Use of Fathometers and Electrical-Conductivity Probes 

to Monitor Riverbed Scour at Bridge Piers

By D.C. Hayes and RE. Drummond

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 94-4164

Prepared in cooperation with the

DELAWARE DEPARTMENT OF TRANSPORTATION, 

MARYLAND DEPARTMENT OF TRANSPORTATION, and 

VIRGINIA DEPARTMENT OF TRANSPORTATION

Richmond, Virginia 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For additional information write to:

District Chief 

U.S. Geological Survey 

3600 West Broad Street 

Room 606 

Richmond, VA 23230

Copies of this report can be purchased from:

U.S. Geological Survey

Earth Science Information Center

Open-File Reports Section

Box 25286, MS 517

Denver Federal Center

Denver, CO 80225
CONTENTS

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

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

Purpose and scope............................................................................... 2

Description of study sites ......................................................................... 2

Related studies.................................................................................. 2

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

Fathometer method of monitoring scour .................................................................. 4

System design and operation....................................................................... 4

Analysis and results.................... ......................................................... 7

Electrical-conductivity probe method of monitoring xour .................................................... 10

System design and operation....................................................................... 10

Analysis and results.............................................................................. 13

Summary and conclusions ............................................................................. 16

References cited ..................................................................................... 17

FIGURES

1. Map showing locations of bridge-scour study sites in Delaware, Maryland, and Virginia ........................ 3

2-3. Schematic diagrams showing:

2. Pier C3 and gage at bridge 2-12B over the Leipsic River at Leipsic, Delaware .............................. 6

3. Pier and gage at bridge 23018 over Sinepuxent Bay near Ocean City, Maryland............................. 7

4-6. Graphs showing:

4. Fathometer data for the Leipsic River at Leipsic, Delaware, October 9,1990, through October 29,1990 ......... 8

5. Fathometer data for the Leipsic River at Leipsic, Delaware, July 28, 1992, through August 16,1992............. 8

6. Fathometer data for Sinepuxent Bay near Ocean City, Maryland, February 9, 1993, through March 5, 1993. ...... 9

7. Schematic diagram of four-electrode conductivity probe.................................................. 11

8. Diagram of conductivity probe...................................................................... 12

9. Schematic diagram of pier and conductivity probe at bridge 6918 over the Pamunkey River near

Hanover, Virginia................................................................................ 13

10. Graphs showing electrical-conductivity probe readings and discharge for the Pamunkey River near

Hanover, Virginia, June 28, 1991, through August 8, 1991. ............................................... 14

TABLE

1. Location, equipment, and bed-material size of bridge-scour study sites in Delaware, Maryland,

and Virginia..................................................................................... 4

Contents III
CONVERSION FACTORS AND ABBREVATIONS

Multiply By To obtain

inch (in.)

millimeter (mm)

foot (ft)

mile (mi)

foot per second (ft/s) 

cubic foot per second (ft3/s)

25.4 millimeter (mm)

0.03937 inch (in.)

0.3048 meter

1.609 kilometer

0.3048 meter per second

0.02832 cubic meter per second

Water temperature expressed in degrees Fahrenheit (°F) can be converted to degrees Celsius (°C) by the following equation:

°C = 0.56 (°F - 32)

IV Contents
Use of Fathometers and Electrical-Conductivity Probes 

to Monitor Riverbed Scour at Bridge Piers

By Donald C. Hayes and Fitzgerald E. Drummond

Abstract

Scour is the lowering of a river channel by erosion 

and is the leading cause of bridge failure. Monitoring the 

riverbed elevation at a bridge where scour is a potential 

problem provides for public safety as a temporary countermeasure until structural improvements can be implemented, or in some situations, as a permanent 

countermeasure. Monitoring of the riverbed elevation 

accurately during high flows using existing streamflow 

measuring equipment is difficult because of strong currents and large amounts of debris. Two systems, a fathometer system and an electrical-conductivity probe 

system, were developed to monitor riverbed elevations at 

bridge piers. The scour-monitoring systems consisted of a 

sensor (fathometer or electrical-conductivity probe), 

power supply, data logger, relay, and system program.

The fathometer system was installed and tested at a 

bridge over the Leipsic River at Leipsic, Delaware, and at 

a bridge over Sinepuxent Bay near Ocean City, Maryland. 

The fathometer calculates the distance from a transducer 

to the riverbed by measuring the two-way travel time of a 

reflected, acoustic wave.

Field data collected indicate that fathometers can be 

used to identify and monitor the riverbed elevation if post 

processing of the data and trends in the data are used to 

determine the riverbed location in relation to the transducer. The accuracy of the system is approximately the 

same as the resolution of the fathometer. Signal scatter, 

caused by electronic or physical interference, can be a 

major source of error. During periods of interference, the 

distance from the transducer to the riverbed can be difficult to determine. Other problems found with the use of a 

fathometer are identifying the exact location of the 

reflected pulse, sequencing multiple fathometers or other 

electronic equipment to prevent electronic interference, 

and protecting the system from damage by debris.

An electrical-conductivity probe system was installed 

and tested at a bridge over the Pamunkey River near 

Hanover, Virginia. The approximate elevation of the 

riverbed is determined by comparing conductivities of the 

surface-water flow with conductivities of submerged bed 

material from equally spaced sensors mounted on the 

probe. The ratio of the voltage across a known resistor 

and the voltage across a variable resistor or sample was 

measured using a six-wire full-bridge circuit, and is proportional to the conductivity of the surrounding surface 

water or submerged bed material.

Field data indicate that an electrical-conductivity 

probe, as tested, has limited usefulness in identifying 

and monitoring the riverbed elevation during high flows. 

As the discharge increases, the concentration of sediment 

in the surface-water flow increases, especially near the 

riverbed. Voltage ratios, measured at the sensors in the 

surface-water flow could not be distinguished from voltage ratios measured at the shallowest sensor in the submerged bed material. However, the voltage ratios of the 

sensors in the surface-water flow and the shallowest sensor in the submerged bed material vary much more than 

the voltage ratios of sensors located deeper in the submerged bed material. Other problems found with the use 

of an electrical-conductivity probe are a gradual decrease 

in readings with time, possibly because of corrosion of the 

electrodes or changes in moisture of the wooden probe, 

and vulnerability of the system to damage by debris.

INTRODUCTION

Many bridges are built over rivers that change over 

time as a result of natural causes and the effects of human 

activities. Scour is an active processes of the river system 

and is defined as the lowering of a river channel by erosion. Scour is a primary concern in bridge design in

Introduction 1
alluvial channels, and limited knowledge of the hydraulics related to scour can result in inadequate bridge design 

for current or future riverbed conditions.

Streambed scour is the leading cause of bridge damage and failure (U.S. Federal Highway Administration, 

1990). Highway departments periodically inspect bridges 

for damage (existing or potential) that results from scour. 

When a scour problem is detected, highway departments 

need a method to monitor the streambed until scour countermeasures can be implemented. Also, a monitor system 

can be used to identify if a potentially dangerous bridgescour situation is occurring.

In 1988, the U.S. Geological Survey (USGS) began a 

study in cooperation with the Delaware Department of 

Transportation, the Maryland Department of Transportation, and the Virginia Department of Transportation to 

improve knowledge of hydraulics and scour at bridges as 

part of a National program to improve bridge design and 

safety. As part of this study, scour-monitoring systems 

were developed and evaluated to monitor riverbed elevations at bridge piers.

Purpose and Scope

This report describes two methods to monitor riverbed elevations at bridge piers. Semiportable scourmonitoring systems that use a fathometer or an electricalconductivity probe to monitor riverbed elevation were 

developed and evaluated. Study sites are described, 

theory of operation for each system is discussed, and 

design methods are evaluated.

Description of Study Sites

Three sites were used to develop and test scourmonitoring systems (fig. 1 and table 1), bridge 2-12B 

on Delaware State highway 9 over the Leipsic River 

(U.S. Geological Survey surface-water gaging station 

01483530), bridge 23018 on Maryland State highway 

611 over Sinepuxent Bay (U.S. Geological Survey 

surface-water gaging station 01484727), and bridge 

6918 on Virginia State highway 614 over the Pamunkey 

River (U.S. Geological Survey surface-water gaging 

station 01673000).

Bridge 2-12B over the Leipsic River consists of 12 

piers (4 of which are located within or near the main channel of the river) that are constructed from!6-in. square, 

concrete piles. The Leipsic River is an estuary at the

bridge site and marshes are found along both banks. 

Much of the tidal area is covered with marsh mud, except 

where stream velocities are high, such as around the piers. 

Marsh mud is removed and an indurated sand, gravel, or 

clay is exposed in the channel where the stream velocities 

are high.

Bridge 23018 over the Sinepuxent Bay consists of 30 

piers (all located within the main channel of the bay) that 

are constructed from 18-in. round, concrete piles with 

steel casings. The Sinepuxent Bay is a tidal bay that connects the northern part of the Chincoteague Bay with the 

Atlantic Ocean and the southern part of the Assawoman 

Bay. The bed material in the area primarily is a loose, 

medium sand. Mud and a grassy vegetation cover the bottom of the bay where depths are shallow and stream 

velocities are low.

Bridge 6918 over the Pamunkey River consists of 

three piers. The piers are constructed from a solid, 

tapered webb, 4 ft wide at the base and 3 ft wide at the 

top. The webb is mounted on piles and is aligned with the 

flow at most stages. The river is not tidal at this location. 

The bed material is loose, medium-to-coarse sand.

Related Studies

Several cooperative studies between the USGS and 

different State Departments of Transportation and Federal 

agencies are ongoing (1994). Butch (1991) has instrumented a pier in New York State with a fathometer system. Crumrine (1992) is using fathometers to monitor 

riverbed scour before and during construction of a bridge 

over the Alsea River Estuary at Waldport, Oreg. In 

Kentucky, the USGS, in cooperation with the U.S. Army 

Corps of Engineers, is instrumenting a 3-mi reach of the 

Ohio River with fathometers to monitor sediment deposition during dam construction. Fathometer systems also 

are being used in USGS cooperative programs in North 

Carolina, Florida, Alabama, and South Dakota.

The Virginia Department of Transportation Highway 

Research Council has instrumented a pier known to have 

scour problems with a series of fathometers that are 

accessed by a portable cellular telephone. Dr. Ronald 

Erchul at Virginia Military Institute has designed and successfully operated a electrical-conductivity probe in the 

laboratory. The National Cooperative Highway Research 

Program has funded studies on fathometer systems and 

several types of electrical probes.

2 Use of Fathometers and Electrical-Conductivity Probes to Monitor Riverbed Scour at Bridge Piers
LJJ

2

(0

jo 

o> Qc

TJ

"w

0) 

O)

Introduction 3
Table 1. Location, equipment, and bed-material size of bridge-scour study sites in Delaware, Maryland, and Virginia

[Latitude and longitude are reported in degrees, minutes, seconds; DE, Delaware State Highway; MD, Maryland State Highway; VA, Virginia State Highway; 

F, site instrumented with a fathometer, C, site instrumented with an electrical-conductivity probe; D50, median grain size of bed material at pier; and mm, 

millimeter]

Station

number

01483530

01484727

01673000

Name

Leipsic River at Leipsic, Del.

Sinepuxent Bay near Ocean City, Md.

Pamunkey River near Hanover, Va.

Latitude

391444

381441

374603

Longitude

0753105

0750923

0771957

Bridge

number

2-1 2B

23018

6918

Road

DE9

MD611

VA614

Equipment

F

F

C

Flow

conditions

Tidal

Tidal

Non-tidal

D50

(mm)

8.0

.47

.43

Acknowledgments

Personnel from the Departments of Transportation in 

Delaware, Maryland, and Virginia are gratefully acknowledged for their support of this project and their assistance 

in providing site recommendations, bridge plans, and 

technical advice. Milo Crumrine, from the USGS in Portland, Oreg., is gratefully acknowledged for his assistance 

in installing and programming the fathometer system used 

for this study.

FATHOMETER METHOD OF MONITORING 

SCOUR

The fathometer transmits a discrete, acoustic wave 

from a transducer through a column of water and measures the two-way travel time of the reflected wave. The 

distance to any object that reflects the acoustic wave is 

computed by use of a predetermined speed of sound 

through water. The distance calculated is the distance 

from the face of the transducer to the acoustically reflective object.

The fathometer consists of a signal processor and 

transducer. The signal processor controls the electrical 

signal sent to the transducer and interprets the return signal. The transducer converts the electrical signal sent by 

the signal processor into a discrete, acoustic wave or 

pulse, and transmits the pulse through the column of 

water. The transducer receives the reflected pulse, and 

converts the pulse back into an electrical signal that is 

processed by the signal processor.

The riverbed elevation is monitored at a desired location, such as the face of a pier or footer, by mounting the 

transducer at a fixed location near the area of interest and 

directing the pulse into the center of the area. As the 

riverbed scours or fills, the distance measured by the

fathometer will vary accordingly. The elevation of the 

riverbed can be computed if the elevation of the transducer and the angle that the pulse is transmitted are 

known. Several locations can be monitored simultaneously by installing multiple transducers and directing 

the pulse from each transducer at a different riverbed 

location.

System Design and Operation

The speed of sound through water varies with the 

density of the water, which is affected by temperature, 

salinity, and sediment. Some fathometers can be calibrated for different water densities; however, most lowcost commercial fathometers cannot be calibrated by the 

user. The associated distance error is usually small. For 

example, the speed of sound through water at 50 °F is 

4,758 ft/s, and the speed of sound through water at 70 °F 

is 4,879 ft/s. If the two-way travel time of the pulse is 

0.0040 seconds, the distances calculated are 9.52 ft and 

9.76 ft, respectively. The difference between the distances calculated is 0.24 ft, which is less than the accuracy 

of many commercial fathometers. The fathometer used 

for this study was not corrected for different water 

densities.

The resolution, which is the ability to separate objects 

or accurately measure the distance to the riverbed, is 

determined by the length of the acoustic pulse. Short 

pulse lengths have good object definition but are limited 

to short ranges, whereas long pulse lengths have poor 

object definition but provide for long ranges. The fathometer used in this study has a resolution of 1 ft and a maximum range of 700 ft.

The acoustic pulse transmitted by the transducer 

expands in a manner similar to the light emitted from a 

flashlight. The farther the pulse travels before being

4 Use of Fathometers and Electrical-Conductivity Probes to Monitor Riverbed Scour at Bridge Piers
reflected, the larger the area that is covered. Narrowangle transducers (8°) will cover approximately a 14-ftdiameter circle at a 100 ft distance, and wide-angle transducers (20°) will cover approximately a 36-ft-diameter 

circle at a 100 ft distance. Narrow-angle transducers concentrate more energy to a small area and, therefore, have 

longer ranges than the wide-angle transducers. Commercial fathometers are normally sold with wide-angle transducers because they can cover a large area in shallowdepth and medium-depth water. The narrow-angle transducer was preferred for this study because the energy is 

concentrated to a small area. However, only wide-angle 

transducers were available with the selected unit

Fathometers are subject to signal scatter caused by 

electronic or physical interference. Electronic interference is produced when the unit interprets electronic noise, 

either from outside the unit or from the unit itself, as 

actual data. Examples of equipment that can create electronic noise and cause electronic interference include boat 

motors, radio transmissions, and power lines. When electronic interference occurs, depth values can be random 

and unreliable. Physical interference is produced when an 

obstruction in the water column other than the intended 

object reflects or absorbs the acoustic pulse. Examples of 

obstructions that can cause physical interference include 

seaweed or grasses, air bubbles, and suspended sediment. When physical interference occurs, there can be 

either no depth determination or the depth values will be 

less than the actual values (Crumrine, 1992). Multiple 

reflections can occur when the pulse is reflected off the 

riverbed to the water surface and then back again to the 

riverbed. Several multiple reflections can be observed if 

the signal is strong enough. When the transducer is 

located near the water surface, the depth of the multiple 

reflections will approximate multiples of the actual riverbed depth.

The fathometer was mounted at a fixed site to monitor 

the same location on the riverbed. The two-way travel 

time of the acoustic pulse was measured by the fathometer 

and transferred to the data logger to be filtered, converted 

into distance, and stored. Sites were instrumented so that 

the transducer would be below the water surface most of 

the time, directly above the area of interest, and perpendicular to the riverbed. The transducer was placed inside 

the structural framework or between piles to protect it 

from ice and debris, and was mounted on a removable 

arm or near the water surface so that it could be cleaned 

without diving. The signal processor was mounted close 

to the transducer, but above expected high water to protect 

it from damage by debris.

The scour-monitoring system consisted of a data logger, data-storage module, relay, fathometer, stage 

recorder, batteries, and solar panel. The data logger 

stored the program, operated the system, and stored the 

data. The normal sequence for sampling the data was for 

the program to instruct the data logger to open a relay that 

turned on the fathometer. The fathometer remained on for 

1 minute to stabilize the signal prior to any measurements 

being recorded. Two-way travel times were measured by 

the fathometer and transferred to the data logger. Values 

were checked for reasonableness (travel time had to be 

greater than the travel time associated with a distance of 

4 ft but less than the travel time associated with a distance 

of 60 ft), converted into distance, and transferred to and 

stored in the data-storage module. The fathometer was 

then turned off and the stage data were read and stored in 

the data-storage module.

Two measuring procedures were used to collect and 

process the fathometer data. The initial measuring procedure used one temporary register to process the data and 

one final register to store the data at each time interval. 

The reading consisted of measuring the depth five times. 

Each measurement was checked for reasonableness in 

which the measurement had to be greater than 4 ft and less 

than 60 ft. The measurement was stored in the temporary 

register if the criteria were met. Each additional measurement that met the criteria overwrote the previous measurement. After the fifth measurement was processed, the 

content of the temporary register was copied to the final 

register and saved as the reading for that time interval. 

Five measurements were collected to ensure that one measurement met the criteria, but only the last measurement 

meeting the criteria was stored. A total of five measurements was collected at each time interval that resulted in 

one stored data value.

A refined measuring procedure was later developed in 

which five temporary registers were used to process the 

fathometer data, and five final registers were used to store 

the data at each time interval. Five measurements were 

collected, checked to meet the above criteria, and processed through the temporary registers for each final register. A total of 25 measurements was collected at each 

time interval that resulted in 5 measurements stored as 

data values. The median value of the five stored values 

was used as the final data value.

The equipment was initially installed on bridge 2-12B 

over the Leipsic River at Leipsic, Del. (fig. 2). The transducer was mounted approximately 1.5 ft upstream from 

the upstream pile of pier C3. The bottom of the transducer was located below the low-tide water-surface

Fathometer Method of Monitoring Scour 5
APPROXIMATE HIGH 

TIDE ELEVATION

APPROXIMATE LOW 

TIDE ELEVATION

NOT TO SCALE

PILES

Figure 2. Schematic diagram of pier 03 and gage at bridge 2-12B over the Leipsic River at Leipsic, Delaware.

elevation. The signal processor, relay, stage recorder, data 

logger, and batteries were mounted in the gage shelter 

under the bridge deck and above any expected high water. 

Threaded galvanized steel rods were used to clamp 

wooden posts to the bridge piles. The gage shelter and 

transducer structure were bolted to the wooden post, and 

the solar panel was clamped to the bridge rail on the 

bridge deck. This type of gage structure was used to prevent damage to the bridge structure from the gage installation. The initial sampling procedure was used to collect 

the data. The system was operated from October 4,1990, 

through December 4, 1990, when it was removed to prevent ice damage. Depth readings were collected every 15 

minutes.

The system was reinstalled at the Leipsic River site 

and operated from July 13, 1992, through September 6, 

1992. The transducer was mounted at a slightly higher 

elevation during the second installation because of structural modifications to the transducer mount. The refined

sampling procedure was used in an attempt to reduce scatter and better define the riverbed location. Depth readings 

were collected every 15 minutes. The equipment was 

removed from the Leipsic site because the riverbed 

around the piles was extremely hard with only minor 

movement of any bed material.

The equipment was installed on bridge 23018 on 

Maryland highway 611 over Sinepuxent Bay near Ocean 

City, Md. (fig. 3). The Sinepuxent Bay site was selected 

because it had previous scour problems and a mobile, 

sandy riverbed. A similar system was used to clamp 

wooden posts to the bridge piles to prevent damage to the 

bridge structure. The gage shelter and transducer structure were mounted to the wooden post. The refined sampling procedure was used at the Sinepuxent Bay site. The 

system was operated from August 20,1992, through 

May 24, 1993. Depth readings were collected every 15 

minutes.

6 Use of Fathometers and Electrical-Conductivity Probes to Monitor Riverbed Scour at Bridge Piers
CONCRETE

PILE WITH STEEL

COLLAR

WOODEN f± 

SUPPORT

NOT TO SCALE

Figure 3. Schematic diagram of pier and gage at bridge 

23018 over Sinepuxent Bay near Ocean City, Maryland.

During each installation and periodically during 

the gage operation, soundings were collected with a 

weighted line to verify the depth readings from the 

fathometer.

Analysis and Results

Field data collected indicate that the fathometer can 

be used to identify and monitor the riverbed elevation if 

post processing of the data and trends in the data are used 

to determine the riverbed location in relation to the transducer. Problems associated with the fathometer are signal 

scatter, determining the exact location of the reflected 

pulse, sequencing additional fathometers or other electrical equipment, and protecting the system from damage by 

debris.

The data collected at the Leipsic River site from 

October 9,1990, through October 29, 1990, using the 

initial sampling procedure are shown in figure 4. The 

sample data are characteristic of the data collected at the 

site from October 4, 1990, through December 4, 1990. 

Three groups of data are evident in figure 4. One group of 

data is a band of depth measurements that tends to define 

the riverbed location. These data are concentrated 

between 11.5 and 12.5 ft below the transducer. This 

approximate 1 ft variation in values defining the riverbed 

location is consistent with the 1-ft resolution of the fathometer. Other possible sources for the variation within 

the 11.5- and 12.5-ft depth measurements are pulse reflections from an uneven bottom, and electrical or physical 

interference. Another group of data are the depth measurements that are less than the depth to the riverbed or 

less than 11.5 ft below the transducer. These data indicate 

the existence of some type of physical interference, such 

as bubbles or grasses in the surface-water flow between 

the transducer and riverbed. Physical interference causes 

the acoustic pulse to be reflected before reaching the 

riverbed, and the distance calculated is less than the actual 

distance from the transducer to the riverbed. The final 

group of data are the depth measurements that are greater 

than the depth to the riverbed or greater than 12.5 ft below 

the transducer. The data values are not multiples of the 

measured depths; therefore, the values do not represent 

the travel time of pulses reflected from the water surface. 

The large data values could be caused by electronic interference, however, or by reflections from combinations of 

objects, such as the bridge structure, the riverbed, and the 

water surface. Because of the extremely hard bottom 

exposed to the surface-water flow around the piers, only 

slight changes in the riverbed elevation are apparent 

during the period of record. Soundings confirmed the 

location of the riverbed between 11.5 and 12.5 ft below 

the transducer.

Fathometer Method of Monitoring Scour 7
i 

i i i i i i i i i i i i i i i i i i i i

0 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

October 1990

Figure 4. Fathometer data for the Leipsic River at Leipsic, 

Delaware, October 9, 1990, through October 29,1990.

26 29 30 31 1 2 3 4 56 7 8 9 10 11 12 13 14 15 16

July August

1992

Figure 5. Fathometer data for the Leipsic River at Leipsic, 

Delaware, July 28,1992, through August 16, 1992.

The data collected at the Leipsic River site from July 

28,1992, through August 16,1992, using the refined sampling procedure are shown in figure 5. The sample data 

are characteristic of the data collected at the site from July 

13, 1992, through September 6, 1992. Again, three 

groups of data are evident. One group of data is a band 

of depth measurements that tends to define the riverbed 

location. These data are concentrated between 10.8 and 

11.3 ft below the transducer. The variation in values 

(0.5 ft) defining the riverbed is approximately half the 

variation in values (1.0 ft) observed in the initial sampling 

procedure and is less than the 1-ft resolution of the fathometer. Another group of data are the depth measurements that are less than the depth to the riverbed or less 

than 10.8 ft below the transducer. These data also indicate 

the existence of some type of physical interference. The 

final group of data are the depth measurements that are 

greater than the depth to the riverbed or greater than 

11.3 ft below the transducer. The density of data in this 

group is less than the density of data in the same group in 

figure 4, which indicates that much of the electronic interference or reflection from the structure could have been 

filtered by the post processing of the data. The small

variation in values between readings that define the riverbed and few individual readings greater than the distance 

to the riverbed indicate that the post processing can 

reduce much of the interference. Because of the 

extremely hard bottom, no significant change in elevation 

of the riverbed was apparent during the period of record. 

Soundings confirmed the location of the riverbed between 

10.8 and 11.3 ft below the transducer.

The data collected at the Sinepuxent Bay site from 

February 9, 1993, through March 5, 1993, using the 

refined sampling procedure are shown in figure 6. The 

sample data are characteristic of the data collected at the 

site from August 20,1992, through May 24,1993. Again, 

three groups of data are evident. One group of data is a 

band of depth measurements that tends to define the riverbed location. These measurements are concentrated 

between 8.5 and 9.0 ft below the transducer. The variation in values (0.5 ft) defining the riverbed is approximately half the variation in values (1.0 ft) observed in the 

initial sampling procedure used at the Leipsic River site, 

and similar to the variation in values (0.5 ft) observed in 

the refined sampling procedure used at the Leipsic River 

site. Another group of data are the depth measurements

8 Use of Fathometers and Electrical-Conductivity Probes tp Monitor Riverbed Scour at Bridge Piers
\ \ I I I I I I I I I I I I I I I I I I I I I I

 ' . ....?

 r * ' - ;; « ->.

I I I I I I I I I I I I I I I I

B 10 11 12 13 14 15 18 17 18 19 20 21 22 23 24 25 26 27 28 1 2 34 5

February March

1993

Figure 6. Fathometer data for Sinepuxent Bay near 

Ocean City, Maryland, February 9,1993, through 

March 5, 1993.

that are less than the depth to the riverbed or less than 

8.5 ft depth below the transducer. The density of data in 

this group is similar to the density of data in the same 

group at the Leipsic River site, using both the initial sampling procedure and refined sampling procedure; therefore, the physical interference may be similar at both sites. 

The final group of data are the depth measurements that 

are greater than the depth to the riverbed or greater than 

9.0 ft below the transducer. The density of the data in this 

group is greater than the density of the data in the same 

group at the Leipsic River site, using both the initial sampling procedure or the refined sampling procedure. These 

comparisons indicate that much of the electronic interference or reflection from the structure is not being filtered 

by the post processing of the data, or that there is additional interference. Soundings confirmed the location of 

the riverbed between 8.5 and 9.0 ft below the transducer; 

however, there appears to be slight bed movement shown 

in figure 6 on February 14, 1993, where the readings 

change from approximately 8.0 ft below the transducer to 

9.0 ft below the transducer. No physical measurements 

were made to confirm or contradict the bed movement.

During two intervals February 11-13, 1993, and 

February 23-27, 1993, the fathometer could have been 

influenced by electronic interference (fig. 6). The location 

of the riverbed could not be accurately determined during 

these periods. The data appear to be random and range 

from 4 ft below the transducer to 24 ft below the transducer. Similar problems were encountered at both sites 

during all three test periods. The duration of the problem 

was from several hours to several days. Attempts to 

determine the cause of the problem were unsuccessful. 

All connections, batteries, relays, and programming were 

checked. The probable causes of the problem are the 

quality of the system and the field conditions in which the 

system was used, or possible discrepancies in the interface to the fathometer, which allowed the data to be read 

by the data logger. The data indicate minor changes in 

elevation of the riverbed during the period of record with 

the maximum changes in riverbed elevation of 1.5 ft. The 

elevation changes appeared to be bed forms moving 

through the area.

Field data collected at the two sites indicate that fathometers can be used to identify and monitor the riverbed 

elevation; however, individual readings cannot be used to 

identify or monitor the riverbed elevation. Signal scatter, 

possibly associated with electronic or physical interference, necessitates the use of multiple-depth measurements, post processing, and analysis of trends in the data 

to determine the riverbed location. Much of the error in 

the measurements is caused by the location of the transducer and quality of the system. The transducer needs to 

be placed close to the riverbed to reduce the physical 

interference, but the transducer also needs to be accessible 

for cleaning. In addition, the transducer needs to be 

mounted below the depth where surface debris could 

destroy the system and above the depth where debris rolling along the bottom could destroy the system. The system resolution needs to be able to define the riverbed 

elevation to the accuracy required by the highway department, and the system needs to be cost effective because it 

will be exposed to a harsh environment. Less than high 

quality fathometer systems also require multiple readings 

and post processing to define the riverbed elevation. 

Multiple fathometers or electronic shielding can reduce 

the need for post processing of the data, and small cone 

angles can reduce the physical interference.

Other problems associated with the fathometer are 

that the exact location of the reflected pulse is not easily 

located, and additional fathometers or other electronic 

equipment can interfere with the signal. Seldom is the 

riverbed flat around bridge piers; therefore, when the

Fathometer Method of Monitoring Scour 9
fathometer pulse is directed toward the area of interest, 

determining what part of the reflective area is returning 

the signal that is being processed can be difficult. In addition, multiple fathometers need to operate on different frequencies, or the sampling must be sequenced by time, to 

eliminate interference between units. Also, if other electronic equipment is used, such as velocity meters, the 

other equipment must also be sequenced to prevent electronic interference.

ELECTRICAL-CONDUCTIVITY PROBE 

METHOD OF MONITORING SCOUR

Electrical conductance or conductivity is the ability 

of a material to conduct an electric current (Hem, 1985, 

p. 66). Fluids conduct electrical current from ion to ion, 

while most solids conduct electrical current from atom to 

atom. Both the surface water and the riverbed will conduct electricity.

Dissolved ions allow water to conduct electricity; 

therefore, pure water has a very low electrical conductivity. As the concentration of ions increases, the electrical 

conductivity of the water increases. Natural surface water 

contain different types and amounts of dissolved solids, 

suspended sediment, and dissolved ions, which have different effects on the electrical conductivity of water. The 

electrical conductivity of the surface water also varies 

according to its chemical characteristics.

The electrical conductivity of the bed material 

depends upon the composition of the parent material. In 

alluvial channels, the riverbed can consist of particles 

from several types of parent material, each of which can 

have different electrical conductivities. When the riverbed is submerged, the water below the riverbed will contain different types and amounts of dissolved solids and 

dissolved ions. The electrical conductivity of the riverbed 

varies according to the physical and chemical characteristics of the bed material and water.

The riverbed elevation is monitored at a desired location by using an electrical-conductivity probe to measure 

the conductivity of the submerged bed material and the 

surface-water flow at selected elevations. An electricalconductivity probe consists of multiple conductivitymeasuring sensors spaced at equal increments along a rod. 

The rod is driven or jetted vertically into the riverbed at a 

location where the elevation of the riverbed is to be monitored, such as the face of a pier footer. The majority of the 

sensors are placed below the riverbed, but at least one 

sensor remains above the riverbed in the surface-water

flow. Sensors located above the riverbed measure the 

conductivity of the surface-water flow. The sensors 

located below the riverbed measure the conductivity of 

the bed material and water. Additional sensors will be 

exposed to the surface-water flow if the riverbed scours at 

the probe. These sensors then measure the conductivity 

of the surface-water flow rather than the conductivity of 

the submerged bed material. The elevation of the riverbed 

can be estimated if the electrical conductivities of the 

riverbed and surface water differ sufficiently. Changes in 

the riverbed elevation are estimated by comparing historical riverbed and surface-water conductivities with current 

conductivity readings at each sensor.

System Design and Operation

Either conductivity or resistivity can be measured to 

estimate the riverbed elevation. Several methods to measure conductivity or resistivity are available, such as a 

four-electrode conductivity sensor, a two-electrode resistivity sensor, and the use of ac (alternating current) or dc 

(direct current). Multiple programming options are available with each method.

Conductivity is the inverse of resistivity; thus, by 

measuring either conductivity or resistivity, the other can 

be determined. Electrical conductivity is determined by 

passing electrical current through two electrodes and measuring the voltage drop across a separate pair of electrodes. The voltage drop is measured at the end of the 

electrodes or load cell; therefore, differences between the 

electrodes are not a large source of error. Convention dictates that a resistivity measurement passes a fixed voltage 

through two electrodes and the voltage drop across the 

electrodes is measured. Resistivity is measured by passing electrical current through, and sampling from, the 

same electrodes; therefore, any differences between the 

electrodes, such as cable length, can cause a noticeable 

difference in the voltage reading (Campbell Scientific, 

Inc., 1986). In order to determine which method to use, a 

test was performed using sand and water collected at the 

test site. An uncalibrated, four-electrode conductivity 

sensor was immersed in sample water and submerged 

sand. The differences between the measured values were 

noted. The measured value was 50 percent higher in the 

sample water than in the submerged sand. An uncalibrated, two-electrode resistivity sensor also was immersed 

in the same sample water and submerged sand, and the 

differences between measured values were noted. The 

measured value was 20 percent lower in the sample water

10 Use of Fathometers and Electrical-Conductivity Probes to Monitor Riverbed Scour at Bridge Piers
v,

 s/W* 

R 2

NOT TO SCALE

EXPLANATION

V, VOLTAGE DROP ACROSS MIDDLE ELECTRODES

R, ELECTRICAL RESISTANCE OF SAMPLE MATERIAL

I, CURRENT THROUGH SAMPLE MATERIAL

V2 VOLTAGE DROP ACROSS R 2

R 2 RESISTOR OF KNOWN VALUE

I 2 CURRENT THROUGH R 2 , ALSO TOTAL CURRENT 

THROUGH SAMPLE MATERIAL

O MEASUREMENT POINTS

Figure 7. Schematic diagram of four-electrode conductivity probe.

than in the submerged sand. The sample water could be 

more easily differentiated from the submerged sand by 

using the four-electrode conductivity sensor than by using 

the two-electrode resistivity sensor. In addition, the variation between multiple readings of resistivity was greater 

than the variation between multiple readings of conductivity. The four-electrode conductivity measurement was 

selected because of a greater difference in readings 

between the submerged sand and surface-water flow than 

the two-electrode resistivity measurement and smaller difference between consecutive readings.

Conductivity can be measured using either ac or dc; 

however, several problems are associated with using dc. 

Because dc flows in only one direction, the ion flow is 

unidirectional, and ions plate onto the electrodes. Plating 

causes changes in the data values as a function of time and 

can cause acute malfunction. In addition, polarization can 

occur in which the electrodes develop a charged field that 

repels the desired current flow. The direction of ac flow 

switches continually and, thus, does not cause plating or 

polarization; therefore, ac was selected as the best available method.

Three programs for measuring conductivity were 

available with the data logger, a differential voltage, a 

voltage ratio for a four-wire full-bridge circuit, and a voltage ratio for a six-wire full-bridge circuit. The programs 

for the differential voltage and four-wire full-bridge circuit were not used because both measure the voltage at the 

power source, as in the resistivity measurement, and not at 

the load cell. The program for the six-wire full-bridge circuit with excitation compensation program (Campbell 

Scientific, Inc., 1986), measures the voltage drop across

both a fixed resistance and a variable resistance. Excitation compensation refers to the measurement of voltage 

across both resistances at once. The six-wire full-bridge 

circuit program was chosen because it uses ac and measures voltage at the load cell and not at the power supply.

A schematic diagram of the four-electrode sensor 

used to measure conductivity is illustrated in figure 7. 

The electrical conductivity of the sampled material, K, 

can be determined by using the following equation:

K =

R2I2

(1)

where C\ is a constant associated with the particular 

conductivity sensor, /j is the current through the sampled 

material at the electrodes, R2 is the series resistor of 

known value, I2 is the current through R2 and also the 

total current through the sampled material, V2 is the voltage drop across R2 caused by I2, and YI is the voltage 

drop across the sampled material caused by I\. If the 

geometry of the sensor does not change, Ci/i//?2/2 is a 

constant that can be determined by measuring the voltage 

ratio while the sensor is immersed in a solution of known 

conductivity (J.H. Ficken, U.S. Geological Survey, 

written commun., 1991).

The data logger was used to store the sampling 

program, execute the program, and store the data. The 

program written for the electrical-conductivity probe used 

a self-contained function of the data logger to compute the 

ratio of the voltage across the variable resistor or sample 

(V]) and the voltage across the fixed resistor (V2) in a sixwire full-bridge circuit (Campbell Scientific, Inc., 1986).

Electrical-Conductivity Probe Method of Monitoring Scour 11
Because the value C\l\IR^i is a constant for each sensor, 

only the values of the ratio V^JV\ at each sensor were 

compared.

Wood was selected for construction of the electrical - 

conductivity probe because it supplied adequate room for 

the electrodes and wires, simplified manufacturing of the 

probe, and had sufficient strength to withstand the forces 

of driving or jetting. Standard 2 by 4 in. White Spruce 

lumber was used for the mounting plate and back plate 

(fig. 8). Each sensor consisted of four electrodes made 

from no. 10, 3/4-in. round-head brass wood screws. A 

total of four sensors were mounted on the probe at 4 in. 

intervals. The electrodes were connected to the data logger by 4-conductor, 22-gage, tinned, copper wire.

Above each sensor, a horizontal channel was cut into 

the wood to protect the wires and electrical connections 

from being damaged during installation. Filling this 

channel with non-conductive caulk further protected the 

wires and prevented short circuiting between the electrodes. In the center of each channel, a hole was drilled to 

carry the wires from the face of the mounting plate to a 

vertical channel cut on the back side of the mounting 

plate. The vertical channel was at the center of the probe 

when the mounting plate and cover plate were bolted 

together. Because driving was the preferred option for 

probe placement, the wires could not extend out of the top 

of the probe and were directed out of a hole in the cover 

plate. A flexible conduit (garden hose) was used to protect the wires from the probe to the rigid conduit at the 

pier face. The mounting plate and cover plate were bolted 

together with 4- by 3/16-in. machine bolts. The bolts 

were counter-sunk to prevent snagging during driving. 

The tip of the probe was cut on a 45° angle to provide a 

point for driving.

The conductivity probe was driven into the riverbed 

approximately 3 ft upstream from the center pile cap 

(fig. 9) at bridge 6918 in the Pamunkey River, near 

Hanover, Va. The probe could not be driven closer to the 

pile cap because of either buried debris or an upstream 

extension of the pile cap beneath the riverbed. Measurements at the test site show that the riverbed in front of the 

pile cap was approximately 1.8 ft below the top edge of 

the pile cap. The distance from the front edge of the pile 

cap to the pier was approximately 1.2 ft. The top sensor 

was mounted so that the top of the probe was at the same 

elevation as the top of the pile cap when the top sensor 

was approximately 2 in. above the riverbed. In such a 

position, the wires from the probe were protected between 

the probe and a rigid conduit attached to the pier. The 

probe was driven to a depth where the top of the riverbed

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 ELECTRODE

Figure 8. Diagram of conductivity probe.

was halfway between sensor 1 and sensor 2. Wire leads 

ran from the electrodes, through the probe and flexible 

conduit, into the rigid conduit and up the face of the pier. 

The wire leads exited the rigid conduit on top of the pier. 

The wires were spliced at this location to provide for a 

breakaway point and for assistance in the installation. 

The wires were placed under the bridge deck to the abutment and then in a shallow trench to the data logger

12 Use of Fathometers and Electrical-Conductivity Probes to Monitor Riverbed Scour at Bridge Piers
WATER SURFACE

CONDUIT

CONDUCTIVITY 

PROBE

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PILE CAP

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Figure 9. Schematic diagram of pier and 

conductivity probe at bridge 6918 over 

the Pamunkey River near Hanover, 

Virginia.

located in the gage house. The total length of wire from 

the data logger to each electrode was approximately 

400ft.

The electrical-conductivity probe was installed on 

June 27, 1991, and operated from June 28 through August 

8, 1991. The individual sensors were read every hour 

with a 2 second delay between deactivation of one sensor 

and activation of the next sensor. The 2 second delay was 

necessary to prevent electrical interference between the 

sensors. The data logger could only measure three sensors; therefore, the top three sensors were monitored 

while the bottom sensor (sensor 4) was sampled only during site visits and for 3 days at the end of the field test.

Analysis and Results

Electrical-conductivity probe readings and discharge 

data collected at the Pamunkey River near Hanover, Va., 

are shown in figure 10. Sensor readings were normalized 

at the end of the test when the probe was removed. The

proportional value of Cj/i//?2/2 * n ecl- 1 was determined 

for each sensor. Readings were collected from each sensor after the probe was removed from the riverbed but was 

still submerged. Readings from sensors 1 and 3 were 

greater than 1 v/v (volt per volt) (1.290 and 1.898 v/v, 

respectively), and readings from sensors 2 and 4 were less 

than 1 v/v (0.556 and 0.654 v/v, respectively). Because 

the readings from the four sensors bounded 1.0, they were 

normalized to that number (K=\). All readings collected 

by sensor 1 were multiplied by 0.775, readings from sensor 2 were multiplied by 1.799, readings from sensor 3 

were multiplied by 0.527, and readings from sensor 4 

were multiplied by 1.529. Normalizing the probe readings eliminated variation because of manufacture of the 

sensors. The normalization values were determined at the 

end of the field test because initial laboratory testing indicated that the sensor readings could be affected either by 

corrosion of the electrodes or by the wood absorbing 

moisture, both of which could interfere with the measurement of voltage potential across the electrodes. Because 

equation 1 was solved for the assumed value of K=l, the 

sensor readings are proportional to the actual conductivities and are comparable.

Discharge data were collected during two small floods 

(peak discharges of 3,740 ft3/s on July 8, and 2,750 ft3/s 

on July 15) except for the period July 12-14, where probe 

data are missing because of a programming error. The 

bed elevation could not be physically monitored during 

the floods because of the depth of the water and velocity 

of the water at the probe. Divers observed the probe on 

July 18 and discovered that sensor 1 (the top sensor) had 

been covered with approximately 1 ft of loose, coarse 

sand and plant debris. On July 26, attempts to uncover the 

top sensor were unsuccessful; however, removal of most 

of the loose material that covered the probe caused the 

readings from sensors 1 and 2 to decrease.

The electrical-conductivity readings and discharge 

data indicate that the electrical-conductivity probe as 

tested has limited usefulness in identifying and monitoring the elevation of the riverbed during high flows (fig. 

10). Problems associated with the electrical-conductivity 

probe are the inability to locate the riverbed from sensor 

readings, decreasing readings with time (possibly as a 

result of corrosion of the electrodes or from the wooden 

probe absorbing moisture), and protecting the system 

from damage by debris.

The plot of readings from sensor 1, which was initially located above the riverbed, shows that the conductivities measured in the surface-water flow increased as 

the discharge increased and decreased as the discharge

Electrical-Conductivity Probe Method of Monitoring Scour 13
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decreased. This observation is consistent with the simultaneous increase and decrease of sediment concentration 

and discharge results by Guy (1970). Variations in consecutive readings indicate that the sensor was located 

where conductivities change rapidly. This observation 

indicates that the sensor was near the interface of the 

riverbed and surface-water flow where there can be movement of bed material. Inspections following the two 

floods showed that approximately 14 in. of loose sand and 

debris were deposited at the site of the probe, which covered sensor 1 with approximately 1 ft of bed material.

The plot of readings from sensor 2, which was 

installed 2 in. below the riverbed, shows that the sensor 

remained buried until the discharge exceeded 3,000 ft3/s. 

Readings decreased with very little variation between 

consecutive readings until midday on July 7, at which 

time, the readings varied with discharge similar to readings from sensor 1. This variation indicates that the riverbed had scoured and uncovered sensor 2, or the interface 

of the riverbed and surface-water flow was near sensor 2. 

During the second flood, the readings from sensor 2 

increased with discharge indicating that the sensor was 

above the riverbed; however, the readings did not 

decrease as the discharge decreased, indicating that the 

sensor was buried. Variation in the readings for sensor 2 

during the floods indicates that the sensor was exposed, 

or the sensor was near the interface of the riverbed and 

surface-water flow where there could have been movement of the bed material.

The plot of readings from sensor 3 indicates that the 

sensor remained buried during the entire test. The readings decreased as discharge increased during the first 

flood and then remained fairly constant during the remainder of the test. The initial decrease in readings could have 

resulted from the increased flow, inducing a vibration in 

the probe and compacting the bed material at the sensor's 

electrodes. Readings from sensor 3 did not vary with discharge, nor was there much variation in consecutive readings. This indicates that sensor 3 remained buried and 

was not affected by the movement of bed material.

The plot of readings from sensor 4 indicates that the 

sensor remained buried during the entire test. The readings remained constant throughout the test. Readings 

from sensor 4 did not vary with discharge, nor was there 

much variation in consecutive readings. This indicates 

that sensor 4 remained buried and was not affected by the 

movement of bed material.

Laboratory tests using bed material and water from 

the Pamunkey River at the test site indicated that sensor 

readings could be higher in the surface-water flow than in

the submerged bed material. Field data plotted in figure 

10 shows higher seasor readings in the submerged bed 

material than in the surface-water flow. The figure also 

shows large differences between readings for all sensors 

in the submerged bed material. The installation method 

(driving) could have caused bed material or debris to 

lodge against the electrodes in such a manner that the 

readings were not characteristic of the surrounding bed 

material, or possibly, the bed material at the probe location is not uniform with depth.

Sensor 1 was initially located in the surface-water 

flow approximately 2 in. above the riverbed, and the readings (0.2 v/v) represent conductivities of the surface-water 

flow before the first flood. Sensors 2, 3, and 4 were initially buried and the readings represent conductivities of 

the water and bed material surrounding the electrodes (0.4 

to 1.4 v/v before the first flood). The readings from sensor 

1 increased from 0.2 to 0.9 v/v during the first high water 

and from 0.3 to 0.6 v/v during the second flood. All readings collected from sensor 1 during the floods were within 

the range of values measured from the sensors that were 

buried. Conductivities of the surface-water flow near the 

riverbed during a flood are similar to conductivities of the 

bed material, therefore, conductivities cannot be used to 

differentiate between the surface-water flow and bed 

material.

Readings from sensors 1 and 2 rapidly converge 

toward 1.0 volt per volt during the floods and are more 

variable than sensor 3. The data indicate that sensors 1 

and 2 were located where conductivities change rapidly, 

such as in the surface-water flow or near the riverbed and 

surface-water flow interface where there can be movement of bed material. However, after the second peak discharge, readings from sensor 1 continued to decrease to 

0.2 v/v, while readings from sensor 2 decreased gradually. 

The variations between consecutive readings continued 

when sensor 1 was buried by 1.0 ft of loose sand and 

debris and sensor 2 was covered by 1.5 ft of loose sand 

and debris. Therefore, rapid changes in conductivities 

cannot be used to differentiate between the surface-water 

flow and bed material.

Readings output from all four sensors decreased 

steadily throughout the test except during the floods. 

Tests in the laboratory also showed a decrease in readings 

with time from a similar type probe. Gradual corrosion of 

the brass electrodes can be one possible explanation for 

the decrease in readings. Another possible explanation 

for the decrease in readings could be that the brass electrodes were mounted in wood. As the submerged wood 

absorbs moisture, the moisture content of the wood

Electrical-Conductivity Probe Method of Monitoring Scour 15
changes and causes a change in conductivity of the wood. 

Readings from the sensors that were buried were less than 

readings from the same sensors when they were in the surface-water flow only; therefore, if the riverbed scoured 

and exposed the sensors, the sensor readings could represent the higher conductivities of the surface-water flow. 

Normalization of the sensor readings was not accomplished until the test was completed because the effect of 

the decrease in readings was not known. Sensors were 

not calibrated to actual conductivities for the same reason.

SUMMARY AND CONCLUSIONS

Scour is the lowering of a river channel by erosion 

and is the leading cause of bridge failure. Monitoring the 

riverbed elevation at a bridge where scour is a potential 

problem provides for public safety as a temporary countermeasure until structural improvements can be implemented, or in some situations, as a permanent countermeasure. Monitoring of the riverbed elevation accurately 

during high flows is difficult because of surface-water 

flow conditions. Two methods, a fathometer system and 

an electrical-conductivity probe system, were developed 

to monitor riverbed elevations at bridge piers. The scourmonitoring systems consisted of a sensor (fathometer or 

electrical-conductivity probe), power supply, data logger, 

relay, and system program.

A fathometer system was installed at bridges over two 

tidal sites, the Leipsic River at Leipsic, Del., and the 

Sinepuxent Bay near Ocean City, Md. The fathometer 

calculates the distance from a transducer to the riverbed 

by measuring the two-way travel time of a reflected, 

acoustic wave. Two sampling procedures were used to 

collect and process the data. The initial procedure measured the depth below the transducer five times, checked 

each reading for reasonableness, and stored the last valid 

reading in the final register of the data logger. A total of 

five readings were collected at each time interval to 

ensure that one reading met the criteria. The refined procedure measured the depth below the transducer five 

times, checked each reading for reasonableness, and 

stored the last valid reading in one of five final registers of 

the data logger. The process was repeated for each of the 

five final registers. A total of 25 readings were collected

at each time interval to ensure that one reading met the 

criteria for each of 5 final registers. The median value 

was used to analyze the performance of the method.

Field data indicate that fathometers can be used to 

identify and monitor the riverbed elevation if post processing of the data and trends in the data are used to determine the riverbed location in relation to the transducer. 

Use of better quality fathometers, multiple fathometers, or 

electronic shielding may reduce the need for post processing of the data. The accuracy of the system is approximately the same as the resolution of the fathometer. 

Signal scatter, caused by electronic or physical interference, is a major source of error. During periods of interference, measured distance values from the transducer to 

the riverbed are erroneous. Other problems associated 

with the use of a fathometer are determining the exact 

location of the reflected pulse, sequencing multiple fathometers or other electronic equipment to prevent electronic interference, and protecting the system from 

damage by debris.

An electrical-conductivity probe system was installed 

at a bridge over the Pamunkey River near Hanover, Va. 

The approximate elevation of the riverbed is determined 

by comparing conductivities of the surface-water flow 

with conductivities of submerged bed material. The probe 

was driven into the riverbed such that three of four 

equally spaced sensors were buried. The ratio of the voltage across a known resistor and the voltage across a variable resistor was measured using a six-wire full-bridge 

circuit, and was proportional to the conductivity of the 

surrounding surface water or bed material.

Field data indicate that an electrical-conductivity 

probe, as tested, has limited usefulness in identifying and 

monitoring the riverbed elevation during high flows. As 

the discharge increases, the concentration of sediment in 

the surface-water flow increases, especially near the riverbed. Voltage ratios measured at the sensors in the surfacewater flow could not be distinguished from voltage ratios 

measured at the shallowest sensor in the bed material. 

Other problems associated with the use of an electricalconductivity probe are a gradual decrease in readings with 

time, possibly because of corrosion of the electrodes or 

changes in moisture of the wooden probe, and potential 

damage to the system by debris. A conductivity probe 

with a different design, a probe measuring temperature 

rather than conductivity, or a probe sensing flow movement may improve results.

16 Use of Fathometers and Electrical-Conductivity Probes to Monitor Riverbed Scour at Bridge Piers
REFERENCES CITED

Butch, G.K., 1991, Measurement of bridge scour at selected

sites in New York, excluding Long Island: U.S. Geological

Survey Water-Resources Investigations Report 91-4083,

17 p. 

Campbell Scientific, Inc., 1986, [1988], CR10 measurement

and control module operator's manual: Logan, Utah,

p. 9-8. 

Crumrine, M.D., 1992, Bridge-scour data for the highway

101 bridge over Alsea River Estuary at Waldport,

Oregon 1988-90: U.S. Geological Survey Open-File

Report 91-531,48 p. 

Guy, H.P., 1970, Fluvial sediment concepts: U.S. Geological

Survey Techniques of Water-Resources Investigations,

book 3, chap. C 1,55 p. 

Hem, J.D., 1985, Study and interpretation of the chemical

characteristics of natural water (3d ed.): U.S. Geological

Survey Water-Supply Paper 2254, 263 p. 

U.S. Federal Highway Administration, 1990, High priority

National program area McLean, Va.: Federal Highway

Administration Report No. FHWA-RD-90-054, 8 p.

References Cited 17