In cooperation with the Texas Department of Transportation 

Potential for Bed-Material Entrainment 

in Selected Streams of the Edwards 

Plateau—Edwards, Kimble, and 

Real Counties, Texas, and Vicinity 

Scientific Investigations Report 2008–5017 

(TxDOT Research Report 0–4695–4) 

U.S. Department of the Interior 

U.S. Geological Survey 
—Technical Report Documentation Page—

1. Report No.

FHWA/TX–08/0–4695–4

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Potential for Bed-Material Entrainment in Selected Streams of the 

Edwards Plateau—Edwards, Kimble, and Real Counties, Texas, 

and Vicinity

5. Report Date

June 2008

6. Performing Organization Code

7. Author(s)

Franklin T. Heitmuller and William H. Asquith

8. Performing Organization Report No.

USGS SIR 2008–5017

9. Performing Organization Name and Address

U.S. Geological Survey

Water Resources Division

8027 Exchange Drive

Austin, Texas 78754

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

Project 0–4695

12. Sponsoring Agency Name and Address

Texas Department of Transportation

Research and Technology Implementation Office

P.O. Box 5080

Austin, Texas 78731

13. Type of Report and Period Covered

 Technical report on research from 2003 to 2008

14. Sponsoring Agency Code

15. Supplementary Notes

Project conducted in cooperation with the Texas Department of Transportation and the Federal Highway Administration.

16. Abstract

The Texas Department of Transportation spends considerable money for maintenance and replacement of low-water crossings of 

streams in the Edwards Plateau in Central Texas as a result of damages caused in part by the transport of cobble- and gravel-sized 

bed material. An investigation of the problem at low-water crossings was made by the U.S. Geological Survey in cooperation with 

the Texas Department of Transportation, and in collaboration with Texas Tech University, Lamar University, and the University of 

Houston. The bed-material entrainment problem for low-water crossings occurs at two spatial scales—watershed scale and channelreach scale. First, the relative abundance and activity of cobble- and gravel-sized bed material along a given channel reach becomes 

greater with increasingly steeper watershed slopes. Second, the stresses required to mobilize bed material at a location can be attributed to reach-scale hydraulic factors, including channel geometry and particle size. The frequency of entrainment generally increases 

with downstream distance, as a result of decreasing particle size and increased flood magnitudes. An average of 1 year occurs 

between flows that initially entrain bed material as large as the median particle size, and an average of 1.5 years occurs between 

flows that completely entrain bed material as large as the median particle size. The Froude numbers associated with initial and complete entrainment of bed material up to the median particle size approximately are 0.40 and 0.45, respectively.

17. Key Words

Bed-material entrainment, Edwards Plateau, gravel transport, lowwater crossings, Texas

18. Distribution Statement

No restrictions.

This document is available to the public through the National

Technical Information Service, Springfield Virginia, 22161, 

www.ntis.gov

19. Security Classif. (of report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of pages

76

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

Cover: Yates Crossing of the Llano River along Ranch Road 385 in Kimble County, Texas, following the 

November 2000 flood. Photo courtesy of the Texas Department of Transportation (TxDOT).
Potential for Bed-Material Entrainment 

in Selected Streams of the Edwards 

Plateau—Edwards, Kimble, and 

Real Counties, Texas, and Vicinity

By Franklin T. Heitmuller and William H. Asquith

In cooperation with the Texas Department of Transportation

Scientific Investigations Report 2008–5017

(TxDOT Research Report 0–4695–4)

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

For product and ordering information:

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natural hazards, and the environment:

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Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Heitmuller, F.T., and Asquith, W.H., 2008, Potential for bed-material entrainment in selected streams of the Edwards 

Plateau—Edwards, Kimble, and Real Counties, Texas, and vicinity: U.S. Geological Survey Scientific Investigations 

Report 2008–5017, 76 p. 
iii

Contents

Abstract .........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Purpose and Scope .............................................................................................................................2

Description of Study Area ..................................................................................................................2

Review of Existing Data ......................................................................................................................4

Acknowledgments ...............................................................................................................................4

Methods ..........................................................................................................................................................6

GIS Analysis ..........................................................................................................................................6

Watersheds ..................................................................................................................................6

Watershed and Channel Slopes ...............................................................................................6

Flood-Frequency Analysis ..................................................................................................................7

Site Reconnaissance ........................................................................................................................21

Field Surveys .......................................................................................................................................21

Channel Geometry ....................................................................................................................22

Low-Water-Crossing Geometry ..............................................................................................23

Bed-Material Particle Size ......................................................................................................23

Bed-Material Entrainment Potential ........................................................................................................41

Relative Abundance and Activity of Bed Material .......................................................................41

Stresses Needed to Mobilize (Entrain) Bed Material ..................................................................45

Risk-Oriented Interpretation Using Flood Frequency ..................................................................52

Summary .......................................................................................................................................................57

References Cited .........................................................................................................................................57

Appendix 1 ....................................................................................................................................................59

Figures 1.1—1.23. Photographs showing: 

1.1. Downstream end of culvert pipes at Ben Williams Crossing of the Nueces 

River on Ranch Road 335 ...........................................................................................61

1.2. Ben Williams Crossing of the Nueces River on Ranch Road 335...........................61

1.3. View downstream from damaged low-water crossing of Cedar Creek at 

Ranch Road 336 following June 2004 flood .............................................................62

1.4. Upper Ranch Road 1120 crossing of the Frio River following the peak of the 

June 2004 flood ............................................................................................................62

1.5. View downstream from cross section 2 of South Llano River at Baker Ranch ...63

1.6. View downstream from the South Llano River at U.S. Highway 377 near 

Rocksprings ..................................................................................................................63

1.7. View across the floodplain of the South Llano River at 700 Springs Ranch 

at cross section 2 ........................................................................................................64

1.8. First Crossing of the South Llano River along U.S. Highway 377 in Kimble 

County ...........................................................................................................................64

1.9. View upstream from the South Llano River at South Llano River State Park 

near cross section 1 ...................................................................................................65

1.10. View across the channel and channel bar of South Llano River at Texas 

Tech University–Junction at cross section 3 ..........................................................65

1.11. Cobble and gravel lenses within relatively fine-grained Holocene floodplain 

deposits in bank of South Llano River at Texas Tech University–Junction .......66
iv

1.12. View across the South Llano River at Texas Tech University–Junction 

toward vegetated Holocene floodplain ...................................................................66

1.13. View across a gravel bar and floodplain of North Llano River near 

Roosevelt at cross section 3 .....................................................................................67

1.14. View across a cobble and gravel bar of North Llano River near Junction at 

cross section 1 .............................................................................................................67

1.15. View upstream from Johnson Fork at Lowlands Crossing at cross section 2 ...68

1.16. View across the channel and low-water crossing of Johnson Fork at 

Guzman Crossing .........................................................................................................68

1.17. View upstream from Johnson Fork at Paks Crossing ............................................69

1.18. View of the bedrock channel bed of Llano River near Junction at cross 

section 1 ........................................................................................................................69

1.19. View across Llano River near Junction at cross section 3 ..................................70

1.20. View across Llano River near Junction at cross section 2 ..................................70

1.21. View downstream from Llano River near Ivy Chapel at cross section 2 ............71

1.22. View upstream from Llano River near Ivy Chapel at cross section 3 .................71

1.23. Reconstruction of Ranch Road 337 low-water crossing of the Frio River near 

Leakey, December 2004 ..............................................................................................72

Appendix 2 ....................................................................................................................................................73

Figures 2.1—2.2:

2.1. Map showing channel resistivity transect at Nueces River at Ben Williams 

Crossing, Edwards-Real County line, Texas .............................................................75

2.2. Resistivity profile showing field results of capacitively-coupled data 

(actual resistivity) at common resistivity scale for (A) Johnson Fork at 

Lowlands Crossing and (B) Nueces River at Ben Williams Crossing ..................75

Figures

1. Photograph showing Flatrock Crossing of the South Llano River in Kimble County, 

Texas ..............................................................................................................................................1

2. Map showing location of study area—Edwards, Kimble, and Real Counties, Texas, 

and vicinity ....................................................................................................................................3

3–10. Maps showing channel slope of the: 

3. North Llano River Subbasin, Kimble County, Texas, and vicinity ................................7

4. South Llano River Subbasin, Edwards, Kimble, and Real Counties, Texas, and 

vicinity ...................................................................................................................................8

5. Llano River Subbasin, Kimble County, Texas, and vicinity ...........................................9

6. Nueces Headwaters Subbasin, Edwards and Real Counties, Texas, and 

vicinity .................................................................................................................................10

7. West Nueces River Subbasin, Edwards County, Texas, and vicinity .......................11

8. Upper Frio River Subbasin, Real County, Texas, and vicinity ....................................12

9. Dry Devils River Subbasin, Edwards County, Texas, and vicinity .............................13

10. Sycamore Creek Subbasin, Edwards County, Texas, and vicinity ............................14

11–23. Longitudinal profile of: 

11. The North Llano River, Kimble County, Texas, and vicinity ........................................15

12. The South Llano River, Edwards and Kimble Counties, Texas ..................................15
v

13. The Llano River, Kimble County, Texas, and vicinity ...................................................16

14. Johnson Fork, Kimble County, Texas, and vicinity .......................................................16

15. The James River, Kimble County, Texas, and vicinity .................................................17

16. The Little Devils River, Kimble County, Texas, and vicinity ........................................17

17. The Nueces River, Edwards and Real Counties, Texas, and vicinity .......................18

18. The West Nueces River, Edwards County, Texas, and vicinity .................................18

19. The West Frio River, Real County, Texas .......................................................................19

20. The East Frio River, Real County, Texas, and vicinity ..................................................19

21. The Frio River, Real County, Texas, and vicinity ...........................................................20

22. The Dry Frio River, Real County, Texas, and vicinity ...................................................20

23. Sycamore Creek, Edwards County, Texas, and vicinity ..............................................21

24. R-code used for partial-duration flood-frequency analyses ..............................................23

25–34. Map and graphs of channel cross-section transects at: 

25. North Llano River near Roosevelt, Kimble County, Texas ..........................................25

26. North Llano River near Junction, Kimble County, Texas ............................................26

27. South Llano River at Baker Ranch, Edwards County, Texas ......................................27

28. South Llano River at U.S. Highway 377 near Rocksprings, Edwards County, 

Texas ...................................................................................................................................28

29. South Llano River at 700 Springs Ranch, Edwards County, Texas ............................29

30. South Llano River at South Llano River State Park, Kimble County, Texas .............30

31. South Llano River at Texas Tech University–Junction, Kimble County, Texas .......31

32. Johnson Fork at Lowlands Crossing, Kimble County, Texas ......................................32

33. Llano River near Junction, Kimble County, Texas .......................................................33

34. Llano River near Ivy Chapel, Kimble County, Texas ....................................................34

35. Cross-section graphs of selected low-water crossings in study area, Kimble 

County, Texas ..............................................................................................................................35

36. Photograph showing underwater sampling grid and particle-size analyzer on 

gravel and cobble bed of South Llano River, Kimble County, Texas ..................................35

37–46. Bed-material particle-size distribution of: 

37. North Llano River near Roosevelt, Kimble County, Texas ..........................................36

38. North Llano River near Junction, Kimble County, Texas ............................................36

39. South Llano River at Baker Ranch, Edwards County, Texas ......................................37

40. South Llano River at U.S. Highway 377 near Rocksprings, Edwards County, 

Texas ...................................................................................................................................37

41. South Llano River at 700 Springs Ranch, Edwards County, Texas ............................38

42. South Llano River at South Llano River State Park, Kimble County, Texas .............38

43. South Llano River at Texas Tech University–Junction, Kimble County, Texas .......39

44. Johnson Fork at Lowlands Crossing, Kimble County, Texas ......................................39

45. Llano River near Junction, Kimble County, Texas .......................................................40

46. Llano River near Ivy Chapel, Kimble County, Texas ....................................................40

47. Graph showing mean particle size descriptors (d16, d50, d84) and downstream 

distance for study sites where Wolman pebble counts were made, Edwards and 

Kimble Counties, Texas .............................................................................................................43

48–50. Photographs showing: 

48. Nueces River at Vance Crossing, Edwards-Real County line, Texas .......................43

49. South Llano River at South Llano River State Park, Kimble County, Texas .............44
vi

50. South Llano River at Baker Ranch, Edwards County, Texas ......................................44

51. Boxplot showing distribution of mean watershed slope and qualitative bedmaterial abundance and activity classification level ..........................................................45

Tables

1. List of study sites and estimates of drainage area and slope of site-specific 

watersheds in the study area, Edwards, Kimble, and Real Counties, Texas, and 

vicinity ............................................................................................................................................5

2. Flood magnitudes of U.S. Geological Survey and Lower Colorado River Authority 

gaging stations in the study area, Edwards, Kimble, and Real Counties, Texas, 

and vicinity, calculated by partial-duration flood-frequency analyses based on 

the kappa distribution ...............................................................................................................22

3. Flood magnitudes of ungaged sites in the study area, Edwards, Kimble, and Real 

Counties, Texas, and vicinity, computed by regional regression equations ....................24

4. Channel slopes computed for study sites using measured cross-sectional data, 

Edwards and Kimble Counties, Texas ....................................................................................41

5. Bed-material particle-size descriptors computed for study sites, Edwards and 

Kimble Counties, Texas .............................................................................................................42

6. Qualitative observations and classification of coarse bed-material abundance 

and activity at selected study sites, Edwards, Kimble, and Real Counties, Texas .........46

7. Computations for initial bed-material entrainment at sites with particle-size data, 

Edwards and Kimble Counties, Texas ....................................................................................47

8. Computations for complete bed-material entrainment at sites with particle-size 

data, Edwards and Kimble Counties, Texas ..........................................................................48

9. Computations for initial bed-material entrainment at sites with particle-size data, 

Edwards and Kimble Counties, Texas, using average-condition approach based 

on median channel slope, downstream decrease of particle size, and median 

Manning’s n ................................................................................................................................50

10. Computations for complete bed-material entrainment at sites with particle-size 

data, Edwards and Kimble Counties, Texas, using average-condition approach 

based on median channel slope, downstream decrease of particle size, and 

median Manning’s n ..................................................................................................................51

11. Froude numbers of initial and complete bed-material entrainment at sites with 

particle-size data, Edwards and Kimble Counties, Texas, for site-by-site analyses ......53

12. Froude numbers of initial and complete bed-material entrainment at sites with 

particle-size data, Edwards and Kimble Counties, Texas, using average-condition 

approach based on median channel slope, downstream decrease of particle size, 

and median Manning’s n ..........................................................................................................54

13. Approximate return periods for initial and complete bed-material entrainment 

at sites with particle-size data, Edwards and Kimble Counties, Texas, based on 

site-by-site individual measurements or estimates .............................................................55

14. Approximate return periods for initial and complete bed-material entrainment 

at sites with particle-size data, Edwards and Kimble Counties, Texas, based 

on median channel slope, downstream decrease of particle size, and median 

Manning’s n ................................................................................................................................56
vii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Density

pound per cubic foot (lb/ft3) 16.02 kilogram per cubic meter (kg/m3)

pound per cubic foot (lb/ft3) 0.01602 gram per cubic centimeter (g/cm3)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2) 247.1 acre

square meter (m2) 10.76 square foot (ft2) 

square kilometer (km2) 0.3861 square mile (mi2)

Flow rate

cubic meter per second (m3/s) 70.07 acre-foot per day (acre-ft/d) 

meter per second (m/s) 3.281 foot per second (ft/s) 

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

cubic meter per second (m3/s) 22.83 million gallons per day (Mgal/d) 

Force

newton (N) ((kg×m)/s2) 0.225 pound (lb)
Blank Page
Abstract 

The Texas Department of Transportation spends considerable money for maintenance and replacement of lowwater crossings of streams in the Edwards Plateau in Central 

Texas as a result of damages caused in part by the transport 

of cobble- and gravel-sized bed material. An investigation 

of the problem at low-water crossings was made by the U.S. 

Geological Survey in cooperation with the Texas Department of Transportation, and in collaboration with Texas Tech 

University, Lamar University, and the University of Houston. The bed-material entrainment problem for low-water 

crossings occurs at two spatial scales—watershed scale and 

channel-reach scale. First, the relative abundance and activity 

of cobble- and gravel-sized bed material along a given channel 

reach becomes greater with increasingly steeper watershed 

slopes. Second, the stresses required to mobilize bed material 

at a location can be attributed to reach-scale hydraulic factors, 

including channel geometry and particle size. The frequency 

of entrainment generally increases with downstream distance, 

as a result of decreasing particle size and increased flood 

magnitudes. An average of 1 year occurs between flows that 

initially entrain bed material as large as the median particle 

size, and an average of 1.5 years occurs between flows that 

completely entrain bed material as large as the median particle 

size. The Froude numbers associated with initial and complete 

entrainment of bed material up to the median particle size 

approximately are 0.40 and 0.45, respectively.

Introduction

The Texas Department of Transportation (TxDOT) 

commonly builds and maintains low-water crossings (LWCs) 

over streams (fig. 1) in the Edwards Plateau in Central Texas. 

LWCs are low-height structures, typically constructed of 

concrete and asphalt, that provide acceptable passage over 

seasonal rivers or streams with relatively low normal-depth 

flow and are designed to accommodate high-flow events by 

roadway overtopping. The streams of the Edwards Plateau are 

characterized by cobble- and gravel-sized bed material and 

highly variable flow regimes. Low base-flow conditions occurring most of the time occasionally are interrupted by severe 

floods. The floods entrain and transport substantial loads of 

bed material in the stream channels (Baker, 1977). As a result, 

LWCs in the Edwards Plateau are bombarded and abraded by 

bed material during high-flow events and periodically must be 

maintained or even replaced.

TxDOT spends considerable money for maintenance 

and replacement of LWCs on relatively low-traffic-volume 

highways in the Edwards Plateau. Based on a review of 

selected TxDOT internal budget reports in San Angelo, Texas, 

in 2004, on average as much as $1 million per year is spent 

on maintenance and repair of LWCs. TxDOT commonly must 

reconstruct LWCs because of site-specific constraints against 

bridging the stream, which might be the optimum solution. 

As a compromise for maintenance and repair, some LWCs are 

constructed more elaborately than others. Structural failure or 

maintenance of LWCs as a result of high-flow-induced bedmaterial transport has occurred as recently as the November 

Potential for Bed-Material Entrainment 

in Selected Streams of the Edwards 

Plateau—Edwards, Kimble, and 

Real Counties, Texas, and Vicinity

By Franklin T. Heitmuller and William H. Asquith

Figure 1. Flatrock Crossing of the South Llano River in Kimble 

County, Texas. 
2000, July 2002, June 2004, and March 2007 floods on the 

Edwards Plateau.

This research study, done by the U.S. Geological Survey 

(USGS), in cooperation with TxDOT, and in collaboration 

with Texas Tech University, Lamar University, and the University of Houston, was designed to quantify the potential for 

bed-material entrainment in selected streams in the Edwards 

Plateau as part of TxDOT Research Project 0–4695: Guidance for Design in Areas of Extreme Bed-Load Mobility. 

The study included Geographic Information System (GIS) 

analyses, flood-frequency analyses, field investigations, and 

computations of bed-material entrainment. The results of the 

current (2008) study are expected to complement numerical 

and physical modeling of bed-material mobility done by Texas 

Tech University, Lamar University, and the University of 

Houston, and to facilitate LWC design and assignation of risk 

to channel reaches subject to bed-material mobility.

Purpose and Scope

The purpose of this report is to assess the potential for 

bed-material entrainment in selected streams of the Edwards 

Plateau in a study area comprising Edwards, Kimble, and 

Real Counties and vicinity. The methods used to address 

bed-material entrainment in the report are (1) GIS analyses of 

drainage area and slopes, (2) partial-duration flood-frequency 

analyses, (3) site reconnaissance, and (4) field surveys of 

channel geometry and bed-material particle size. Using data 

generated by these methods, computations of bed-material 

entrainment are given for selected sites, and the results are 

generalized for the study area.

Description of Study Area

The study area for this report comprises Edwards, Kimble, and Real Counties and some parts of adjacent counties in 

south-central Texas (fig. 2); however, the bed-material entrainment problem in the LWC context extends to other counties in 

the Edwards Plateau. An investigation of the potential for cobble- and gravel-bed entrainment could be focused anywhere 

in the Edwards Plateau, but the research team, in conjunction 

with TxDOT personnel involved in the project, decided on the 

three counties. The three-county study area contains a number 

of streams appropriate for bed-material entrainment research, 

including the Llano River, Frio River, Nueces River, and tributaries. The streams originate in relatively western and southern 

parts of the Edwards Plateau, an elevated lower-Cretaceous 

limestone and dolomite tableland. The streams in the study 

area are incised into the plateau, resulting in the removal of 

considerable amounts of lower-Cretaceous carbonate rock. 

Much of this material has been transported downstream toward 

the Gulf of Mexico, but large quantities remain and some 

of this material occurs as valley fill and bed material in the 

streams of the plateau.

The history of sediment transport and sedimentation 

in the Edwards Plateau is important to understanding the 

contemporary rates and processes of bed-material entrainment 

in the study area. Blum and Valastro (1989, 1992), Blum and 

others (1994), and Mear (1995) document episodes of fluvial 

geomorphic development and sedimentation in the Edwards 

Plateau for the last 20,000 years.

In summary, flood regimes were more moderate and 

frequent during the last glacial maximum (about 20,000 to 

14,000 years ago), and soil formation in the uplands was 

accompanied by lateral migration of stream channels and 

storage of sediment in floodplains. The end of the Pleistocene 

Epoch (about 14,000 to 10,500 years ago) was characterized 

by less humid conditions. Streams removed sediment from 

valleys, became incised into bedrock, and caused upland soil 

erosion (Cooke and others, 2003). Complete upland soil erosion and an episode of valley widening and gradual aggradation occurred during the early- to middle-Holocene Epoch 

(about 10,500 to 5,000 years ago). The period from about 

5,000 to 2,500 years ago was characterized by the driest conditions across the Edwards Plateau during the last 20,000 years. 

The supply of sediment exceeded the capacity of streams to 

transport the material, and a decrease in floods resulted in 

relatively little floodplain activity. More humid conditions and 

frequent floods characterized the period from about 2,500 to 

1,000 years ago. Broad, shallow stream channels continued to 

fill with coarse sediment during this period. Finally, conditions 

during the last 1,000 years have been comparatively dry, but 

are punctuated by extreme floods. Stream channels are incised 

into the alluvium, and older floodplains are abandoned and 

stranded as inactive terrace deposits.

The present study area is in a climatic transition area 

between semiarid conditions in the west and subhumid conditions in the east (Larkin and Bomar, 1983). More importantly 

for bed-material entrainment, however, is that the study area 

is characterized by a high flash-flood potential (Beard, 1975; 

Caran and Baker, 1986).

Flash floods in the Edwards Plateau are generated by 

diverse climatic mechanisms. Convergent low-pressure 

troughs or cut-off systems have the potential to intercept moist 

air masses from either the Pacific Ocean to the southwest or 

the Gulf of Mexico to the southeast. Additionally, tropical disturbances and hurricanes from the Gulf of Mexico can result 

in widespread, intense rainfall. As a result of steep slopes and 

shallow soils incapable of absorbing much rainfall, streams 

in the study area rapidly respond to intense rainfall, and flash 

floods are common.

The study area is mostly rural ranchland that supports 

hunting leases and limited grazing. The absence of thick 

upland soils constrains minor irrigated agriculture to wide 

valley reaches. Vegetation is characterized by oak and juniper 

woods in the uplands and open to wooded groves of mesquite, 

oak, pecan, cypress, and cottonwood trees in the river valleys. 

Steep slopes are common throughout the study area, especially 

along the walls adjacent to alluvial valleys.

2 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 2. Location of study area—Edwards, Kimble, and Real Counties, Texas, and vicinity. 

Introduction 3

100°30' 100°00' 99°30'

30°30'

30°00'

29°30'

Edwards 

Plateau (Griffith and

 others, 2004)

TEXAS

Study

area

LOCATION MAP

JUNCTION

ROCKSPRINGS

LEAKEY

EDWARDS 

COUNTY

REAL

COUNTY

KIMBLE

COUNTY

Base modified from Texas Natural Resources 

Information System (TNRIS) digital data at

1:24,000 (2005)

Albers equal area projection parameters Central meridian: -100.0

Standard parallel 1: 34.91666667

Standard parallel 2: 27.41666667

Latitude of origin: 31.16666667

2 13

21

20

23

24

25 26

22

1

3 4

5

6

7

8

9

10

11

12

14

15

16

17 18

19

1

3

4

5

6

7

8

9

10

11

12

14

15

16

17

18

19

13 08150000 Llano River near Junction 08196000 Dry Frio River near Reagan Wells

08195000 Frio River at Concan

08190000 Nueces River at Laguna

08190500 West Nueces River near Brackettville

EXPLANATION

North Llano River near Roosevelt

South Llano River at Baker Ranch

South Llano River at U.S. Highway 377 near Rocksprings

South Llano River at 700 Springs Ranch

South Llano River at First Crossing

South Llano River at South Llano River State Park

South Llano River at Texas Tech University–Junction

South Llano River at Flatrock Crossing

Johnson Fork at Guzman Crossing

Johnson Fork at Paks Crossing

Johnson Fork at Lowlands Crossing

Llano River near Ivy Chapel

Llano River at Yates Crossing

Nueces River at Ben Williams Crossing

Nueces River at Vance Crossing

Cedar Creek at Ranch Road 336

Frio River at Upper Ranch Road 1120 Crossing

20

21

23

24

25

26

Johnson Fork near Junction

James River near Mason

22 08150700 Llano River near Mason

Site name

Site

number

(table 1) Site name

Site

number

(table 1)

2 08148500 North Llano River near Junction

Boundary of study area

Boundary of Edwards Plateau

Detailed study site and site number

Low-water crossing study site and site number

Lower Colorado River Authority streamflow-gaging station and site number

U.S. Geological Survey streamflow-gaging station and site number

Detailed study site at U.S. Geological Survey streamflow-gaging station and site number 2

17

21

22

3

0 10 20 30 KILOMETERS

0 3 10 20 0 MILES

§ ̈¦

277

£¤

£¤

£¤

£¤

£¤

£¤

West Nueces

River

Creek

Sycamore

River Nueces

Dry

Frio

River

Frio

River

River Llano

Llano River

North

South Llano

River

Johnson

Fork

River River

Little

Devils

James

Frio

Frio

East

West

River

River

Dry Devils River
Only one small reservoir is located in the study area and 

occurs on the South Llano River, in Junction. Although both 

floods and base flows spill over the top of the control structure, the impoundment effectively traps coarse bed material. 

The ubiquitous presence of cobble and gravel bars just downstream, however, shows that the North Llano River supplies 

sufficient sediment to the main-stem Llano River.

The study sites (fig. 2) primarily were chosen to characterize downstream changes in channel geometry and bedentrainment potential, contrasting with an alternative approach 

to compare separate drainage basins. For this reason, all 

detailed study sites are in the Llano River Basin, chosen 

because most of the problematic LWCs identified by TxDOT 

personnel are located there. A summary of the study sites is 

provided:

•	 South Llano River at Baker Ranch (site 3) represents 

an ephemeral, upper headwater channel. The channel occurs in relatively fine-grained valley fill and is 

a sequence of water-filled pools that connect during 

storms that produce runoff.

•	 South Llano River at U.S. Highway 377 near Rocksprings (site 4) is a larger, bedrock, ephemeral channel just downstream from the incision point into the 

plateau.

•	 South Llano River at 700 Springs Ranch (site 5) and 

North Llano River near Roosevelt (site 1) represent 

channels with mixed bedrock and cobble and gravel 

beds, each having disconnected floodplain units 

between the channel and valley wall.

•	 South Llano River at South Llano River State Park 

(site 7) and at Texas Tech University–Junction (site 8) 

have a meandering channel within a Holocene floodplain containing lenses of cobble- and gravel-sized 

material wedged between relatively fine-grained sediment. The Holocene floodplain occurs within a larger, 

elevated late Pleistocene or early Holocene terrace containing cobble- and gravel-sized material, commonly 

cemented by calcite near the surface.

•	 North Llano River near Junction (site 2) is relatively 

straight and has a cobble and gravel bed. The channel is closely bounded by the late Pleistocene or early 

Holocene terrace, with a few areas of recent floodplain 

deposits.

•	 Johnson Fork at Lowlands Crossing (site 12) is similar 

to North Llano River near Junction (site 2), except the 

meander bends are enlarged as a result of Holocene 

hydrologic change and contemporary decreased rates 

of lateral migration (Blum and Valastro, 1989).

•	 Llano River near Junction (site 13) and near Ivy Chapel 

(site 14) also have enlarged meanders and gradual transitions between the older terrace deposits and modern 

floodplain units. 

Additional study sites shown in figure 2 were not surveyed in detail and are not used in computations of bedmaterial entrainment. A summary of these sites is provided:

•	 Nueces River at Ben Williams Crossing (site 16) and 

at Vance Crossing (site 17) is a cobble- and gravelchoked meandering channel within Holocene floodplain units between the channel and valley wall. Cedar 

Creek at Ranch Road 336 (site 18) is similar to the 

Nueces River but at a smaller scale.

•	 Frio River at the Upper Ranch Road 1120 Crossing 

(site 19) is a meandering, cypress-tree-lined, cobble- to 

gravel-bed channel within a Holocene floodplain. 

•	 South Llano River at First Crossing (site 6), South 

Llano River at Flatrock Crossing (site 9), Johnson Fork 

at Guzman Crossing (site 10), Johnson Fork at Paks 

Crossing (site 11), and Llano River at Yates Crossing 

(site 15) only were visited to document and inspect the 

LWC structure.

Review of Existing Data 

The topic of bed-material entrainment and bedload transport of gravel-bed streams has received considerable attention. 

Heitmuller and others (2005), an earlier product of TxDOT 

research project 0–4695, serves as a literature review for this 

project. The literature associated with sediment transport in 

gravel-bed streams and application of theory to stream management is extensive, global, and multidisciplinary. Investigations focus on data derived from natural channels or experimental flumes, and approaches vary from deterministic to 

empirical. A variety of methods are used to study gravel-bed 

stream characteristics, including surveying channel geometry, 

particle tracking, bed-material and bedload sampling, and 

developing sediment-discharge rating curves, among others. 

Regardless of the method implemented to understand the sediment regime of gravel-bed streams, a thorough knowledge of 

watershed characteristics, local processes controlling the transport, and collection of data characterizing sediment mobility 

are essential for the success of any management practice. 

Heitmuller and others (2005) conclude that an investigation 

of bed-material mobility needs to include robust geographic, 

field, and laboratory assessments. For other studies on bedmaterial entrainment, refer to Gustavson (1978) and Elliott 

(2002).

Acknowledgments

The authors acknowledge the following colleagues of 

TxDOT for their considerable professional and technical support of this work and their critical feedback on study direction: 

The authors also thank Julia Brown (0–4695 Program Coordinator, 2007–2008), Lewis Nowlin (0–4695 Project Director, 

2007–2008), George R. Herrmann (0–4695 Project Advisor), 

and Walter McCullough (0–4695 Project Advisor), and others. 

4 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Introduction 5

Table 1. List of study sites and estimates of drainage area and slope of site-specific watersheds in the study area, Edwards, Kimble, 

and Real Counties, Texas, and vicinity.

[Drainage area and slope estimates were derived from a Geographic Information System; mi2, square miles; km2, square kilometers; LCRA, Lower Colorado 

River Authority; --, not computed]

Study site 

number 

(fig. 2)

Study site identifier or USGS station 

number and identifier

Purpose of site for bedentrainment project1Drainage area 

[km2 (mi2)]

Mean and standard 

deviation of 

watershed slope 

(percent)

1 North Llano River near Roosevelt Survey 1,145 (442) 4.06; 3.87

2 08148500 North Llano River near 

Junction

Flood frequency and 

survey

2,335 (902) 5.35; 5.78

3 South Llano River at Baker Ranch Survey 417 (161) 2.63; 1.92

4 South Llano River at U.S. Highway 377 

near Rocksprings

Survey 1,134 (438) 3.88; 3.33

5 South Llano River at 700 Springs 

Ranch

Survey 1,352 (522) 4.46; 4.19

6 South Llano River at First Crossing Low-water crossing survey 2,040 (788) 4.95; 4.76

7 South Llano River at South Llano River 

State Park

Survey 2,256 (871) 5.36; 5.50

8 South Llano River at Texas Tech 

University–Junction

Survey 2,271 (877) 5.41; 5.64

9 South Llano River at Flatrock Crossing Low-water crossing survey 2,279 (880) 5.45; 5.72

20 Johnson Fork near Junction (LCRA) Flood frequency 758 (293) --;--

10 Johnson Fork at Guzman Crossing Qualitative channel-bed 

observation only2

-- (--) --;--

11 Johnson Fork at Paks Crossing Qualitative channel-bed 

observation only2

-- (--) --;--

12 Johnson Fork at Lowlands Crossing Survey 778 (300) 5.30; 5.82

13 08150000 Llano River near Junction Flood frequency and 

survey

4,815 (1,859) 5.50; 5.98

14 Llano River near Ivy Chapel Survey 5,939 (2,293) 5.68; 6.38

15 Llano River at Yates Crossing Low-water crossing survey 6,078 (2,347) 5.74; 6.47

22 08150700 Llano River near Mason Flood frequency 8,410 (3,247) --;--

21 James River near Mason (LCRA) Flood frequency 845 (326) --;--

16 Nueces River at Ben Williams Crossing Qualitative channel-bed 

observation only

578 (223) 15.3; 13.3

17 Nueces River at Vance Crossing Qualitative channel-bed 

observation only

642 (248) 16.4; 14.2

24 08190000 Nueces River at Laguna Flood frequency 1,908 (737) --;--

23 08190500 West Nueces River near 

Brackettville

Flood frequency 1,800 (695) --;--

18 Cedar Creek at Ranch Road 336 

Crossing

Qualitative channel-bed 

observation only

30.5 (11.8) 31.4; 18.6

19 Frio River at Upper Ranch Road 1120 

Crossing

Qualitative channel-bed 

observation only

620 (239) 19.8; 17.1

26 08195000 Frio River at Concan Flood frequency 1,009 (390) --;--

25 08196000 Dry Frio River near Reagan 

Wells

Flood frequency 327 (126) --;--

1 Surveys include cross-sectional channel geometry and bed-material particle-size analyses. Low-water crossing surveys include only cross-sectional 

depictions of the roadway surface. Flood-frequency analyses were made at sites with gaging stations. Qualitative channel-bed observations were made at sites to 

assess bed-material abundance and activity.

2 These sites were not included in bed-material abundance and activity analyses, because of proximity to Johnson Fork at Lowlands Crossing, which was used.
The authors are indebted to our research colleagues Theodore G. Cleveland (University of Houston), Xing Fang (Lamar 

University), David B. Thompson (Texas Tech University), and 

Keh-Han Wang (University of Houston) for their collaboration. Additionally, the authors recognize Kyle E. Juracek 

(USGS) for helpful reviews. Finally, we thank those who provided access to public and private properties in the study area, 

without which the work would not have been possible.

Methods

The methods used to obtain data necessary for computation of bed-material entrainment potential can be categorized 

under four headings: (1) GIS analyses, (2) flood-frequency 

analyses, (3) site reconnaissance, and (4) field surveys. Multiple sites for data collection are included in the study (fig. 2; 

table 1), and the specific methods used at each individual site 

varied. GIS mapping software was used to delineate watersheds, determine drainage areas, and model watershed and 

channel slopes. Partial-duration flood-frequency analyses 

were done to estimate the return period of floods for USGS 

and Lower Colorado River Authority (LCRA) gaging stations 

in the study area. Regional regression equations were used to 

estimate flood frequency at ungaged study sites (Asquith and 

Slade, 2007). Site reconnaissance was done with assistance 

from TxDOT personnel knowledgeable about the locations of 

sites with extreme bed-material mobility and damage to transportation infrastructure. Finally, field surveys were designed 

to obtain the parameters needed to apply equations to compute 

bed-material entrainment potential. Field surveys included 

measurements of channel geometry, LWC geometry, and bedmaterial particle sizes.

GIS Analysis

Watersheds

Watersheds are important when accounting for factors 

that contribute to surface-water hydraulics or sediment transport at spatial scales greater than the channel reach. Watersheds were delineated for all sites in the study area using ESRI 

ArcGIS 9.1 (Maidment, 2002). Before GIS analysis, 10-meter 

digital elevation models (DEMs) of the study area were downloaded from the USGS Seamless Data Distribution System 

(U.S. Geological Survey, 2005). Using GIS, the individual 

DEM panels were tiled into one raster dataset. The DEM 

mosaic subsequently was adjusted for sinks to ensure that 

the model would have continuous flow from higher to lower 

elevations appropriate for dendritic drainage networks. Next, 

the filled DEM was processed into flow-direction and flowaccumulation raster datasets. Pour points at study sites were 

located on the flow-accumulation raster dataset, and watersheds were constructed using the flow-direction raster dataset. 

Finally, individual watershed raster datasets were converted to 

vector polygon feature classes, reprojected from a geographic 

coordinate system to the Albers equal area coordinate system, 

and drainage areas (table 1) were computed.

Watershed and Channel Slopes

Mean watershed slope, determined by DEM-analysis 

and defined here as the average of all land-surface slopes 

in a drainage basin regardless of directional orientation, is 

considered an important parameter for understanding the 

potential for sediment delivery to stream channels in the study 

area. Theoretically, for watersheds of similar size, shape, and 

land-cover characteristics, greater mean watershed slopes will 

promote increased rates of hillslope erosion and subsequent 

delivery of sediment to channel networks. For the purpose of 

this investigation, mean watershed slope acts as a surrogate 

for the variety of processes occurring at spatial scales larger 

than the channel reach. Mean watershed slope was computed 

using GIS. The previously mentioned 10-meter DEM mosaic 

was used to generate a percentage-slope raster dataset. The 

percentage-slope raster dataset subsequently was reprojected 

from a geographic coordinate system to the Albers coordinate system. Using watershed polygons, zonal statistics were 

applied in GIS to calculate mean watershed slope (table 1) 

from the percentage-slope raster dataset.

Visualization of the distribution of channel slopes (conceptually distinct from mean watershed slope) is an important 

tool for TxDOT engineers and other practitioners operating at 

watershed and channel-reach scales. For this reason, channel slopes were mapped in the study area using GIS. First, 

high-resolution hydrography data for subbasins in the study 

area were downloaded from the USGS National Hydrography 

Dataset (NHD) (U.S. Geological Survey, 2006). Hydrography 

data were reprojected from a geographic coordinate system 

to the Albers coordinate system. Mean channel slopes were 

assigned to individual flowline segments using zonal statistics 

applied to the aforementioned percentage-slope raster dataset. Visualization is maximized because NHD flowline data 

originally are divided into small vector lengths. The final maps 

for NHD subbasins in the study area are presented in figures 3 

through 10.

An alternative method for visualization of channel slope 

is through the creation of longitudinal profiles. A longitudinal 

profile is a graph of channel-bed elevation with downstream 

distance and is limited to one channel. Longitudinal profiles 

can be used to (1) estimate channel slope for reaches that were 

not surveyed in the field or (2) locate relatively steep channel 

reaches where greater energy conditions exist for sediment 

entrainment and transport. Longitudinal profiles for selected 

streams in the study area were created using GIS. Individual 

channel reaches, beginning at the headwaters, were selected by 

feature name from NHD high-resolution datasets of subbasins 

in the study area and were merged into one continuous line 

segment. The channel-reach line segment was converted to a 

three-dimensional (3-D) feature class using the filled 10-meter 

DEM mosaic. Next, the 3-D channel-reach feature class was 

converted to a route feature class that stores downstream 

6 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
distance. The route feature class line segment was converted 

into equidistant points separated by 500 meters of downstream 

distance, and the points were used to extract downstream 

distance from the route feature class. Finally, elevations were 

extracted to the points using the filled 10-meter DEM. Some 

points were selectively removed if discrepancies in the DEM 

caused downstream elevations to be higher than upstream 

elevations. Longitudinal profiles are presented in figures 11 

through 23.

Flood-Frequency Analysis

The potential for bed-material entrainment can be computed for a site using GIS- and field-collected data. However, 

the only way to estimate the temporal frequency of bed-material entrainment is through association with flood frequency. 

For this reason, partial-duration flood-frequency analyses 

were done for USGS and LCRA gaging stations in the study 

area (table 2). The partial-duration series uses discharges 

Figure 3. Channel slope of the North Llano River Subbasin, Kimble County, Texas, and vicinity. 

Methods 7

£¤377

£¤377

£¤83

§ ̈¦10

FM 2291

FM 1674

EXPLANATION

Mean channel slope,

in percent

0–2

2–5

5–10

10–20

20–50

N

KIMBLE

COUNTY

U.S. Geological Survey 10-meter Digital Elevation Model

Albers equal area projection parameters

Central meridian: -96.0

Standard parallel 1: 29.5

Standard parallel 2: 45.5

Latitude of origin: 23.0

Hydrography from

National Hydrography Dataset

(U.S. Geological Survey, 2006) 0 3 6 9 12 KILOMETERS

0369 12 MILES

Area enlarged NORTH LLANO RIVER SUBBASIN

INDEX MAP OF STUDY AREA
of measured and estimated flood peaks above a designated 

threshold (referred to as the base discharge).

Partial-duration series are preferred over annual-maximum series to estimate flood frequency of relatively small 

or moderate events (1- to 10-year return period) or for gaging stations with a relatively short period of record (Soong 

and others, 2004). This advantage is particularly important to 

estimate the frequency of bed-material entrainment, because 

sediment commonly is mobilized by moderate events, even 

those below bankfull stage. Additionally, the episodic nature 

of floods in the study area causes the annual-maximum series 

to have low outliers for dry years, whereas the partial-duration 

series avoids low outliers.

Base discharges for USGS gaging stations are published 

in annual water-data reports; for example U.S. Geological 

Survey (2007). Some published base discharges have either 

increased or decreased over time. As a result, some earlyrecord peaks above base are missing for gaging stations with 

Figure 4. Channel slope of the South Llano River Subbasin, Edwards, Kimble, and Real Counties, Texas, and vicinity. 

8 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

0 3 6 9 12 KILOMETERS

0369 12 MILES

State Hwy 55

State Hwy 41

£¤377

§ ̈¦10

£¤83

EXPLANATION

Mean channel slope,

in percent

0–2

2–5

5–10

10–20

20–75

N

EDWARDS

COUNTY

REAL

COUNTY

KIMBLE

COUNTY

U.S. Geological Survey 10-meter Digital Elevation Model

Albers equal area projection parameters

Central meridian: -96.0

Standard parallel 1: 29.5

Standard parallel 2: 45.5

Latitude of origin: 23.0

Hydrography from

National Hydrography Dataset

(U.S. Geological Survey, 2006)

Area enlarged SOUTH LLANO RIVER SUBBASIN

INDEX MAP OF STUDY AREA
Figure 5. Channel slope of the Llano River Subbasin, Kimble County, Texas, and vicinity. 

Methods 9

U.S. Geological Survey 10-meter Digital Elevation Model

Albers equal area projection parameters

Central meridian: -96.0

Standard parallel 1: 29.5

Standard parallel 2: 45.5

Latitude of origin: 23.0

Hydrography from

National Hydrography Dataset

(U.S. Geological Survey, 2006)

0 5 10 KILOMETERS

0 5 10 MILES

EXPLANATION

Mean channel slope,

in percent

0–2

2–5

5–10

10–20

20–80

N

£¤377

£¤377

£¤83 § ̈¦10

£¤290

£¤83

FM 385

FM 1871

FM 2169

KIMBLE

COUNTY

INDEX MAP OF STUDY AREA

Area enlarged

LLANO RIVER SUBBASIN
Figure 6. Channel slope of the Nueces Headwaters Subbasin, Edwards and Real Counties, Texas, and vicinity. 

10 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

U.S. Geological Survey 10-meter Digital Elevation Model

Albers equal area projection parameters

Central meridian: -96.0

Standard parallel 1: 29.5

Standard parallel 2: 45.5

Latitude of origin: 23.0

Hydrography from

National Hydrography Dataset

(U.S. Geological Survey, 2006)

EXPLANATION

Mean channel slope,

in percent

0–2

2–5

5–10

10–20

20–60

N

55

55

2630

£¤377 41

FM 337

FM 335

EDWARDS

COUNTY

REAL

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

Area enlarged

NUECES HEADWATERS SUBBASIN

INDEX MAP OF STUDY AREA
Figure 7. Channel slope of the West Nueces River Subbasin, Edwards County, Texas, and vicinity.

Methods 11

N

EXPLANATION

Mean channel slope,

in percent

0–2

2–5

5–10

10–20

20–70

0 5 10 KILOMETERS

0 5 10 MILES

FM 674

£¤377

EDWARDS

COUNTY

U.S. Geological Survey 10-meter Digital Elevation Model

Albers equal area projection parameters

Central meridian: -96.0

Standard parallel 1: 29.5

Standard parallel 2: 45.5

Latitude of origin: 23.0

Hydrography from

National Hydrography Dataset

(U.S. Geological Survey, 2006)

Area enlarged

WEST NUECES RIVER SUBBASIN

INDEX MAP OF STUDY AREA
Figure 8. Channel slope of the upper Frio River Subbasin, Real County, Texas, and vicinity.

12 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

N

EXPLANATION

Mean channel slope,

in percent

0–2

2–5

5–10

10–20

20–65

0 5 10 KILOMETERS

0 5 10 MILES

FM 336

£¤83

FM 337

FM 1120

41

REAL 39

COUNTY

U.S. Geological Survey 10-meter Digital Elevation Model

Albers equal area projection parameters

Central meridian: -96.0

Standard parallel 1: 29.5

Standard parallel 2: 45.5

Latitude of origin: 23.0

Hydrography from

National Hydrography Dataset

(U.S. Geological Survey, 2006)

Area enlarged

UPPER FRIO RIVER SUBBASIN

INDEX MAP OF STUDY AREA
Figure 9. Channel slope of the Dry Devils River Subbasin, Edwards County, Texas, and vicinity.

Methods 13

N

EXPLANATION

Mean channel slope,

in percent

0–2

2–5

5–10

10–20

20–50

0 5 10 KILOMETERS

0 5 10 MILES

FM 2523

£¤377

£¤277

55

EDWARDS

COUNTY

U.S. Geological Survey 10-meter Digital Elevation Model

Albers equal area projection parameters

Central meridian: -96.0

Standard parallel 1: 29.5

Standard parallel 2: 45.5

Latitude of origin: 23.0

Hydrography from

National Hydrography Dataset

(U.S. Geological Survey, 2006)

Area enlarged

DRY DEVILS RIVER SUBBASIN

INDEX MAP OF STUDY AREA
Figure 10. Channel slope of the Sycamore Creek Subbasin, Edwards County, Texas, and vicinity. 

14 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

N

EXPLANATION

Mean channel slope,

in percent

0–2

2–5

5–10

10–20

20–30

0 5 KILOMETERS

0 5 MILES

U.S. Geological Survey 10-meter Digital Elevation Model

Albers equal area projection parameters

Central meridian: -96.0

Standard parallel 1: 29.5

Standard parallel 2: 45.5

Latitude of origin: 23.0

Hydrography from

National Hydrography Dataset

(U.S. Geological Survey, 2006)

FM 2523

£¤377

EDWARDS

COUNTY

Area enlarged

SYCAMORE CREEK SUBBASIN

INDEX MAP OF STUDY AREA
Figure 11. Longitudinal profile of the North Llano River, Kimble County, Texas, and vicinity. 

Figure 12. Longitudinal profile of the South Llano River, Edwards and Kimble Counties, Texas. 

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

Methods 15
Figure 13. Longitudinal profile of the Llano River, Kimble County, Texas, and vicinity. 

Figure 14. Longitudinal profile of Johnson Fork, Kimble County, Texas, and vicinity. 

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

16 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 15. Longitudinal profile of the James River, Kimble County, Texas, and vicinity. 

Figure 16. Longitudinal profile of the Little Devils River, Kimble County, Texas, and vicinity. 

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

Methods 17
Figure 17. Longitudinal profile of the Nueces River, Edwards and Real Counties, Texas, and vicinity.

Figure 18. Longitudinal profile of the West Nueces River, Edwards County, Texas, and vicinity. 

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

18 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 19. Longitudinal profile of the West Frio River, Real County, Texas.

Figure 20. Longitudinal profile of the East Frio River, Real County, Texas, and vicinity.

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

Methods 19
Figure 21. Longitudinal profile of the Frio River, Real County, Texas, and vicinity.

Figure 22. Longitudinal profile of the Dry Frio River, Real County, Texas, and vicinity. 

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

20 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
a recent decrease in base discharge. Additionally, water-data 

reports for Texas since the 2001 water year only publish 

the annual peak discharge, and other peaks above the base 

discharge are not available. LCRA data were used to fill the 

recent gap for gaging stations in the Llano River Basin. The 

partial-duration flood-frequency analyses in this study used 

the entire period of record for all streamflow-gaging stations 

because the periods with missing peaks are short. The periods with missing peaks are not expected to greatly influence 

flood-frequency analyses, because the periods of record are 

sufficiently long (table 2).

Partial-duration flood peaks were entered into spreadsheets, ranked according to magnitude and recurrence intervals for the period of record were computed. The flood peak 

discharges and recurrence intervals were read into the R 

environment for statistical computing (R Development Core 

Team, 2004) (fig. 24) and the four-parameter kappa distribution curve fit the L-moments (Hosking and Wallis, 1997) of 

observed data. Computations of arrival rates and return periods 

of the partial-duration series are given in Stedinger and others 

(1993), and the techniques described therein were used to 

transform average arrival rates into annual exceedance probabilities. Flood magnitudes for the 1-, 1.5-, 2-, 5-year, 10-, 25-, 

50-, and 100-year return periods (table 2) were calculated from 

the kappa distribution. Flood magnitude estimates have less 

accuracy for gaging stations with short periods of record.

Flood frequency also was estimated for (ungaged) study 

sites not coincident with streamflow-gaging stations using 

regional regression equations (Asquith and Slade, 1997). 

Regional regression equations are available for the 2-, 5-, 10-, 

25-, 50-, and 100-year return periods (table 3). For ungaged 

study sites in the Llano River Basin, equations for Texas 

hydrologic region 4 (based on drainage area and channel 

slope) were used; and for sites in the Nueces River Basin, 

equations for Texas hydrologic region 5 (based on drainage 

area and a basin shape factor) were used.

The estimated flood magnitudes for ungaged sites in the 

Llano River Basin are considerably less than flood magnitudes 

computed from partial-duration flood-frequency analyses at 

USGS gaging stations 08148500 (North Llano River near 

Junction) and 08150000 (Llano River near Junction) (table 2).

Site Reconnaissance

Site reconnaissance trips were made to the study area 

from September 2003 to July 2004. Most trips were guided by 

TxDOT personnel familiar with LWC problems in the study 

area. The reconnaissance trips were necessary to (1) identify 

sites appropriate for field research, (2) qualitatively examine 

channel-bed morphology and sediment, (3) devise appropriate 

methods for field research, (4) examine the design of LWCs 

relative to the channel bed, and (5) document the condition of 

channels and LWCs following the June 2004 flood. Appendix 1 contains photographs of streams and LWCs made during 

site reconnaissance and survey trips to the study area.

Field Surveys

Field surveys at selected sites in the study area were done 

to (1) measure channel-reach geometry, (2) measure LWC 

geometry, and (3) measure particle size of bed material. The 

Figure 23. Longitudinal profile of Sycamore Creek, Edwards County, Texas, and vicinity. 

DOWNSTREAM DISTANCE, IN KILOMETERS

0 25 50 75 100 125 150 175 200

ELEVATION, IN METERS, ABOVE NAVD 88

150

200

300

400

500

600

700

750

Methods 21
first field objective was accomplished at 10 sites in the study 

area. Channel-reach geometry was measured by surveying 

cross sections. These data are used to compute channel crosssectional area and local slope, parameters required for bedmaterial entrainment computations. The second field objective 

was accomplished at four sites in the study area. One of the 

four sites, South Llano River at U.S. Highway 377 near Rocksprings, also has channel-reach geometry surveys. LWC cross 

sections, although not used in computations of bed-material 

entrainment in this study, permit visual comparison to natural 

channel cross sections in the study area. The third field objective was accomplished at 10 sites, the same locations for channel-reach geometry surveys, in the study area. Particle-size 

data are required for computations of bed-material entrainment 

potential. Field surveys were limited to the Llano River Basin 

because (1) most of the LWCs of interest to TxDOT are there, 

and (2) it was desirable to analyze systematic downstream 

trends in bed-material entrainment potential without introducing drainage-basin variability, specifically watershed and 

channel slope differences between the Llano River Basin and 

the Frio, Nueces, and West Nueces River Basins.

Channel Geometry

An electronic total station was used to measure elevation 

and horizontal distance of channel cross sections at 10 selected 

sites in the study area (figs. 25–34), all located in the Llano 

River subbasin. The total station functioned with a coaxial 

22 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

Table 2. Flood magnitudes of U.S. Geological Survey and Lower Colorado River Authority gaging stations in the study area, Edwards, 

Kimble, and Real Counties, Texas, and vicinity, calculated by partial-duration flood-frequency analyses based on the kappa distribution.

[m3/s, cubic meters per second; ft3/s, cubic feet per second; LCRA, Lower Colorado River Authority]

Study 

site 

number 

(fig. 2)

Station number 

or name, 

or both

Period of 

record used

Return period

1-year 

[m3/s (ft3/s)]

1.5-year 

[m3/s (ft3/s)]

2-year 

[m3/s (ft3/s)]

5-year 

[m3/s (ft3/s)]

10-year 

[m3/s (ft3/s)]

25-year 

[m3/s (ft3/s)]

50-year 

[m3/s (ft3/s)]

100-year 

[m3/s (ft3/s)]

2 08148500 North 

Llano River near 

Junction, Tex.

09/14/1915–

09/30/1977; 

06/13/2001–

09/30/2005

131 

(4,610)

331 

(11,700)

524 

(18,500)

1,250 

(44,100)

1,780 

(62,700)

2,370 

(83,600)

2,730 

(96,400)

3,030 

(107,000)

13 08150000 Llano 

River near 

Junction, Tex.

09/13/1915–

01/31/2007

218 

(7,690)

476 

(16,800)

716 

(25,300)

1,740 

(61,300)

2,720 

(96,000)

4,280 

(151,000)

5,660 

(200,000)

7,250 

(256,000)

22 08150700 Llano 

River near 

Mason, Tex.

03/07/1968–

01/31/2007

408 

(14,400)

793 

(28,000)

1,133 

(40,000)

2,495 

(88,100)

3,738 

(132,000)

5,663 

(200,000)

7,306 

(258,000)

9,118 

(322,000)

20 Johnson Fork 

near Junction 

(LCRA)

09/25/2000–

01/31/2007

14 

(510)

23 

(810)

84 

(2,960)

1,250 

(44,300)

2,580 

(91,300)

3,650 

(129,000)

3,960 

(140,000)

4,080 

(144,000)

21 James River near 

Mason (LCRA)

04/15/1999–

01/31/2007

29 

(1,010)

101 

(3,570)

189 

(6,690)

592 

(20,900)

920 

(32,500)

1,300 

(46,000)

1,540 

(54,300)

1,730 

(61,100)

24 08190000 Nueces 

River at Laguna, 

Tex.

10/25/1923–

09/30/2005

138 

(4,890)

326 

(11,500)

513 

(18,100)

1,400 

(49,500)

2,410 

(85,000)

4,300 

(152,000)

6,320 

(223,000)

9,010 

(318,000)

23 08190500 

West Nueces 

River near 

Brackettville, 

Tex.

10/01/1939–

09/30/1950; 

04/01/1956–

09/30/2005

25 

(900)

184 

(6,490)

368 

(13,000)

1,210 

(42,700)

2,080 

(73,500)

3,600 

(127,000)

5,040 

(178,000)

6,850 

(242,000)

26 08195000 Frio 

River at Concan, 

Tex.

10/26/1923–

09/30/1929; 

10/01/1930–

09/30/2005

161 

(5,670)

331 

(11,700)

484 

(17,100)

1,110 

(39,100)

1,690 

(59,800)

2,580 

(91,100)

3,400 

(120,000)

4,280 

(151,000)

25 08196000 Dry 

Frio River near 

Reagan Wells, 

Tex.

09/01/1952–

09/30/2005

69 

(2,450)

144 

(5,080)

212 

(7,490)

521 

(18,400)

861 

(30,400)

1,510 

(53,200)

2,200 

(77,500)

3,120 

(110,000)
phase-contrast measuring system used to triangulate position 

and elevation from horizontal and vertical angles and distance 

to the target. The data logger connected to the total station was 

set to alert the surveyor when any tolerances were exceeded 

following a shot. No tolerances were greatly exceeded during 

the course of any survey done for the project; therefore, reliable position and elevation measurements were acquired.

Sites selected for measurement of channel geometry 

range from upland headwater reaches with limited cobble- and 

gravel-sized bed material to main-stem river channels with 

considerable amounts of coarse bed material. Two sites are at 

USGS gaging stations and surveyed elevations were associated 

with the established stage datum at those stations. The 

remaining eight sites were assigned an arbitrary elevation 

datum. Horizontal coordinates were based on an arbitrary 

datum at all sites, and global positioning system coordinates 

were noted at each station setup. In this manner, potential 

future studies could closely follow the same cross-sectional 

transect.

Channel-geometry survey data were entered into spreadsheets, and channel slopes were computed for sites using 

elevations at edges of water (water surface), thalwegs, or 

floodplains, in decreasing preferential order (table 4). Channel 

slopes computed using measured cross-sectional data are preferable to DEM-computed slopes for reach-scale estimations of 

bed-material entrainment. Cross-sectional geometric data were 

imported into WinXSPRO Version 3.0, a free U.S. Department 

of Agriculture Forest Service (2005) software package, for 

computation of cross-sectional areas and hydraulic radiuses 

for different depths.

Low-Water-Crossing Geometry

An electronic total station was used to measure elevation 

and horizontal distance of LWC cross sections at four selected 

sites in the study area (figs. 28 [bridge center-line], 35), all 

in the Llano River Basin. The four sites were assigned an 

arbitrary elevation and horizontal datum. LWC cross sections 

enable computations of cross-sectional areas and hydraulic 

radiuses for different depths.

Bed-Material Particle Size

Bed material was sampled for particle size at 10 sites 

following the same cross-sectional transects surveyed for 

channel-reach geometry. Particle-size sampling was not done 

at cross sections where coarse bed material was not present. 

Samples were collected along transects to account for different 

geomorphic surfaces of the channel bed, including longitudinal bars, the thalweg, and riffles and pools. The technique used 

along transects commonly is referred to as the Wolman pebble 

count (Wolman, 1954).

Figure 24. R-code used for partial-duration flood-frequency analyses. 

Methods 23

library(lmomco)

Tp <- c(1,1.5,2,5,10,25,50,100)

myfile <- file.choose()

Q <- read.table(file=myfile, header=TRUE, sep="\t", fill=TRUE)

names(Q)

attach(Q)

lmr <- lmoms(Qcfs)

Qsort <- sort(Qcfs)

PP <- pp(Qsort)

plot(PP, log10(Qsort))

lmrdia <- lmrdia()

plotlmrdia(lmrdia, xlim=c(-0.1, 0.6), ylim=c(-0.1, 0.6))

points(lmr$ratios[3], lmr$ratios[4])

ls()

length(Qsort)

para <- parkap(lmr)

plot(PP, log10(Qsort))

lines(PP, log10(quakap(PP, para)),lwd=2)

lambda <- length(Qsort)/5.8

qe <- 1/(Tp*lambda)

G <- 1-qe

SOLUTION <- quakap(G,para)

detach(Q)

H <- list(event=SOLUTION,Tp=Tp,G=G,file=myfile)

points(H$G,log10(H$event),pch=16,col=2,cex=3)

return(H)
The Wolman pebble count was facilitated through the 

use of a sampling grid and a particle-size analyzer (fig. 36). 

The sampling grid measures 50 by 50 centimeters and 

contains intersections spaced every 10 centimeters, resulting 

in 25 intersections per placement. The sampling grid is 

placed on a gravel or cobble bed, and the 25 particles that 

lie directly beneath each intersection are selected. The b-axis 

of each particle is passed through the smallest possible 

opening of the size analyzer. The b-axis is the short axis 

along the same dimensional plane as the longest axis of the 

particle. The diameter of the size-analyzer opening that 

passes the particle is noted. Occasionally by accident, 

slightly more or less than 25 particles are counted, but that 

should not reduce data quality. Using the count data, cumulative particle-size distribution curves can be constructed 

(figs. 37–46), and common particle-size descriptors can 

be computed (table 5). These descriptors are d16, d50, and 

d84, and represent the diameters at which 16, 50, and 84 

percent, respectively, of the sample is finer. The particle-size 

descriptors are required in computations of bed-material 

entrainment.

The Wolman pebble count technique (Wolman, 1954) 

differs from other methods that rely on volume or weight of 

particles to infer a size distribution. One disadvantage of the 

Wolman pebble count is that practitioners tend to select the 

largest particles. The sampling grid described above is used to 

minimize such biases, but larger particles still can be preferentially selected. Nevertheless, the consistency of the method 

permits acceptable comparisons between sites. An additional 

disadvantage of the Wolman pebble count is that only particles 

at the surface are accounted for. Particles buried beneath the 

surface are not included, although they certainly are mobilized 

during very high flows. A gradation in particle size with depth 

beneath the bed surface is not accounted for in studies relying 

on the pebble-count method. A further disadvantage involves 

the neglect of particle shape, as only the b-axis is measured. 

Long, narrow cobbles and gravels are represented by the 

shortest plane, and the assumption of a perfectly round particle 

effectively reduces the recorded particle size. Finally, the 

pebble count does not include differences in particle density, 

which means that particle weight is not propagated through to 

computations of bed-material entrainment.

24 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

Table 3. Flood magnitudes of ungaged sites in the study area, Edwards, Kimble, and Real Counties, Texas, and vicinity, computed by 

regional regression equations (Asquith and Slade, 1997).

[m3/s, cubic meters per second; ft3/s, cubic feet per second]

Study site 

number 

(fig. 2)

Study site

Return period

2-year 

[m3/s (ft3/s)]

5-year 

[m3/s (ft3/s)]

10-year 

[m3/s (ft3/s)]

25-year 

[m3/s (ft3/s)]

50-year 

[m3/s (ft3/s)]

100-year 

[m3/s (ft3/s)]

1 North Llano River near 

Roosevelt

95.7 

(3,380)

283 

(10,000)

490 

(17,300)

841 

(29,700)

1,170 

(41,300)

1,570 

(55,400)

3 South Llano River at Baker 

Ranch

15.6 

(552)

47.9 

(1,690)

86.1 

(3,040)

153 

(5,410)

219 

(7,740)

300 

(10,600)

4 South Llano River at U.S. 

Highway 377 near 

Rocksprings

48.1 

(1,700)

139 

(4,910)

240 

(8,490)

411 

(14,500)

569 

(20,100)

767 

(27,100)

5 South Llano River at 700 

Springs Ranch

60.6 

(2,140)

173 

(6,120)

297 

(10,500)

507 

(17,900)

697 

(24,600)

934 

(33,000)

6 South Llano River at First 

Crossing

106

 (3,760)

300 

(10,600)

504 

(17,800)

844 

(29,800)

1,150 

(40,600)

1,530 

(54,000)

7 South Llano River at South 

Llano River State Park

118 

(4,180)

331 

(11,700)

555 

(19,600)

926 

(32,700)

1,260 

(44,400)

1,670 

(58,900)

8 South Llano River at Texas 

Tech University–Junction

111 

(3,920)

309 

(10,900)

518 

(18,300)

864 

(30,500)

1,170 

(41,400)

1,550 

(54,900)

9 South Llano River at Flatrock 

Crossing

116 

(4,100)

323 

(11,400)

544 

(19,200)

903 

(31,900)

1,230 

(43,400)

1,630 

(57,500)

12 Johnson Fork at Lowlands 

Crossing

123 

(4,330)

382 

(13,500)

674 

(23,800)

1,170 

(41,300)

1,650 

(58,300)

2,240 

(79,000)

14 Llano River near Ivy Chapel 365 

(12,900)

971 

(34,300)

1,570 

(55,400)

2,510 

(88,800)

3,310 

(117,000)

4,300 

(152,000)

15 Llano River at Yates Crossing 362 

(12,800)

960 

(33,900)

1,550 

(54,700)

2,480 

(87,600)

3,260 

(115,000)

4,250 

(150,000)

16 Nueces River at Ben Williams 

Crossing

158 

(5,570)

620 

(21,900)

1,210 

(42,900)

2,460 

(87,000)

3,850 

(136,000)

5,720 

(202,000)
Figure 25. Map and graphs of channel cross-section transects at North Llano River near Roosevelt (see fig. 2 for site location), Kimble 

County, Texas. 

Methods 25

EXPLANATION

Cross-section 

transect

0 75 150 225 300 METERS

0 250 500 750 1,000 FEET

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared

Flow direction

Cross section 1

Cross section 3

Cross section 2

N

20

25

30

35

40

45

50

55

60

20

25

30

35

40

45

50

55

60

-

20

25

30

35

40

45

50

55

60

Survey points at 

arbitrary vertical 

datum

EXPLANATION

-50 0 50 100 150 200 250 300 350 400 450

DISTANCE FROM LEFT, IN METERS

ARBITRARY HEIGHT, IN METERS

Cross section 1

Cross section 2

Cross section 3
Figure 26. Map and graphs of channel cross-section transects at North Llano River near Junction (see fig. 2 for site location), Kimble 

County, Texas. Elevations are based on U.S. Geological Survey gage datum. 

Cross section 3

Cross section 2

Cross section 1

Cross section 0

Interstate 10

FM 1674

Flow direction

EXPLANATION

Cross-section 

transect

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared

0 75 150 225 300 METERS

0 250 500 750 1,000 FEET

N

-1

0

1

2

3

4

5

6

7

8

9

10

-1

0

1

2

3

4

5

6

7

8

9

10

-1

0

1

2

3

4

5

6

7

8

9

10

-1

0

1

2

3

4

5

6

7

8

9

10

-50 0 50 100 150 200 250

DISTANCE FROM LEFT, IN METERS

HEIGHT ABOVE GAGE DATUM, IN METERS

Survey points at gage datum

EXPLANATION

Cross section 1

Cross section 2

Cross section 3

Cross section 0

26 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 27. Map and graphs of channel cross-section transects at South Llano River at Baker Ranch (see fig. 2 for site location), 

Edwards County, Texas.

Survey points at arbitrary vertical datum

EXPLANATION

Cross section 1

Cross section 2

Cross section 3

Interpolated pool cross section

-50 0 50 100 150 200 250 300 350 400 450

DISTANCE FROM LEFT, IN METERS

ARBITRARY HEIGHT, IN METERS

27

28

30

32

34

36

37

27

28

30

32

34

36

37

27

28

30

32

34

36

37

27

28

30

32

34

36

37

Flow direction

Cross-section 1

Cross-section 2

Cross-section 3

Interpolated pool cross section

EXPLANATION

Cross-section

transect

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared 

0 75 150 225 300 METERS

0 250 500 750 1,000 FEET

N

Methods 27
Figure 28. Map and graphs of channel cross-section transects at South Llano River at U.S. Highway 377 near Rocksprings (see fig. 2 

for site location), Edwards County, Texas. 

U.S. Highway 377

Cross section 1

Cross section 2

Flow direction

N

0 75 150 225 300 METERS

0 250 500 750 1,000 FEET

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared

EXPLANATION

Cross-section 

transect

20

25

30

35

40

20

25

30

35

40

20

25

30

35

40

Survey points at arbitrary vertical datum

EXPLANATION

DISTANCE FROM LEFT, IN METERS

ARBITRARY HEIGHT, IN METERS

Cross section 1

Cross section 2

Bridge center-line

-50 0 50 100 150 200 250 300

28 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 29. Map and graphs of channel cross-section transects at South Llano River at 700 Springs Ranch (see fig. 2 for site location), 

Edwards County, Texas.

Flow direction

Cross section 1

Cross section 2

Cross section 3

EXPLANATION

Cross-section

transect

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared

15

20

25

30

35

40

45

15

20

25

30

35

40

45

15

20

25

30

35

40

45

ARBITRARY HEIGHT, IN METERS

Cross section 1

Cross section 2

Cross section 3

-50 0 50 100 150 200 250 300 350

DISTANCE FROM LEFT, IN METERS

Survey points at arbitrary vertical datum

EXPLANATION

N

0 75 150 225 300 METERS

0 250 500 750 1,000 FEET

Methods 29
Figure 30. Map and graphs of channel cross-section transects at South Llano River at South Llano River State Park (see fig. 2 for site 

location), Kimble County, Texas. Cross section 1 is associated with a local elevation datum different from that of cross section 3. 

U.S. Highway 377

Park Road 73

0 125 250 375 500 METERS

0 375 750 1,125 1,500 FEET

Flow direction

N

Cross section 1

Cross section 3

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared

EXPLANATION

Cross-section 

transect

23

25

30

35

40

23

25

30

35

40

Cross section 1

Cross section 3

ARBITRARY HEIGHT, IN METERS

Survey points at arbitrary vertical datum

EXPLANATION

-50 0 50 100 150 200 250 300 350 400 450

DISTANCE FROM LEFT, IN METERS

30 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 31. Map and graphs of channel cross-section transects at South Llano River at Texas Tech University–Junction (see fig. 2 for 

site location), Kimble County, Texas.

0 75 150 225 300 METERS

0 250 500 750 1,000 FEET

N

Cross section 1

Cross section 2

Cross section 3

Cross section 4

Flow direction

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared

EXPLANATION

Cross-section 

transect

Survey points at arbitrary vertical datum

EXPLANATION

ARBITRARY HEIGHT, IN METERS

25

30

35

40

25

30

35

40

25

30

35

40

25

30

35

40

Cross section 2

Cross section 3

Cross section 4

Cross section 1

DISTANCE FROM LEFT, IN METERS

-50 0 50 100 150 200 250 300 350 400

Methods 31
Figure 32. Map and graphs of channel cross-section transects at Johnson Fork at Lowlands Crossing (see fig. 2 for site location), 

Kimble County, Texas.

FM 2169

Cross section 1

Cross section 2

Cross section 3

Line1

N

EXPLANATION

Cross-section 

transect

Resistivity

transect

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared

Flow direction

0 75 150 225 300 METERS

0 250 500 750 1,000 FEET

25

30

35

40

43

25

30

35

40

43

25

30

35

40

43

Cross section 1

Cross section 2

Cross section 3

Survey points at arbitrary vertical datum

EXPLANATION

-50 0 50 100 150 200 250 300

DISTANCE FROM LEFT, IN METERS

ARBITRARY HEIGHT, IN METERS

32 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 33. Map and graphs of channel cross-section transects at Llano River near Junction (see fig. 2 for site location), Kimble County, 

Texas. Elevations are based on U.S. Geological Survey gage datum.

Cross section 1

Cross section 2

Cross section 3

FM 2169

EXPLANATION

Cross-section 

transect

Flow direction

N

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared

0 75 150 225 300 METERS

0 250 500 750 1,000 FEET

-5

0

5

10

15

-5

0

5

10

15

-5

0

5

10

15

Survey points at gage datum

EXPLANATION

-25 0 25 50 100 125 150 175 200 225 250

DISTANCE FROM LEFT, IN METERS

75

HEIGHT ABOVE GAGE DATUM, IN METERS

Cross section 1

Cross section 2

Cross section 3

Methods 33
Figure 34. Map and graphs of channel cross-section transects at Llano River near Ivy Chapel (see fig. 2 for site location), Kimble 

County, Texas.

N

Flow direction

Cross section 1

Cross section 2

Cross section 3

0 75 150 225 300 METERS

0 250 500 750 1,000 FEET

EXPLANATION

Cross-section 

transect

Projection: Transverse Mercator

Zone: 14 North

Spheroid: WGS84

Imagery: 2004 color infrared

25

30

35

40

45

25

30

35

40

45

25

30

35

40

45

ARBITRARY HEIGHT, IN METERS

Cross section 1

Cross section 2

Cross section 3

Survey points at arbitrary vertical datum

EXPLANATION

-50 0 50 100 150 200 250 300

DISTANCE FROM LEFT, IN METERS

34 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 35. Cross-section graphs of selected low-water crossings in study area, Kimble County, Texas. 

Figure 36. Underwater sampling grid and particle-size analyzer on gravel and cobble bed of South Llano River, Kimble County, Texas. 

-50 0 50 100 150 200 250 300 350 400 450

DISTANCE FROM LEFT, IN METERS

ARBITRARY HEIGHT, IN METERS

18

20

25

30

35

18

20

25

30

35

18

20

25

30

35

Survey points at arbitrary vertical datum

EXPLANATION

South Llano River at First Crossing

South Llano River at Flatrock Crossing

Llano River at Yates Crossing

Methods 35
Figure 37. Bed-material particle-size distribution of North Llano River near Roosevelt, Kimble County, Texas. Most of the channel bed 

is bedrock. The distribution plot is for a transitional area between the channel and floodplain and one in-channel gravel bar at cross 

section 3.

Figure 38. Bed-material particle-size distribution of North Llano River near Junction, Kimble County, Texas. 

36 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 2

Cross section 3

EXPLANATION

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 0

Cross section 2

Cross section 3

EXPLANATION
Figure 39. Bed-material particle-size distribution of South Llano River at Baker Ranch, Edwards County, Texas. Most of the bed 

material at the site is fine-grained (less than 2 millimeters). The distribution graph is for one isolated area of angular cobbles and gravel.

Figure 40. Bed-material particle-size distribution of South Llano River at U.S. Highway 377 near Rocksprings, Edwards County, Texas. 

Most of the channel bed is bedrock. The distribution plot is for one isolated area of cobbles and gravel.

Methods 37

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 3

EXPLANATION

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 3

EXPLANATION
Figure 41. Bed-material particle-size distribution of South Llano River at 700 Springs Ranch, Edwards County, Texas. Most of the 

channel bed is bedrock. The distribution plot is for a transitional area between the channel and floodplain and one in-channel gravel bar 

at cross section 1.

Figure 42. Bed-material particle-size distribution of South Llano River at South Llano River State Park, Kimble County, Texas. 

38 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 1

Cross section 2

Cross section 3

EXPLANATION

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 1

Cross section 3

EXPLANATION
Figure 43. Bed-material particle-size distribution of South Llano River at Texas Tech University–Junction, Kimble County, Texas.

Figure 44. Bed-material particle-size distribution of Johnson Fork at Lowlands Crossing, Kimble County, Texas. 

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 1

Cross section 2

Cross section 3

EXPLANATION

Methods 39

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 1

Cross section 2

Cross section 3

EXPLANATION
Figure 45. Bed-material particle-size distribution of Llano River near Junction, Kimble County, Texas. Most of the channel bed at cross 

section 2 is bedrock, except for a small side-channel bar.

Figure 46. Bed-material particle-size distribution of Llano River near Ivy Chapel, Kimble County, Texas. 

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 2

Cross section 3

EXPLANATION

PARTICLE SIZE, IN MILLIMETERS

1 10 100 1,000

CUMULATIVE PERCENT FINER

0

20

40

60

80

100

Cross section 2

Cross section 3

EXPLANATION

40 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Particle size generally decreases downstream, although 

variability is evident (fig. 47). Variability is the result of tributary inputs of coarse bed material or localized incision into 

older Pleistocene or Holocene terrace deposits. The generalized downstream decrease in bed-material particle size is the 

result of abrasion during episodes of entrainment, decreases in 

channel slope, and, possibly, solution of submerged particles.

Bed-Material Entrainment Potential

Bed-material entrainment refers to the incipient motion 

of sediment along a channel bed. During low-flow conditions 

in cobble- and gravel-bed rivers, coarse bed material remains 

in place. However, moderate- to high-flow conditions exert 

greater stresses on the channel bed, and coarse bed material 

initially mobilizes at some threshold stress, which can be associated with flow depth, flow velocity, and discharge. Computations of bed-material entrainment rely on channel-geometry 

and particle-size data.

The bed-material entrainment problem for TxDOT LWCs 

is examined at two spatial scales. First, the relative supply 

and abundance of cobble- and gravel-sized bed material at 

a site can be attributed to watershed-scale characteristics, 

specifically watershed slopes. Second, the stresses needed to 

mobilize bed material at a site can be attributed to reach-scale 

hydraulic factors. The two scale-dependent approaches are 

discussed separately below.

Relative Abundance and Activity of Bed 

Material

The relative abundance and activity of bed material at 

a site in the Edwards Plateau depends on watershed-scale 

factors, including lithology, historical climate patterns, land 

use, drainage area, and slope. For this investigation, lithology, historical climate patterns, and land use are considered 

approximately equal for all study sites. By default, the relative abundance and activity of bed material in the study area 

depends on drainage area and watershed slopes. It is expected 

that larger drainage areas have the potential to contribute 

greater amounts of sediment to downstream channel sites, and 

the rate at which material is supplied is controlled by watershed slopes.

During site reconnaissance trips, notes were taken to 

qualitatively describe the abundance and recent activity 

of cobble- and gravel-sized bed material at each study site 

(table 6). The observed channel size was considered when 

describing bed-material abundance, and local TxDOT personnel provided information about the activity of the bed during 

recent high flows. At some sites, coarse bed material dominated the channel composition, but other sites were characterized by bedrock channel beds and an absence of bed material.

Bed-Material Entrainment Potential 41

Table 4. Channel slopes computed for study sites using measured cross-sectional data, Edwards and Kimble Counties, Texas.

Study site number 

(fig. 2) Study site Channel slope (dimensionless)

Computation 

method

1 North Llano River near Roosevelt 0.0021 Water surface

2 North Llano River near Junction .0029 Thalweg

3 South Llano River at Baker Ranch .0016 Thalweg

.0014 Floodplain

4 South Llano River at U.S. Highway 377 near Rocksprings .0020 Thalweg

5 South Llano River at 700 Springs Ranch .0008 Water surface

.0012 Thalweg

7 South Llano River at South Llano River State Park 10 Water surface

.0012 Longitudinal profile

8 South Llano River at Texas Tech University–Junction .0005 Water surface

12 Johnson Fork at Lowlands Crossing .0053 Water surface

13 Llano River near Junction .0071 Water surface

14 Llano River near Ivy Chapel .0002 Water surface

.0017 Thalweg

1Water surface between cross sections 1 and 2 was ponded, and no difference in elevation was detected.
As a measure of relative bed-material abundance and 

activity, a three-level classification was used in this investigation: 

1. Level I describes channels that are clogged by bed material and undergo substantial reconfiguration during high 

flows (fig. 48).

2. Level II describes channels that contain individual lobes 

or bars of bed material that undergo some reconfiguration 

during high flows (fig. 49).

3. Level III describes channels that do not contain appreciable amounts of coarse bed material and do not undergo 

reconfiguration during high flows (fig. 50).

Mean watershed slope, computed using GIS, accounts 

for both drainage area and slope and is an appropriate 

parameter to predict relative abundance and activity of 

coarse bed material in the study area. The relation between 

mean watershed slope and the qualitative classification of 

coarse bed-material abundance and activity shows that steeper 

42 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

Table 5. Bed-material particle-size descriptors computed for study sites, Edwards and Kimble Counties, Texas.

[d16, d50, and d84, descriptors that represent diameters at which 16, 50, and 84 percent, respectively, of the sample is finer than; mm, millimeters]

Study site 

number 

(fig. 2)

Study site Cross 

section

Total particles 

measured

d16

(mm)

d50

(mm)

d84

(mm)

1 North Llano River near Roosevelt 2 102 26.9 39.9 66.7

3 101 16.5 31.0 55.4

2 North Llano River near Junction 0 250 8.5 22.4 44.7

2 125 15.3 29.5 56.9

3 100 11.2 19.0 35.7

3 South Llano River at Baker Ranch 3 50 20.0 39.2 74.4

4 South Llano River at U.S. Highway 377 near 

Rocksprings

1 25 27.3 42.2 64.0

5 South Llano River at 700 Springs Ranch 1 95 33.4 52.3 79.6

2 75 17.2 38.1 70.5

3 50 28.2 51.3 84.2

7 South Llano River at South Llano River State Park 1 200 11.0 27.6 54.5

3 200 11.6 26.4 58.0

8 South Llano River at Texas Tech University–

Junction

1 150 9.0 21.8 44.5

2 75 13.5 27.0 64.0

3 100 9.8 24.2 56.4

12 Johnson Fork at Lowlands Crossing 1 249 5.4 24.0 60.4

2 175 9.8 25.5 56.1

3 75 12.0 29.5 70.5

13 Llano River near Junction 2 25 5.6 9.9 20.4

3 145 11.1 26.8 86.9

14 Llano River near Ivy Chapel 2 75 10.3 26.7 55.4

3 125 11.8 22.8 44.1
0 20 40 60 80 100 120 140 160

DOWNSTREAM DISTANCE, IN KILOMETERS

PARTICLE SIZE, IN MILLIMETERS

0

10

20

30

40

50

60

70

80

90

d84

d50

d16

dn refers to the particle size, in 

millimeters, at which n percent

of the sample is finer.

Regression line

EXPLANATION

d84 = -0.201x + 77.665

r2 = .45

d50 = -0.170x + 45.892

r2 = .38

d16 = -0.119x + 26.165

r2 = .32

Figure 47. Mean particle size descriptors (d16, d50, d84) and downstream distance for study sites where Wolman pebble counts were 

made, Edwards and Kimble Counties, Texas. Particle sizes represent the mean of all cross-sectional transects at each study site. Eleven 

rather than ten sites are shown because the distance between cross sections 1 and 3 at South Llano River State Park is far enough 

apart to consider them separate sites. Linear regression equations and r-squared values are provided. 

Figure 48. Nueces River at Vance Crossing, Edwards-Real County line, Texas. Cobble- and gravel-sized bed material clogs the channel 

and undergoes substantial reconfiguration during high flows.

Bed-Material Entrainment Potential 43
Figure 49. South Llano River at South Llano River State Park, Kimble County, Texas. Cobble- and gravel-sized bed material form 

longitudinal channel bars and undergo some reconfiguration during high flows.

Figure 50. South Llano River at Baker Ranch, Edwards County, Texas. The channel bed is composed of fine-grained (diameter less than 

2 millimeters) bed material mixed with isolated angular cobbles and gravel. 

44 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
watershed slopes contribute to the increased availability and 

frequent reconfiguration of cobble- and gravel-sized bed 

material (fig. 51).

One exception to the relation between mean watershed slope and the qualitative classification previously 

described is Johnson Fork at Lowlands Crossing. This site has 

a channel that is known for considerable quantities of coarse 

bed material and reconfiguration. Although overall mean 

watershed slope is not relatively high (5.3 percent), the site 

is in the lower part of the watershed, characterized by increasing numbers of steep tributaries. The site shows that exceptions to the relation of mean watershed slope to bed-material 

abundance and activity can be explained by proximity to the 

highest slopes of the watershed.

Stresses Needed to Mobilize (Entrain) Bed 

Material

Site-specific computations of bed-material entrainment 

are based on the relation between the critical shear stress 

(τc) required to initially mobilize sediment and the boundary 

shear stress (τo) exerted by a given flow. Boundary shear stress 

(Elliott, 2002) can be computed from:

τo = γ RS, (1)

where 

τo is the boundary shear stress, in newtons per square meter;

γ is the specific weight of water at 20 degrees Celsius 

(9,789 newtons per cubic meter);

Figure 51. Distribution of mean watershed slope and qualitative bed-material abundance index and activity classification level. The 

statistical visualization represents mean watershed slopes, in percent, of sites classified by bed-material abundance and activity. Level 

I describes channels that are clogged by bed material and undergo substantial reconfiguration during high flows. Level II describes 

channels that contain individual lobes or bars of bed material that undergo some reconfiguration during high flows. Level III describes 

channels that do not contain appreciable amounts of coarse bed material and do not undergo reconfiguration during high flows. 

(4)

(8)

(2)

(2)

EXPLANATION

Sample size

Data point

Mean

Median

Bed-material abundance and activity classification level

Cobble and gravel-clogged channel bed

Cobble and gravel bars in channel bed

Little or no coarse bed material

I

II

III

BED-MATERIAL ABUNDANCE AND ACTIVITY CLASSIFICATION LEVEL

I II III

MEAN WATERSHED SLOPE, IN PERCENT

0

5

10

15

20

25

30

35

 

Bed-Material Entrainment Potential 45
R is the hydraulic radius, in meters; mean depth commonly 

is used as a substitute; and

S is the energy gradient, in meters per meter; channel slope 

commonly is used as a substitute.

Critical shear stress (Elliott, 2002) can be computed 

from:

τc = τ*(γs-γ)dn, (2)

where 

τc is the critical shear stress, in newtons per square meter;

τ* is the dimensionless shear stress or Shields parameter 

(0.045);

γs is the specific weight of the calcite sediment (26,430 

newtons per cubic meter); and

dn is the particle size, in meters, at which n percent of the 

sediment is finer.

The dimensionless shear stress (τ*) (Shields, 1936) can 

be modified according to bed-material particle size and bed 

roughness. Elliott (2002) uses a τ* value of 0.030 for channel 

beds composed of coarse material, appropriate for hazard-mitigation investigations because shallow flow depths and lesser 

discharges will result in bed-material entrainment (Shen and 

Julien, 1993). Others have found that a τ* value of 0.030 is a 

minimum associated with sand-sized beds (Knighton, 1998), 

and surfaces with a relatively greater roughness height commonly attain a τ* value of 0.045 (Komar, 1988; Robert, 2003). 

For purposes of this investigation, a τ* value of 0.045 is used 

in computations of critical shear stress because cobble- and 

gravel-sized beds support hydraulically rough conditions at 

moderate to high flows. The particle size (dn) used in computations of critical shear stress depends on the purpose 

of the investigation. For a variety of applications, d50, the 

median particle size, is used in computations of bed-material 

entrainment (see Elliott, 2002).

The critical shear stress represents the hydraulic conditions at which incipient motion of bed particles of size n occur, 

but mobilization is limited to a few particles. Elliott (2002) 

also computes a complete entrainment threshold at which 

the boundary shear stress is twice the critical shear stress for 

the given particle size. The complete entrainment threshold 

describes the conditions at which widespread mobilization of 

bed sediment occurs. Factors other than shear stress have been 

linked to bed-material entrainment, including turbulence and 

channel size (Knighton, 1998), but this investigation did not 

consider those factors.

For this investigation, critical shear stress values were 

computed (tables 7 and 8) using equation 2 for all cross 

sections in the study area that have particle-size data (see 

46 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

Table 6. Qualitative observations and classification of coarse bed-material abundance and activity at selected study sites, Edwards, 

Kimble, and Real Counties, Texas.

[I, cobble- and gravel-choked channel bed, substantial reconfiguration during high flows; II, cobble and gravel bars in channel bed, moderate reconfiguration 

during high flows; III, little or no coarse bed material, little to no reconfiguration during high flows]

Study site number 

(fig. 2) Study site Qualitative bed-material abundance and activity classification level

1 North Llano River near Roosevelt II

2 North Llano River near Junction II

3 South Llano River at Baker Ranch III

4 South Llano River at U.S. Highway 377 near Rocksprings III

5 South Llano River at 700 Springs Ranch II

7 South Llano River at South Llano River State Park II

8 South Llano River at Texas Tech University–Junction II

12 Johnson Fork at Lowlands Crossing I

13 Llano River near Junction II

14 Llano River near Ivy Chapel II

16 Nueces River at Ben Williams Crossing I

17 Nueces River at Vance Crossing I

18 Cedar Creek at Ranch Road 336 I

19 Frio River at Upper Ranch Road 1120 Crossing II
Table 7. Computations for initial bed-material entrainment at sites with particle-size data, Edwards and Kimble Counties, Texas.

[d16, d50, and d84, descriptors that represent diameters at which 16, 50, and 84 percent, respectively, of the sample is finer than; τc, critical shear stress; N/m2, 

newtons per square meter; Q, discharge; m3/s, cubic meters per second; ft3/s, cubic feet per second; --, not computed]

Study site 

number 

(fig. 2)

Study site Cross 

section

d16 τc

(N/m2)

d50 τc

(N/m2)

d84 τc

(N/m2)

Manning’s 

n

d16 Q

[m3/s (ft3/s)]

d50 Q

[m3/s (ft3/s)]

d84 Q

[m3/s (ft3/s)]

1 North Llano River 

near Roosevelt

2 20.14 29.88 49.95 0.040 18.1 

(639)

54.4 

(1,920)

162 

(5,700)

3 12.36 23.21 41.49 .035 10.7 

(379)

47.6 

(1,680)

684 

(24,100)

2 North Llano River 

near Junction

0 6.37 16.77 33.47 .040 1.74 

(61.4)

14.2 

(502)

53.3 

(1,880)

2 11.46 22.09 42.61 .035 18.9 

(668)

63.8 

(2,250)

234 

(8,260)

3 8.39 14.23 26.73 .035 6.48 

(229)

19.4 

(686)

69.5 

(2,450)

3 South Llano River 

at Baker Ranch

3 14.98 29.36 55.72 .027 105 

(3,710)

626 

(22,100)

-- 

(--)

4 South Llano River 

at U.S. Highway 

377 near 

Rocksprings

1 20.44 31.60 47.93 .027 98.9 

(3,490)

242 

(8,530)

542 

(19,100)

5 South Llano River 

at 700 Springs 

Ranch

1 25.01 39.17 59.61 .035 292 

(10,300)

923 

(32,600)

2,680 

(94,800)

2 12.88 28.53 52.79 .030 52.8 

(1,860)

568 

(20,000)

2,490 

(88,000)

3 21.12 38.42 63.05 .030 309 

(10,900)

1,000 

(35,300)

3,200 

(113,000)

7 South Llano River 

at South Llano 

River State Park

1 8.24 20.67 40.81 .040 7.18 

(253)

202 

(7,130)

1,330 

(46,900)

3 8.69 19.77 43.43 .040 15.9 

(563)

222 

(7,830)

1,090 

(38,600)

8 South Llano River 

at Texas Tech 

University–

Junction

1 6.74 16.33 33.32 .035 86.2 

(3,040)

733 

(25,900)

3,970 

(140,000)

2 10.11 20.22 47.93 .035 220 

(7,760)

1,180 

(41,800)

-- 

(--)

3 7.34 18.12 42.24 .035 54.4 

(1,920)

864 

(30,500)

-- 

(--)

12 Johnson Fork 

at Lowlands 

Crossing

1 4.04 17.97 45.23 .035 .11 

(4.0)

5.95 

(210)

146 

(5,160)

2 7.34 19.10 42.01 .035 .34 

(12.0)

14.4 

(509)

126 

(4,470)

3 8.99 22.09 52.79 .035 1.62 

(57.1)

11.8 

(416)

210 

(7,430)

13 Llano River near 

Junction

2 4.19 7.41 15.28 .035 .2 

(7.2)

.3 

(10.5)

1.58 

(55.7)

3 8.31 20.07 65.08 .035 .58 

(20.6)

4.42 

(156)

253 

(8,950)

14 Llano River near 

Ivy Chapel

2 7.71 19.99 41.49 .035 8.82 

(311)

140 

(4,940)

637 

(22,500)

3 8.84 17.07 33.02 .035 2.24 

(79.1)

103 

(3,640)

402 

(14,200)

Bed-Material Entrainment Potential 47
48 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

Table 8. Computations for complete bed-material entrainment at sites with particle-size data, Edwards and Kimble Counties, Texas.

[d16, d50, and d84, descriptors that represent diameters at which 16, 50, and 84 percent, respectively, of the sample is finer than; τc, critical shear stress; N/m2, 

newtons per square meter; Q, discharge; m3/s, cubic meters per second; ft3/s, cubic feet per second; --, not computed]

Study site 

number 

(fig. 2)

Study site Cross 

section

d16 τc

(N/m2)

d50 τc

(N/m2)

d84 τc

(N/m2)

Manning’s 

n

d16 Q

[m3/s (ft3/s)]

d50 Q

[m3/s (ft3/s)]

d84 Q

[m3/s (ft3/s)]

1 North Llano River 

near Roosevelt

2 40.29 59.76 99.90 0.040 104 

(3,670)

243 

(8,570)

1,090 

(38,500)

3 24.71 46.43 82.97 .035 271 

(9,570)

859 

(30,300)

2,370 

(83,800)

2 North Llano River 

near Junction

0 12.73 33.55 66.95 .040 7.34 

(259)

53.4 

(1,890)

224 

(7,920)

2 22.92 44.18 85.22 .035 65.4 

(2,310)

269 

(9,490)

877 

(31,000)

3 16.77 28.46 53.47 .035 33.6 

(1,190)

81.7 

(2,880)

258 

(9,120)

3 South Llano River 

at Baker Ranch

3 29.95 58.71 111.43 .027 635 

(22,400)

-- 

(--)

-- 

(--)

4 South Llano River 

at U.S. Highway 

377 near 

Rocksprings

1 40.89 63.20 95.85 .027 403 

(14,200)

988 

(34,900)

2,580 

(91,100)

5 South Llano River 

at 700 Springs 

Ranch

1 50.02 78.33 119.22 .035 1,790 

(63,100)

5,180 

(183,000)

-- 

(--)

2 25.76 57.06 105.59 .030 456 

(16,100)

3,020 

(107,000)

-- 

(--)

3 42.24 76.83 126.11 .030 1,200 

(42,500)

5,220 

(184,000)

-- 

(--)

7 South Llano River 

at South Llano 

River State Park

1 16.47 41.34 81.63 .040 103 

(3,640)

1,340 

(47,300)

6,740 

(238,000)

3 17.37 39.54 86.87 .040 179 

(6,310)

813 

(28,700)

-- 

(--)

8 South Llano River 

at Texas Tech 

University–

Junction

1 13.48 32.65 66.65 .035 454 

(16,000)

3,840 

(136,000)

-- 

(--)

2 20.22 40.44 95.85 .035 1,180 

(41,800)

-- 

(--)

-- 

(--)

3 14.68 36.24 84.47 .035 411 

(14,500)

-- 

(--)

-- 

(--)

12 Johnson Fork 

at Lowlands 

Crossing

1 8.09 35.95 90.46 .035 1.51 

(53.2)

57.8 

(2,040)

521 

(18,400)

2 14.68 38.19 84.02 .035 2.42 

(85.5)

91.6 

(3,230)

539 

(19,000)

3 17.97 44.18 105.59 .035 5.64 

(199)

164 

(5,810)

858 

(30,300)

13 Llano River near 

Junction

2 8.39 14.83 30.55 .035 .35 

(12.5)

1.55 

(54.6)

17.4 

(614)

3 16.62 40.14 130.15 .035 2.32 

(81.9)

85.1 

(3,010)

892 

(31,500)

14 Llano River near 

Ivy Chapel

2 15.43 39.99 82.97 .035 56.0 

(1,980)

585 

(20,700)

2,540 

(89,700)

3 17.67 34.15 66.05 .035 116 

(4,110)

439 

(15,500)

1,580 

(55,900)
table 5). Critical shear stress values were computed for 

three particle-size descriptors: d16, d50, and d84. Complete 

entrainment thresholds were computed by doubling the critical 

shear stress values. Hydraulic radius (R) values were backcomputed by equating the critical shear stress to the boundary 

shear stress in equation 1. For sites with cross-sectional geometric data, hydraulic radius (R) values were associated with 

channel cross-sectional areas (A) by using output files from 

cross-sectional analyses in WinXSPro. Manning’s n roughness 

coefficients, based on suggestions in Arcement and Schneider 

(1989) and the authors’ judgment, were assigned to individual 

cross sections. Flow velocity (U) values for combinations of 

critical shear stress, complete entrainment, and particle-size 

descriptors at each cross section were computed from (Shen 

and Julien, 1993):

U = (1/n)R2/3 S1/2, (3)

where 

U is flow velocity in meters per second;

n is Manning’s roughness coefficient;

R is the hydraulic radius, in meters; and

S is channel slope, in meters per meter.

Next, discharge (Q) values for combinations of critical 

shear stress, complete entrainment, and particle-size descriptors at each cross section were computed from:

Q = UA, (4)

where 

Q is discharge in cubic meters per second; and

A is cross-sectional area in square meters.

Finally, the Froude number, a dimensionless number 

quantifying the resistance of an object moving through water, 

was computed for combinations of critical shear stress, 

complete entrainment, and particle-size descriptors at each 

cross section. The Froude number is computed from Sturm 

(2001):

F = U/(gR)

1/2, (5)

where 

F is the dimensionless Froude number; and

g is the gravitational constant (9.80 meters per square 

second).

For convenience, the hydraulic radius (R) substitutes for 

depth (D).

Critical shear stress values show variation from site to 

site as a result of differences in particle-size descriptors (d16, 

d50, and d84). Greater variability becomes evident in backcomputations of hydraulic radiuses necessary for initial or 

complete entrainment as a result of differences in channel 

slope. Finally, further variability in the cross-sectional areas 

associated with computed hydraulic radiuses lead to considerably different discharges responsible for bed-material 

entrainment. The results in tables 7 and 8 show that site-bysite assessments probably cannot support broad regional interpretations of bed-material entrainment frequency, especially 

when data are collected at sites that might not reflect the average condition of most channel reaches.

An alternative to the site-by-site approach is using an 

average- or expected-condition approach. Parameters that can 

be generalized in computations of bed-material entrainment 

are channel slope and particle size. The median channel slope 

of all study sites is 0.0017, appropriate for main channels in 

the study area because of relatively straight or linear longitudinal profiles (figs. 11–14). Linear regression models were 

applied to predict the particle-size descriptors d16, d50, and d84, 

for a given downstream distance from the headwater source. 

Figure 47 shows a general downstream decrease in particle 

size for the study sites, described by the following regressionfitted equations:

d16 = -0.119x + 26.165, (6)

d50 = -0.170x + 45.892, and (7)

d84 = -0.201x + 77.665, (8)

where 

dn is the particle size, in millimeters, at which n percent of 

sediment is finer; and

x is downstream distance, in kilometers.

R-squared values of .32 apply to equation 6, .38 to equation 7, 

and .45 to equation 8.

The median channel slope, 0.0017, and the particle-size 

descriptors computed from equations 6 to 8 are used in equations 2 and 3. A median n of 0.035 is used to compute flow 

velocity in equation 3. Finally, equation 4 is used to compute 

the discharges required to entrain the expected particle size 

for a given downstream distance.

The average- or expected-condition approach limits the 

variability inherent in site-by-site analysis and represents general downstream conditions. In general, the average-condition 

approach slightly reduced the discharges required to entrain 

bed material in upper channel reaches and slightly increased 

the discharges required to entrain bed material in lower channel reaches. The results are desirable, because the site-bysite approach overestimated discharges required for bed-material entrainment in upper channel reaches and underestimated 

discharges required for bed-material entrainment in lower 

channel reaches. The average-condition approach should 

only be applied along channel reaches that have slopes within 

one magnitude and a relatively consistent downstream reduction of bed-material particle size. The results from application 

of the average-condition approach are presented in tables 9 

and 10.

Bed-Material Entrainment Potential 49
50 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

Table 9. Computations for initial bed-material entrainment at sites with particle-size data, Edwards and Kimble Counties, Texas, using 

average-condition approach based on median channel slope, downstream decrease of particle size, and median Manning’s n.

[d16, d50, and d84, descriptors that represent diameters at which 16, 50, and 84 percent, respectively, of the sample is finer than; τc, critical shear stress; N/m2, 

newtons per square meter; Q, discharge; m3/s, cubic meters per second; ft3/s, cubic feet per second; --, not computed]

Study site 

number 

(fig. 2)

Study site Cross 

section

d16 τc

(N/m2)

d50 τc

(N/m2)

d84 τc

 (N/m2)

d16 Q

[m3/s (ft3/s)]

d50 Q

[m3/s 

(ft3/s)]

d84 Q

[m3/s (ft3/s)]

1 North Llano River near 

Roosevelt

2 14.60 27.18 49.65 17.3

 (610)

60.5 

(2,130)

190 

(6,730)

3 14.60 27.18 49.65 12.4 

(439)

54.7 

(1,930)

798 

(28,200)

2 North Llano River near 

Junction

0 11.68 22.99 44.71 3.26 

(115)

21.9 

(774)

80.8 

(2,850)

2 11.68 22.99 44.71 20.9 

(739)

71.6 

(2,530)

264 

(9,330)

3 11.68 22.99 44.71 8.84 

(312)

29.2 

(1,030)

107 

(3,780)

3 South Llano River at Baker 

Ranch

3 17.00 30.63 53.69 86.4 

(3,050)

380 

(13,400)

-- 

(--)

4 South Llano River at U.S. 

Highway 377 near 

Rocksprings

1 13.85 26.14 48.45 42.7 

(1,510)

169 

(5,960)

567 

(20,000)

5 South Llano River at 700 

Springs Ranch

1 12.81 24.64 46.65 176 

(6,220)

640 

(22,600)

2,150 

(75,900)

2 12.81 24.64 46.65 42.5 

(1,500)

416 

(14,700)

1,860 

(65,500)

3 12.81 24.64 46.65 179 

(6,320)

601 

(21,200)

2,120 

(74,800)

7 South Llano River at South 

Llano River State Park

1 10.26 21.04 42.39 8.96 

(316)

220 

(7,780)

1,470 

(51,800)

3 10.18 20.82 42.16 19.1 

(675)

248 

(8,740)

1,160 

(40,800)

8 South Llano River at Texas 

Tech University–Junction

1 9.81 20.29 41.56 90.3 

(3,190)

691 

(24,400)

3,760 

(133,000)

2 9.81 20.29 41.56 176 

(6,200)

968 

(34,200)

-- 

(--)

3 9.81 20.29 41.56 53.8 

(1,900)

760 

(26,800)

-- 

(--)

12 Johnson Fork at Lowlands 

Crossing

1 14.45 27.03 49.50 .32 

(11.4)

9.44 

(333)

188 

(6,630)

2 14.45 27.03 49.50 .64 

(22.7)

22.0 

(776)

171 

(6,020)

3 14.45 27.03 49.50 2.68 

(94.7)

16.3 

(575)

244 

(8,600)

13 Llano River near Junction 2 8.76 18.87 39.84 .42 

(14.9)

.7 

(24.9)

3.79 

(134)

3 8.76 18.87 39.84 .77 

(27.1)

5.38 

(190)

232 

(8,190)

14 Llano River near Ivy Chapel 2 7.11 16.47 36.99 8.36 

(295)

123 

(4,340)

590 

(20,800)

3 7.11 16.47 36.99 1.94 

(68.5)

101 

(3,550)

433 

(15,300)
Bed-Material Entrainment Potential 51

Table 10. Computations for complete bed-material entrainment at sites with particle-size data, Edwards and Kimble Counties, Texas, 

using average-condition approach based on median channel slope, downstream decrease of particle size, and median Manning’s n.

[d16, d50, and d84, descriptors that represent diameters at which 16, 50, and 84 percent, respectively, of the sample is finer than; τc, critical shear stress; N/m2, 

newtons per square meter; Q, discharge; m3/s, cubic meters per second; ft3/s, cubic feet per second; --, not computed]

Study site 

number 

(fig. 2)

Study site Cross 

section

d16 τc

(N/m2)

d50 τc

(N/m2)

d84 τc

(N/m2)

d16 Q

[m3/s (ft3/s)]

d50 Q

[m3/s (ft3/s)]

d84 Q

[m3/s (ft3/s)]

1 North Llano River near 

Roosevelt

2 29.21 54.37 99.30 99.4 

(3,510)

270 

(9,530)

1,290 

(45,400)

3 29.21 54.37 99.30 314 

(11,100)

988 

(34,900)

2,770 

(97,900)

2 North Llano River near 

Junction

0 23.36 45.98 89.41 13.7 

(485)

82.3 

(2,910)

340 

(12,000)

2 23.36 45.98 89.41 72.4 

(2,560)

302 

(10,600)

990 

(35,000)

3 23.36 45.98 89.41 45.8 

(1,620)

123 

(4,340)

398 

(14,000)

3 South Llano River at Baker 

Ranch

3 34.00 61.26 107.39 527 

(18,600)

-- 

(--)

-- 

(--)

4 South Llano River at U.S. 

Highway 377 near 

Rocksprings

1 27.71 52.27 96.90 189 

(6,660)

659 

(23,300)

2,720 

(96,100)

5 South Llano River at 700 

Springs Ranch

1 25.61 49.27 93.31 1,080 

(38,100)

3,590 

(127,000)

-- 

(--)

2 25.61 49.27 93.31 367 

(13,000)

2,220 

(78,200)

-- 

(--)

3 25.61 49.27 93.31 697 

(24,600)

3,140 

(111,000)

-- 

(--)

7 South Llano River at South 

Llano River State Park

1 20.52 42.09 84.77 129 

(4,540)

1,460 

(51,600)

7,450 

(263,000)

3 20.37 41.64 84.32 214 

(7,570)

908 

(32,000)

-- 

(--)

8 South Llano River at Texas 

Tech University–Junction

1 19.62 40.59 83.12 476 

(16,800)

3,620 

(128,000)

-- 

(--)

2 19.62 40.59 83.12 947 

(33,400)

-- 

(--)

-- 

(--)

3 19.62 40.59 83.12 406 

(14,300)

-- 

(--)

-- 

(--)

12 Johnson Fork at Lowlands 

Crossing

1 28.91 54.07 99.00 4.26 

(150)

91.7 

(3,240)

669 

(23,600)

2 28.91 54.07 99.00 4.60 

(162)

140 

(4,930)

727 

(25,700)

3 28.91 54.07 99.00 9.36 

(331)

227 

(8,030)

994 

(35,100)

13 Llano River near Junction 2 17.52 37.74 79.68 .73 

(25.8)

3.66 

(129)

41.8 

(1,480)

3 17.52 37.74 79.68 3.05 

(108)

104 

(3,660)

817 

(28,800)

14 Llano River near Ivy Chapel 2 14.23 32.95 73.99 53.1 

(1,870)

515 

(18,200)

2,350 

(83,100)

3 14.23 32.95 73.99 101 

(3,560)

429 

(15,200)

1,710 

(60,300)
The parameter showing the least variability for interpretation of bed-material entrainment is the Froude number. The 

Froude numbers computed for all combinations of critical 

shear stress, complete entrainment, and particle-size descriptors at each cross section (tables 11 and 12) are less than 1, 

implying that subcritical flows are responsible for bed-material 

entrainment in the study area. Variability in the Froude number 

is greater for the site-by-site analysis, but is greatly reduced 

for the average-condition approach. Using the averagecondition approach, Froude numbers of 0.40 and 0.45 are 

associated with initial and complete entrainment, respectively, 

of bed material as large as the median particle size (d50). For 

culvert or LWC design, use of the Froude number at which 

bed material is mobilized is advantageous because structures 

can be designed to accommodate certain flow velocities at 

depth.

Risk-Oriented Interpretation Using Flood 

Frequency

Discharges necessary to initially or completely entrain 

bed material at study sites (tables 7–10) can be associated 

with flood-frequency analyses to estimate the potential for 

bed-material entrainment in the study area. Partial-duration 

flood-frequency analyses of gaging-station data and regional 

regression flood-frequency analyses are used as appropriate 

for estimating entrainment potential at individual sites.

•	 Three sites, North Llano River near Junction, Johnson Fork at Lowlands Crossing, and Llano River near 

Junction, are at or very close to gaging stations and, 

therefore, are appropriate for use of partial-duration 

flood-frequency analyses.

•	 Two sites, South Llano River at South Llano River 

State Park and South Llano River at Texas Tech 

University–Junction, have drainage areas and watershed slopes similar to North Llano River near Junction, 

thus the partial-duration flood-frequency analysis for 

the North Llano River near Junction gage is applicable 

for those sites.

•	 One site, Llano River near Ivy Chapel, occurs between 

the Llano River near Junction and the Llano River near 

Mason. For that site, a drainage-area ratio of the Llano 

River near Junction and Llano River near Mason gages 

is used to estimate flood frequency.

•	 Four sites, South Llano River at Baker Ranch, South 

Llano River at U.S. Highway 377 near Rocksprings, 

South Llano River at 700 Springs Ranch, and North 

Llano River near Roosevelt, have drainage areas or 

watershed slopes that are not similar to those of 

available gaging stations and, therefore, are appropriate 

for use of regional regression flood-frequency 

analyses. 

Tables 13 and 14 show the return periods of the discharges responsible for initial and complete entrainment of 

bed material, based on site-by-site assessments and generalized channel slope and particle size, respectively. No values 

indicate that the hydraulic radius necessary to entrain bed 

material exceeded the vertical extent of the available crosssectional geometric data.

The return periods for initial and complete bed-material 

entrainment vary greatly. In general, the return period for 

entrainment decreases with downstream distance, both as 

a function of particle size and the method used to compute 

flood frequency. As particle size gradually decreases with 

downstream distance, the shear stress necessary to entrain bed 

material decreases. The most influential factor in the observed 

decrease, however, is the method used to determine flood 

frequency. As mentioned previously, the regional regression 

method underestimates peak streamflow in the study area 

and, therefore, the streamflow necessary to entrain material is 

predicted to occur less frequently than at sites for which flood 

frequency was computed using the partial-duration series. The 

most accurate estimates of return periods for bed-material 

entrainment are for sites that use the partial-duration floodfrequency analyses.

It is emphasized that the computation of critical 

shear stress, streamflow, and return periods for bed-material 

entrainment only serve as a predictive model for bedmaterial entrainment in the study area. The model indicates 

that bed material is more likely to be entrained as particle 

size decreases and channel slope and hydraulic radius increase, 

essentially as resistance decreases and hydraulic energy 

increases. The applicability of the model is increased, at the 

expense of a reduction in accuracy, by associating hydraulic 

radius with a measured cross-sectional area and a userdefined Manning’s n to compute the streamflow necessary 

for bed-material entrainment, and then by associating that 

streamflow with a flood-frequency model. The final results 

provide limited guidance in prediction of bed-material 

entrainment in the study area, but some interpretations can be 

gleaned.

First, the frequency of bed-material entrainment generally 

increases with downstream distance as a result of decreasing 

particle size and increased flood magnitudes. Second, an 

average of 1 year occurs between flows that initially entrain 

bed material as large as the median particle size, and an 

average of 1.5 years occurs between flows that completely 

entrain bed material as large as the median particle size. The 

return periods tend to match qualitative observations in the 

study area. Floods and bed-material entrainment are not uniformly distributed through time. Floods and associated 

bed-material entrainment frequently occur with decreased 

intervals of time between them during wet periods; and they 

occur less frequently during relatively normal to dry periods. 

The frequent entrainment of cobble- and gravel-sized bed 

material underscores the difficulties TxDOT faces with maintenance of LWCs in the study area.

52 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Bed-Material Entrainment Potential 53

Table 11. Froude numbers of initial and complete bed-material entrainment at sites with particle-size data, Edwards and Kimble 

Counties, Texas, for site-by-site analyses.

[F, Froude number; d16, d50, and d84, descriptors that represent diameters at which 16, 50, and 84 percent, respectively, of the sample is finer than; initial, initial 

entrainment; complete, complete entrainment]

Study site 

number 

(fig. 2)

Study site Cross 

section

F 

(initial d16)

F 

(initial d50)

F 

(initial d84)

F 

(complete d16)

F 

(complete d50)

F 

(complete d84)

1 North Llano River 

near Roosevelt

2 0.36 0.39 0.42 0.41 0.44 0.48

3 .38 .43 .47 .43 .48 .53

2 North Llano River 

near Junction

0 .34 .39 .44 .38 .44 .50

2 .42 .47 .53 .47 .53 .59

3 .40 .44 .49 .45 .49 .55

3 South Llano River 

at Baker Ranch

3 .46 .51 .57 .52 .58 .64

4 South Llano River 

at U.S. Highway 

377 near 

Rocksprings

1 .53 .57 .61 .60 .64 .69

5 South Llano River 

at 700 Springs 

Ranch

1 .36 .39 .41 .40 .43 .47

2 .37 .43 .47 .42 .48 .53

3 .41 .45 .49 .46 .50 .55

7 South Llano River 

at South Llano 

River State Park

1 .26 .30 .34 .29 .34 .38

3 .26 .30 .34 .30 .34 .39

8 South Llano River 

at Texas Tech 

University–

Junction

1 .22 .25 .28 .24 .28 .32

2 .23 .26 .30 .26 .29 .34

3 .22 .25 .29 .25 .28 .33

12 Johnson Fork 

at Lowlands 

Crossing

1 .43 .56 .65 .49 .63 .73

2 .48 .56 .64 .54 .63 .72

3 .50 .58 .67 .56 .65 .75

13 Llano River near 

Junction

2 .48 .53 .60 .54 .59 .67

3 .54 .63 .76 .61 .70 .85

14 Llano River near 

Ivy Chapel

2 .33 .39 .44 .37 .44 .49

3 .34 .38 .42 .38 .42 .47
54 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

Table 12. Froude numbers of initial and complete bed-material entrainment at sites with particle-size data, Edwards and Kimble 

Counties, Texas, using average-condition approach based on median channel slope, downstream decrease of particle size, and median 

Manning’s n.

[F, Froude number; d16, d50, and d84, descriptors that represent diameters at which 16, 50, and 84 percent, respectively, of the sample is finer than; initial, initial 

entrainment; complete, complete entrainment]

Study 

site 

number 

(fig. 2)

Study site Cross 

section

F 

(initial d16)

F 

(initial d50)

F 

(initial d84)

F 

(complete d16)

F 

(complete d50)

F 

(complete d84)

1 North Llano River 

near Roosevelt

2 0.37 0.41 0.45 0.41 0.46 0.51

3 .37 .41 .45 .41 .46 .51

2 North Llano River 

near Junction

0 .35 .40 .44 .40 .45 .50

2 .35 .40 .44 .40 .45 .50

3 .35 .40 .44 .40 .45 .50

3 South Llano River at 

Baker Ranch

3 .38 .42 .46 .42 .47 .51

4 South Llano River at 

U.S. Highway 377 

near Rocksprings

1 .36 .41 .45 .41 .46 .50

5 South Llano River at 

700 Springs Ranch

1 .36 .40 .45 .40 .45 .50

2 .36 .40 .45 .40 .45 .50

3 .36 .40 .45 .40 .45 .50

7 South Llano River at 

South Llano River 

State Park

1 .35 .39 .44 .39 .44 .49

3 .35 .39 .44 .39 .44 .49

8 South Llano River 

at Texas Tech 

University–

Junction

1 .34 .39 .44 .39 .44 .49

2 .34 .39 .44 .39 .44 .49

3 .34 .39 .44 .39 .44 .49

12 Johnson Fork 

at Lowlands 

Crossing

1 .37 .41 .45 .41 .46 .51

2 .37 .41 .45 .41 .46 .51

3 .37 .41 .45 .41 .46 .51

13 Llano River near 

Junction

2 .34 .38 .44 .38 .43 .49

3 .34 .38 .44 .38 .43 .49

14 Llano River near Ivy 

Chapel

2 .33 .38 .43 .37 .42 .48

3 .33 .38 .43 .37 .42 .48
Bed-Material Entrainment Potential 55

Table 13. Approximate return periods for initial and complete bed-material entrainment at sites with particle-size data, Edwards and 

Kimble Counties, Texas, based on site-by-site individual measurements or estimates.

[d16, d50, and d84, descriptors that represent diameters at which 16, 50, and 84 percent, respectively, of the sample is finer than; yr, years; >, greater than; --, crosssectional area not possible to compute with available geometric data; <, less than]

Study 

site 

number 

(fig. 2)

Study site Cross 

section

Return 

period for 

initial 

entrainment 

of d16

(yr)

Return 

period for 

initial 

entrainment 

of d50

(yr)

Return 

period for 

initial 

entrainment 

of d84

(yr)

Return 

period for 

complete 

entrainment 

of d16

(yr)

Return 

period for 

complete 

entrainment 

of d50

(yr)

Return 

period for 

complete 

entrainment 

of d84

(yr)

1 North Llano River near 

Roosevelt

2 <1 <1.5 3.1 2.1 4.4 44

3 <1 <1.5 18 4.8 26 >100

2 North Llano River near 

Junctiona

0 <1 <1 <1 <1 <1 1.2

2 <1 <1 1.3 <1 1.3 3.5

3 <1 <1 <1 <1 <1 1.3

3 South Llano River at 

Baker Ranch

3 14 >100 -- >100 -- --

4 South Llano River at 

U.S. Highway 377 

near Rocksprings

1 3.7 10 46 24 >100 >100

5 South Llano River at 

700 Springs Ranch

1 9.8 98 >100 >100 >100 --

2 1.8 33 >100 21 >100 --

3 11 >100 >100 >100 >100 --

7 South Llano River at 

South Llano River 

State Park

1 <1 1.2 5.7 <1 5.8 >100

3 <1 1.2 4.4 1.1 3.2 --

8 South Llano River 

at Texas Tech 

University–Junction

1 <1 2.9 >100 1.8 >100 --

2 1.2 4.7 -- 4.7 -- --

3 <1 3.4 -- 1.7 -- --

12 Johnson Fork at 

Lowlands Crossing

1 <1 <1 2.2 <1 1.8 3.1

2 <1 1.0 2.1 <1 2.0 3.2

3 <1 <1 2.3 <1 2.2 4.0

13 Llano River near 

Junction1

2 <1 <1 <1 <1 <1 <1

3 <1 <1 1.1 <1 <1 2.5

14 Llano River near Ivy 

Chapel

2 <1 <1 1.5 <1 1.4 6.6

3 <1 <1 1.1 <1 1.2 3.5

1

 Partial-duration flood-frequency site.
56 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau

Table 14. Approximate return periods for initial and complete bed-material entrainment at sites with particle-size data, Edwards and 

Kimble Counties, Texas, based on median channel slope, downstream decrease of particle size, and median Manning’s n.

[d16, d50, and d84, descriptors that represent diameters at which 16, 50, and 84 percent, respectively, of the sample is finer than; yr, years; >, greater than; --, crosssectional area not possible to compute with available geometric data; <, less than]

Study 

site 

number 

(fig. 2)

Study site Cross 

section

Return 

period 

for initial 

entrainment 

of d16

(yr)

Return 

period 

for initial 

entrainment 

of d50

(yr)

Return 

period 

for initial 

entrainment 

of d84

(yr)

Return 

period for 

complete 

entrainment 

of d16

(yr)

Return 

period for 

complete 

entrainment 

of d50

(yr)

Return 

period for 

complete 

entrainment 

of d84

yr)

1 North Llano River 

near Roosevelt

2 <1 <1.5 3.5 2.1 4.8 65

3 <1 <1.5 23 5.7 36 >100

2 North Llano River 

near Junction1

0 <1 <1 <1 <1 <1 1.5

2 <1 <1 1.3 <1 1.4 3.9

3 <1 <1 <1 <1 <1 1.7

3 South Llano River at 

Baker Ranch

3 10 >100 >100 >100 -- --

4 South Llano River at 

U.S. Highway 377 

near Rocksprings

1 1.8 6.5 50 7.5 73 >100

5 South Llano River 

at 700 Springs 

Ranch

1 5.1 42 >100 >100 >100 --

2 1.5 18 >100 15 >100 --

3 5.2 37 >100 50 >100 --

7 South Llano River at 

South Llano River 

State Park

1 <1 1.2 7.1 <1 7.0 >100

3 <1 1.3 4.6 1.2 3.6 --

8 South Llano River 

at Texas Tech 

University–

Junction

1 <1 2.7 >100 1.9 >100 --

2 1.1 3.8 -- 3.7 -- --

3 <1 3.0 -- 1.7 -- --

12 Johnson Fork 

at Lowlands 

Crossing

1 <1 <1 2.3 <1 2.0 3.5

2 <1 1.5 2.2 <1 2.2 3.7

3 <1 1.1 2.4 <1 2.4 4.3

13 Llano River near 

Junction1

2 <1 <1 <1 <1 <1 <1

3 <1 <1 1.0 <1 <1 2.3

14 Llano River near Ivy 

Chapel

2 <1 <1 1.4 <1 1.3 5.7

3 <1 <1 1.2 <1 1.2 3.8

1

 Partial-duration flood-frequency site. 
References Cited 57

Summary

The Texas Department of Transportation (TxDOT) 

spends considerable money for maintenance and replacement 

of low-water crossings (LWCs) of streams in the Edwards Plateau in Central Texas as a result of the transport of cobble- and 

gravel-sized bed material. TxDOT Research Project 0–4695 is 

focused on research of bed-material entrainment as related to 

LWC maintenance issues in the Edwards Plateau. This study 

was done by the U.S. Geological Survey (USGS), in cooperation with TxDOT, and in collaboration with Texas Tech University, Lamar University, and the University of Houston, to 

quantify the potential for bed-material entrainment in selected 

streams of the plateau in a study area comprising Edwards, 

Kimble, and Real Counties, Texas, and vicinity.

The study area contains a number of streams appropriate for bed-material entrainment research, including the Llano 

River, Frio River, Nueces River, and tributaries. The streams 

originate in relatively western and southern parts of the 

Edwards Plateau, an elevated lower-Cretaceous limestone and 

dolomite tableland. The streams in the study area are incised 

into the plateau, resulting in removal of considerable amounts 

of lower-Cretaceous carbonate rock. Climatic fluctuations during the last 20,000 years have resulted in periods of variable 

streamflow and associated sediment transport rates. Currently 

(2007), much of the alluvial material occurs as valley fill and 

bed material in the streams of the plateau.

The study sites primarily were chosen to characterize 

downstream changes in channel geometry and bed-material 

entrainment potential. For this reason, all detailed study sites 

are in the Llano River Basin, chosen because (1) most of the 

problematic LWCs identified by TxDOT personnel are located 

there and (2) it was desirable to analyze systematic downstream trends in bed-material entrainment potential without 

introducing drainage-basin variability.

Study sites range from ephemeral, bedrock, upper headwater channels to relatively wide, perennial channels occurring 

within Pleistocene and Holocene alluvial terraces and active 

floodplains. Other sites in the Frio and Nueces River Basins 

were visited and qualitatively examined for problems related 

to bed-material entrainment.

A variety of methods were used to obtain data necessary 

for computation of bed-material entrainment potential and 

included:

•	 GIS mapping software was used to delineate watersheds, determine drainage areas, and model watershed 

and channel slopes.

•	 Partial-duration flood-frequency analyses were done to 

determine the return periods of floods for USGS and 

Lower Colorado River Authority gaging stations in the 

study area. Regional regression equations were used to 

estimate flood frequency at ungaged study sites.

•	 Site reconnaissance was done with assistance from 

TxDOT personnel knowledgeable about the locations 

of sites with extreme bed-material mobility and damage to transportation infrastructure.

•	 Field surveys were designed to obtain the parameters 

needed to apply equations to compute bed-material 

entrainment potential. Field surveys included measurements of channel geometry, LWC geometry, and bedmaterial particle sizes.

Results indicate that the bed-material entrainment problem for TxDOT LWCs occurs at two spatial scales—watershed 

scale and channel-reach scale.

First, the relative abundance and activity of cobble- and 

gravel-sized bed material at a site can be attributed to watershed characteristics, specifically watershed slope. The relative 

abundance and activity of bed material along a given channel 

reach becomes greater with increasingly steeper watershed 

slopes, based on analyses of digital elevation models and 

qualitative observations made during field-reconnaissance trips 

to the study area.

Second, the stresses needed to mobilize bed material at 

a site can be attributed to reach-scale hydraulic factors. The 

propensity for initial and complete entrainment of bed material depends on cross-sectional geometry and particle size. 

Flood-frequency analyses are associated with computations of 

bed-material entrainment to estimate the frequency of bedmaterial entrainment. In general, the frequency of entrainment 

increases with downstream distance, as a result of decreasing 

particle size and increased flood magnitudes. An average of 1 

year occurs between flows that initially entrain bed material 

as large as the median particle size (d50 [diameter at which 50 

percent of the sample is finer]), and an average of 1.5 years 

occurs between flows that completely entrain bed material up 

to the median particle size (d50). The Froude numbers associated with initial and complete entrainment of bed material as 

large as the median particle size (d50) are about 0.40 and 0.45, 

respectively.

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Appendix 1

Appendix 1—Selected Photographs of Sites and Low-Water 

Crossings in Edwards, Kimble, and Real Counties, Texas
Blank Page
Figure 1.1. Downstream end of culvert pipes at Ben Williams Crossing of the Nueces River on Ranch Road 335. Many of the culverts 

are clogged with cobbles and gravel.

Figure 1.2. Ben Williams Crossing of the Nueces River on Ranch Road 335. 

Appendix 1 61
Figure 1.3. View downstream from damaged low-water crossing of Cedar Creek at Ranch Road 336 following June 2004 flood. 

Figure 1.4. Upper Ranch Road 1120 crossing of the Frio River following the peak of the June 2004 flood.

62 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 1.5. View downstream from cross section 2 of South Llano River at Baker Ranch.

Figure 1.6. View downstream from the South Llano River at U.S. Highway 377 near Rocksprings. Notice the depth of channel incision 

into the Edwards Plateau on the right bank. 

Appendix 1 63
Figure 1.7. View across the floodplain of the South Llano River at 700 Springs Ranch at cross section 2. The depth of incision into the 

Edwards Plateau is greater than that at South Llano River at U.S. Highway 377 near Rocksprings. 

Figure 1.8. First Crossing of the South Llano River along U.S. Highway 377 in Kimble County. 

64 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 1.9. View upstream from the South Llano River at South Llano River State Park near cross section 1.

Figure 1.10. View across the channel and channel bar of South Llano River at Texas Tech University–Junction at cross section 3. 

Appendix 1 65
Figure 1.11. Cobble and gravel lenses within relatively fine-grained Holocene floodplain deposits in bank of South Llano River at Texas 

Tech University–Junction.

Figure 1.12. View across the South Llano River at Texas Tech University–Junction toward vegetated Holocene floodplain. 

66 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 1.13. View across a gravel bar and floodplain of North Llano River near Roosevelt at cross section 3.

Figure 1.14. View across a cobble and gravel bar of North Llano River near Junction at cross section 1.

Appendix 1 67
Figure 1.15. View upstream from Johnson Fork at Lowlands Crossing at cross section 2. 

Figure 1.16. View across the channel and low-water crossing of Johnson Fork at Guzman Crossing.

68 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 1.17. View upstream from Johnson Fork at Paks Crossing.

Figure 1.18. View of the bedrock channel bed of Llano River near Junction at cross section 1.

Appendix 1 69
Figure 1.19. View across Llano River near Junction at cross section 3. Notice the downstream imbrication of boulders of the channel 

bar deposit. 

Figure 1.20. View across Llano River near Junction at cross section 2.

70 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Figure 1.21. View downstream from Llano River near Ivy Chapel at cross section 2.

Figure 1.22. View upstream from Llano River near Ivy Chapel at cross section 3.

Appendix 1 71
Figure 1.23. Reconstruction of Ranch Road 337 low-water crossing of the Frio River near Leakey, December 2004.

72 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
Appendix 2

Appendix 2—Two-Dimensional Geophysical Resistivity 

Investigation
Blank Page
Two-dimensional (2-D) geophysical resistivity investigations were made at Johnson Fork at Lowlands Crossing and Nueces 

River at Ben Williams Crossing. The resistivity surveys were done to understand the composition of bed material with depth 

beneath the surface, but results were limited and not directly applicable to computations of bed-material entrainment.

2D resistivity surveys were made at Ben Williams Crossing of the Nueces River on the Edwards and Real County line and 

Lowlands Crossing of Johnson Fork in Kimble County. Surface and geophysical methods provide a relatively fast and inexpensive means to characterize the subsurface of channel beds (Powers and others, 1999). Electrical surface-geophysical methods can 

be used to detect variations in the electrical properties of the subsurface. The electrical properties of soil and rock are controlled 

by water content, salt content, porosity, and the presence of metallic minerals. Variations in the electrical properties of soils and 

rocks, either vertically or horizontally, produce variations in the electrical signatures measured by surface geophysical tools 

(Shah and Stanton, 2006). Changes in the received signal can be related to changes in the composition, extent, and physical 

properties of subsurface soil and rock, to the extent that differences in lithology or rock type are accompanied by differences in 

resistivity (U.S. Army Corps of Engineers, 1995). According to Lucius and others (2007), dry sand- and gravel-deposits have 

high resistivity (200 ohm-meters [ohm-m] or more), and saturated sand- and gravel-deposits are characterized by low resistivity (50 ohm-m or less). Carbonate bedrock also is characterized by high resistivity, and commonly is greater than 1,000 ohm-m. 

Specifically, the Geometrics OhmMapper TR5, a mobile resistivity measuring system, was used to complete the survey. The 

OhmMapper TR5 is a capacitively coupled instrument that measures apparent resistivity of the earth using antennas in a dipoledipole array (Geometrics, 2005).

The data for two resistivity profiles traversing Johnson Fork at Lowlands Crossing (fig. 32) and Nueces River at Ben Williams Crossing (fig. 2.1) are described and graphically presented in the form of 2D sections that show contoured resistivity in 

gradational colors (fig. 2.2). The two resistivity sections are shown in figure 2.2 at a common resistivity scale.

Line 1 (fig. 2.2A), at Johnson Fork, traverses northwest to southeast toward Ranch Road 2169, aligned roughly parallel to 

the channel. Line 1 shows five distinct pockets of higher resistivity throughout the section at similar depths. At the mid-to-western end of the line are three pockets of relatively high resistivity (1,500 to 2,000 ohm-m) that extend to about 0.9 to 1.8 meters (3 

to 6 feet) below the surface. The remaining two pockets of high resistivity range from about 1,000 to 1,300 ohm-m. The remainder of the section shows a relatively lower resistivity (higher conductivity) range from about 100 to 800 ohm-m.

Line 2 (fig. 2.2B) is perpendicular to the Nueces River at Ben Williams Crossing. The capacitively-coupled data were collected at a shallower depth than in line 1 probably because of a more conductive environment. Line 2 shows a pattern of high 

resistivity (greater than 1,000 ohm-m) close to the surface throughout the section. Most of these resistivity zones start at the 

surface and extend to a depth of approximately 1.5 meters (5 feet). Highest resistivity occurs in two small zones about 0.5 to 0.9 

meters (1.5 to 3 feet) below the surface near the southeastern end of the line.

To accurately interpret these data, more ground-truthing data, such as well construction, lithologic, or borehole geophysical 

logs are needed. Also, additional surface-geophysical methods such as direct-current resistivity or time-domain electromagnetic 

techniques (Lucius and others, 2007) could penetrate deeper below land surface. The minimal variations in the sections were not 

sufficient to identify the content of the interstitial space between coarse alluvial particles. With appropriate verification using 

complementary methods, high conductivity (low resistivity) at the surveyed crossings could indicate the presence of either clay 

or water in the interstitial space, as opposed to sand or air, which can affect channel-substrate stability at low-water crossings.

Appendix 2 75
EXPLANATION

RESISTIVITY, IN OHM-METERS

100 500 1,000 1,500 2,000

A

B

DEPTH, IN FEET BELOW LAND SURFACE

0

1

3

5

7

0

2

4

6

8

0 165 330 495 660

0 245 490 735 980

DISTANCE, IN FEET

(Line 1, fig. 32)

(Line 2, fig. 2.1)

Figure 2.2. Field results of capacitively-coupled data (actual resistivity) at common resistivity scale for (A) Johnson Fork at Lowlands 

Crossing and (B) Nueces River at Ben Williams Crossing.

Flow direction

Line 2

FM 335 EXPLANATION

Resistivity

transect

0 75 150 225 300 METERS

0 240 480 720 960 FEET

Projection: Transverse Mercator

Zone: 14 North

Spheroid: NAD83

Imagery: 2004 color infrared

N

Figure 2.1. Channel resistivity transect at Nueces River at Ben Williams Crossing (see fig. 2 for site location), Edwards-Real County 

line, Texas.

76 Potential for Bed-Material Entrainment in Selected Streams of the Edwards Plateau
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