U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

EXPERIMENTAL TESTING OF FLEXIBLE BARRIERS 

FOR CONTAINMENT OF DEBRIS FLOWS

by

Jay S. DeNatale1 

Richard M. Iverson2

Jon |. Major2 

Richard G. LaHusen2

Gregg L. Fiegel1

John D. Duffy1

1 California Polytechnic State University 

Civil and Environmental Engineering Department

1 Grand Avenue 

San Luis Obispo, California 93407

2 United States Geological Survey

David A. Johnston Cascades Volcano Observatory

5400 MacArthur Boulevard

Vancouver, Washington 98661

OPEN-FILE REPORT 99-205

Vanvcouver, Washington 

1999
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBIT, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government.

For additional information 

write to:

U.S. Geological Survey 

Cascades Volcano Observatory 

5400 MacArthur Boulevard 

Vancouver, Washington 98661

Copies of this report 

can be purchased from:

U.S. Geological Survey 

Branch of Distribution 

Federal Center, Box 25286 

Denver, CO 80225
CONTENTS

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

Introduction................................................................................................................ 3

Barrier configuration................................................................................................... 6

Barrier performance .................................................................................................... 14

Conclusions................................................................................................................ 38

References.................................................................................................................. 39

in
ILLUSTRATIONS

Figure Page

1. Schematic plan view of the barrier placement on the run-out pad ................ 5

2. Barrier 1 (test 1) ......................................................................................... 8

3. Post-flow condition of barrier 1 (test 1) ....................................................... 8

4. Maximum run-up on barrier 2 (test 2).......................................................... 9

5. Post-flow condition of barrier 2 (test 2)....................................................... 9

6. Collapse of right column and overtopping of barrier 2 (test 3)...................... 10

7. Post-flow profile view of barrier 2 (test 3).................................................... 10

8. Barrier 3 (test 4) ........................................................................................... 11

9. Post-flow frontal view of barrier 3 (test 4) .................................................... 11

10. Maximum run-up on barrier 3 (test 5) ........................................................... 12

11. Debris deposition behind barrier 3 (test 5)..................................................... 12

12. Barrier 4 (test 6) ........................................................................................... 13

13. Post-flow condition of barrier 4 (test 6) ......................................................... 13

14. Net displacement data.................................................................................... 18

15. Comparison of gradations retained and passing debris (test 2)........................ 20

16. Comparison of gradations retained and passing debris (test 4)........................ 21

17. Comparison of gradations retained and passing debris (test 6)........................ 22

18. Comparison of debris passing the chicken-wire (test 4) and

silt-screen (test 6) net liners ...................................................................... 23

19. Distribution of debris passing beneath or through the barrier (test 6) ............... 24

20. Contours of equal deposit thickness (in centimeters)

For the debris flow of test 1 ...................................................................... 25

21. Contours of equal deposit thickness (in centimeters)

For the debris flow of test 2 ...................................................................... 26

22. Contours of equal deposit thickness (in centimeters)

For the debris flow of test 3 ...................................................................... 27

23. Contours of equal deposit thickness (in centimeters)

For the debris flow of test 4 ...................................................................... 28

24. Contours of equal deposit thickness (in centimeters)

For the debris flow of test 5 ...................................................................... 29

25. Contours of equal deposit thickness (in centimeters)

For the debris flow of test 6 ...................................................................... 30

26. Anchor cable forces (test 1) ............................................................................ 31

27. Anchor cable forces (test 2) ............................................................................ 32

28. Anchor cable forces (test 3) ............................................................................ 33

29. Anchor cable forces (test 4) ............................................................................ 34

30. Anchor cable forces (test 5) ............................................................................ 35

31. Anchor cable forces (test 6) ............................................................................ 36

32. Comparison of right tie-back anchor cable forces ............................................ 37

IV
TABLES

Table Page

1. Comparison of debris volume, flow velocity, and net deflection data................. 17

2. Comparison of barrier effectiveness in terms of debris containment................... 19
CONVERSION FACTORS

Multiply by To obtain

millimeter (mm) 

centimeter (cm) 

meter (m)

square meter (m2)

cubic meter (m3)

meter per second (m/s)

kilogram (kg) 

metric ton (Mg)

Newton (N) 

kiloNewton (kN)

Length

0.03937

0.3937

3.281

Area

10.76

1.196

Volume

35.31

1.308

Speed

3.281

2.237

Mass

0.06854

2.205

1.102

Force

0.2248

0.1124

inch 

inch 

foot

square foot 

square yard

cubic feet 

cubic yard

foot per second 

mile per hour

slug

pound (mass)

ton (mass)

pound (force) 

ton (force)

VI
ABSTRACT

In June 1996, six experiments conducted at the U.S. Geological Survey Debris 

Flow Flume demonstrated that flexible, vertical barriers constructed of wire rope netting 

can stop small debris flows. All experimental debris flows consisted of water-saturated 

gravelly sand with less than two percent finer sediment by weight. All debris flows had 

volumes of about 10 cubic meters, masses of about 20 metric tons, and impact velocities 

of 5 to 9 meters per second. In four experiments, the debris flow impacted pristine, 

undeformed barriers of varying design; in the other two experiments, the debris flow 

impacted barriers already loaded with sediment from a previous flow.

Differences in barrier design led to differences in barrier performance. 

Experiments were conducted with barriers constructed of square-mesh wire-rope netting 

with 30 centimeter, 20 centimeter, and 15 centimeter mesh openings as well as 

30 centimeter diameter interlocking steel rings. In all cases, sediment cascading 

downslope at the leading edge of the debris flows tended to spray through the nets. Nets 

fitted with finer-mesh chain link or chicken wire liners contained more sediment than did 

unlined nets, and a ring net fitted with a synthetic silt screen liner contained nearly 

100 percent of the sediment. Irreversible net displacements of up to 2 meters and friction 

brake engagement on the support and anchor cables dissipated some of the impact energy. 

However, substantial forces developed in the steel support columns and the lateral and 

tie-back anchor cables attached to these columns. As predicted by elementary mechanics, 

the anchor cables experienced larger tensile forces when the support columns were
hinged at the base rather than bolted rigidly to the foundation. Measured loads in the 

lateral anchor cables exceeded those in the tie-back anchor cables and the load cell 

capacity of 45 kilo-Newtons. Measurements also indicated that the peak loads in the tieback anchors were highly transient and occurred at the points of maximum momentum 

impulse to the net.
INTRODUCTION

During the winter of 1994-95, a flexible wire rope rock net installed along 

California State Route 41 in San Luis Obispo County stopped and contained several 

rainfall-induced debris flows having a combined volume of approximately 60 m3 (cubic 

meters) (Duffy and DeNatale, 1996). In June 1996, a limited experimental study was 

undertaken to establish, in a more quantitative fashion, the response of such flexible 

systems to debris-flow loading.

Experiments were performed at the United States Geological Survey (USGS) 

Debris Flow Flume located within the H. J. Andrews Experimental Forest near Blue 

River, Oregon. The flume is a reinforced concrete channel 95 m (meters) long, 2 m wide, 

and 1.2 m deep. The uppermost 88 m of the flume bed lie at an angle of 31 degrees and 

the lowermost 7 m gradually flatten to a run-out slope of 3 degrees. A concrete run-out 

pad at this same 3 degree slope extends 25 m beyond the mouth of the flume. Up to 

20 m3 of water-saturated sediment can be discharged from the mouth of the flume at 

velocities of approximately 10 m/s (meters per second) (Iverson and others, 1992). The 

"debris" employed during the June 1996 experiments consisted of a poorly graded, clean, 

gravelly sand that had been used in previous USGS debris flow studies (Iverson, 1997; 

Major and others, 1997).

Massive 1.22 m wide concrete panels can be placed on the concrete run-out pad to 

confine the flow beyond the mouth of the flume. In the June 1996 experiments, eight 

confinement panels were placed to provide an additional 5 m of channelized flow. This
arrangement ensured that the flow would not spread laterally prior to its impact with the 

flexible barrier. A schematic plan view of the experiment configuration is shown in 

figure 1.

In the June 1996 experiments, a load cell and two piezometers were placed in a 

port within the run-out pad (1m behind the flexible barrier) to record the vertical total 

stress and pore fluid stress that developed at the base of the deposit. An ultrasonic flow 

depth sensor was attached to a support beam that crossed this same section. An 

extensometer was attached to the front of the net to record the history of net deformation 

in the direction of flow. Load cells were installed in two of the four barrier anchor cables 

to measure the history of tensile force sustained by these key structural components. A 

high-frequency digital data acquisition system permitted continuous, synchronous 

recording of all instrument signals. The kinematics of the flow and barrier response were 

documented through still and video camera surveillance during each flow. A topographic 

survey of the deposit was done at the conclusion of each flow.

This report summarizes basic experimental data that bear on the performance of 

the flexible barrier systems. The data lead to the conclusion that some such systems can 

effectively contain debris flows with volumes of about 10 m3 and speeds less than 10 m/s.
CONCRETE CDNFINEMENT PANELS

FLUME MDUTH TIEBACK ANCHOR

X WIRE RDPE NETTING 

SUPPORT COLUMN

2 M

*LATERAL ANCHOR

Figure 1. Schematic plan view of the barrier placement on the run-out pad.
BARRIER CONFIGURATION

Six tests were conducted with four different flexible barriers. All barriers had a 

height of 2.4 m and a length of 9.1 m, sufficient to completely span the flume's concrete 

run out pad, as shown in figure 1.

The first barrier system (test 1) employed 30 cm by 30 cm wire rope netting 

overlaid with a 5.1 cm chain link liner to reduce the size of the largest clear opening. The 

net panel was supported by 1.9 cm diameter perimeter and anchor cables and two vertical 

W 4 x 13 structural steel columns rigidly fixed to concrete anchor pads. W 4 x 13 is the 

American Institute for Steel Construction (AISC) designation for a wide-flange section 

having a nominal depth of 4 inches and a weight of 13 pounds/lineal foot. Photographs 

taken before and after the debris flow are included as figures 2 and 3.

The second barrier system (tests 2 and 3) employed 20 cm by 20 cm wire rope 

netting overlaid with a 5 cm chain link liner. An additional chicken wire liner was 

attached to the middle third of the net to further reduce the size of the largest clear 

opening. The net panel was supported by 1.9 cm diameter perimeter and anchor cables 

and hinged-base steel columns. The barrier was subjected to two consecutive debris 

flows. Photographs taken during and after the debris flow are included as figures 4-7.

The third barrier system (tests 4 and 5) employed 15 cm by 15 cm wire rope 

netting overlaid with full-width chain link and chicken wire liners. The net panel was 

supported by 1.9 cm diameter perimeter and anchor cables and W 8 x 48 fixed-base steel
columns. The barrier was subjected to two consecutive debris flows. Photographs taken 

before, during and after the debris flow are included as figures 8-11.

The fourth barrier system (test 6) employed 30 cm diameter interlocking rings 

overlaid with full-width chain link and silt screen liners. The net panel was supported by 

1.9 cm diameter perimeter and anchor cables and W 8 x 48 fixed-base steel columns. 

Photographs taken before and after the debris flow are included as figures 12 and 13.

The rope nets used in tests 1 -5 were fabricated using galvanized wire rope with a 

diameter of 0.8 cm and a nominal breaking strength of 40.9 kN. Each "ring" of the ring 

net used in test 6 consisted of a bundle of four steel rings, each with a cross-sectional 

diameter of 0.3 cm.
Figure 2. Barrier 1 (test 1).

Figure 3. Post-flow condition of barrier 1 (test 1).
Figure 4. Maximum run-up on barrier 2 (test 2).

Figure 5. Post-flow condition of barrier 2 (test 2).
Figure 6. Collapse of right column and overtopping of barrier 2 (test 3).

Figure 7. Post-flow profile view of barrier 2 (test 3).

10
Figure 8. Barrier 3 (test 4).

Figure 9. Post-flow frontal view of barrier 3 (test 4).

11
Figure 10. Maximum run-up on barrier 3 (test 5).

Figure 11. Debris deposition behind barrier 3 (test 5).

12
Figure 12. Barrier 4 (test 6).

Figure 13. Post-flow condition of barrier 4 (test 6).

13
BARRIER PERFORMANCE

Debris volume, flow velocity, and barrier deflection data measured during the six 

experiments are compared in table 1. Flow velocities were derived from the timestamped videotape records acquired with an overhead camera. The concrete run-out pad 

is marked with a set of one meter square gridlines. Velocities were obtained by noting 

the time required for certain readily identifiable debris (such as the gravel spray, chunks 

of foam sealant, and/or wave fronts) to pass from one grid line to another.

Barrier deflection in the direction of flow was recorded by a cable extensometer 

that was attached to the net at mid-span, at a point 50 cm above the run-out pad. 

Deflection data are compared in figure 14. The three successful barrier systems 

experienced comparable degrees of outward deflection during the first loading cycle 

(tests 2, 4, and 6). In all three cases, the maximum deflection was between 1.5 and 2.0 m. 

A second release of debris into an already loaded net (tests 3 and 5) produced less than 

one-half meter of additional outward movement.

One objective of the June 1996 field testing program was to establish the degree 

to which relatively open and porous flexible barriers could stop and contain rapidly 

moving, staged debris flows. Hence, measurements were made of the volume and spatial 

distribution of the material that passed beneath or through each net.

A comparison of barrier effectiveness in terms of debris containment is presented 

in table 2. The first two barrier systems incorporated chain link net panel liners to reduce 

the size of the largest net opening. The first barrier system (test 1) experienced a support

14
column failure and collapsed during the flow. The second barrier system performed well 

during the initial debris flow (test 2) but experienced an anchor cable connection failure 

and partially collapsed during the subsequent debris flow (test 3). The last two barrier 

systems incorporated both chain link and chicken wire (tests 4 and 5) or silt screen 

(test 6) net panel liners. As shown in table 2, each barrier stopped and contained most of 

the flowing debris. Essentially full containment was realized in test 6, where less than 

0.05 percent of the sediment passed beneath or through the net.

Laboratory sieve analyses were done to determine the gradation characteristics of 

sediment samples recovered at the conclusion of each debris flow. The dry mass of these 

samples ranged from 2.6 to 17.0kg (kilograms). Grain size distribution curves of the 

passed and retained materials are compared in figures 15-18. In tests 4 and 6, the passed 

material consisted almost entirely of gravel-sized particles (fig. 16 and 17). As expected, 

somewhat more sand passed through the chicken wire than the finer-mesh silt screen 

(fig. 18). Photographs reveal that a very diffuse spray of gravel always precedes the first 

wave of flowing debris. It is this diffuse spray (and only this spray) that passed beneath 

and through the net. Most of the passed material came to rest very quickly. In test 6, for 

example, only 12.5 percent of the passed material (0.006 percent of the initial volume) 

flowed more than 3 m beyond the net (fig. 19).

In general, there was little or no lateral flow around the left and right edges of the 

barrier. Contours of the standing debris deposits derived from the post-flow topographic 

survey data are shown in figures 20-25.

15
Tensile forces in the lateral and tie-back (upslope) anchor cables were measured 

by installing 45 kN load cells between the anchor cables and the ground anchors (rock 

bolts). The anchor cable forces measured during tests 1-6 are shown in figures 26-31, 

respectively. In the first three experiments, one cell was installed in the right lateral 

anchor cable and one cell was installed in the right tie-back anchor cable.

In test 1 (fig. 26), the signal gain was set too high and cable forces in excess of 

10 kN could not be logged. The gain was set lower in all subsequent tests. In test 2 

(fig. 27), the lateral anchor force peaked at 40 kN (90 percent of the cell's capacity). In 

test 3 (fig. 27), the lateral anchor force reached 42 kN (95 percent of the cell's capacity) 

before the threaded bolt pulled out from the cell base. Although frustrating, this 

connection failure may have been serendipitous, for it prevented the cell from being 

overloaded and permanently damaged. Prior to test 4, the cell was removed from the 

right lateral anchor cable and placed in the left tie-back anchor cable.

Elementary mechanics reveals that the anchor force in a cable-stayed column can 

be reduced by increasing the flexural rigidity of the column or by increasing the moment 

resistance of the base plate assembly. The experimental anchor force data shown in 

figure 32 are entirely consistent with elementary theory. The largest tie-back forces 

occurred during test 1 (which employed fixed-base but relatively flexible W 4 x 13 

columns) and tests 2 and 3 (which employed hinged-base columns). The smallest tie 

back forces occurred during tests 4, 5 and 6 (which used fixed-base and relatively stiff 

W 8 x 48 wide-flange columns).

16
Table 1. Comparison of debris volume, flow velocity, and net deflection data

[m3 , cubic meters; m/sec, meters per second; m, meters]

Characteristic

Debris volume (m3)

Impact velocity of gravel spray (m/sec)

Impact velocity of debris (m/sec)

Outward deflection of net panel (m)

Testl

9.8

SM

SM

SM

Test 2

9.6

12.5

9.0

1.46

TestS

10.3

NS

6.5

0.30

Test 4

10.4

10.0

8.0

1.93

TestS

10.4

NS

6.0

0.40

Test 6

10.1

10.0

5.0

1.50

SM = Data acquisition system malfunctioned 

NS = No gravel spray due to presence of intervening sediment deposited during previous test

17
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Test 2 

Tests 

Test 4 

Tests 

Test 6

15.0 17.5 20.0

TIME, IN SECONDS

Figure 14. Net displacement data.

22.5 25.0

18
Table 2. Comparison of barrier effectiveness in terms of debris containment

[m3 , cubic meters]

Characteristic

Initial debris volume (m3) 

Volume passing through net (m3 ) 

Percent passing (%)

Test 1

9.8 

SC 

SC

Test 2 Test 3

9.6 10.3 

0.46 SC 

4.8 SC

Test 4

10.4 

0.12 

1.2

TestS

10.4 

.046 

0.44

Test 6

10.1 

.0046 

0.05

SC = System collapsed

19
CO

1

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100

90

80

70

60

50

40

30

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100

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-o- Passing chicken wire

-a Passing chain link only

10 1 0.1 

GRAIN SIZE, IN MILLIMETERS

0.01

Figure 15. Comparison of gradations of retained and passing debris

(test 2).

20
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100

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Passing chicken wire 

Retained by net

10 1 0.1 

GRAIN SIZE, IN MILLIMETERS

0.01

Figure 16. Comparison of gradations of retained and passing debris

(test 4).

21
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DISTANCE BEYOND BARRIER, IN METERS

14 16

Figure 19. Distribution of debris passing beneath or through 

the barrier (test 6).

24
Figure 20. Contours of equal deposit thickness (in centimeters) for the debris flow of

test 1.

25
Figure 21. Contours of equal deposit thickness (in centimeters) for the debris flow of

test 2.

26
Figure 22. Contours of equal deposit thickness (in centimeters) for the debris flow of

test3.

27
-30 J

Figure 23. Contours of equal deposit thickness (in centimeters) for the debris flow of

test 4.

28
Figure 24. Contours of equal deposit thickness (in centimeters) for the debris flow of

test 5.

29
Figure 25. Contours of equal deposit thickness (in centimeters) for the debris flow of

test 6.

30
ou

40

30

g 20 

O 

j> 10

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0 15 20 25 3( 

TIME, IN SECONDS

...................

" Right tie-back

anchor Signal off-scale

over 1 U kN

10 15 20

TIME, IN SECONDS

25 30

Figure 26. Anchor cable forces (Test #1).

31
CO

z 

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in 

z

50

40

30

20

10

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Right lateral 

anchor

15 20

TIME, IN SECONDS

15 20

TIME, IN SECONDS

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TENSILE FORC 50 

40 

30 

20 

10 

0

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Right tie-back 

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1 .... 1 .... 1 ....

30

Figure 27. Anchor cable forces (Test #2).

32
TENSILE FORCE, IN KILO-NEWTONS

ou 

40 

30 

20 

10 

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1

50 

40 

30 

20 

10 

0

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TIME, IN SECONDS

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Figure 28. Anchor cable forces (Test #3).

33
50

40

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TIME, IN SECONDS

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TIME, IN SECONDS

25 30

Figure 29. Anchor cable forces (Test #4).

34
CO

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Figure 30. Anchor cable forces (Test #5).

35
TENSILE FORCE, IN KILO-NEWTONS

ou 

40 

30 

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TIME, IN SECONDS

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10 15 20

TIME, IN SECONDS

25 30

Figure 31. Anchor cable forces (Test #6).

36
CO

Testl 

Test 2 

Test3 

Test 4 

Tests 

TestG

15.0 17.5 20.0

TIME, IN SECONDS

22.5 25.0

Figure 32. Comparison of right tie-back anchor cable forces.

37
SUMMARY

The experimental study described in this report permitted the evaluation of four 

flexible debris-flow mitigation systems. The moderate variety of options studied and the 

inherent variability of even a well-controlled, staged debris flow make it difficult to 

isolate the effect of any single parameter during a six-test study. Nevertheless, the testing 

program did provide much useful data. Both the 15 cm wire rope barrier and the 30 cm 

interlocking ring net barrier effectively stopped and contained the rapidly flowing 

sediment. Essentially full containment was realized in test 6, where less than 

0.05 percent of the sediment passed beneath or through the net. These final two barrier 

systems may have the energy absorption and retention characteristics to effectively 

mitigate many small, natural debris flows.

38
REFERENCES

Duffy, J.D., and DeNatale, J.S., 1996, Debris flow mitigation using flexible barriers, 

Proceedings of the 47th Annual Highway Geology Symposium, p. 243-252.

Iverson, R.M., 1997, The physics of debris flows, Reviews of Geophysics, v. 35, no. 3, 

p. 245-296.

Iverson, R.M., LaHusen, R.G., and Costa, I.E., 1992, Debris-flow flume at H.J. Andrews 

Experimental Forest, Oregon, U.S. Geological Survey Open-File Report 92-483, 2 p.

Major, J.J., Iverson, R.M., McTigue, D.F., Macias, S., and Fiedorowicz, B.K., 1997, 

Geotechnical properties of debris-flow sediments and slurries, Proceedings of the 

First International Conference on Debris-Flow Hazards Mitigation, p. 249-259.

39