U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3047

June 2008

In cooperation with the Texas Department of Transportation

Bed-Material Entrainment and Associated Transportation Infrastructure 

Problems in Streams of the Edwards Plateau, Central Texas 

Printed on recycled paper

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. They are designed to 

accommodate flow by roadway overtopping during highflow events. The streams of the Edwards Plateau are characterized by cobble- and gravel-sized bed material and 

highly variable flow regimes. Low base flows that occur 

most of the time occasionally are interrupted by severe 

floods. The floods entrain and transport substantial loads 

of bed material in the stream channels. As a result, LWCs 

over streams in the Edwards Plateau are bombarded and 

abraded by bed material during floods and periodically 

must be maintained or even replaced.

This research study, done by the U.S. Geological Survey (USGS), in cooperation with TxDOT, 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, floodfrequency analyses, field investigations, and computations of bed-material entrainment. Results are expected 

to complement numerical and physical modeling of 

bed-material mobility at Texas Tech University, the 

University of Houston, and Lamar University, all collaborators on this study, and to facilitate LWC design 

and assignation of risk to channel reaches subject to 

bed-material mobility. 

Methods Applied to Quantify BedMaterial Entrainment

Visualization of the distribution of channel slopes 

at watershed and channel-reach scales is important for 

TxDOT engineers and other practitioners. Accordingly, 

channel slopes were mapped in the study area (fig. 2) 

using GIS. An example of a final slope map for the North 

Llano River subbasin is shown in figure 3.

An alternative method for visualization of channel 

slope involves longitudinal profiles. A longitudinal profile 

is a graph of channel-bed elevation with downstream 

distance for one stream. 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. 

One example of a longitudinal profile is shown in figure 4 

for the Llano River.

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 selected gaging stations in the study area. 

The partial-duration series uses discharges of measured 

and estimated flood peaks above a designated threshold 

(base discharge). In combination with flood-frequency 

analyses, field surveys of cross-sectional channel geometry and bed-material particle size (fig. 5) were done to 

obtain the data to apply equations that quantify bedmaterial entrainment.

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

County, Texas.
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 streams, coarse bed 

material remains in place (fig. 6). However, moderate- to 

high-flow conditions exert greater stresses on the channel bed, and coarse bed material initially mobilizes at a 

critical shear stress, which can be associated with flow 

depth, flow velocity, and discharge. Computations of bedmaterial entrainment (table 1) rely on channel-geometry 

and particle-size data. 

The computed critical shear stresses, streamflows, 

and return periods for bed-material entrainment serve 

only as a predictive model for bed-material entrainment 

in the study area. The results thus provide limited guidance for predicting actual bed-material entrainment, but 

some interpretations are possible: 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 

100°30' 100°00' 99°30'

30°30'

30°00'

29°30'

Edwards

Plateau

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

1 3

5

2

4

6

EXPLANATION

Boundary of study area

Boundary of Edwards Plateau

Detailed study site and site number (table 1)

Low-water crossing study site and site number (table 1)

Lower Colorado River Authority streamflow-gaging station (Heitmuller and Asquith, 2008)

U.S. Geological Survey streamflow-gaging station (Heitmuller and Asquith, 2008)

Detailed study site at U.S. Geological Survey streamflow-gaging station and site number (table 1) 3

6

4

0 10 20 30 KILOMETERS

0 3 10 20 0 MILES

§ ̈¦10

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£¤377

£¤377

£¤83

£¤87

£¤290

£¤83

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

Figure 2. Location of study area—Edwards, Kimble, and Real Counties, Texas, and vicinity. 
Figure 3. Channel slope of the North Llano River subbasin, Kimble County, Texas, and vicinity. 

£¤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 Hydrography from U.S. Geological Survey

National Hydrography Dataset

0 3 6 9 12 KILOMETERS

0 369 12 MILES

Area enlarged NORTH LLANO RIVER SUBBASIN

INDEX MAP OF STUDY AREA

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.

Figure 4. Longitudinal profile of the Llano 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
Table 1. Computations for bed-material entrainment and return periods at sites in Edwards and Kimble Counties, Texas.

[d50, descriptor that represents diameter at which 50 percent of sample is finer than; τc, critical shear stress; N/m2, newtons per square meter; Q, streamflow; m3/s, cubic 

meters per second; ft3/s, cubic feet per second; yr, years; <, less than]

Study 

site 

number 

(fig. 2)

Study site Cross 

section

d50 τc

initial 

(N/m2)

d50 Q

initial 

[m3/s (ft3/s)]

d50 τc

complete 

(N/m2)

d50 Q

complete 

[m3/s (ft3/s)]

Return period 

for initial 

entrainment of d50

(yr)

Return period 

for complete 

entrainment of d50

(yr)

1 North Llano River 

near Junction

0 22.99 21.9 (774) 45.98 82.3 (2,910) <1 <1

2 22.99 71.6 (2,530) 45.98 302 (10,600) <1 1.4

3 22.99 29.2 (1,030) 45.98 123 (4,340) <1 <1

2 Johnson Fork at Lowlands Crossing

1 27.03 9.44 (333) 54.07 91.7 (3,240) <1 2.0

2 27.03 22.0 (776) 54.07 140 (4,930) 1.5 2.2

3 27.03 16.3 (575) 54.07 227 (8,030) 1.1 2.4

3 Llano River near 

Junction

2 18.87 .70 (24.9) 37.74 3.66 (129) <1 <1

3 18.87 5.38 (190) 37.74 104 (3,660) <1 <1

4 Llano River near Ivy 

Chapel

2 16.47 123 (4,340) 32.95 515 (18,200) <1 1.3

3 16.47 101 (3,550) 32.95 429 (15,200) <1 1.2

Figure 5. Underwater sampling grid and particle-size analyzer on 

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

Figure 6. 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. 

Reference

This fact sheet is based on the following USGS report:

Heitmuller, F.T., and Asquith, W.H., 2008, Potential for bedmaterial 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. 

—Franklin T. Heitmuller and William H. Asquith

Any use of trade, product, or firm names is for descriptive 

purposes only and does not imply endorsement by the U.S. 

Government.

Information on technical reports and hydrologic data related to 

this study can be obtained from:

Director

USGS Texas Water Science Center

World Wide Web: http://tx.usgs.gov/

E-mail: gs-w-txpublic-info@usgs.gov