USGS


Use of a Ground-Penetrating Radar System to Detect Pre- and Post-Flood Scour at Selected Bridge Sites in New Hampshire, 1996-98

Water-Resources Investigations Report 00-4035

 

By Joseph R. Olimpio

 

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 re-surveyed 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 ground-penetrating-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.

CONTENTS

Abstract

Introduction

Purpose and Scope

Description of the Study Area

Use of Terms

Acknowledgments

Ground-penetrating Radar

Equipment

Data-collection Methods

Scour Estimates at Bridge Sites

Route 9 Bridge

Downstream Bridge-face Cross Section

Column-set Cross Section, Upstream to Downstream

Route 18 Bridge

Route 13 Bridge

Upstream Bridge-face Cross Section

Downstream Bridge-face Cross Section

Routes 202 and 9 Bridge

Interstate 89 Southbound Bridge

Interstate 89 Northbound Bridge

Routes 3 and 25 Bridge

Route 13 Bridge

Downstream Bridge-face Cross Section

Right-bank-pier Cross Section

Summary and Conclusions

References Cited

FIGURES

  1. Map showing the location of ground-penetrating-radar data-collection sites at seven bridge sites in New Hampshire
  2. Photograph of ground-penetrating radar data-collection equipment
  3. -13. Figures showing ground-penetrating-radar profiles of the:
  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. 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.
  11. 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.

TABLES

  1. Selected descriptive characteristics of seven bridge sites in New Hampshire
  2. Approximate ground-penetrating radar two-way travel times for selected materials
  3. Flood-flow data at stream-gaging stations near seven bridges in New Hampshire.
  4. Ground-penetrating-radar scour data for seven bridges in New Hampshire.

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