Prepared in cooperation with the Federal Highway Administration

Assessment of Bridge Scour Countermeasures

at Selected Bridges in the United States, 2014–18

Scientific Investigations Report 2019–5080

U.S. Department of the Interior

U.S. Geological Survey
Cover: (Top) Combined lidar and bathymetry image of the bridge over the Upper Iowa River at IA-76 near Dorchester, Iowa (site 030). (Bottom) 

Bridge over the Mississippi River near Caruthersville, Missouri (site 004). Photo by Richard Huizinga, U.S. Geological Survey.
Assessment of Bridge Scour Countermeasures at Selected Bridges in the

United States, 2014–18

By Thomas P. Suro, Richard J. Huizinga, Ryan L. Fosness, and Taylor J. Dudunake

Prepared in cooperation with the Federal Highway Administration

Scientific Investigations Report 2019–5080

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation:

Suro, T.P., Huizinga, R.J., Fosness, R.L., and Dudunake, T.J., 2020, Assessment of bridge scour countermeasures at 

selected bridges in the United States, 2014–18: U.S. Geological Survey Scientific Investigations Report 2019–5080, 

29 p., https://doi.org/10.3133/sir20195080.

Associated data for this publication:

Dudunake, T.J., 2017, Geospatial data for bridge scour countermeasure assessments at select bridges in the United 

States, 2014 to 2016: U.S. Geological Survey data release, https://doi.org/10.5066/F71R6NQ2.

Dudunake, T.J., 2019, Geospatial data for bridge scour countermeasure assessments at select bridges in the United 

States, 2016–18: U.S. Geological Survey data release, https://doi.org/10.5066/F7WW7G4W.

ISSN 2328-0328 (online)
iii

Acknowledgments

The authors would like to express their appreciation to the Departments of Transportation in 

the 11 cooperating States (Connecticut, Florida, Idaho, Illinois, Iowa, Missouri, Montana, New 

Jersey, Pennsylvania, South Carolina, and Tennessee), to Daniel Ghere, David Henderson, 

Kornel Kerenyi, and the Hydraulics Research and Development Program Staff at the Federal 

Highway Administration who assisted in the study. The authors would like to also express their 

appreciation and gratitude to their U.S. Geological Survey (USGS) colleagues Peter Cinotto, 

Chad Wagner, Rich Akins, Justin Boldt, Keith Cobb, Jerilyn Collenburg, Brenda Densmore, 

Ben Dietsch, Jarrett Ellis, Wladmir Guimaraes, Steve Holnbeck, Justin Krahulik, Sean 

Lawlor, Kathryn Lee (former USGS employee), Mike Linhart (former USGS employee), Michal 

Niemoczynski, Ben Rivers, Patricia Sadler, Jason Shvanda, Kevin Townsend, and Ben Sleeper 

(former USGS employee).

v

Contents

Acknowledgments........................................................................................................................................iii

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

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

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

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

General Methods ...........................................................................................................................................5

Site Selection..................................................................................................................................................6

Procedures for Survey Data Collection and Site Evaluation..................................................................7

Types of Countermeasures Evaluated........................................................................................................7

Riprap Countermeasure Evaluation...................................................................................................8

A-Jacks and Cabled Concrete Mattress Countermeasure Evaluation........................................8

Gabion Baskets and Articulating Concrete Block Countermeasure Evaluation......................13

Procedures for Bathymetric and Topographic Data Collection and Processing..............................16

Equipment.............................................................................................................................................16

Processing ...........................................................................................................................................17

Error Estimation...................................................................................................................................18

Initial Imaging......................................................................................................................................18

High-Resolution Data Collection and Processing .........................................................................20

Additional Data Collection with Tripod-Mounted Terrestrial Lidar Systems ............................20

Flood History at Study Sites .......................................................................................................................25

Peak Streamflow History Since Countermeasures Were Installed ...........................................25

Summary........................................................................................................................................................27

References Cited..........................................................................................................................................28
vi

Figures

1. Map showing bridge scour countermeasure assessment sites used in this 

study throughout the United States, 2014–18...........................................................................3

2. Oblique view showing the topographic point cloud data from the terrestrial 

lidar survey of site 031, the Wapsipinicon River at structure 20740 on U.S. 

Highway 30 near Wheatland, Iowa, on December 17, 2017 ..................................................6

3. Photograph showing view of downstream side of bridge at site 011 over 

Grand River at Route A near McFall, Missouri, showing riprap on right 

bank, debris on right pier, and deposited sand bar with minor scour hole 

on left bank ....................................................................................................................................8

4. Photograph showing side-scan sonar image of base of riprap on right 

bank at site 011, Grand River at Route A near McFall, Missouri ..........................................9

5. Wolman Particle Count chart for riprap on right bank at site 011, Grand 

River at Route A near McFall, Missouri ..................................................................................10

6. Photograph showing A-Jack countermeasures interlocked along 

abutment slope at site 006 on the Snake River in Idaho ......................................................11

7. Photograph showing A-Jack countermeasures remain relatively 

unchanged and effective since installation in 2002 at site 006, Snake River 

at Shelley West River Road near Shelley, Idaho ...................................................................11

8. Photographs showing A, A-Jack countermeasures installed around pier 2 

at site 006 are visible with underwater videography and appear intact and 

effective; B, A-Jack countermeasures installed on toe of abutment slope 

and around piers 1 and 3 at site 006 along the Snake River in Idaho are 

mostly covered or embedded with gravel and sediment.....................................................12

9. Photographs showing distressed cabled-concrete mattress 

countermeasures installed at site 019, Gallatin River at I-90 near 

Manhattan, Montana .................................................................................................................14

10. Photographs showing A, Gabion baskets installed around upstream right 

side of Site 028, a bridge over Smith Branch along Clement Road in 

Columbia, South Carolina, showing some signs of shifting near toe of 

embankment; B, Gabion baskets installed around upstream left side of 

bridge over Smith Branch showing more signs of shifting and the edge of 

the baskets was scoured, lifted, and folded over itself........................................................15

11. Photograph showing sanitary sewer pipe line across Smith Branch, 

site 028, Columbia, South Carolina, just upstream of original upstream extent 

of Gabion mattress countermeasure installation..................................................................16

12. Photograph showing vessel mounted Teledyne-Optech ILRIS High Density 

Enhanced Range laser scanner...............................................................................................17

13. Oblique view of the bathymetric survey of the scour countermeasures 

near A, upstream pier 3, and B, downstream pier 5 of site 007 at structures 

097-0003/0004 on Interstate 64 over the Wabash River near Grayville, 

Illinois, showing original resolution data from the May 2, 2016 survey.............................19

14. Oblique view of the bathymetric survey of the scour countermeasures 

at site 007 of structures 097-0003/0004 on Interstate 64 over the Wabash River 

near Grayville, Illinois, near upstream pier 3 showing A, original resolution data 

from the May 2, 2016 survey, and B, high-resolution data from the August 27, 2018 

survey, and near downstream pier 5 showing C, original resolution data from the 

May 2, 2016 survey, and D, high-resolution data from the August 27, 2018 survey.........21
vii

15. Photographs showing A, Teledyne-Optech ILRIS High Density Enhanced 

Range laser scanner, B, FARO Focus 3D X130 laser scanner setup, and C, 

Trimble VX Stationary three-dimensional single-laser scanner, all shown 

mounted on tripods.....................................................................................................................22

16. Oblique view of the terrestrial lidar scanned data merged with channel 

bathymetry survey data of the bridge and river channel at site 004, 

Interstate 155 over the Mississippi River near Caruthersville, Missouri, 

from May 3, 2016 .........................................................................................................................24

17. Plot of 1-percent annual exceedance probability discharge and maximum 

recorded discharge since countermeasures were installed at 34 study sites ................27

Tables

1. List of bridge scour sites, category, and countermeasure type used in 

this study throughout the United States, 2014–18 ...................................................................4

2. List of bridge scour countermeasure assessment sites where 

terrestrial lidar was used to scan above water features.......................................................5

3. Peak streamflow since scour countermeasures were installed, 

1-percent annual exceedance probability, and 1-percent annual 

exceedance probability discharge at 34 bridge-scour 

countermeasure assessment sites used in this study .........................................................26
viii

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

cubic foot (ft3) 28.32 cubic decimeter (dm3) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

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

cubic foot per second per square 

mile ([ft3/s]/mi2) 0.01093 cubic meter per second per square kilometer ([m3/s]/km2)

International System of Units to U.S. customary units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

Area

square meter (m2) 0.0002471 acre 

square kilometer (km2) 247.1 acre

square meter (m2) 10.76 square foot (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 0.0002642 million gallons (Mgal) 

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 1.308 cubic yard (yd3) 

Flow rate

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) 

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

°F = (1.8 × °C) + 32.

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

°C = (°F – 32) / 1.8.
ix

Datum

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).

Elevation, as used in this report, refers to distance above the vertical datum.

Abbreviations

AEP annual exceedance probability

DOT Department of Transportation

FHWA Federal Highway Administration

HEC Hydraulic Engineering Circular

INS Inertial navigation system

MBES multibeam echo sounder

NBI National Bridge Inventory

NWIS U.S. Geological Survey National Water Information System

OFR Open-File Report

RI recurrence interval

RTK-GNSS real-time kinematic-global navigation satellite system

SBES single beam echosounder

T-lidar terrestrial light detection and ranging technology

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey

Assessment of Bridge Scour Countermeasures at Selected 

Bridges in the United States, 2014–18

By Thomas P. Suro, Richard J. Huizinga, Ryan L. Fosness, and Taylor J. Dudunake

Abstract

Erosion of the streambed, known also as scour, around 

pier 3 of the New York State Thruway bridge over Schoharie 

Creek caused the pier to fail, which ultimately resulted in 

bridge failure during the flooding event of April 5, 1987. 

The Federal Highway Administration (FHWA) responded 

to the need for better guidance on the evaluation of bridge 

scour and the selection and installation of scour countermeasures with the release of several Hydraulic Engineering Circulars. Although this information has been available, used, and updated over the years, an evaluation of 

the current conditions of scour countermeasures has not 

been performed. Therefore, the U.S. Geological Survey, in 

cooperation with the FHWA, began a study in 2013 to assess 

the current conditions of bridge scour countermeasures at 

selected sites around the country. The bridge scour countermeasure site assessments included reviewing countermeasure design plans, field inspections, traditional surveys, 

motion-compensated terrestrial light detection and ranging 

technology (lidar), high-resolution multi-beam bathymetry 

scanning, underwater video imaging, and a review of the 

peak and daily streamflow history for the associated river or 

stream. A total of 34 bridge scour countermeasure sites were 

selected in 11 states for this study. The types of countermeasures installed at the bridge scour study sites ranged from 

riprap, the most common countermeasure in the study, to 

A-Jacks and cabled-concrete mattresses.

The installed countermeasures were generally exposed 

to hydraulic forces from floods that equaled or exceeded 

the 1-percent, and even the 0.2-percent, annual exceedance 

probability at some of the study sites, but not all. The field 

inspections and countermeasure evaluations identified areas 

of shifting, slumping, and some scour holes and damage 

or washouts to the countermeasures, but generally most 

remained in place. The high-resolution laser scanner data, 

photo imaging and traditional survey data, and field notes 

were provided to the FHWA for expert evaluation of the 

bridge scour countermeasure performance. 

Introduction

Although the science and engineering community were 

aware of the physical process of scour in rivers and streams, 

the catastrophic bridge failure of the New York State Thruway 

bridge over Schoharie Creek on April 5, 1987, transformed 

the understanding of how much risk is associated with scour 

around bridges. This tragic event resulted in the loss of 10 lives 

(Zembrzuski and Evans, 1989) and unfortunately, another 

bridge failure due to scour happened 2 years later, on April 1, 

1989, along US-51 over the Hatchie River in Tennessee (Bryan, 

1989). These catastrophic bridge failures prompted the Federal 

Highway Administration (FHWA) to publish Hydraulic Engineering Circulars (HECs) 18, 20, and 23 to provide guidance on 

the evaluation of bridge scour (Federal Highway Administration, 2009, 2012a, 2012b). The FHWA also initiated a national 

program, under state jurisdiction, to evaluate the susceptibility 

of existing bridges to scour. Many states conducted different 

levels of bridge assessments using the methods presented in 

HEC 18 (Federal Highway Administration, 2012a).

Recognizing that potential scour can be estimated by use 

of these methods, different types of scour countermeasures were 

employed to abate pier and abutment scour around bridges. 

Bridge scour countermeasures protect transportation infrastructure by reducing the potential of sediment scour at locations susceptible to scour around bridge structures. After many 

years of relying on HEC 18, the FHWA understood the need to 

provide better guidance on scour countermeasure selection and 

design. In 2001, the FHWA published HEC No. 23, which was 

revised to a third edition in 2009 (HEC-23; Federal Highway 

Administration, 2009) to provide specific selection, design, and 

implementation guidelines for bridge scour and stream instability countermeasures. Although additional guidelines have been 

released and updated over the years, the effectiveness of countermeasures installed at bridges since FHWA HEC 23 guidelines were released has not been evaluated. In 2013, the U.S. 

Geological Survey (USGS), in cooperation with the FHWA, 

began this study to assess the current condition of bridge scour 

countermeasures at selected sites across the United States.
2 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

Photograph of scour countermeasures along right bank of the Mississippi River near Caruthersville, Missouri. Photograph by Richard Huizinga, U.S. 

Geological Survey. 

Purpose and Scope

This report summarizes the countermeasure site assessment data collected from 2014 through 2018 at 34 selected 

bridges across the United States. The drainage areas of the 

selected bridge sites ranged from about 10 square miles (mi2) 

to more than 29,000 mi2. The assessment included site visits to 

survey and document the current conditions of installed countermeasures, an analysis of the computed peak flow at each 

site since the installation of countermeasures, and a review of 

the daily streamflow history at a nearby USGS streamgage. 

The project generally focused on local pier and abutment 

scour countermeasures, and utilized multi-beam bathymetry 

surveys, side-scan sonar, underwater cameras, and traditional 

survey and probing techniques. The bridge site assessment 

data collected during this study are published in two USGS 

Open-File Reports (Dudunake and others, 2017, 2019) and 

related data are available in companion data releases available 

at the USGS ScienceBase repository (Dudunake, 2017, 2019).

Description of Study Area

The study includes assessments of 34 bridge sites in 

11 States: Connecticut, Florida, Idaho, Illinois, Iowa, Missouri, Montana, New Jersey, Pennsylvania, South Carolina, 

and Tennessee (fig. 1, table 1). These sites represent various 

river and bridge sizes, streamflow magnitudes, and countermeasure types. Bridge sites for this study were selected from a 

combination of sources including the National Bridge Inventory (NBI) and State Department of Transportation databases. 

Bridge selection was restricted to sites that met the following 

criteria: (1) the site had bridge scour countermeasures designed 

according to HEC 23 guidelines already installed at the site, 

(2) the site was near an existing streamgage with available 

daily and peak streamflow record, and (3) the site had experienced a significant streamflow event after the countermeasure 

was installed, defined as a peak streamflow with an estimated 

annual exceedance probability (AEP) of less than or equal to 

about 4 percent (25-year or greater recurrence interval).
OREGON

WASHINGTON

IDAHO

MONTANA

WYOMING

NORTH DAKOTA

SOUTH DAKOTA

UTAH COLORADO

NEBRASKA

KANSAS

ARIZONA

NEW MEXICO

CALIFORNIA

NEVADA

TEXAS

OKLAHOMA

ARKANSAS

MISSOURI

MISSISSIPPI ALABAMA

TENNESSEE

MINNESOTA

WISCONSIN

MICHIGAN

IOWA

ILLINOIS

INDIANA

KENTUCKY

OHIO

PENNSYLVANIA

NEW YORK

VIRGINIA

NORTH CAROLINA

GEORGIA

FLORIDA

LOUISIANA

9

8

7

6

5

4

3

2 1

3

32

31

30

28 29

27

26

24 25

23

18

17

16

15

14

12

11

10

22

21 20

19

120°

110°

100°

80°

90°

40°

30°

EXPLANATION

10 Bridge Scour Study Sites

0 125 250 500 KILOMETERS

0 125 250 500 MILES

120°

110°

100°

80°

90°

30°

Figure 1. Bridge scour countermeasure assessment sites used in this study throughout the United States, 2014–18.
Table 1. List of bridge scour sites, category, and countermeasure type used in this study throughout the United States, 2014–18. [FL, Florida; MO, Missouri; IL, Illinois; TN, Tennessee; ID, Idaho; IN, Indiana; PA, Pennsylvania; CT, Connecticut; MT, Montana; SC, South Carolina; IA, Iowa; NJ, New Jersblock; CCM, cabled-concrete mattress] Site identifier Category Watercourse State Site name 

1 1, 2 Apalachicola River FL Apalachicola River at I-10 near Chattahoochee, FL 2 5 Spring Creek FL Spring Creek at US-231 near Campbellton, FL 3 1, 2 Mississippi River MO/IL Mississippi River at US-54 at Louisiana, MO 4 1, 2 Mississippi River MO/TN Mississippi River at I-155 near Caruthersville, MO 5 1, 2 Snake River ID Snake River at Ferry Butte Rd (W 500 S), Bingham County, ID 6 1, 2 Snake River ID Snake River at Shelley West River Rd (E 1250 N) near Shelley, ID 7 1, 2 Wabash River IL/IN Wabash River at I-64 near Grayville, IL 8 3, 4 Thompson River MO Thompson River at MO-6 near Trenton, MO 9 5, 6 Fox River MO Fox River at US-61 near Wayland, MO 

10 8 Perkiomen Creek and Mill Race PA Perkiomen Creek and Mill Race at SR 1024 (Salford Station Rd) near Perkiomenvill

11 3, 4 Grand River MO Grand River at Rte-A near McFall, MO 

12 8 WB Brandywine Creek PA West Branch Brandywine Creek at SR 3062 (Strasburg Rd) near Coatesville, PA 13 8 Byram River CT Byram River at Sherwood Ave at Greenwich, CT 14 8 Bitterroot River MT Bitterroot River at US-93 near Hamilton, MT 15 3 Bitterroot River MT Bitterroot River at Bell Crossing near Victor, MT 16 3 Blackfoot River MT Blackfoot River at I-90 at Bonner, MT 17 8 Clark Fork River MT Clark Fork River at Turah Rd near Bonner, MT 18 8 Beaverhead River MT Beaverhead River at MT-41 at Twin Bridges, MT 19 8 Gallatin River MT Gallatin River at I-90 near Manhattan, MT 20 8 Jefferson River MT Jefferson River at MT-2 near Three Forks, MT 21 8 Madison River MT Madison River at I-90 near Three Forks, MT 22 8 Gallatin River MT Gallatin River at S-205 near Manhattan, MT 23 9 Judith River MT Judith River at MT-81 near Lewistown, MT 24 9 Musselshell River MT Musselshell River at S-300 at Ryegate, MT 25 8 Tongue River MT Tongue River at I-94 at Miles City, MT 26 9 Two Medicine River MT Two Medicine River at US-89 near Browning, MT 27 3 Yellowstone River MT Yellowstone River at Hwy 312 at Huntley, MT 28 5, 6 Smith Branch SC Smith Branch at S-126 (Clement Rd) at Columbia, SC 29 3, 4 Black River SC Black River at US-52 at Kingstree, SC 30 3, 4 Upper Iowa River IA Upper Iowa River at IA-76 near Dorchester, IA 31 3, 4 Wapsipinicon River IA Wapsipinicon River at US-30 near Wheatland, IA 32 5, 6 Old Man’s Creek IA Old Man’s Creek at IA-1 near Iowa City, IA 33 5, 6 Yellow Brook NJ Yellow Brook at NJ-34 at Colts Neck Township, NJ 34 5 Saddle River NJ Saddle River at NJ-17 at Ridgewood, NJ 
General Methods 5

General Methods

After the April 1987 collapse of the New York State 

Thruway Bridge over Schoharie Creek, a National Scour 

Evaluation Program was established. The original Technical 

Advisory was superseded in 1991 to provide more guidance 

on the development and implementation of procedures for 

evaluation of bridge scour. Since 1991, many advances have 

been made in estimating scour at bridge crossings and in the 

design of scour countermeasures (Federal Highway Administration, 2012a, 2012b), but there has not been a comprehensive 

evaluation of the long-term performance of these bridge scour 

countermeasures. This study focused on collecting data to 

assess the current condition of bridge scour countermeasures, 

primarily those used to armor the streambed or streambank 

around bridge structures. Several types of countermeasures 

were included in this study, including riprap, articulated 

concrete blocks, concrete armor units, and Gabion mattresses. 

Data collection was standardized using consistent field forms 

and procedures for collecting topographic surveys and bathymetric surveys, as well as Wolman particle counts (Federal 

Highway Administration, 2016) at sites that used riprap as the 

primary countermeasure. Traditional survey methods were 

used at smaller bridge sites, but larger bridges were surveyed 

using laser imaging technology (table 2). Field crews utilized 

motion-compensated terrestrial light detection and ranging 

technology (T-lidar) to capture high-resolution topography 

data for areas above the water surface and below the estimated 

peak-flow stage at these larger bridges. T-lidar technology uses 

rapidly moving laser pulses transmitted from the survey instrument that reflect off the structure or landscape and back to the 

instrument. The instrument then calculates the distance of the 

returned pulse based on the incoming velocity (Kimbrow and 

Lee, 2013). Field crews utilized a fixed tripod mount system 

and a motion-compensated, boat-mounted T-lidar system to 

collect data for this study. An example of T-lidar data collection at site 031, Wapsipinicon River at structure 20740 on U.S. 

Highway 30 near Wheatland, Iowa, is shown in figure 2.

Bridges that spanned large, deep rivers were identified as 

Category 1 and 2 (Dudunake and others, 2017), requiring the 

use of a manned boat to collect bathymetric data. The depths 

at these sites generally equaled or exceeded 15 feet (ft), which 

made them candidates for using a multibeam echosounder 

(MBES) to acquire bathymetric data. The MBES was coupled 

with real-time kinematic global navigation satellite systems 

Table 2. List of bridge scour countermeasure assessment sites where terrestrial lidar was used to scan above water features.

[FL, Florida; MO, Missouri; IL, Illinois; SC, South Carolina; IA, Iowa]

Site 

identifier Site name Category Countermeasure type

1 Apalachicola River at I-10 near Chattahoochee, FL 1, 2 Riprap

2 Spring Creek at US-231 near Campbellton, FL 5 Gabion

3 Mississippi River at US-54 at Louisiana, MO 1, 2 Riprap

4 Mississippi River at I-155 near Caruthersville, MO 1, 2 Riprap

7 Wabash River at I-64 near Grayville, IL 1, 2 Riprap

8 Thompson River at MO-6 near Trenton, MO 3, 4 Riprap

9 Fox River at US-61 near Wayland, MO 5, 6 Riprap

11 Grand River at Rte-A near McFall, MO 3, 4 Riprap

28 Smith Branch at S-126 (Clement Rd) at Columbia, SC 5, 6 Gabion 

29 Black River at US-52 at Kingstree, SC 3, 4 Gabion 

30 Upper Iowa River at IA-76 near Dorchester, IA 3, 4 Riprap

31 Wapsipinicon River at US-30 near Wheatland, IA 3, 4 Riprap

32 Old Man’s Creek at IA-1 near Iowa City, IA 5, 6 Riprap
6 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

Figure 2. Oblique view (from downstream left [southeast]) showing the topographic point cloud data from the terrestrial lidar survey of 

site 031, the Wapsipinicon River at structure 20740 on U.S. Highway 30 near Wheatland, Iowa, on December 17, 2017. Topographic points 

are colored based on concurrent photos taken during the survey.

(RTK-GNSS) to provide reference position and elevation data 

for the surveys. If a bridge site had clear water conditions during the survey, gridded underwater camera systems were available to provide images of the submerged countermeasures. 

Details of MBES data processing and imaging of submerged 

countermeasures are provided in a later section of this report, 

titled “Procedures for Bathymetric and Topographic Data Collection and Processing.”

A Wolman particle size count analysis was performed at 

21 of the study sites used in this report to determine the riprap 

gradation. This is a manual process that involves measuring the size of random particles (rocks or riprap) along equal 

sampling intervals until a minimum of 100 particles have been 

measured. Once the particles have been measured, a gradation 

curve can be developed based on the frequency distribution of 

various sizes of rock measured.

A site-selection criterion was established to limit the 

study to countermeasure sites that were designed using FHWA 

guidelines, where a history of daily and peak-streamflow 

records were available or could be reliably estimated. The 

following section describes the general site selection criterion 

used for this study.

Site Selection

The bridge scour countermeasure sites for this study were 

selected jointly by the USGS and FHWA. Sites were selected 

from the NBI and State Departments of Transportation databases using the following criteria:

1. The site had bridge scour countermeasures in place that 

were designed according to HEC 23 guidelines.

2. The site spanned a stream that had an existing continuous-record streamgage, with daily and peak streamflow 

records that preceded the installation of the countermeasures, for which flood-frequency statistics had been 

computed and published. In situations where a good 

bridge scour site did not have a streamgage located at the 
Types of Countermeasures Evaluated 7

bridge, flood-frequency statistics were not computed at 

the scour site, therefore, the site would need to meet the 

drainage area ratio guidelines in the local USGS flood 

frequency report for adjusting the computed statistics to 

the bridge scour site.

3. The site had experienced a significant streamflow event 

since the countermeasure was installed. A significant 

streamflow event was generally defined as a peak 

streamflow corresponding to an AEP of 4 percent or less 

(25-year or greater recurrence interval).

Criterion 1. Although the study objective was 

to assess the quality and overall effectiveness of 

countermeasures designed to FHWA HEC 23 guidelines, some exceptions were made for sites with 

installed countermeasures designed according to 

earlier versions of FHWA guidelines. For example, 

site 004, Mississippi River at I-155 near Caruthersville, Missouri, was designed and built using earlier 

guidelines. The installed countermeasure remained 

structurally sound around main channel piers even 

though it was exposed to several substantial floods. 

Extensive details provided in the bridge scour countermeasure plans made it an adequate candidate for 

this study.

Criterion 2. The daily and peak streamflow 

records for a nearby streamgage were reviewed to 

determine the flood history after countermeasures 

were installed. Historical streamflow records were 

obtained from the USGS National Water Information 

System (U.S. Geological Survey, 2016a), whereas 

published flood-frequency statistics were obtained 

from the StreamStats web application or generated using the USGS StreamStats web application 

(https://streamstats.usgs.gov/; U.S. Geological Survey, 2016b), and the U.S. Army Corps of Engineers 

River Gages website (http://rivergages.mvr.usace.

army.mil/WaterControl/new/layout.cfm; U.S. Army 

Corps of Engineers, 2014).

Criterion 3. The 4-percent AEP was selected 

to ensure the installed countermeasure had been 

exposed to a relatively rare flood. Flood-frequency 

statistics downloaded from StreamStats for USGS 

streamgages were computed following the guidelines presented in Bulletin 17B (Interagency 

Advisory Committee on Water Data, 1982) and 

only loaded into StreamStats after they had been 

reviewed, approved, and published in a USGS 

Scientific Investigations Report for that specific 

state. Flood-frequency statistics were not available for a couple of sites on the Mississippi River 

due to complex variable flow control conditions, 

therefore, frequency statistics computed by the U.S. 

Army Corps of Engineers (USACE) as part of their 

Upper Mississippi River System Flow Frequency 

Study (U.S. Army Corps of Engineers, 2004) were 

provided to the USGS to help estimate the rareness 

of the peak flow conditions since countermeasures 

were installed. At some other sites, countermeasures 

that experienced peak flows less than the 4-percent 

AEP were considered for the study if the bridges 

were in mountainous regions. For example, sites on 

a high-gradient stream that experienced streamflows 

less than the 4-percent AEP were determined to have 

a stream power and complexity of hydraulics that 

caused scouring comparable to that of a 4-percent 

AEP streamflow on a stream of lesser gradient. 

These sites were selected when the bankfull streamflow (typically 0.5-percent AEP) produced scouring 

comparable to the 4-percent AEP streamflow given 

a specific set of basin characteristics (Holnbeck and 

McCarthy, 2009). Most of the selected sites in Montana met the bankfull streamflow criteria.

Procedures for Survey Data Collection 

and Site Evaluation

Traditional and GNSS survey data collection detailed 

bridge geometry, including pier and abutment data, river 

cross sections, flood plains, and countermeasures using a 

total station, survey grade level, or GNSS surveying techniques. Traditional survey techniques include measurements 

of angles, distances, and elevations from a benchmark with 

known horizontal position and elevation. The RTK-GNSS 

surveys followed the techniques and methods described in 

Rydlund and Densmore (2012). Digital pictures and video also 

were collected to document and evaluate the conditions of 

the bridges, abutments, stream channel, and countermeasures. 

More advanced techniques for topographic and bathymetric 

imaging using T-lidar systems for topography and MBES systems for bathymetry also were implemented and are described 

later in this report.

Types of Countermeasures Evaluated

The most common bridge scour countermeasure used 

among the sites selected for this study was riprap. Combining 

the use of riprap for pier scour, bank armoring, and abutment 

protection into one category identified 17 of the 34 bridges 

using riprap as the primary scour countermeasure design. Of 

the remaining 17 sites, 4 (all located on the East Coast) used 

Gabion baskets, 3 (all in Idaho) used A-Jacks, 3 used articulated concrete block, and 7 used cabled-concrete mattresses.
8 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

Riprap Countermeasure Evaluation

The assessment of the 17 bridges in this study that utilized riprap as a scour countermeasure indicated that, in most 

cases, the riprap remained intact even after experiencing peak 

streamflows of magnitudes exceeding that of the 4-percent 

AEP (25-year recurrence interval) and, in some cases, streamflows exceeding the 1-percent AEP (100-year recurrence 

interval). At some locations, bed sediments deposited around 

the bridge during floods buried the installed countermeasures, 

making it more difficult to determine the condition of the 

riprap. Site 11 over the Grand River at Route A near McFall, 

Missouri, is an example of the challenges of determining the 

presence and condition of installed riprap countermeasures. 

The downstream side of the bridge with riprap clearly visible 

on the right bank, debris caught on the right pier, and a scour 

hole in the sediments on the downstream side of the left pier 

are shown in figure 3. At this site, side-scan sonar was used 

to inspect the base of the riprap along the right bank (fig. 4). 

The sand bar on the left bank was assumed to have covered 

the countermeasures installed along the left bank. In addition to visual inspection of the riprap and sonar imaging to 

inspect underwater countermeasures, a Wolman particle count 

(Federal Highway Administration, 2016) also was performed 

to determine the size distribution based on a random sampling 

of the in-place riprap. The right bank riprap matrix was found 

to have a D50, median particle size of riprap, of 11.7 inches 

(fig. 5).

A-Jacks and Cabled Concrete Mattress 

Countermeasure Evaluation

Three of the sites studied used A-Jacks (engineered concrete armoring units designed to interlock together and reduce 

the forces of scour) and seven used cabled-concrete block 

Figure 3. View of downstream side of bridge at site 011 over Grand River at Route A near McFall, Missouri, showing riprap on right 

bank, debris on right pier, and deposited sand bar with minor scour hole on left bank. Photograph by Richard Huizinga, U.S. Geological 

Survey.
Types of Countermeasures Evaluated 9

0 32 64 96 128 160 192 224 256

Side-scan sonar signal return strength

EXPLANATION

Figure 4. Side-scan sonar image of base of riprap on right bank at site 011, Grand River at Route A near McFall, Missouri.

mattresses combined with riprap. Most of these sites showed 

no indications of scour, although most were exposed to flooding conditions that have a statistical probability of occurrence 

of 4 to 10 percent each year. A-Jacks act similarly to riprap in 

the sense that they are still individual units that can be placed 

randomly to reduce scour forces, but they have a unique characteristic of being shaped like a “jack.” This design feature 

allows them to be interlocked or banded together to form a 

larger mass or matrix but still be flexible enough to conform 

to curved channel features (fig. 6). When interlocked together 

as shown in figure 6, the resulting A-Jack mattress or interconnected structure is the scour countermeasure being evaluated 

rather than the individual A-Jack module. Study site 006, a 

bridge over the Snake River at Shelly West River Road near 

Shelly, Idaho, utilized engineered A-Jacks as a scour countermeasure along the abutment slopes and around piers to prevent 

erosion and scour. In this example, A-Jacks were placed in a 

horizontal mattress configuration along the toe of the slope to 

prevent slope failure and around piers 1–3 to prevent scour. 

The A-Jacks have been in place since 2002 and have experienced scour forces from floods that have an estimated 10-percent AEP. Although the countermeasures have not experienced 

flood magnitudes in excess of a 2-percent or 1-percent flood 

discharge, measurements made by the USGS at streamgage 

13060000, Snake River near Shelley, Idaho, at flows ranging 

from 26,700 cubic feet per second (ft3/s) to 40,200 ft3/s indicated an average channel velocity of 6–7 feet per second (ft/s). 

The USGS Snake River near Shelley streamgage is located 

about 5.7 miles (mi) upstream of site 006, and thus the average 

channel velocity provides some indication about the potential 

velocities exerted on the countermeasures. The site inspection 

indicated that the A-Jacks have experienced little damage that 

would reduce their overall effectiveness (fig. 7). Observations 

from underwater videography indicate that the A-Jack modules placed along the abutments and around piers have generally remained intact and undamaged. The A-Jacks in the center 

of the channel (pier 2) remain mostly clear, whereas along the 

abutments and at piers 1 and 3, they are nearly 100 percent 

embedded with gravel and sediment (fig. 8).

Cabled-concrete mattresses were used as the primary 

scour countermeasure at seven of the sites in this report. 

Site 019, Gallatin River at I-90 near Manhattan, Montana, 

is an example of a cabled-concrete mattress that showed 

some signs of distress in the form of movement or slumping. The scour countermeasure plans indicated that cabledconcrete mattresses were to be installed at piers 2, 3, and 

4 of Interstate 90 (eastbound and westbound) in 2006. Site 

inspection as part of this study indicated about 1 ft of mattress 
10 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

D15 5.8 inches 20.7 inches

D50 11.7 inches 48.0 inches

0

10

20

30

40

50

60

70

80

90

100

1

Cumulative percent smaller

Particle size, in inches

10 100

D85

D100

 

 

Particle 

size 

(inches)

Particle 

size range 

(inches)

Particle 

count

Cumulative 

particle 

count

Cumulative 

percent 

smaller

3.00

4.25

6.00

8.50

12.00

17.00

24.00

36.00

48.00

68.00

96.00

3 < x ≤ 4.25

4.25 < x ≤ 6

6 < x ≤ 8.5

8.5 < x ≤ 12

12 < x ≤ 17

17 < x ≤ 24

24 < x ≤ 36

36 < x ≤ 48

48 < x ≤ 68

68 < x ≤ 96

≤ 3

5

8

16

20

25

15

7

1

0

0

3

8

16

32

52

77

92

99

100

100

100

3

8.0

16.0

32.0

52.0

77.0

92.0

99.0

100.0

100.0

100.0

3.0

Total 100

Figure 5. Wolman Particle Count chart for riprap on right bank at site 011, Grand River at Route A near McFall, Missouri.
Types of Countermeasures Evaluated 11

Figure 6. A-Jack countermeasures interlocked along abutment slope at site 006 on the Snake River in Idaho. Photograph by Lotwick 

Reese, U.S. Geological Survey.

Figure 7. A-Jack countermeasures remain relatively unchanged and effective since installation in 2002 at site 006, Snake River at 

Shelley West River Road near Shelley, Idaho. Photograph by Daniel Ghere, Federal Highway Administration.
12 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

B

A

Figure 8. A, A-Jack countermeasures installed around pier 2 (center pier) at site 006 are visible with underwater videography and 

appear intact and effective; B, A-Jack countermeasures installed on toe of abutment slope and around piers 1 and 3 at site 006 along 

the Snake River in Idaho are mostly covered or embedded with gravel and sediment. Photographs by Ryan Fosness, U.S. Geological 

Survey.
Types of Countermeasures Evaluated 13

Photograph of articulated concrete block countermeasures along the banks of the Saddle River at Ridgewood, New Jersey. Photograph by Jason 

Shvanda, U.S. Geological Survey.

separation in the longitudinal direction and some slumping 

towards the center of the channel (fig. 9).

Gabion Baskets and Articulating Concrete 

Block Countermeasure Evaluation

At four of the study sites, Gabion baskets were installed 

as the primary scour countermeasure. Two of the sites were 

in South Carolina, one was in Florida, and one was in New 

Jersey. Most sites showed minimal, if any, scour, but shifting 

of the baskets was noted at several sites. Site 028, a bridge 

over Smith Branch along Clement Road in Columbia, South 

Carolina, is an example of a site that utilized an extensive layout of Gabion baskets around the bridge and across the channel in the approach to and exit from the bridge (fig. 10). The 

Gabion baskets at this site were installed in 1998, and major 

floods that exceeded the 1-percent AEP were documented by 

the USGS in July 2013, October 2015, and August 2016. Site 

inspection during this study indicated that the Gabion baskets 

generally showed shifting and slumping in the vicinity of the 

bridge, but some areas experienced more extensive damage. 

Part of the original Gabion baskets were lifted and folded 

downstream over themselves during a past flood event, when 

scour along the edge of the Gabion baskets allowed flood 

waters to force the baskets downstream resulting in the damaged countermeasures shown in figure 10. A sanitary sewer 

line is present immediately upstream of the mattress along the 

stream bottom that may have complicated the flow hydraulics in this area (fig. 11). Articulated concrete block (ACB) 

countermeasures were installed at site 013, Byram River at 

Sherwood Ave. at Greenwich, Connecticut; site 015, Bitterroot River at Bell Crossing near Victor, Montana; and site 034, 

Saddle River at NJ-17 at Ridgewood, New Jersey. The evaluation of sites 013 and 034 showed the countermeasures were 

generally intact. The ACBs were covered with sediment in 

some areas but remained generally visible, but some notes 

indicated that some blocks had begun to separate near the 

edges. The ACBs at site 015 were submerged and not visible 

at the time of the site evaluation, but riprap along the banks 

was noted as being intact without signs of failure. Additional 

data about this site and others used in this study can be found 

in two USGS Open-File Reports (Dudunake and others, 

2017, 2019) and the companion data releases (Dudunake, 

2017, 2019).
14 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

B

A

Figure 9. Distressed cabled-concrete mattress countermeasures installed at site 019, Gallatin River at I-90 near Manhattan, Montana. 

A, looking upstream at I-90 bridge towards left bank at distressed countermeasures; B, at upstream side of bridge looking down at 

distressed countermeasure showing large separation to the right towards main channel. Photographs by Stephen Holnbeck, U.S. 

Geological Survey.
Types of Countermeasures Evaluated 15

B

A

Figure 10. A, Gabion baskets installed around upstream right side of Site 028, a bridge over Smith Branch along Clement Road in 

Columbia, South Carolina, showing some signs of shifting near toe of embankment; B, Gabion baskets installed around upstream left 

side of bridge over Smith Branch showing more signs of shifting and the edge of the baskets was scoured, lifted, and folded over itself. 

Photographs by Richard Huizinga, U.S. Geological Survey.
16 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

Figure 11. Sanitary sewer pipe line across Smith Branch, site 028, Columbia, South Carolina, just upstream of original upstream extent 

of Gabion mattress countermeasure installation. Photograph by Richard Huizinga, U.S. Geological Survey.

Procedures for Bathymetric and 

Topographic Data Collection and 

Processing

The bathymetric and topographic data at each of the sites 

in Categories 1 and 2 were collected using a high-resolution 

marine-based mobile mapping system. Topographic data at 

several other sites were collected using a T-lidar system. A 

marine-based mobile mapping system is an integration of 

several individual components: a MBES, a T-lidar system, 

an inertial navigation system (INS), and a data-collection 

computer. A brief description of the equipment follows; more 

in-depth descriptions of the various system components and 

types of methods used in this study are available in reports 

by Huizinga (2010), Huizinga and others (2010), Lee (2013), 

Densmore and others (2013), Kimbrow and Lee (2013), and 

Huizinga and Wagner (2019).

Equipment

Bathymetric data were collected using four different 

MBES systems: the R2Sonic 2024 (site 001), the Teledyne 

RESON SeaBat 7125-SV2 (sites 003, 004, and 007), the Teledyne Odom ES3 240kHz (sites 005 and 006), and the Norbit 

iWBMSh (site 030). The MBES data generally were collected 

in longitudinal transects with approximately 50-percent overlap of subsequent survey swaths to ensure complete coverage 

of the channel bed and to minimize sonic “shadows.” Substantial overlap was achieved for many of the surveyed swaths, 

except in wide, shallow areas near the channel banks or near 

spur dikes, other in-flow structures, debris rafts, or moored 

barges. At several sites, the areas near the bridge piers and 

along the banks also were surveyed in an upstream direction 

with the MBES head tilted (mechanically or electronically) at 

30 degrees to either port or starboard to increase the acquisition of bathymetric data in the shallow areas, and higher on 
Procedures for Bathymetric and Topographic Data Collection and Processing 17

the banks and the sides of the piers. To limit potential damage 

to the MBES head, most shallow areas having water less than 

approximately 6 ft deep were not surveyed.

Topographic data of the above-water part of several 

Category 1 and 2 sites (sites 001, 003, 004, and 007) were 

collected using a vessel-mounted T-lidar system (fig. 12), a 

Teledyne-Optech ILRIS High Density (HD) Enhanced Range 

(ER) laser scanner (Teledyne-Optech, 2012). The topographic 

data were collected along a single vertical line in the field of 

view of the instrument, and data essentially were collected in 

a vertical “fan” along that line as the survey vessel moved. 

Topographic data were obtained along each bank, along the 

upstream and downstream faces of the bridge, and on both 

sides of each pier at the site of interest, often in multiple 

passes with different aspect angles and speeds to capture 

data from multiple directions in an attempt to maximize the 

coverage.

The bathymetric and topographic data collected with 

the MBES and T-lidar components of the marine-based 

mobile mapping system were accurately represented in 

three-dimensional space by use of an INS. The INS that was 

used in surveys of Category 1 and 2 sites was the Applanix 

Position Orientation Solution for Marine Vessels (POS MV) 

system (Applanix Corporation, 2006). The INS uses an inertial 

measurement unit (IMU) coupled with two GNSS antennae to 

provide position in three-dimensional space and measure the 

heave, pitch, roll, and heading of the vessel and, thereby, the 

MBES and T-lidar units.

Processing

The navigation information from the INS was postprocessed using the POS-Pac Mobile Mapping Suite (MMS) 

software (Applanix Corporation, 2009) to mitigate the effects 

of degraded positional accuracy of the vessel while near or 

under a bridge. POS-Pac MMS provides tools to identify and 

compensate for sensor and environmental errors and computes 

an optimally blended navigation solution from the GNSS 

and IMU raw data. The blended navigation solution (called a 

“smoothed best estimate of trajectory” or “SBET” file) generated by post-processing the navigation data provides centimeter- to decimeter-level positional accuracy and was applied to 

the survey at a given bridge to improve the real-time position 

information of the survey and minimize the effects of the 

GNSS outages while surveying under the bridge.

The bathymetric data from the MBES and INS components were processed and integrated into cohesive datasets for 

cleanup and visualization. A computer on the survey vessel ran the HYPACK/HYSWEEP data acquisition software 

(HYPACK, Inc., 2015) that was used to prepare for the bathymetric and topographic surveys and collect the survey data. 

After completing the surveys, the acquired bathymetric data 

were further processed to remove data spikes and other spurious points in the multibeam swath trace, georeferenced using 

the navigation and position solution data from the SBET file 

from POS-Pac MMS, and visualized in HYPACK/HYSWEEP 

as a triangulated irregular network surface or a point cloud. 

Figure 12. Vessel mounted Teledyne-Optech ILRIS High Density (HD) Enhanced Range (ER) laser scanner. Photograph by Richard 

Huizinga, U.S. Geological Survey.
18 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

The edited bathymetric data were projected to a three-dimensional grid at a resolution of 1.64 ft (0.5-meter) using the 

Combined Uncertainty Bathymetry Estimator (CUBE) method 

(Calder and Mayer, 2003), as implemented in the MB-MAX 

processing package of the HYPACK/HYSWEEP software 

(HYPACK, Inc., 2015). Uncertainty in the overall bathymetric survey was estimated by computing the total propagated 

uncertainty (TPU) for each survey grid cell in the bathymetric 

surface of each survey area, also using the CUBE method. 

The bathymetric data near piers and scour countermeasures 

were output without any data reduction to provide increased 

resolution near these features of interest. The final reduced or 

unreduced bathymetric data were output to a space-delimited 

XYZ data file and were compiled and included with metadata 

in Dudunake (2017, 2019).

Similarly, the topographic data from the T-lidar and 

INS components were parsed, processed, and integrated into 

cohesive datasets for visualization and cleanup. The T-lidar 

data were parsed in the Teledyne-Optech Parser executable 

to apply necessary offset and orientation parameters, and to 

georeference the data using the navigation and position solution data from the SBET file from POS-Pac MMS. The T-lidar 

data were output to multiple XYZ datasets with intensity, one 

for each survey line. The various XYZ files for a given survey 

were combined into a single topographic dataset and visualized in the PolyWorks IM-Align and IM-Survey software 

packages and were cleaned of spurious points and reflections 

off the water. The processed and edited data were output to 

a space-delimited XYZ dataset with intensity, one file for 

each site, and are included with metadata in the data releases 

(Dudunake, 2017, 2019).

Error Estimation

The errors associated with the collection of bathymetric 

and topographic data can be classified as systematic or random. Systematic errors are those that can be measured or modeled through calibration (Byrnes and others, 2002). Random 

errors are a result of the limitations of the measuring device 

and an inability to perfectly model the systematic errors. 

Because the channel bed generally was not visible in the 

bathymetric data collection, random errors associated with the 

limitations of the MBES are difficult to quantify. To minimize 

these errors, quality assurance assessments were performed or 

verified in real time during the surveys. A series of patch tests 

was performed to measure variations in the orientation and 

timing of the MBES with respect to the INS and real-world 

coordinates (HYPACK, Inc., 2015). The patch tests are used to 

determine timing offsets caused by latency between the MBES 

and the INS, and angular offsets to roll (longitudinal axis), 

pitch (lateral axis), and yaw (rotation around vertical axis) 

caused by the alignment of the transducer head. The results 

of these patch tests were accounted for when processing the 

bathymetric data with the HYPACK/HYSWEEP software.

Similarly, a boresight calibration test is used to measure 

variations in the orientation of the T-lidar unit with respect to 

the INS and real-world coordinates (Teledyne-Optech, 2012). 

The angular offsets to roll, pitch, and yaw caused by the alignment of the T-lidar unit and the INS on the survey vessel, as 

well as the positional (lever arm) offsets of the T-lidar with 

relation to the INS were determined for each scan direction 

(port or starboard) of the T-lidar unit on each vessel. The lever 

arm and angular offsets determined in the boresight calibration 

test were applied when parsing and processing the data collected from a given T-lidar survey.

Initial Imaging

Bathymetric data were output with no data reduction near 

the principal features of interest in the study, namely the scour 

countermeasures. The higher-resolution data near the countermeasures provide a more accurate image of the countermeasure condition, compared to the designed countermeasure. 

The data collected at the first Category 1 and 2 site was used 

to develop data-collection procedures that were applied at 

subsequent sites. At site 001 on the Apalachicola River near 

Chattahoochee, Florida, the flow conditions were nearly ideal 

for high-resolution bathymetric data collection, since the river 

is relatively wide in a tidal zone with low-velocity flow at the 

time of the survey. The survey vessel was able to maintain a 

low rate of speed while maintaining good navigation control, 

particularly near the bridge piers and countermeasures. Multiple survey passes were made on both sides of each pier, using 

different settings and swath angles of the MBES to collect data 

and characterize the condition of the countermeasures near 

these piers. These multiple passes would, in theory, provide 

high resolution of the countermeasures when overlain.

The defined data-collection methods were generally 

followed at the subsequent Category 1 and 2 sites. However, 

guidance on the speed of the survey vessel had not been specified at the first site; therefore, data collected at subsequent 

sites was controlled and varied according to local conditions. 

The subsequent sites were on the Mississippi and Wabash 

Rivers, each subject to substantially higher flow velocities 

than those on the Apalachicola River, and data collection was 

further complicated by the presence of debris rafts and other 

navigational hazards, particularly near the piers. Nevertheless, 

multiple survey passes were collected over the countermeasures, with the intent of overlaying the data if needed.

The resolution of the resulting datasets was not sufficient 

to classify the size of individual riprap stones, which was 

necessary to determine the particle-size distribution of the in 

situ countermeasures. Whereas the data initially appeared to 

be of sufficiently high resolution to accomplish this, closer 

examination of the data implied the distance between points 

was too large to accurately resolve the various sizes of the 

stone (fig. 13A, original image resolution). Furthermore, it 

was discovered that because of minor positional variations in 
Procedures for Bathymetric and Topographic Data Collection and Processing 19

EXPLANATION

112

111

110

109

108

107

106

105

104

103

102

101

Channel-bed elevation, 

in meters above North 

American Vertical 

Datum of 1988

B. Downstream—Pier 5, original resolution

A. Upstream—Pier 3, original resolution

Figure 13. Oblique view (from downstream right [southwest]) of the bathymetric survey of the scour countermeasures near A, 

upstream pier 3, and B, downstream pier 5 of site 007 at structures 097-0003/0004 on Interstate 64 over the Wabash River near Grayville, 

Illinois, showing original resolution data from the May 2, 2016 survey.
20 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

the data in each pass, combining multiple survey passes led to 

“fuzziness” in the resultant combined point cloud data. It was 

also determined that although the positional accuracy of the 

MBES data was generally good, errors resulting from GNSS 

signal outages under the bridge led to minor discrepancies in 

position of the vessel and the resultant MBES data. Therefore, 

better methods for collecting high-resolution MBES images of 

the scour countermeasures were pursued.

High-Resolution Data Collection and Processing

Once it had been determined that the resolution of 

most of the data collected with the MBES was insufficient 

to determine the riprap particle-size distribution, the methods by which MBES data were obtained were re-evaluated. 

The desired resolution was a nominal point distance of 

0.02 meter (m) that could be achieved without requiring multiple survey passes. It was determined that the Apalachicola 

River data were close to this resolution in the original survey, 

so the unreduced data near the piers were simply saved to individual files rather than as a combined dataset of overlain lines. 

However, the other Category 1 and 2 sites required a modified 

method of data collection.

Trigonometry and beam configuration were evaluated to 

achieve the desired resolution between points in each ping. 

(A “ping” as used herein is a short acoustic pulse transmitted 

into the water from the MBES). The distance between points 

in the MBES swath (in a given ping) was computed based on 

the swath angle, the distance to the channel bed, and the number of discrete points (512) in each ping within the swath. The 

swath angle could then be adjusted to give a nominal distance 

between points of 0.02 m, depending on the depth of the water 

over the countermeasure during the survey.

The distance between pings along the survey line was 

more difficult to control because it is controlled by the ping 

rate of the MBES (the number of pings per second) and the 

forward speed of the vessel over ground (distance traveled per 

second). The ping rate, in turn, is controlled by the ping pulse 

length and range settings of the MBES. If the pulse length is 

set too high, the MBES is required to “listen” longer for the 

full pulse of the return ping; therefore, it was beneficial to set 

the pulse length to the minimum value. However, the range 

setting is dependent on the depth of the channel; as the depth 

increases, the range must be increased accordingly to capture 

data in the entire swath angle, which decreases the ping rate 

as the MBES is again required to “listen” longer for the return 

ping.

The forward speed of the vessel was then adjusted 

according to the achievable ping rate to maintain a nominal 

longitudinal point spacing of 0.02 m. Given the depth typically 

observed at the Mississippi River (10–15 m), a swath angle 

of 40–60 degrees and a range setting of 15–20 m would result 

in a ping rate of around 20 pings per second (20 hertz, or Hz). 

At this ping rate, a forward speed of approximately 1 knot 

(0.51 meter per second, or m/s) would result in the desired 

resolution. A great deal of caution was required to navigate 

the survey vessel near the bridge piers at such a low speed, as 

turbulence near the pier was often substantial. Nonetheless, by 

use of this approach, a single pass of high-resolution data was 

able to be collected for the scour countermeasures on either 

side of each pier at sites 003, 004, and 007.

The data from the high-resolution collection efforts were 

processed in the same way as the original data (as detailed 

in the “Processing” section above). In addition to the highresolution data near the piers and scour countermeasures, a 

survey of the whole site was conducted, as the channel-bed 

conditions during the high-resolution survey were different 

from those during the original survey. The overall survey data 

were processed and reduced to a resolution of 1.64 ft (0.5 m) 

using the CUBE method as with the original survey, and 

uncertainty in the overall bathymetric survey was estimated by 

computing the TPU for each survey grid cell in the bathymetric surface of each survey area, also using the CUBE method. 

The high-resolution bathymetric data were output without any 

data reduction to provide maximum resolution near features of 

interest. The final reduced or unreduced bathymetric data were 

output to a space-delimited XYZ file and were compiled and 

included with metadata in data releases by Dudunake (2017, 

2019) along with the original data.

Using the modified data-collection procedures, highresolution data were collected at the Category 1 and 2 sites. 

A sample of the original and high-resolution data is shown in 

figure 14. Using similar processes, high-resolution bathymetric data also were collected at site 030, one of the Category 3 

and 4 sites where one of the scour countermeasures was submerged riprap near the piers.

Additional Data Collection with Tripod-Mounted 

Terrestrial Lidar Systems

At several of the Category 1 through 7 sites, the condition 

of the above-water part of the surveyed reach at each site was 

determined using a tripod-mounted T-lidar system. The same 

Teledyne-Optech ILRIS HD ER instrument that was operated 

in motion-compensated mode to survey the Category 1 and 

2 sites also was used to survey the Category 3 and 4 sites in 

Missouri, but was mounted to a tripod and equipped with a 

pan/tilt base, allowing the scanner’s standard 40 x 40 degree 

field of view to be rotated 360 degrees and also tilted vertically, if necessary (fig. 15A). Topographic data were obtained 

by rotating the T-lidar unit in discrete steps on the pan/tilt base 

at several tripod setup points in the surveyed reach. Reference 

spheres were placed throughout the reach in the field of view 

of the lidar unit at each tripod setup to assist in aligning each 

scan with the others. The locations of these spheres were also 

surveyed using GNSS to georeference the survey.

At the three Category 3 and 4 sites in Iowa, T-lidar data 

were collected using a FARO Focus3D X130 laser scanner 

(fig. 15B). The FARO scanner is substantially smaller than 

the ILRIS HD ER with a more limited range; however, it 
A. Upstream—Pier 3, original resolution B. Upstream—Pier 3, high resolution C. Downstream—Pier 5, original resolution D. Downstream—Pier 5, high resolution

Figure 14. Oblique view (from downstream right [southwest]) of the bathymetric survey of the scour countermeasures at site 007 of structures 097-0over the Wabash River near Grayville, Illinois, near upstream pier 3 showing A, original resolution data from the May 2, 2016 survey, and B, high-resolu27, 2018 survey, and near downstream pier 5 showing C, original resolution data from the May 2, 2016 survey, and D, high-resolution data from the Aug
22 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

A

B

C

Figure 15. A, Teledyne-Optech ILRIS High Density

(HD) Enhanced Range (ER) laser scanner, B, FARO 

Focus 3D X130 laser scanner setup, and C, Trimble VX 

Stationary three-dimensional single-laser scanner, 

all shown mounted on tripods. Photographs 15A and 

15B by Richard Huizinga, U.S. Geological Survey. 

Photograph 15C by Ryan Fosness, U.S. Geological 

Survey.
Procedures for Bathymetric and Topographic Data Collection and Processing 23

U.S. Geological Survey field crew Stephen Holnbeck and Sean Lawlor surveying bathymetry near two large piers at bridge over Blackfoot River at I-90 

at Bonner, Montana (site 016). Photo by Taylor Dudunake, U.S. Geological Survey.

can rapidly collect data in an approximately 130-m sphere 

around the unit. The smaller size and rapid data-collection 

ability make the FARO scanner useful for surveying smaller 

bridge sites. Data were obtained by allowing the FARO unit to 

rotate 360 degrees at several (up to 12) different tripod setups 

throughout the surveyed reach, using reference spheres to 

align the scan from each setup with other scans.

The Trimble VX, a stationary three-dimensional single-laser scanner (Trimble, 2010), also was used at some 

Category 1 and 2 sites (005 and 006) to collect topographic 

data (fig. 15C). Topographic data were collected within a 

selected area and scanned at a target resolution of about 0.3 to 

1.0 m along the banks, and 0.1 to 0.3 m along the piers.

For the tripod-mounted T-lidar systems, field procedures 

similar to those described in Kimbrow and Lee (2013) were 

followed during data collection. A surveying methodology was 

employed with the intent of one-sided coverage of features on 

the banks of the surveyed river reach.

As with the Category 1 and 2 sites, the data from the tripod T-lidar were parsed, processed, and integrated into cohesive datasets for visualization and cleanup. The ILRIS T-lidar 

data were parsed in the Teledyne-Optech Parser executable to 

process the raw data into multiple XYZ datasets with signal 

return intensity, one for each pivot step at a given tripod setup. 

The FARO T-lidar data were processed in the FARO Scene 

software, which allows multiple FARO scan files to be aligned 

using reference targets, and common topographic features 

between the scan files and output to multiple XYZ datasets 

with intensity, one for each scanner setup. Additionally, with 

the FARO T-lidar data, photographs taken concurrently with 

the lidar data can be used to apply a three-value red-green-blue 

(RGB) color to each point in the lidar point cloud. These colorized point clouds also were output to multiple XYZ datasets 

with RGB color, one for each scanner setup.

The various XYZ files for a given survey were combined into a single topographic dataset and visualized in the 

PolyWorks IM-Align and IM-Survey software packages. The 

data were aligned and georeferenced using coordinates for 

the various reference targets and spheres in the surveyed area. 

The processed and edited data collected with the ILRIS HD 

ER unit were output to a space-delimited XYZ dataset with 

intensity, one file for each survey, and are included with the 

metadata in data releases by Dudunake (2017, 2019). The 

processed and cleaned data collected with the FARO unit 

were output to two space-delimited XYZ datasets, one with 

intensity and the other with RGB color (one of each per survey), and are also included with the metadata in the two data 

releases (Dudunake, 2017, 2019). A sample of these colored 

T-lidar data are shown in figure 2. At sites where available 

(001, 003, 004, 007, and 030), bathymetric data were combined with the T-lidar data obtained at each site (fig. 16).
Channel-bed elevation, in meters above North American Vertical Datum of 1988 49 53 57 61 65 69 73 77 0 32 64 96 128 160 192 224 256 Lidar signal return strength EXPLANATION

Figure 16. Oblique view (from downstream [southeast]) of the terrestrial lidar scanned data merged with channel bathymetry survey data of the brid004, Interstate 155 over the Mississippi River near Caruthersville, Missouri, from May 3, 2016. 
Flood History at Study Sites 25

The alignment error for each of the tripod-mounted 

T-lidar surveys was determined based on the difference in the 

relative easting, northing, and elevation values obtained for the 

center of the reference spheres or targets around the site, using 

the position of the virtual reference points at the center of 

each target to align the survey with real-world coordinates for 

those targets, determined by use of GNSS and conventional 

surveying methods. The alignment and georeferencing errors 

are documented in the various metadata files in data releases 

(Dudunake, 2017, 2019).

Flood History at Study Sites

In most cases, the flood history at each site was derived 

from streamflow records of the USGS streamgage along 

the same stream that the bridge spanned. In a few cases, a 

streamgage on another nearby stream was used to estimate 

the flood conditions on the river or stream passing under the 

study bridge because direct streamflow data were not available. At these sites, the regional flood frequency equations in 

StreamStats were used to determine the AEP of the estimated 

peak streamflows. Countermeasures were installed at the 

34 selected bridges from as early as 1973 for site 004, Mississippi River at I 55 (A1700) near Caruthersville, Missouri, to 

as recently as 2011 for site 002, Spring Creek at US-231 near 

Cambellton, Florida. Flood frequency statistics were estimated 

using methods from published reports for each state where a 

study site was located.

The published USGS flood frequency methods that were 

used typically conform to the guidelines set forth in Bulletin 

17B (Interagency Advisory Committee on Water Data, 1982). 

In 2018, Bulletin 17C (England and others, 2018) was released 

with new guidelines for determining flood-flow frequency, but 

since this study uses previously published statistics developed using Bulletin 17B guidelines, the new guidelines were 

generally not referenced in this report. Frequency analysis of 

annual peak streamflow at streamgages provides a means of 

estimating the probability of occurrence of a given streamflow. Flood frequency is commonly expressed in terms of the 

probability of being exceeded, or a recurrence interval (the 

recurrence interval is the reciprocal of the AEP). For example, 

a flood having an annual AEP of 0.01, which is equivalent to 

a 1-percent chance of being equaled or exceeded in any given 

year, is often referred to as a “100-year flood.” The guidelines 

described in Bulletin 17B include methods for computing atsite statistics, use of regional regression equations to estimate 

statistics at ungaged locations, and methods for computing 

weighted statistics for locations on gaged streams. At gaged 

locations, or at ungaged locations within allowable limits 

along a gaged stream, the weighted statistics are considered the most reliable values, therefore, whenever possible, 

weighted flood frequency statistics were used to estimate the 

AEPs of the highest three floods known to have occurred since 

the countermeasures were installed.

Peak Streamflow History Since 

Countermeasures Were Installed

A summary of the maximum streamflows known to have 

occurred since countermeasures were installed at sites in this 

study are presented in table 3. Additional data are available 

in Dudunake and others (2017, 2019). The initial criteria for 

study site selection included peak streamflows that equaled 

or exceeded the 4-percent AEP since countermeasures were 

installed. Although only 22 of 34 sites met the 4-percent AEP 

requirement, all sites met the revised requirement of peak 

streamflows greater than or equal to the 10-percent AEP. At 

13 of the 34 sites, an extreme flooding event was documented 

after countermeasures were installed, with peak streamflows 

that equaled or exceeded the 1-percent AEP (fig. 17), and a 

subset of those sites experienced peak streamflows with an 

AEP of less than 0.2 percent (greater than the 500-year recurrence interval). At 20 of 34 sites, the maximum streamflows 

after countermeasures were installed occurred in 2011 and 

ranged in exceedance probability from 20 percent to less 

than 0.2 percent. Some scour holes and shifting and distress 

to countermeasures were noted, but field inspections did not 

indicate any major failures of scour countermeasures. Evaluation of the conditions of the countermeasures indicated that 

they generally remained in place as intended during moderate flooding, and in some cases, during floods of magnitudes 

greater than the 1-percent AEP. Detailed analysis of the 

performance of scour countermeasures during flood flows was 

beyond the scope of this study, but the data collected were 

provided to the FHWA for use in evaluating the performance 

of scour countermeasures and to assist in evaluating scour 

protection guidelines. 
Table 3. Peak streamflow since scour countermeasures were installed, 1-percent annual exceedance probability, and 1-percent annual exceedancbridge-scour countermeasure assessment sites used in this study. [AEP, annual exceedance probability, ft3/s, cubic feet per second; FL, Florida; MO, Missouri; ID, Idaho; IL, Illinois; PA, Pennsylvania; CT, Connecticut; MT, Montana; IA, Iowaable; <, less than] Site identifier Site name Year countermeasure was installed Peak streamflow since countermeasures were installed (ft3/s) Year of peak discharge 

1 Apalachicola River at I-10 (SR 8), near Chattahoochee, FL 2000 159,000 2005 2 Spring Creek at US-231, near Cambellton, FL 2011 10,000 2013 3 Mississippi River at US-54, (K0932) at Louisiana, MO 1992 456,000 2008 

4 Mississippi River at I-155 (A1700), near Caruthersville, MO 1973 2,040,000 2011 5 Snake River at Ferry Butte Road (W 500 S), Bingham County, ID 2002 28,700 2011 6 Snake River at Shelley West River Road (E 1250 N) near Shelley, ID 2002 32,300 2011 7 Wabash River at I-64 (097-0003/0004), near Grayville, IL 2009 270,000 2011 8 Thompson River at MO-6 (A0906), near Trenton, MO 2006 78,200 2014 9 Fox River at US-61 (A4584), near Wayland, MO 2009 26,600 2011 

10 Perkiomen Creek and Mill Race at Salford Station Road (SR1024) near Perkiomenville, PA 2012 26,600 2011 

11 Grand River at Rte-A (P0250) near McFall, MO 2001 55,000 2007 

12 West Branch Brandywine Creek at Strasburg Road (SR 3062) near Coatesville, PA 2008 7,000 2014 

13 Byram River at Sherwood Ave (05018) at Greenwich, CT 2010 1,700 2011 14 Bitterroot River at US-93 near Hamilton, MT 2004 12,000 2009 15 Bitterroot River at Bell Crossing near Victor, MT 2004 13,700 2011 16 Blackfoot River at I-90 at Bonner, MT 2004 17,200 2011 17 Clark Fork River at Turah Road, near Bonner, MT 2006 13,400 2011 18 Beaverhead River at MT-41 at Twin Bridges, MT 2009 3,100 2011 19 Gallatin River at I-90, near Manhattan, MT 2006 9,360 2011 20 Jefferson River at MT-2 near Three Forks, MT 2008 17,700 2011 21 Madison River at I-90 near Three Forks, MT 2004 8,050 2011 22 Gallatin River at S-205, near Manhattan, MT 2006 9,360 2011 23 Judith River at MT-81, near Lewistown, MT 2006 11,690 2011 24 Musselshell River at S-300, at Ryegate, MT 2006 9,190 2011 25 Tongue River at I-94, at Miles City, MT 2001 15,300 2011 26 Two Medicine River at US-89, near Browning, MT 2008 7,940 2011 27 Yellowstone River at Hwy 312 at Huntley, MT 2008 73,700 2011 28 Smith Branch at S-126 (Clement Rd) at Columbia, SC 1998 5,030 2015 29 Black River at US-52 at Kingstree, SC 1998 83,700 2015 30 Upper Iowa River at IA-76 near Dorchester, IA 2001 38,000 2016 31 Wapsipinicon River at US-30 near Wheatland, IA 2000 37,200 2014 32 Old Man’s Creek at IA-1 near Iowa City, IA 2007 13,900 2013 33 Yellow Brook at NJ-34 at Colts Neck Township, NJ 2008 2,030 2011 34 Saddle River at NJ-17 at Ridgewood, NJ 2009 6,770 2011 

1Annual exceedance probability (AEP) discharges provided by U.S. Army Corps of Engineers from Upper Mississippi River System flow frequency study (U.S. Army Corps o21-percent annual exceedance probability (AEP) discharge determined from regional flood frequency equations (U.S. Geological Survey, 2016b).
Summary 27

100

1,000

10,000

100,000

1,000,000

10,000,000

0 5 10 15 20 25 30 35

Discharge, in cubic feet per second

Study site number

EXPLANATION

1-percent annual exceedance probability 

discharge

Peak streamflow since countermeasures 

were installed

Figure 17. Plot of 1-percent annual exceedance probability discharge and maximum recorded discharge since countermeasures were 

installed at 34 study sites.

Summary

The catastrophic bridge failure of the New York State 

Thruway bridge over Schoharie Creek in 1987 transformed 

the science and engineering communities’ understanding 

of risk associated with scour around bridges. The Federal 

Highway Administration (FHWA) Hydrologic Engineering 

Circulars (HECs) 18, 20, and 23 provided guidance on bridge 

scour, stream instability, and the selection and design of scour 

countermeasures, however, even after years of implementation, a broad evaluation of installed countermeasures at bridge 

sites had not been conducted. In 2013, the U.S. Geological 

Survey (USGS), in cooperation with the FHWA, began a study 

to assess the condition of bridge scour countermeasures at 

selected sites across the United States.

This study collected data at 34 bridge sites with scour 

countermeasures installed following guidelines presented in 

FHWA HEC 18 and HEC 23. In addition to following FHWA 

guidelines for the design and installation of countermeasures, 

study site selection criteria included a requirement for annual 

peak streamflow data and at least one documented streamflow 

that equaled or exceeded the 4-percent annual exceedance 

probability (AEP) after installation of the countermeasures. 

This criterion could not be met at some sites, so some exceptions were made to accept sites with streamflows that equaled 

or exceeded the 10-percent AEP. The USGS used traditional 

survey methods, global navigation satellite systems, terrestrial 

light detection and ranging technology scanning, underwater 

cameras, and single- and multibeam sonar technology to collect data that describe the stream channel conditions, bridges, 

and countermeasures. Furthermore, adjustments were made 

to procedures and setup parameters of the multibeam sonar 

systems to facilitate the collection of riprap stone sizes at 

sites with submerged countermeasures. Using these methods, 

installed countermeasures were inspected to determine existing placement, general quantity, and visible condition relative 

to installation plans. The presence of any notable scour holes 

around the bridge abutments, piers, and countermeasures, 

washouts, slumping, or movement were documented. The 

countermeasures that were evaluated can be combined into 
28 Assessment of Bridge Scour Countermeasures at Selected Bridges in the United States, 2014–18

five general categories: riprap, A-Jacks, cabled-concrete 

mattresses, articulated concrete block matting, and Gabion 

baskets. Data for the 34 study sites were published in two 

USGS Open-File Reports and associated data releases. Site 

evaluation reports documented that most of the countermeasures evaluated seemed to generally remain in place, but some 

scour holes, movement, and slumping were found. Streamflow 

data were used to determine conditions at countermeasure sites 

since they were installed to estimate potential scour conditions from flood flows. It was estimated that at least 13 of the 

34 bridge study sites experienced peak streamflows that were 

less than the 1-percent AEP (greater than a 100-year recurrence interval), and 22 of the 34 study sites experienced peak 

streamflows that met or exceeded the 4-percent AEP (greater 

than a 25-year recurrence interval). 

All of the data collected during this study were provided 

to the FHWA to aid in assessing the performance of the scour 

countermeasures. Continued inspection and evaluation of 

scour countermeasures will provide valuable information to 

assist the FHWA with improving existing guidelines.

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For additional information, contact:

Director, New Jersey Water Science Center

U.S. Geological Survey 

3450 Princeton Pike, Suite 110

Lawrenceville NJ 08648

Or visit our website at:

https://www.usgs.gov/centers/nj-water

Publishing support provided by the 

West Trenton Publishing Service Center
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