. Stream power (W/m^{2})
was introduced by Bagnold (1966) to assess sediment transport, and can be
defined as:
ω
= UρgdS, where
ω
is stream power per unit bed area (W/m^{2}),
U
is flow velocity (m/s),
ρ
is the density of water (1000 kg/m^{3}),
g
is gravitational acceleration (9.80 m/s^{2}),
d
is flow depth (m), and
S
is channel slope (m/m).
Stream power (W/m^{2})
can also be thought of as the product of bed shear stress (N/m^{2}) and
flow velocity (m/s).
An initial attempt to model stream power using GIS should
begin by assigning slope to channel segments. First, the raw 10-meter DEM
mosaic was reprojected to the U.S. Contiguous Albers Equal Area projection.
Using the reprojected raw 10-meter DEM mosaic of the
Figure 14.
Slope grid generated by the Spatial Analyst toolbar in ESRI ArcMap 9.1. The
Zonal Statistics function was then used to assign mean slope values to selected
river and stream reaches.
Figure 15.
Map of mean percent channel slope of the North Llano River, created in ESRI
ArcMap 9.1 using the Zonal Statistics function and a 10-meter slope grid.
The other parameters needed to model stream power (W/m^{2})
are flow velocity (m/s) and flow depth (m). These parameters can represent any
given flow within the range of possible discharges of the river or stream. An
initial approach toward the application of these parameters to model stream
power would be to choose a flow of interest, such as bankfull discharge (m^{3}/s).
Field surveys of the channel reaches would also be necessary to characterize
bankfull channel depth (m). Assumptions of bankfull channel depth (m) may be
empirically estimated from a plot of bankfull channel width (m) and depth (m).
Finally, mean flow velocity (U) (m/s)
could be estimated from a flow resistance equation, such as the Darcy–Weisbach
friction factor (f), defined as
(Robert 2003):
f = 8gdS/U^{2}, and
1/√f = 2.11 + 2.03log_{10}(d/k_{s}),
and
k_{s} ≈
6.8D_{50}, where
f is the
dimensionless Darcy–Weisbach friction factor,
k_{s} is
the equivalent sand roughness height (m), and
D_{50} is
the median bed-particle size (m).
Again, field surveys would
be necessary to make appropriate assumptions of bed- particle size (m) for
channel reaches. Generally, a downstream decrease in bed-particle size (m) is
observed, although tributaries and changes in lithology can reverse this trend.
CONCLUSIONS
An analysis of channel geometry in the
REFERENCES
Bagnold, R.A. 1966. An approach to the sediment transport from general physics.
Bagnold, R.A. 1977. Bed load transport by natural
rivers. Water Resources Research
13:303–312.
Carson, M.A., and Griffiths, G.A. 1987. Bedload
transport in gravel channels. Journal of
Hydrology (
Church, M. 1992. Channel morphology and typology.
In The river handbook—Volume 1, eds. P. Calow and G.E. Petts, 126–143:
Leopold, L.B., and Langbein, W.B. 1962. The concept of entropy in landscape
evolution.
Leopold, L.B., and Maddock, T. 1953. The hydraulic geometry of stream channels
and some physiographic implications.
Leopold, L.B., and Wolman, M.G. 1957. River channel patterns—Braided, meandering,
and straight.
Maidment, D.L., ed. 2002. Arc Hydro—GIS for water resources.
Morris, P.H., and Williams, D.J. 1997. Exponential
longitudinal profiles of streams. Earth
Surface Processes and Landforms 22:143–163.
Robert, A. 2003. River processes.
Rosgen, D.L. 1994. A classification of natural
rivers. Catena 22:169–199.
Snow, R.S., and Slingerland, R.L. 1987.
Mathematical modeling of graded river profiles. Journal of Geology 95:15–33.
van den Berg, J.H. 1995. Prediction of alluvial
channel pattern of perennial rivers. Geomorphology
12:259–279.