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

December 9, 2005

 

            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 Central Texas.

            A study of the mutual adjustment of channel pattern and channel shape of streams draining the Edwards Plateau and Llano Uplift of the Texas Hill Country, including the Llano River and Sandy Creek basins (Figure 1), offers an opportunity to examine controls of channel adjustment in a unique setting. The regional climate is characterized by a transition from eastern subhumid conditions to western semiarid conditions, and the hydrologic regime of the region is noted for low perennial flows punctuated by extreme floods (Figure 2). Additionally,       river sediment dramatically varies as a result of two distinct lithologies, Cretaceous carbonate rocks associated with the Edwards Plateau and Paleozoic sedimentary and igneous rocks associated with the Llano Uplift. The flashy hydrologic regime has important implications to adjustment of river channels in the region, particularly in examining the validity of bankfull discharge as a control on channel geometry. Moreover, the sharply contrasting lithology provides an opportunity to observe how changes in sediment load affect channel shape and pattern.

 

Figure 1. Map of study area.

 

 

 

Figure 2. Annual peak streamflow (m3/s) for the Llano River at Llano, Texas and comparison with other rivers in similarly sized drainage basins.

 

 

            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; Ferguson 1987; Rosgen 1994); however the three planes seldom are integrated with one another in studies of mutual adjustment (Figure 4).

 

 

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 Llano River and Sandy Creek basins should be approached from a spatial context and organized in a spatial framework. A wide variety of fluvial geomorphic parameters rely on spatial measurements, including, but not limited to, drainage area (km2), channel width (m), sinuosity [channel length (m)/valley axis length (m)], radius of curvature (m), and slope (m/m). Clearly, the organization and analysis of spatial data are integral to assessing fluvial geomorphic forms and processes. Currently, geographic information systems (GIS) not only serve as the most widely accepted platform for storage and organization of fluvial geomorphic data, but are also useful for a variety of geomorphic analyses.

 

 

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 Llano River and Sandy Creek basins in Central Texas. Specifically, sections below are devoted to: (1) the data and procedures necessary for stream and watershed delineation, (2) the creation of longitudinal profiles, or plots of elevation with distance downstream, in the study area, and (3) an assessment of the potential to model stream power (W/m2) in the study area.

            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 Llano River and Sandy Creek basins (Figure 5). NHD data contain reach codes, feature names, and other codes to uniquely identify river and stream reaches.

 

Lake Buchanan and L.B.J.

 

South Llano River

 

North Llano River

 

Llano River

 

 

Figure 5. High resolution (1:24,000) National Hydrography Dataset (NHD) subbasins for Llano River and Sandy Creek basins.

 

 

            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 Llano River and Sandy Creek basins. Unfortunately, the 10-meter DEM contained vertical stripes of no data, stretching across the entire study area from north to south. A second attempt to download the 10-meter DEM was made, yielding the same result. Jean Parcher, National Mapping Division Liason of the USGS in Austin, was approached about the no-data stripe. As a result, the correct data were provided on an external hard drive from the USGS Earth Resources Observation and Science (EROS) Data Center. Rectangular DEM mosaics were created for both the 30- and 10-meter DEM panels using the Raster Calculator in Spatial Analyst for ESRI ArcMap 9.1. The 30-meter DEM mosaic is 110 megabytes and required 2 individual panels; the 10-meter DEM mosaic is 1.81 gigabytes and required 12 individual panels.

 

 

Figure 6. 10-meter DEM mosaic spanning the Llano River and Sandy Creek basins.

 

 

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 Llano River was missing (Figure 8). This area was replaced by using the Dissolve command (ESRI ArcToolbox 9.1) on the missing subwatersheds and added to the 10-meter watershed with the Append command (ESRI Toolbox 9.1). Undesired vertices inside the appended section were manually removed during an edit session in ESRI ArcMap 9.1.

 

Figure 7. Drainage divides from the NHD and processed from 30- and 10-meter DEMs using Arc Hydro Tools. The northeastern edge of the Llano River basin represents the most exaggerated deviation of the three data sources.

 

 

10-meter DEM

30-meter DEM

 

 

Figure 8. Missing portion of the Llano River watershed created from the 10-meter DEM.

 

 

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/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 Llano River, was selected to investigate a process for creating longitudinal profiles in ESRI ArcMap 9.1. The high-resolution NHD dataset was used for channel positions. First, the stream segments comprising the longest distance from the drainage divide to the outlet of Beaver Creek were combined into one continuous arc using the Merge function in ESRI ArcToolbox 9.1. Next, the arc for Beaver Creek was converted to points using the XTools Pro extension toolbar, downloadable at http://www.xtoolspro.com (Figure 9). The function for this conversion is Convert Features to Points, which allows an arc to be converted to points at a user-specified distance between points. For Beaver Creek, the distance between points was set at 100 meters. Unfortunately, two problems associated with the points complicated assigning a distance downstream: (1) unique ObjectID numbers were not assigned in order from one end to the other, but instead would occur in ordered strings randomly positioned along the arc, and (2) the distance between ordered strings differed from 100 meters (Figure 10). Because of these problems, manual notes were required to keep track of the total distance downstream at each point (Figure 11). The point table was exported to a .dbf file and Microsoft Excel 2003 was used to expedite populating each point with a distance downstream. Next, elevations from the 10-meter DEM were assigned to the points by using the Extract Values to Points function in ESRI ArcToolbox 9.1. The point feature class with elevation values was joined to the .dbf file with downstream distances using the ObjectID field. Finally, a longitudinal profile was plotted for Beaver Creek (Figure 12). 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).

 

 

Figure 9. ESRI ArcMap 9.1 interface and the XTools Pro toolbar. The arc for the Llano River was converted to points with an interval of 100 meters.

 

 

 

 

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 Llano River and Sandy Creek study area, the Slope function from the Spatial Analyst toolbar in ESRI ArcMap 9.1 was implemented to generate a percent slope grid (Figure 14). Next, the Zonal Statistics function from the Spatial Analyst Toolbar was used to compute mean slope for NHD arc segments of selected river and stream reaches (Figure 15).

 

 

 

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.

 

 

CONCLUSIONS

 

            An analysis of channel geometry in the Llano River and Sandy Creek basins requires hydrologic data, including drainage areas (km2), discharges (m3/s), and channel slope. Arc Hydro Tools in ESRI ArcMap 9.1 is an excellent platform to generate and manage hydrologic data for fluvial geomorphic analyses. DEM processing in Arc Hydro Tools, combined with downloadable NHD data provide the best spatial hydrologic datasets for the study area. Edits to DEM-generated watersheds are common, but corrections are simple and do not compromise data quality. Using DEMs and NHD data, longitudinal profiles of elevation and distance downstream can be produced in ESRI ArcMap 9.1, although tedious manual tracking of downstream distance is involved. Finally, GIS has the potential to model stream power in river and stream reaches. Slope is the parameter that is most easily modeled. Velocity and flow depth parameters require field investigations, flow frequency assessments, and empirical assessments.

 

 

REFERENCES

 

Bagnold, R.A. 1966. An approach to the sediment transport from general physics. U.S. Geological Survey Professional Paper 422–I. Washington, DC: United States Government Printing Office.

 

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 (New Zealand) 26:1–151.

 

Church, M. 1992. Channel morphology and typology. In The river handbook—Volume 1, eds. P. Calow and G.E. Petts, 126–143: Oxford, England, Blackwell.

 

Ferguson, R.I. 1987. Hydraulic and sedimentary controls of channel pattern. In River channels—Environment and process, ed. K.S. Richards, 129–158. Oxford, England: Blackwell.

 

Leopold, L.B., and Langbein, W.B. 1962. The concept of entropy in landscape evolution. U.S. Geological Survey Professional Paper 500–A. Washington, DC: United States Government Printing Office.

 

Leopold, L.B., and Maddock, T. 1953. The hydraulic geometry of stream channels and some physiographic implications. U.S. Geological Survey Professional Paper 252. Washington, DC: United States Government Printing Office.

 

Leopold, L.B., and Wolman, M.G. 1957. River channel patterns—Braided, meandering, and straight. U.S. Geological Survey Professional Paper 282–B:39–85. Washington, DC: United States Government Printing Office.

 

Maidment, D.L., ed. 2002. Arc Hydro—GIS for water resources. Redlands, CA: ESRI Press.

 

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. London: Arnold.

 

Rosgen, D.L. 1994. A classification of natural rivers. Catena 22:169–199.

 

Schumm, S.A. 1963. Sinuosity of alluvial rivers on the Great Plains. Geological Society of America Bulletin 74:1089–1100.

 

Schumm, S.A. 1985. Patterns of alluvial rivers. Annual Review of Earth and Planetary Sciences 13:5–27.

 

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.