Water-Resources Investigations Report 00-4035

IN COOPERATION WITH

NEW HAMPSHIRE DEPARTMENT OF TRANSPORTATION

Use of a Ground-Penetrating Radar 

System to Detect Pre- and Post-Flood 

Scour at Selected Bridge Sites in 

New Hampshire, 1996-98

U.S. Department of the Interior

U.S. Geological Survey
Cover photograph is showing the Peabody River in Gorham, N.H. Mount Washington and the 

Presidential Range is in the background. (Photograph was taken by J.R. Olimpio, U.S. Geological 

Survey)
Use of a Ground-Penetrating Radar 

System to Detect Pre- and Post-Flood 

Scour at Selected Bridge Sites in 

New Hampshire, 1996-98

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 00-4035

By Joseph R. Olimpio

Pembroke, New Hampshire

2000

IN COOPERATION WITH

NEW HAMPSHIRE DEPARTMENT OF TRANSPORTATION
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

New Hampshire/Vermont District

361 Commerce Way

Pembroke, NH 03275-3718

or through our website at

 http://nh.water.usgs.gov
Contents V

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

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

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

Description of the Study Area ..................................................................................................................................... 4

Use of Terms................................................................................................................................................................ 4

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

Ground-penetrating Radar ..................................................................................................................................................... 4

Equipment.................................................................................................................................................................... 5

Data-collection Methods.............................................................................................................................................. 6

Scour Estimates at Bridge Sites............................................................................................................................................. 6

Route 9 Bridge ............................................................................................................................................................. 7

Downstream Bridge-face Cross Section............................................................................................................ 10

Column-set Cross Section, Upstream to Downstream....................................................................................... 10

Route 18 Bridge........................................................................................................................................................... 10

Route 13 Bridge........................................................................................................................................................... 14

Upstream Bridge-face Cross Section................................................................................................................. 14

Downstream Bridge-face Cross Section............................................................................................................ 16

Routes 202 and 9 Bridge.............................................................................................................................................. 16

Interstate 89 Southbound Bridge ................................................................................................................................. 19

Interstate 89 Northbound Bridge ................................................................................................................................. 19

Routes 3 and 25 Bridge................................................................................................................................................ 22

Route 13 Bridge........................................................................................................................................................... 22

Downstream Bridge-face Cross Section............................................................................................................ 24

Right-bank-pier Cross Section........................................................................................................................... 24

Summary and Conclusions.................................................................................................................................................... 24

References Cited.................................................................................................................................................................... 27

FIGURES

1. Map showing the location of ground-penetrating-radar data-collection sites at seven bridge sites 

in New Hampshire ...................................................................................................................................................... 3

2. Photograph of ground-penetrating radar data-collection equipment .......................................................................... 5

3-13. Figures showing ground-penetrating-radar profiles of the:

3. Downstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 post-flood data collection, 

from left to right bank, Route 9 over the Soucook River, Concord, N.H. ......................................................... 11

4. Pier-set cross sections for (A) 1996 pre-flood and (B) 1998 post-flood data collection, from upstream to 

downstream, Route 9 over the Soucook River, Concord, N.H. ......................................................................... 12

 5. Upstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 post-flood data collection, 

from left to right bank, Route 18 over the Connecticut River, Littleton, N.H................................................... 13

6. Upstream bridge-face cross sections for (A) 1997 pre-flood and (B) 1998 post-flood data collection, 

from pier to right bank, Route 13 over the Cold River, Sandwich, N.H............................................................ 15

7. Downstream bridge-face cross sections for (A) 1997 pre-flood and (B) 1998 post-flood data collection, 

from pier to right bank, Route 13 over the Cold River, Sandwich, N.H............................................................ 17

8. Upstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 post-flood data collection, 

from left to right bank, Routes 202 and 9 over the Contoocook River, Hopkinton, N.H. ................................. 18

9. Upstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 post-flood data collection, 

from left to right bank, Interstate 89 southbound over the Warner River, Warner, N.H................................... 20

CONTENTS
VI Contents

10. Downstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 post-flood data collection, 

from left to right bank, Interstate 89 northbound over the Warner River, Warner, N.H................................... 21

11. Upstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 post-flood data collection, 

from left to right bank, Routes 3 and 25 over the Pemigewassett River, Ashland, N.H. .................................. 23

12. Downstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 post-flood data collection, 

from left to right bank, Route 13 over the Souhegan River, Milford, N.H. ...................................................... 25

13. Right-bank pier cross section for (A) 1996 pre-flood and (B) 1998 post-flood data collection, 

from upstream to downstream, Route 13 over the Souhegan River, Milford, N.H........................................... 26

TABLES

1. Selected descriptive characteristics of seven bridge sites in New Hampshire........................................................ 4

2. Approximate ground-penetrating radar two-way travel times for selected materials............................................. 5

3. Flood-flow data at stream-gaging stations near seven bridges in New Hampshire................................................ 7

4. Ground-penetrating-radar scour data for seven bridges in New Hampshire .......................................................... 8

 CONVERSION FACTORS, ABBREVIATIONS, AND SEA LEVEL

Sea Level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929

(NGVD of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets

of both the United States and Canada, formerly called Sea Level Datum of 1929.

ABBREVIATIONS USED IN THIS REPORT

MHz = megahertz

Multiply By To obtain

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

cubic feet per second (ft3/s) 0.02832 cubic meter per second
Abstract 1

Abstract

Ground-penetrating radar was used to 

measure the depth and extent of existing and 

infilled scour holes and previous scour surfaces at 

seven bridges in New Hampshire from April 1996 

to November 1998. Ground-penetrating-radar 

survey techniques initially were used by the 

U.S. Geological Survey to study streambed scour 

at 30 bridges. Sixteen of the 30 bridges were resurveyed where floods exceeded a 2-year 

recurrence interval. A 300-megahertz signal was 

used in the ground-penetrating radar system that 

penetrated through depths as great as 20 feet of 

water and as great as 32 feet of streambed 

materials. 

Existing scour-hole dimensions, infilled 

thickness, previous scour surfaces, and streambed 

materials were detected using ground-penetrating 

radar. Depths to riprap materials and pier footings

were identified and verified with bridge plans. 

Post data-collection-processing techniques were 

applied to assist in the interpretation of the data, 

and the processed data were displayed and printed 

as line plots. Processing included distance 

normalization, migration, and filtering but 

processing was kept to a minimum and some 

interference from multiple reflections was left in 

the record.

Of the 16 post-flood bridges, 22 groundpenetrating-radar cross sections at 7 bridges were 

compared and presented in this report. Existing 

scour holes were detected during 1996 (pre-flood) 

data collection in nine cross sections where scour 

depths ranged from 1 to 3 feet. New scour holes 

were detected during 1998 (post-flood) data 

collection in four cross sections where scour 

depths were as great as 4 feet deep. Infilled scour 

holes were detected in seven cross sections, where

depths of infilling ranged from less than 1 to 

4 feet. Depth of infilling by means of steel rod 

and hammer was difficult to verify in the field 

because of cobble and boulder streambeds or deep

water. 

Previous scour surfaces in streambed 

materials were identified in 15 cross sections and 

the depths to these surfaces ranged from 1 to 

10 feet below the streambed. Riprap materials or 

pier footings were identified in all cross sections. 

Calculated record depths generally agree with 

bridge plans. Pier footings were exposed at two 

bridges and steel pile was exposed at one bridge. 

Exposures were verified by field observations.

INTRODUCTION

In 1993, the New Hampshire Department of 

Transportation (NHDOT) completed scour analysis of 

48 bridges across the State that had previously been 

determined to be scour susceptible. Based on 

recommended procedures outlined in Richardson and 

others (1993), 44 of the 48 bridges evaluated had scour 

estimated to extend below the top of the footings and 

were classified as “scour critical” (Whitman and 

Howard, 1992). Additionally, predicted scour was 

below the elevation of the footings for floods at the 

10-year recurrence interval for 35 of the 44 bridges. 

For New Hampshire bridges that have been in place 

for 50 years or more, scour-related problems were 

appreciably less frequent than results from these 

analyses indicate.

Use of a Ground-Penetrating Radar System to 

Detect Pre- and Post-Flood Scour at Selected Bridge

Sites in New Hampshire, 1996-98

By Joseph R. Olimpio
2 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

Bridges over waterways that are identified as 

scour critical require evaluation and monitoring to 

ensure public safety. However, such evaluation and 

monitoring measures are costly to apply to large 

numbers of bridges. Transportation agencies need 

better information and methods to accurately assess, 

predict, and remediate scour problems in a costeffective manner. Detailed field observation of scour 

processes is needed to achieve these objectives in New 

Hampshire. Measurements of streambed scour and 

deposition at bridge sites are essential to studying the 

scour process; therefore, by understanding this 

process, better construction techniques and scour 

countermeasures can be developed to minimize scour 

effects. Maximum scour at a bridge usually occurs 

near the peak of a flood when measurements are 

difficult to collect. Scour holes are often infilled just 

after the peak flow and during the flow recession. If 

the scour hole has been infilled, data collected after the

flood based on traditional methods (sounding weight) 

may not be able to detect the maximum scour depth. 

Pre- and post-flood ground-penetrating-radar (GPR) 

data collection may be able to detect existing or 

infilled scour holes that occurred during the flood. 

GPR techniques are suitable as an alternative to 

traditional methods of flood-data collection such as 

the installation of fixed instrumentation or the deployment of mobile flood teams. GPR data collected at 

bridge sites contain a continuous profile of the 

streambed and, in some instances, the sub-bed bottom. 

These sub-bed-bottom reflectors can be correlated to 

erosional or depositional surfaces, pier footings, 

previous scour surfaces, or sediment layers (Placzek 

and others,1995). 

A disadvantage of pre- and post-flood GPR data 

collection is that it is not a technique used to protect 

the motorist or the public during a flood and analyses 

of data may be limited if sufficient hydraulic data are 

not available to quantify flow velocities during a flood. 

GPR, however, does provide advantages over other 

methods in that it can be used to identify historical 

maximum scour depths, as well as existing or infilled 

scour holes that occurred during a flood.

In 1995, the U.S. Geological Survey (USGS), in 

cooperation with the NHDOT, began a study to 

provide data needed to improve the understanding of 

bridge-scour processes, scour prediction, and scour 

monitoring by collecting limited-detail scour data at 

selected bridges in New Hampshire. As part of this 

study, GPR was used to measure pre- and post-flood 

scour cross-sectional profiles at upstream, 

downstream, and pier locations at bridge sites. 

Initially, 30 bridge sites were chosen for pre-flood 

GPR data collection. Site-selection criteria for GPR 

data collection were based on (1) physical characteristics of the river and bridge, (2) close proximity of 

USGS stream-gaging stations to bridge sites, and 

(3) the occurrence of a flood. The pre-flood GPR data 

served as a baseline cross-sectional survey for a site. 

Existing scour, infilling of scour holes, and historical 

maximum scour surfaces were identified on the 

records. River-characteristics criteria included the 

presence of permeable streambed materials that the 

radar signal was assumed to be able to penetrate. Sites 

with steep ravines; streambed materials predominantly

consisting of large boulders; bedrock channels; large 

amounts of debris (such as trees, tree stumps, or 

concrete and granite abutment materials from old 

bridges); or in saltwater were not chosen for GPR data 

collection. Close proximity of the bridge to USGS 

stream-gaging stations also was a selection factor 

because streamflow could be associated with a 

recurrence interval. Recurrence interval for floods is 

defined as the average time interval between 

occurrences of a flood of a given or greater magnitude, 

usually expressed in years. When streamflow reached 

the level of a 2-year or greater recurrence interval 

based on USGS streamflow data at a nearby gaging 

station, the bridge was chosen for post-flood GPR data 

collection. 

Purpose and Scope

The purpose of this report is to describe (1) the 

use of GPR to detect pre- and post-flood scour holes 

and previous scour surfaces at bridge sites, and (2) the 

interpretation of data collected from selected bridge 

sites. This report includes descriptions of the GPR 

methods and equipment used during this study. Each 

of the bridge sites are described with interpretations of 

the geophysical records. Of the original 30 evaluated 

bridges, 16 bridges with floods exceeding the 2-year 

recurrence interval were selected for post-flood GPR 

data collection. Of the 16 bridges surveyed, 11 bridge 

sites at 7 bridges (fig. 1) were evaluated for pre- to 

post-flood changes.
INTRODUCTION 3

Figure 1. Location of ground-penetrating-radar data-collection sites at seven bridge sites in 

New Hampshire.
4 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

Description of the Study Area

The study area includes bridges crossing the 

Connecticut, Cold, Pemigewasset, Warner, Soucook, 

Contoocook, and Souhegan Rivers in northwestern to 

southern New Hampshire. Selected descriptive 

characteristics of these bridge sites are shown in 

table 1.

Use of Terms

In this report, the following terminology is used 

to describe various bed-bottom-scour characteristics:

Existing scour hole: The bed-bottom scour 

observed during the 1996 pre-flood cross-sectional 

data collection. Depth is measured in feet below the 

streambed reference surface adjacent to the scour hole.

New scour hole: New or deepened scour 

observed during the 1998 post-flood cross-sectional 

data collection. Depth is measured in feet below the 

streambed reference surface adjacent to the scour hole.

Infilling: The partial or complete filling of an 

existing scour hole observed during the post-flood 

cross-sectional data collection. Extent of infilling is 

measured as the difference in hole depths between the 

pre- and post-flood data collection.

Previous scour surface: Buried scour holes or 

surfaces that are evident in the GPR record and existed 

prior to the pre-flood cross-sectional data collection. 

Depth is measured in feet below the streambed 

reference surface at the time the cross section was 

surveyed.

Multiple: A secondary reflection of the 

streambed surface in a GPR record that appears below 

the streambed and is not indicative of sub-bed-bottom 

reflectors.

Acknowledgments

Information for this study was provided by the 

NHDOT. The author wishes to thank Dave Powellson,

Chief of the Existing Bridge Section of the Bureau of 

Bridge Design, and his staff, for their cooperation and 

assistance during this study.

GROUND-PENETRATING RADAR

GPR was used to provide continuous profiles of 

the streambed surfaces at the seven bridges shown on 

figure 1. GPR surveys have been used for a variety of 

subsurface investigations (Beres and Haeni, 1991; 

Placzek and Haeni, 1995). This investigation used the 

GPR methods reported by Placzek and Haeni (1995) 

to obtain profiles of the shallow stratigraphy of subsurface deposits. Depths to reflectors and types of lithologies were verified by comparing depths to reflectors 

with depths to lithologic units and physical structures 

on bridge-construction plans. 

 

Table 1. Selected descriptive characteristics of seven bridge sites in New Hampshire

[No., number; NHDOT, New Hampshire Department of Transportation; mi2, square mile; ft/ft, foot per foot; ft, foot; NH, New Hampshire; US, United 

States]

Bridge 

site 

No.

(fig. 1)

Town NHDOT No. Route River crossed

Drainage area

(mi2)

Slope

(ft/ft)

Total 

width

(ft)

Water

depth

(ft)

Bed-bottom materials

1 Littleton 109/134 NH 18 Connecticut 1,604 0.0005 530 1-20 Silt, fine sand, some boulders

2 Sandwich 238/092 NH 113 Cold 31 .04 146 1-5 Coarse sand, gravel, cobbles

3 Ashland 076/080 US 3 & NH 25 Pemigewasset 634 .001 800 1-8 Medium sand, cobbles

4 Warner 166/103 I-89 Warner 83 .001 191 1-4 Fine to medium sand, gravel, 

cobbles

5 Concord 160/188 NH 9 Soucook 77 .004 178 1-4 Fine to medium sand, boulders

6 Hopkinton 049/096 NH 9 & US 202 Contoocook 400 .0006 312 1-12 Fine to medium sand, some 

boulders

7 Milford 123/133 NH 13 Souhegan 120 .001 112 1-8 Fine to medium sand, some 

boulders
GROUND-PENETRATING RADAR 5

The GPR-survey system transmits radiofrequency electromagnetic pulses into the water and 

bed-bottom sediments and receives energy reflected 

back from surface and subsurface reflectors. Reflectors can be any subsurface contact between water and 

geologic materials or geologic materials with different 

physical and electrical properties, such as the interface 

between lithologic units or layers within a unit. 

Manmade materials such as concrete and steel also can 

be a subsurface reflector. The surveys were conducted 

with a 300-MHz-center-frequency monostatic 

transmitting and receiving antenna that was near the 

water surface. The profiles can be examined visually 

to locate below water and subsurface features. 

Interpretations of GPR profiles are improved by 

comparison with lithologic logs and bridgeconstruction plans.

Beres and Haeni (1991) provide an interpretation guide for various types of reflector patterns for 

unconsolidated deposits. Sections of the GPR profiles 

with parallel reflectors can indicate the presence of 

laminated fine-grained sediments, such as pondbottom sediments observed in this study. Complex, 

subparallel, and chaotic GPR-profile patterns 

generally indicate coarse-grained sediments. Inverted 

V-shaped patterns are indicative of point reflectors that

could be from cobbles or boulders.

All reported depths to existing and new scour 

holes and bed-bottom and sub-bed-bottom reflectors 

are approximate. These depths are based upon 

estimated ground-penetrating radar two-way travel 

times for water, saturated sands, till, and bedrock 

(table 2).

 

Equipment

A Subsurface Interface Radar System-2 (SIR-2) 

manufactured by Geophysical Survey Systems, Inc. 

was used to investigate scour in this study. Data are 

displayed in real-time on a color display and stored on 

an internal hard drive. For this study, the GPR 

processor and 300-MHz antenna were placed in a 

small inflatable boat (fig. 2). The inflatable boat 

Table 2. Approximate ground-penetrating radar two-way 

travel times for selected materials

Material Approximate two-way travel times, 

in nanoseconds per foot

Water 18

Saturated sands 10

Tills 6.5

Bedrock 5

Figure 2. Ground-penetrating radar data-collection equipment.
6 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

provides a stable platform that has minimal effect on 

radar-signal transmission and reception. In deep water 

(depth is greater than or equal to 4 ft), the operator sat 

in the boat, which was propelled across the water 

surface by an electric trolling motor. In shallow water 

(depth is less than 4 ft), the equipment operator waded 

alongside the boat and pushed the boat by hand in 

water or dragged it across sand, gravel, and cobble 

bars. These methods of equipment deployment were 

fast, safe, and easily transportable from the vehicle to 

the river’s edge by one person. A geophysical record 

was collected at one bridge per day.

Data-collection Methods

Cross-sectional GPR profiles were collected at 

upstream and downstream bridge faces, from the left 

bank to the right bank of the river, directly beneath the 

leading and trailing edges of the bridge, and within 1 ft 

of each pier nose. (Left bank and right bank 

terminology used here is from a facing downstream 

perspective). Profiles also were collected 1 ft from 

and alongside the piers parallel with the axis of the 

river, from downstream to upstream within about 20 to 

40 ft downstream of the nose of the pier, alongside the 

length of the pier, and within about 20 to 40 ft 

upstream of the nose of the pier.

Depths of scour holes, exposed pier footings, 

channels, boulders, and streambeds were measured 

with a surveying rod during data collection. Attempts 

to probe into the streambed with a steel rod to verify 

depths of infilled holes were met with limited success, 

as the presence of cobbles and boulders or deep water 

usually prevented verification. The water-surface 

elevation was measured from a known reference point 

on the bridge before and after data were collected.

Transmission velocities of the radar signal were 

used to interpret depth to a reflector. Approximate 

GPR two-way travel times for selected materials 

(Geophysical Survey Systems, Inc., 1994) used for 

interpretation are summarized in table 2.

GPR data were processed to enhance reflector 

patterns in the data. Distance normalization, 

migration, and signal amplification were used 

selectively to process the data. These processes, as 

performed using commercial computer software, are 

described by Geophysical Survey Systems, Inc. 

(1995). Processing was kept to a minimum to preserve

as much of the original record as possible.

SCOUR ESTIMATES AT BRIDGE SITES

The results of the geophysical surveys presented 

in this section include selected GPR cross sections, 

showing pre- and post-flood-scour data at seven 

bridges. Baseline GPR data were collected at 

30 bridge sites as part of the pre-flood data-collection 

effort. The criteria for post-flood data collection at 

bridge sites were the occurrence of a 2-year or greater 

recurrence interval flood at any of the 30 bridge sites 

during the study. The flood-recurrence interval was 

determined from USGS streamflow data at gaging 

stations located either on each river near the bridge, or 

compared to USGS streamflow data at gaging stations 

that were about 2 to 10 mi away on an adjacent river 

with similar basin characteristics. Flood-flow data at 

stream-gaging stations near bridges in New Hampshire

are presented in table 3. Bridges that had uninterpretable GPR data from the pre-flood survey also were 

eliminated as potential post-flood GPR data-collection 

sites. Thus, 22 pre- and post-flood GPR cross sections 

(11 pre-flood sections and 11 post-flood sections) at 

7 bridge sites were used for this study. Water-surface 

elevations were determined by using a steel tape and 

known reference mark on the bridge deck. The 

vertical distance from the reference point to the water 

surface was measured twice during each site visit. 

This measured distance from the reference point to the 

water surface was used to obtain a water-surface 

elevation for each day GPR data were collected 

(elevation data for the reference point were obtained 

from NHDOT bridge plans). The water-surface 

elevation was used as a point of reference on the GPR 

cross sections. Estimated two-way travel times (in 

nanoseconds per foot, table 2) in water, saturated sand, 

till, and bedrock were used to determine depths to the 

surface of the streambed, existing and new scour 

holes, infilled streambed sections, and subsurface 

reflectors such as previous scour surfaces, pier 

footings, and riprap materials. Depths to reflectors 

were calculated based on the known water-surface 

elevation and travel time through the water or subsurface materials. These calculated depths were 

compared to cross-sectional data on NHDOT bridge 

plans to verify depths to features such as streambed 

and top-of-footing and(or) bottom-of-footing.

The footing outline is superimposed on the GPR 

records. Footing placement on the record is based on 

known dimensions of the footing, the top and bottom 

elevations of the footing, and the two-way travel time 

of the radar signal in the bed-bottom materials (footing

data are from NHDOT bridge plans).
SCOUR ESTIMATES AT BRIDGE SITES 7

Scour holes and depths to exposed bridge 

features (top-of-footing, top-of-steel pile) were 

manually measured during GPR data collection using 

a surveying rod. These measurements also were used 

to verify depths to these features, when identified, in 

the GPR cross sections. A summary of scour 

estimates at each of the 7 bridges and 22 cross sections 

is provided in table 4. A detailed description for each 

bridge site follows this summary.

Route 9 Bridge

A bridge is located on Route 9 and crosses the 

Soucook River (about 100 ft wide). The bridge is 

178 ft long by 44 ft wide, and is supported by two 

6-column-bridge pile bents. Each of the columns are 

3 ft wide and 5 ft long. Each 6-column-bridge pile 

bent is 40 ft long.

Pre- and post-flood GPR data were collected at 

the upstream and downstream bridge faces and along 

 

Table 3. Flood-flow data at stream-gaging stations near seven bridges in New Hampshire

[Bridge site distance is distance in miles from bridge to gaging station. NHDOT reference no., New Hampshire Department of Transportation bridge 

reference number; GPR, Ground-penetrating Radar; ft3/s, cubic feet per second; >, greater than]

Stream-gaging station 

information: 

river, location, 

station identifier

Bridge site, 

river, 

distance, 

NHDOT 

reference no.

1996 Pre-flood 

GPR data 

collection:

Date and flow 

(ft3/s)

1998 Post-flood 

GPR data 

collection: 

Date and flow 

(ft3/s)

Highest flow 

during study: 

Date and flow 

(ft3/s)

Approximate 

recurrence 

interval 

(years)

Soucook River

Concord

01089100

Route 9

Soucook

1.9

160/188

5/20/96

311

11/05/98

17

3/10/98

1,890 5-10

Connecticut River

Dalton

01131500

Route 18

Connecticut

11.2

109/134

8/08/96

1,123

11/10/98

1,500

4/01/98

42,000 25-50

Bearcamp River

Tamworth

01064801

Route 13

Cold

10

238/092

8/12/97

15

11/13/98

129

6/14/98

6,150 >50

Contoocook River

Henniker

01085000

Route 9/202

Contoocook

4.2

049/096

12/04/96

3,430

11/06/98

179

6/18/98

5,740 2-5

Pemigewasset

Plymouth

01076500

Route 3/25

Pemigewasset

5.9

076/080

10/03/96

237

11/16/98

1,640

6/14/98

22,200 2-5

Souhegan River

Merrimack:

01094000

Route 13

Souhegan

3.8

123/133

8/11/96

54

11/20/98

76

10/22/96

6,260 5-10

Contoocook River

Henniker

01085000

I-89

Warner

8.5

166/103-4

12/04/96

3,430

11/06/98

179

6/18/98

5,740 2-5
8 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New

Hampshire,

1996-98

 

Table 4. Ground-penetrating-radar scour data for seven bridges in New Hampshire

[No., number]

Site Section of river and 

figure No.

Groundpenetrating 

radar survey

dates

Existing scour hole 

depth observed 

during study period, 

in feet, and location

New scour hole 

depth observed 

during study 

period, in feet, 

and location

Maximum 

discharge 

observed during 

study period, in 

cubic feet per 

second, and date

Infilling observed

during period, in 

feet, and location

Previous streambed 

surface depth below 

bed bottom, in feet, 

and location

Route 9 over Soucook River in Concord

Concord, Route 9 over 

Soucook River

Downstream cross-section 

(fig. 3)

8/13/96

11/5/98

1.3

Right bank pier

1.7

Right bank pier

1,890

3/10/98

0.5

Left bank pier

1-4

Left and right sides of 

section

Concord, Route 9 over 

Soucook River

Pier-set cross-section 

(fig. 4)

8/13/96

11/5/98

2.8

Column 5

2.0

Column 2

1,890

3/10/98

None 1-5

Between columns

Route 18 over Connecticut River in Littleton

Littleton, Route 18 over 

Connecticut River

Upstream cross-section 

(fig. 5)

8/26/96

11/10/98

2.5

Pier 2

None 42,000

4/1/98

2.0

Pier 2

None

Route 13 over Cold River in Sandwich

Sandwich, Route 13 over 

Cold River

Upstream cross-section 

(fig. 6)

8/12/97

11/13/98

None 3.0

Pier

6,150

1

6/14/98

None 6-13

Middle of river channel

Sandwich, Route 13 over 

the Cold River

Downstream cross-section 

(fig. 7)

8/12/97

11/13/98

None 1.2

Pier

6,150

1

6/14/98

None 6-9

Middle of river channel

Routes 202 and 9 over Contoocook River in Hopkinton

Hopkinton, Routes 202 and 

9 over Contoocook River

Upstream cross-section 

(fig. 8)

12/4/96

11/6/98

2.0

Right bank pier

None 5,740

2

6/18/98

2.0

Right bank pier

None

Interstate I-89 southbound over Warner River in Warner

Warner, I-89 southbound 

over Warner River

Upstream cross-section 

(fig. 9)

12/3/96

11/6/98

1.2

Middle of river channel

None 5,740

2

6/18/98

0.7

Middle of river 

channel

1-8

Middle of river channel

Interstate I-89 northbound over Warner River in Warner

Warner, I-89 northbound 

over Warner River

Downstream cross-section 

(fig. 10)

12/3/96

11/6/98

1.0

Right bank pier

None 5,740

6/18/98

1.0

Right bank pier

4

Middle of river channel
S

C

O

U

R ES

TIM

A

TES

A

T

B

RID

GE SITES 9

Routes 3 and 25 over Pemigewasset River in Ashland

Ashland, Routes 3 and 25 

over Pemigewasset River

Upstream cross-section 

(fig. 11)

10/3/96

11/16/98

3.0/1.0

Left channel/pier

3.5/2.0

Left channel/pier

22,200

6/14/98

2.0

Middle of river 

channel

6-8

Middle of river channel

Route 13 over Souhegan River in Milford

Milford, Route 13 over 

Souhegan River

Downstream cross-section 

(fig. 12)

8/11/96

11/20/98

2.0

Right bank pier

3.0

Right bank pier

6,260

10/22/96

None 2-3

Left bank pier

Milford, Route 13 over 

Souhegan River

Right bank pier cross-section

(fig. 13)

8/11/96

11/20/98

3.0/2.0

us pier/ds pier

4.0/4.0

us pier/ds pier

6,260

10/22/96

4.0

ds pier

None

1Maximum discharge for the Bearcamp River, Tamworth, 4.2 miles east of this site was 6,150 cubic feet per second on June 14, 1998.

2Maximum discharge for the Contoocook River, Henniker, 9.6 miles south of this site was 5,740 cubic feet per second on June 18, 1998.

Table 4. Ground-penetrating-radar scour data for seven bridges in New Hampshire—Continued

[No., number]

Site Section of river and 

figure No.

Groundpenetrating 

radar survey

dates

Existing scour hole 

depth observed 

during study period, 

in feet, and location

New scour hole 

depth observed 

during study 

period, in feet, 

and location

Maximum 

discharge 

observed during 

study period, in 

cubic feet per 

second, and date

Infilling observed

during period, in 

feet, and location

Previous streambed 

surface depth below 

bed bottom, in feet, 

and location
10 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

the right and left bank sides of the pile bents. The 

downstream bridge face and the right-bank pier cross 

sections were interpretable with GPR and were used 

for comparison of pre- and post-flood conditions. 

Water depths range from 1 to 5 ft during the study. 

The streambed materials at this site consist of fine-tocoarse sand with cobbles. Radar-signal-penetration 

depths ranged from 5 ft in water to 13.5 ft in 

streambed sediments. 

A stream-gaging station (01089100, table 3) is 

on the Soucook River 1.9 mi south of the bridge. Preflood GPR data were collected on May 20, 1996, 

(streamflow of 311 ft3/s, table 3) and post-flood GPR 

data were collected on November 5, 1998, (streamflow

of 17 ft3/s, table 3). The highest flood flow during this 

time interval was 1,890 ft3/s and was recorded on 

March 10, 1998. This flow corresponds to a 5- to 

10-year flood. 

Downstream Bridge-face Cross Section

The 1996 (pre-flood) cross section (fig. 3A) 

shows an undulating streambed of strong sand reflectors. Near the left-bank column a small existing scour 

hole measures about 1 ft deep. An existing scour hole 

(1.3 ft deep) is adjacent to the right-bank column. 

Previous scour surfaces (thin faint reflectors in the 

middle left and middle right of the section) are 1 to 

2.5 ft below the streambed. Depth to the top of the 

column footing in this cross section is 7 ft below the 

streambed.

The 1998 (post-flood) cross section (fig. 3B) 

shows a gradual smoothing of the streambed and 

infilling of the small existing scour hole near the leftbank column. This infilling measures 0.5 ft deep. The 

existing scour hole identified in the pre-flood section 

adjacent to the right-bank column now measures 1.7 ft 

deep. The previous scour surface near the right-bank 

column is easily identified in this section as a discontinuous thin subparallel reflector on the left and right 

sides of the record, and is 1.8 to 3.7 ft below the 

present streambed. The scour surface near the leftbank column may represent the potential maximum 

extent of scour that occurred during the intervening 

period (between pre- and post-flood measurements). 

The inverted triangular reflectors near the right-bank 

column are interference patterns from the concrete 

column. Depth to the top of the column footing in this 

cross section remains unchanged at about 7 ft.

Column-set Cross Section, Upstream to 

Downstream

The 1996 (pre-flood) cross section (fig. 4A) 

shows a strong sand reflector representing the 

streambed, across the entire record, in shallow water. 

This shallow reflector dips downward, from left to 

right, toward the downstream end of the record, ending

at an existing scour hole at column 5. The hole depth 

is 2.4 ft. The inverted triangular reflectors just to the 

left of column 2, and to the right of the scour hole at 

column 5, represent reflections off of exposed steelpile sections. Previous scour surfaces (thin faint 

reflectors beneath the hole in the middle right of the 

record) range from 1.6 to 4.8 ft beneath the streambed. 

Riprap protection of the pier footing is represented by 

a strong, thick, parallel-wavy reflector on the right side

of the record above the footing. The position of this 

reflector on the record agrees with measured elevation 

data from the bridge plans. Average depth to the top 

of the pier footing is about 10 ft.

The 1998 (post-flood) cross section (fig. 4B) 

shows the same strong sand reflector representing the 

streambed, from left to right across the entire record, 

but there is a new scour hole 1.2 ft deep and 4 ft wide 

located around column 2. The inverted triangular 

reflectors are still present near columns 2 and 5. The 

shallow streambed reflector follows the same 

downward trend toward the pre-existing scour hole at 

column 5. The hole at column 5 (2.4 ft deep) appears 

to have remained relatively stable. There is no 

evidence of infilling in the record (fig. 4B). The riprap 

protection above the pier footing is still represented in 

the record, but the thick parallel-wavy reflector pattern 

is more obscured in this record by thick wavy reflectors just above it. Average depth to the top of the 

footing remains the same at 10 ft.

Route 18 Bridge

The Route 18 bridge in Littleton crosses the 

450-ft-wide Connecticut River and is supported by 

four piers. Each pier is 6.5 ft wide and 31 ft long. 

Pre- and post-flood GPR data were collected at the 

upstream and downstream bridge faces and along both 

the right and left bank sides of each pier (fig. 5). The 

upstream bridge face cross section was used for 

comparison of pre- and post-flood conditions. Water 

depths ranged from 2 to 32 ft during the study. The 

streambed materials at this site consist of silt, 
SCOUR ESTIMATES AT BRIDGE SITES 11

Figure 3. Ground-penetrating-radar profiles of the downstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 

post-flood data collection, from left to right bank, Route 9 over the Soucook River, Concord, N.H.
12 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

Figure 4. Ground-penetrating-radar profiles of the pier-set cross sections for (A) 1996 pre-flood and (B) 1998 post-flood data 

collection, from upstream to downstream, Route 9 over the Soucook River, Concord, N.H.
SCOUR ESTIMATES AT BRIDGE SITES 13

Figure 5. Ground-penetrating-radar profiles of the upstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 

post-flood data collection, from left to right bank, Route 18 over the Connecticut River, Littleton, N.H.
14 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

fine-to-medium sand, and some boulders (New 

Hampshire Department of Transportation, 1992). 

Radar-signal-penetration depths were about 32 ft in 

water and 8.2 ft in streambed materials.

A stream-gaging station (01131500, table 3) is 

on the Connecticut River 10 mi north of the bridge. 

Pre-flood GPR data were collected on August 8, 1996, 

and the flow at the gaging station was 1,123 ft3/s. 

Post-flood GPR data were collected on November 10, 

1998, and the flow at the gaging station was 

1,500 ft3/s. The highest flood flow during this time 

interval was 42,000 ft3/s recorded on April 1, 1998. 

This flow corresponds to a 25- to 50-year flood. 

The 1996 (pre-flood) cross-section (fig. 5A) 

data were collected in water up to 32 ft deep. The 

GPR signal is attenuated by the water and only able to 

penetrate about 8.2 ft of the streambed sediments. At 

each pier, as seen on the record, a mound of streambed 

material represents the granite riprap protection that 

was installed to prevent streambed scour. At pier 2, 

2.5-ft-deep scour holes exist on both sides of the pier 

(fig. 5A). A small hole (2.3 ft deep) developed to the 

right of pier 3.

The 1998 (post-flood) cross section (fig. 5B) 

was collected in water somewhat shallower than in 

1996 (23 ft deep). The GPR signal is still attenuated 

(but to a lesser degree because the water is shallower) 

and is able to penetrate further into the streambed 

sediments. At pier 2, the hole configuration has 

changed; the scour holes have filled in (about 2 ft 

deep), but the overall depth of streambed material over 

the top of the footing is less (this result may represent 

the settlement of riprap as streambed material is 

scoured away from the toe of the riprap slope). At pier 

3, the mound of riprap is still identifiable on the 

record, and the hole to the right of the pier appears to 

have filled in with 1.2 ft of material. Previous scour 

surfaces were not identified because the radar signal 

was attenuated by the depth of the water and the finegrained streambed materials.

Route 13 Bridge

The Route 13 bridge in Sandwich crosses the 

Cold River and is 146 ft long by 32 ft wide, and has 

2 bridge spans. The river width is about 40 ft. The 

single pier is 3.5 ft wide and 40 ft long.

Pre- and post-flood GPR data were collected at 

the upstream and downstream bridge faces and along 

the right and left bank sides of the pier. The upstream 

and downstream cross sections were used for comparison of pre- and post-flood conditions. Observed 

water depths ranged from 1 to 4.2 ft during the study. 

The streambed materials at this site consist of coarse 

sand, gravel, cobbles, and boulders. Radar-signalpenetration depths were about 4.2 ft in water and 11 ft 

in streambed materials.

A stream-gaging station (01064801, table 3) is 

on the Bearcamp River 4.2 mi east of the bridge. Preflood GPR data were collected on August 12, 1997, 

and the flow at the gaging station was 15 ft3/s. Postflood GPR data were collected on November 13, 1998, 

and the flow at the gaging station was 129 ft3/s. The 

highest flood flow for this time period was 6,150 ft3/s 

and was recorded on June 14, 1998. This corresponds 

to a greater than 50- to 100-year flood.

Upstream Bridge-face Cross Section

The 1997 (pre-flood) cross-section data 

(fig. 6A) were collected in shallow water from 0 to 

2.5 ft deep. The section represents the right half of the 

bridge opening from the pier to the right bank. The 

radar signal did not penetrate the dry sand and cobble 

streambed materials in the left bridge-face opening. 

The streambed appears as a thick continuous reflector 

(in the top middle of the record) that trends downward 

to the deepest part of the streambed at an existing 

depression 2.2 ft deep. This bridge site restricts the 

flow of water during flood flows. This restriction 

causes contraction scour to occur (the 2.2-ft-deep 

depression in the center of the channel). No scour 

holes were observed during this survey. However, thin

wavy reflectors represent previous scour surfaces of 

unknown age about 8 ft below the streambed (fig. 6A). 

The entire right half of the record shows chaotic wavy 

thin reflectors. This is a cobble-boulder section of the 

streambed.

The 1998 (post-flood) cross-section data 

(fig. 6B) were collected in deeper water (up to 4.4 ft 

deep) than in 1997. Similar to the upstream record in 

figure 6A, data for the entire bridge opening were 

collected but only the data for the right bridge face 

opening are shown for comparison to the 1997 data. 

A scour hole about 2 ft deep exists in front of the pier. 

The depth to the top of the pier footing is about 2 ft 

below the streambed. Beneath this hole, parallel 

strong reflectors represent the top of the pier footing. 

Large boulders (riprap) to the right of the pier are 
SCOUR ESTIMATES AT BRIDGE SITES 15

Figure 6. Ground-penetrating-radar profiles of the upstream bridge-face cross sections for (A) 1997 pre-flood and (B) 1998 

post-flood data collection, from pier to right bank, Route 13 over the Cold River, Sandwich, N.H.
16 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

represented by a mound of faint chaotic reflectors. 

The depression in the middle of the channel is 0.6 ft 

deeper than the depression in the 1997 record. This 

depression could be the result of continued contraction 

scour processes occurring during flood flows. 

Previous scour surfaces apparent in the 1997 data 

(fig. 6) are less readily identifiable; they are obscured 

by the interfering inverted triangular reflection 

signature of boulders on the streambed. The right half 

of the record is similar to the 1997 record; chaotic 

wavy reflectors are the dominant signature. 

Streambed-bottom materials on this side of the bridge 

opening appear to be protected from scour processes 

by large boulders. 

Downstream Bridge-face Cross Section

The 1997 (pre-flood) cross-section data 

(fig. 7A) were collected in water from 0 to 2 ft deep. 

The section represents the right half of the bridge 

opening from the pier to the right bank. The radar 

signal did not penetrate the dry sand and cobble 

streambed materials in the left bridge-face opening. 

The streambed appears as a thick continuous reflector 

on the left side of the record trending downward to the 

right. No scour holes were observed in this survey, 

however, thin wavy reflectors represent previous scour

surfaces that are about 6 to 9 ft below the streambed 

and up to 2 ft below the bottom of the footing. On the 

right side, cobbles, boulders, and a coarse sand 

streambed produce chaotic and thin wavy reflection 

patterns. There is approximately 7 ft of streambed 

material above the footing.

The 1998 (post-flood) cross-section data 

(fig. 7B) were collected in water up to about 3 ft deep. 

Similar to the upstream record (fig. 6), data for the 

entire bridge opening were collected but only the data 

for the right bridge-face opening are shown for 

comparison to the 1997 data. The thick, continuous 

reflector sloping toward the right (shown in the 1997 

data), is replaced in 1998 by a discontinuous flat-lying 

reflector (streambed materials have been removed and 

boulders have moved into the section). A new scour 

hole has formed (1.2 ft deep) to the right of the pier. 

Interference patterns from cobbles and boulders 

(inverted triangles) fill much of the record; field 

observations confirm the movement of sand out of the 

section and movement of riprap protection material 

from the upstream bridge-face area to the downstream 

bridge-face area during a recent flood. Thin wavy 

reflectors representing previous scour surfaces are still 

present at about 6 to 9 ft below the streambed. There 

is 6 ft of streambed material above the footing; this 

result indicates that 1 ft of material has been removed 

from the downstream nose of the pier.

Routes 202 and 9 Bridge

The Routes 202 and 9 bridge in Hopkinton that 

crosses the Contoocook River is 312 ft long by 60 ft 

wide, and has 2 piers. The river is approximately 

180 ft wide, and each pier is 5 ft wide and 70 ft long.

Pre- and post-flood GPR data were collected at 

the upstream and downstream bridge faces and along 

the right and left bank sides of both piers. The 

upstream bridge-face cross section was used for 

comparison of pre- and post-flood conditions. 

Observed water depths ranged from 4 to 13 ft during 

the study. The streambed materials at this site consist 

of medium-grained sand (New Hampshire Department 

of Transportation, 1992). Radar-signal-penetration 

depths were 13 ft in water and 4 ft in streambed 

materials.

A stream-gaging station (01085000, table 3) is 

on the Contoocook River 5.9 mi west of the bridge. 

Pre-flood GPR data were collected on December 4, 

1996, and the flow at the gaging station was 

3,430 ft3/s. Post-flood GPR data were collected on 

November 6, 1998, and the flow at the gaging station 

was 179 ft3/s. The highest flood flow for this time 

period was 5,740 ft3/s and was recorded on 

June 18, 1998. This flood flow corresponds to a 

2 to 5-year flood.

The 1996 (pre-flood) cross-section data 

(fig. 8A) were collected in 2 to 13 ft of water. The 

streambed is represented by a continuous thick wavy 

reflector. Below the streambed, thin subparallel 

discontinuous reflectors represent layering of finegrained sediments (silt or sand) and inverted triangular 

interference patterns represent coarse-grained 

materials (cobbles or boulders). At the left bank pier, 

there is 2.5 ft of streambed material above the top of 

the pier footing. An existing 2-ft-deep scour hole is at 

the front-left side of the right bank pier.

 The 1998 (post-flood) cross section (fig. 8B) 

shows a bed-bottom geometry similar to the 1996 data. 

However, 0.75 ft of infilling has occurred at the leftbank pier and there is now 2 ft of infilled material 

above the right-bank pier footing. The thin subparallel 
SCOUR ESTIMATES AT BRIDGE SITES 17

Figure 7. Ground-penetrating-radar profiles of the downstream bridge-face cross sections for (A) 1997 pre-flood and (B) 1998 

post-flood data collection, from pier to right bank, Route 13 over the Cold River, Sandwich, N.H.
18 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

Figure 8. Ground-penetrating-radar profiles of the upstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 

post-flood data collection, from left to right bank, Routes 202 and 9 over the Contoocook River, Hopkinton, N.H.
SCOUR ESTIMATES AT BRIDGE SITES 19

reflectors beneath the streambed in the center of the 

record are not as distinct as the 1996 data; they are 

obscured somewhat by the inverted triangular 

signature of the cobble-boulder sublayer. No previous 

scour surfaces were observed in either the pre- or postflood surveys at this bridge.

Interstate 89 Southbound Bridge

The Interstate 89 Southbound bridge in Warner 

crosses the 90-ft-wide Warner River, is 190 ft long by 

52 ft wide, and has 2 piers. Each pier is 3 ft wide and 

68 ft long.

Pre- and post-flood GPR data were collected at 

the upstream and downstream bridge faces and along 

both the right and left bank sides of the two piers. 

The upstream bridge-face cross section was used for 

comparison of pre- and post-flood conditions. 

Observed water depths ranged from 2 to 10 ft during 

the study. The streambed materials at this site consist 

of sand and gravel. Radar-signal-penetration depths 

ranged from 6 ft in water and 20 ft in substreambed 

materials.

A stream-gaging station (01085000, table 3) is 

on the Contoocook River 9.6 mi south of the bridge. 

This gaging station was used as an indicator for 

potential flood-flows on the Warner River (fig. 1). 

Even though the gage is not on the Warner River, the 

flows are similar. Pre-flood GPR data were collected 

on December 4, 1996, and the flow at the Contoocook 

River gaging station was 3,430 ft3/s. A comparison of 

drainage areas (Davidian, 1984) provided an estimate 

of 1,268 ft3/s at the Warner River site. Post-flood GPR

data were collected on November 6, 1998, and the 

flow at the Contoocook River gaging station was 

179 ft3/s. The estimated discharge at the Warner River 

site was 66 ft3/s. The highest flood flow for the 

Contoocook River in Henniker during this time 

interval was 5,740 ft3/s and was recorded on 

June 18, 1998. This result corresponds to a 2- to 

5-year flood.

The 1996 (pre-flood) cross section (fig. 9A) 

shows a thin wavy streambed reflector extending 

across the entire record. An existing 1.2 ft scour hole 

is in the middle of the river channel. A thin wavy 

subparallel reflector extends across the middle center 

of the record and represents a multiple reflection of the 

streambed. Previous scour surfaces of sand or sand 

and gravel appear as thin subparallel sloping reflectors 

in the center of the record at 1 to 3 ft below the 

streambed. Thin dense wavy reflectors 9 ft below the 

streambed at the left bank pier represent the top of the 

riprap protection for the left-bank pier footing. 

The 1998 (post-flood) cross section (fig. 9B) 

shows a similar thin parallel streambed reflector across 

the entire record but the streambed is now a smooth 

surface. Approximately 0.7 ft of material infilled the 

middle of the river channel. A thin subparallel 

reflector extends across the top-middle of the record 

and represents a similar multiple to that shown in the 

1996 record. Previous scour surfaces of sand or sand 

and gravel appear as discontinuous thin wavy reflectors (obscured somewhat by multiple reflectors of the 

streambed) 7.5 ft below the streambed. These scour 

surfaces may represent the potential maximum extent 

of scour that occurred during the 23 months between 

measurements.

Interstate 89 Northbound Bridge

The Interstate 89 Northbound bridge in Warner 

crosses the 90-ft-wide Warner River, is 190 ft long by 

52 ft wide, and has 2 piers. Each pier is 3 ft wide and 

68 ft long.

Pre- and post-flood GPR data were collected at 

the upstream and downstream bridge faces and along 

the right and left bank sides of the two piers. The 

downstream bridge-face cross section was used for 

comparison of pre- and post-flood conditions. 

Observed water depths ranged from 2 to 10 ft during 

the study. The streambed materials at this site consist 

of sand and gravel. Radar-signal-penetration depths 

were as great as 10 ft in water and 20 ft in streambed 

materials.

A stream-gaging station (01085000, table 3) is 

on the Contoocook River 9.6 mi south of the bridge. 

This gaging station was used as an indicator for 

potential flood-flow events on the Warner River even 

though the gage is not on the Warner River. Pre-flood 

GPR data were collected on December 4, 1996, with a 

flow rate of 3.430 ft3/s. The highest flow of 5,740 ft3/s 

was recorded on June 18, 1998. This flow corresponds

to a 2- to 5-year flood.

The 1996 (pre-flood) cross section (fig. 10A) 

shows a thin wavy streambed reflector extending 

across the entire record. A small existing scour hole 

(1 ft deep) is on the left side of the right pier. A thin 

wavy parallel reflector extends across the middle 
20 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

Figure 9. Ground-penetrating-radar profiles of the upstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 

post-flood data collection, from left to right bank, Interstate 89 southbound over the Warner River, Warner, N.H.
SCOUR ESTIMATES AT BRIDGE SITES 21

Figure 10. Ground-penetrating-radar profiles of the downstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 

post-flood data collection, from left to right bank, Interstate 89 northbound over the Warner River, Warner, N.H.
22 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

center of the record 11.3 ft below the streambed and 

represents a multiple reflection of the streambed. 

Previous scour surfaces of sand or sand and gravel 

appear as multiple thin wavy reflectors at 3.8 ft below 

the streambed across the entire record. Boulder 

protection of the pier footing is identified on the record 

as chaotic wavy reflectors at both piers 11 ft below the 

streambed.

The 1998 (post-flood) cross section (fig. 10B) 

shows the same thin wavy streambed reflector present 

in the 1996 record extending across the entire record. 

The thin wavy parallel reflector extending across the 

middle center of the record represents the same 

multiple reflection of the streambed. Previous scour 

surfaces of sand or sand and gravel appear as similar 

multiple thin wavy reflectors at between 2 and 10 ft 

below the streambed. The deepest scour surfaces 

could potentially represent the potential maximum 

extent of scour that occurred during the intervening 

period between measurements. There are only subtle 

changes in the streambed; a flat streambed at leftcenter changes to a cobble-boulder streambed on the 

right side of the record, and the small existing scour 

hole identified in the pre-flood section has been 

infilled with 1 ft of streambed material.

Routes 3 and 25 Bridge

The Routes 3 and 25 bridge in Ashland crosses 

the Pemigewasset River and is 800 ft long, 24 ft wide, 

and has 6 piers. The flood plain is 500 ft wide. Only 

one of the 6 piers is in the 300-ft-wide river. This 

1 pier is 8 ft wide and 32 ft long.

Pre- and post-flood GPR data were collected at 

the upstream and downstream bridge faces and along 

the right and left bank sides of the pier. The upstream 

bridge-face cross section was used for comparison of 

pre- and post-flood conditions. Observed water depths

during the study ranged from less than 1 to 6 ft. The 

streambed materials at this site consist of fine-tomedium sand and cobbles. Radar-signal-penetration 

depths were as great as 6 ft in water and 7 ft in 

streambed materials.

A stream-gaging station (01076500, table 3) is 

3.8 mi north of the bridge. Pre-flood GPR data were 

collected on October 3, 1996, and the flow at the 

gaging station was 237 ft3/s. Post-flood GPR data 

were collected on November 16, 1998, and the flow at 

the gaging station was 1,640 ft3/s. The highest flood 

flow for this time period was 22,200 ft3/s and was 

recorded on June 14, 1998. This flow corresponds to a 

2- to 5-year flood.

The 1996 (pre-flood) cross-section (fig. 11A) 

data were collected in water 0 to 3 ft deep. An 18 ftwide area around the center pier consisted of a sand 

and gravel bar (less than 0.5 ft water depth) and is 

represented by the flat dark low-frequency band at the 

top-center of the record. Field observations of 

streambed features (ripples, elongated scour channels, 

sand bars) indicate that the streambed consisting of 

sands and gravels is mobile at this bridge site. The 

streambed is represented by the thick subparallel 

reflector that forms a 3-ft-deep hole in the middle of 

the channel on the left side of the pier and a 2-ft-deep 

hole in the middle of the channel on the right side of 

the pier. A 1-ft-deep scour hole exposes a steel pile on 

the left side of the pier. Infilling of sands and gravels 

(2 ft thick) is represented by the highly reflective wavy 

bands in both river bed channels.

The 1998 (post-flood) cross-section (fig. 11B) 

data were collected in water 1-5 ft deep. (The sand 

and gravel bar around the center pier is now 

submerged under 1 ft of water). The streambed is 

represented by the same subparallel reflector identified

in the 1996 record (fig. 11A). A 6-ft-deep depression 

exists in the river channel left of center in the record. 

Contraction scour may be occurring between the pier 

and the left bank and the scour depression may be the 

result of this contraction. A new 3-ft-deep scour hole 

exposes the steel pile on the left side of the pier. This 

scour hole is 2 ft deeper than the hole identified in the 

1996 data. The wavy chaotic streambed reflectors to 

the right of the pier represent an infilling of sand and 

gravel about 2 ft thick. Previous scour surfaces exist 

below the infilled material on the right side of the 

record and may represent the potential maximum 

extent of scour that occurred during the intervening 

period between measurements.

Route 13 Bridge

The Route 13 bridge in Milford crosses the 

Souhegan River and is 112 ft long and 50 ft wide. This 

bridge has one concrete and stone pier that is 10 ft 

wide and 60 ft long. The river width is about 100 ft. 

Pre- and post-flood GPR data were collected at 

the upstream and downstream bridge faces and 

along the right and left bank sides of the pier. The 
SCOUR ESTIMATES AT BRIDGE SITES 23

Figure 11. Ground-penetrating-radar profiles of the upstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 

post-flood data collection, from left to right bank, Routes 3 and 25 over the Pemigewassett River, Ashland, N.H.
24 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

downstream bridge-face cross section and the right 

bank side-pier cross section were used for comparison 

of pre- and post-flood conditions. Observed water 

depths ranged from 1 to 8 ft during the study. The 

streambed materials at this site consist of silt, fine-tomedium sand, and some boulders. Radar-signalpenetration depths were about 8 ft in water and about 

6 ft in streambed materials.

A stream-gaging station (01094000, table 3) is 

8.5 mi east of the bridge. Pre-flood GPR data were 

collected on August 11, 1996, and the flow at the 

gaging station was 54 ft3/s. Post-flood GPR data were 

collected on November 20, 1998, and the flow at the 

gaging station was 76 ft3/s. The highest flood flow for 

this time period was 6,260 ft3/s and was recorded on 

October 22, 1996. This flow corresponds to a 5- to 

10-year flood.

Downstream Bridge-face Cross Section

The 1996 (pre-flood) cross section (fig. 12A) 

shows an irregular streambed represented by a thin 

parallel reflector. A 5-ft-deep channel exists on the 

left side of the record. Chaotic inverted triangular 

reflectors dominate the right side of the record and are 

evidence of boulders in the streambed. A previous 

scour surface on the left side of the pier is represented 

by a thin diagonal reflector 0 to 4 ft below the 

streambed and is overlain by more recent infilled sand 

of the current streambed. A small existing 2-ft-deep 

scour hole is to the right of the pier.

The 1998 (post-flood) cross section (fig. 12B) is 

similar to the 1996 record with some differences. On 

the left side of the record, the scour channel is 1.5 ft 

below the bottom of the footing and almost 1 ft deeper 

than the 1996 scour channel, indicating that sand was 

scoured out of the section. The previous scour surface 

in the top center of the record is still identifiable and 

the sand-bed material above this surface to the left of 

the pier has shifted slightly to the right accumulating 

over the pier footing. This scour surface may 

represent the potential maximum extent of scour that 

occurred during the intervening period between 

measurements. The small existing scour hole to the 

right of the pier and the chaotic reflector signature of 

the streambed boulders on the right side of the record 

remain unchanged.

Right-bank-pier Cross Section

The 1996 (pre-flood) cross section (fig. 13A) 

shows an irregular streambed represented by thin wavy

parallel reflectors. An existing 3-ft-deep scour hole 

(hole 1) is at the upstream center section of the 

footing. Inverted triangular reflectors dominate the 

center of the record and represent interference patterns 

from boulders present in the streambed. Two existing 

scour holes in the streambed (holes 2 and 3) are 1 and 

2 ft deep, respectively, and are at the downstream 

center section of the footing. A strong subparallel 

reflector is across the bottom third of the record and 

represents a multiple reflection of the streambed. 

Radar-signal penetration in streambed sediments is 

blocked by the presence of boulders in the streambed.

The 1998 (post-flood) cross section (fig. 13B) 

shows a similar irregular streambed represented by 

thin wavy parallel reflectors. An additional 1 ft of 

scour has occurred at hole 1 identified in the 1996 

section (left-center at the footing upstream end) and 

extends downward slightly past the top of the pier 

footing. The extent of the two existing scour holes at 

the downstream center section of the footing also have 

changed. Existing scour hole 2 is now 4 ft deep (2 ft 

deeper than in 1996) and hole 3 has been infilled. A 

previous scour surface now exists at hole 3 and is 

overlain by 4 ft of new streambed materials.
SCOUR ESTIMATES AT BRIDGE SITES 25

Figure 12. Ground-penetrating-radar profiles of the downstream bridge-face cross sections for (A) 1996 pre-flood and (B) 1998 

post-flood data collection, from left to right bank, Route 13 over the Souhegan River, Milford, N.H.
26 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

Figure 13. Ground-penetrating-radar profiles of the right-bank pier cross section for (A) 1996 pre-flood and (B) 1998 post-flood 

data collection, from upstream to downstream, Route 13 over the Souhegan River, Milford, N.H.
SUMMARY AND CONCLUSIONS 27

SUMMARY AND CONCLUSIONS

 Ground-penetrating-radar (GPR) techniques 

were used to detect pre- and post-flood scour from 

1996-98 at bridge sites in New Hampshire in a cooperative study with the New Hampshire Department of 

Transportation. The 1996 pre-flood GPR crosssection data were collected at 30 bridges. The 1998 

post-flood GPR cross-section data were collected at 

16 bridges where GPR data were interpretable. Of the 

post-flood data collected at 16 bridges, 7 sites and 

22 profiles were selected for this report. Post-flood 

data-collection efforts at the bridges were based on the 

occurrence of a 2-year or greater flood recurrence 

interval at any of the bridge sites. 

The geophysical equipment was operated from a

small inflatable boat. The boat was easily maneuvered 

by an operator wading beside it and guiding it in 

shallow water. In deep water, an electric trolling 

motor was used to propel and steer the boat by an 

operator inside the boat. This method of equipment 

deployment was fast, safe, and easily transportable 

from the vehicle to the river’s edge by one person. 

Geophysical record was collected at one bridge per 

day.

Water depth and streambed materials affected 

the depth of penetration of the GPR signal. Shallow 

water (less than 3 ft) had a small affect on signal 

penetration of streambed materials, whereas deep 

water (10 ft or greater) attenuated the signal 

preventing signal penetration beyond 1-2 ft of the 

streambed. Cobbles and boulder streambeds stopped 

signal penetration and caused the record to become 

obscured with multiple inverted triangular reflections.

After data collection, the geophysical record 

was processed using commercially available software 

and correlated with data contained in New Hampshire 

Department of Transportation bridge plans. These 

data supplemented the geophysical data and aided in 

the interpretation of depths and locations of footings 

and geologic materials.

Twenty-two pre- and post-flood cross sections at

seven bridge sites are presented and discussed. 

Existing 1996 pre-flood data collection scour holes 

were detected in 14 cross sections. Scour depths 

ranged from 1 to 3 ft. New, 1998 post-flood datacollection scour holes were detected in seven cross 

sections. Scour depths ranged from greater than 1 to 

4 ft. Infilled scour holes were detected in seven of the 

post-flood cross sections. Depths of infilling ranged 

from less than 1 to 4 ft. Field verification of the depth 

of infilling by means of steel rod and hammer was 

difficult because of cobble and boulder streambeds or 

deep water. Previous scour surfaces in streambed 

materials are identified in 15 cross sections and the 

depths to these surfaces ranged from 1 ft to 10 ft below 

the streambed. 

Riprap materials or pier footings were identified 

in all 22 cross sections and calculated record depths 

generally agree with bridge plans. Pier footings were 

exposed at two bridge sites and steel pile was exposed 

at one bridge site. Exposures were verified by field 

observations.

GPR was found to be an effective tool for 

detecting existing scour holes, infilled scour holes, and

previous scour surfaces at bridge sites. The use of preand post-flood GPR surveys was effective in 

measuring the potential maximum extent of scour that 

occurred during the intervening period between 

measurements. If this scour occurred during the 

highest flow in the intervening period, pre- and postflood GPR can be effective in maximizing the number 

of scour measurements at bridges during a flood event. 

Thus, the use of GPR permits augmentation of scourdata collection that is limited by the number of mobile 

teams or fixed instrumentation available during floods. 

GPR also is more versatile than fixed instrumentation 

mounted on bridge piers because data collection is not 

confined to the fixed-instrument location. The GPR 

instrument is mobile and can be positioned anywhere 

beneath the bridge to collect continuous streambed 

data in a cross-sectional format. Care must be taken to 

position the equipment to follow the same traverse 

paths for comparison of pre- and post-flood measurements.
28 Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 

1996-98

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Beres, Milan, Jr., and Haeni, F.P., 1991, Application of 

ground-penetrating-radar methods in hydrogeologic 

studies: Ground Water, v. 29, no. 3, p. 375-386.

Davidian, J., 1984, Computation of water-surface profiles in 

open channels: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 3, chap. A15, 

48 p.

Geophysical Survey Systems, Inc., 1994, SIR System-2 

Operation Manual #MN72-140: Hudson, N.H., 20 p.

———1995, Radan for Windows Manual #MN43-116: 

Hudson, N.H., 55 p.

Haeni, F.P., 1995, Use of ground-penetrating radar and 

continuous seismic-reflection profiling on surfacewater bodies in environmental and engineering studies 

in Bell, R.S. (ed.), Symposium on the application of 

geophysics to engineering and environmental 

problems, Oak Brook Illinois, April 26-29, 1992, 

Proceedings: Golden, Colo., Society of Engineering 

and Mineral Exploration Geophysicists, p. 145-162.

New Hampshire Department of Transportation, 1992, Final 

report for bridge scour evaluation: Whitman and 

Howard, Inc., November, 208 p.

Placzek, Gary, and Haeni, F.P., 1995, Surface-geophysical 

techniques used to detect existing and infilled scour 

holes near bridge piers: U.S. Geological Survey 

Water-Resources Investigations Report 95-4009, 44 p.

Richardson, Dr. E.V., Harrison, L.J., Richardson, Dr. J.R., 

and Davis, S.R., 1993, Evaluating scour at bridges: 

National Highway Institute, Federal Highway 

Administration Technical Report HEC-18, PB93-

186138, 244 p.

Whitman and Howard, Inc., 1992, Final report for bridge 

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11366, 214 p.