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
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 (m3/s)
is bankfull discharge (Qbf),
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.
Annual peak streamflow (m3/s) for the
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
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
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
Drainage divides from the NHD and processed from 30- and 10-meter DEMs using
Arc Hydro Tools. The northeastern edge of the
Missing portion of the
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/m2) and stream power (W/m2). 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
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/m2) 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/m2) 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/m2),
U is flow velocity (m/s),
ρ is the density of water (1000 kg/m3),
g is gravitational acceleration (9.80 m/s2),
d is flow depth (m), and
S is channel slope (m/m).
Stream power (W/m2) can also be thought of as the product of bed shear stress (N/m2) 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/m2) 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 (m3/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/U2, and
1/√f = 2.11 + 2.03log10(d/ks), and
ks ≈ 6.8D50, where
f is the dimensionless Darcy–Weisbach friction factor,
ks is the equivalent sand roughness height (m), and
D50 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.
An analysis of channel geometry in the
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