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USGS2014

The 1946 Unimak Tsunami Earthquake Area: revised tectonic structure in reprocessed seismic images and a suspect near field tsunami source

In 1946 at Unimak Pass, Alaska, a tsunami destroyed the lighthouse at Scotch Cap, Unimak Island, took 159 lives on the Hawaiian Islands, damaged island coastal facilities across the south Pacific, and destroyed a hut in Antarctica. The tsunami magnitude of 9.3 is comparable to the magnitude 9.1 tsunami that devastated the Tohoku coast of Japan in 2011. Both causative earthquake epicenters occurred in shallow reaches of the subduction zone. Contractile tectonism along the Alaska margin presumably generated the far-field tsunami by producing a seafloor elevation change. However, the Scotch Cap lighthouse was destroyed by a near-field tsunami that was probably generated by a coeval large undersea landslide, yet bathymetric surveys showed no fresh large landslide scar. We investigated this problem by reprocessing five seismic lines, presented here as high-resolution graphic images, both uninterpreted and interpreted, and available for the reader to download. In addition, the processed seismic data for each line are available for download as seismic industry-standard SEG-Y files. One line, processed through prestack depth migration, crosses a 10 × 15 kilometer and 800-meter-high hill presumed previously to be basement, but that instead is composed of stratified rock superimposed on the slope sediment. This image and multibeam bathymetry illustrate a slide block that could have sourced the 1946 near-field tsunami because it is positioned within a distance determined by the time between earthquake shaking and the tsunami arrival at Scotch Cap and is consistent with the local extent of high runup of 42 meters along the adjacent Alaskan coast. The Unimak/Scotch Cap margin is structurally similar to the 2011 Tohoku tsunamigenic margin where a large landslide at the trench, coeval with the Tohoku earthquake, has been documented. Further study can improve our understanding of tsunami sources along Alaska’s erosional margins.

Alaska↗
USGS1997

Level II scour analysis for Bridge 125 (MIDBUS00070125) on U.S. Route 7 crossing the Middlebury River, Middlebury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure MIDBUS00070125 on U.S. Route 7 crossing the Middlebury River, Middlebury, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the Green Mountain section of the New England physiographic province in west-central Vermont. The 46.8-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover consists of row crops on the right overbank upstream and downstream, and trees on the left overbank. In the study area, the Middlebury River has a straight channel with a slope of approximately 0.005 ft/ft, an average channel top width of 77 ft and an average channel depth of 4 ft. The predominant channel bed materials are sand and cobbles with a median grain size (D 50 ) of 59.4 mm (0.195 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 18, 1996, indicated that the reach was stable. The U.S. Route 7 crossing of the Middlebury River is a 202-ft-long, two-lane bridge consisting of one 91-foot, and two 55-foot steel-beam spans (Vermont Agency of Transportation, written communication, December 14, 1995). The bridge is supported by vertical, concrete abutment walls with spill-through embankments. The channel is skewed approximately 45 degrees to the opening while the opening-skew-to-roadway is 45 degrees. The scour protection measures at the site were type-2 stone fill (less than 36 inches diameter) on the spill-through embankments of each abutment and type-1 stone fill (less than 12 inches diameter) on the right bank upstream. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.0 to 1.2 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.5 to 11.0 ft. The worst-case abutment scour occurred at the 500-year discharge at the left abutment. Pier scour ranged from 8.3 to 15.9 ft. for each modeled discharge. The worst-case pier scour occurred at the 500-year discharge. In this report, piers are numerically designated “1” and “2” for the left and right piers respectively. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 4 (ARLITH00010004) on Town Highway 1, crossing Warm Brook, Arlington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ARLITH00010004 on Town Highway 1 crossing Warm Brook, Arlington, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the Taconic section of the New England physiographic province in southwestern Vermont. The 12.1-mi 2 drainage area consists of a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is brush except for the upstream and downstream right banks which are covered by brush and grass. In the study area, Warm Brook has an incised, straight channel with a slope of approximately 0.003 ft/ft, an average channel top width of 19 ft and an average bank height of 1 ft. The channel bed material ranges from sand to cobble with a median grain size (D 50 ) of 33.3 mm (0.109 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 30, 1996, indicated that the reach was stable. The Town Highway 1 crossing of Warm Brook is a 49-ft-long, two-lane bridge consisting of one 44-foot steel-beam span (Vermont Agency of Transportation, written communication, January 30, 1996). The bridge is supported by vertical, concrete abutments with wingwalls. The abutments have been placed on top of the previous stone abutments. The channel is skewed approximately 0 degrees to the opening while the opening-skew-to- roadway is 20 degrees. A scour hole approximately 1.0 ft deeper than the mean thalweg depth was observed mid- channel in the upstream reach within 30 ft of the bridge. The only scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) along the upstream left bank approach to the bridge. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.0 to 1.7 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.3 to 11.9 ft. The worst-case abutment scour also occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 23 (WOLCTH00130023) on Town Highway 13, crossing the Wild Branch of the Lamoille River, Wolcott, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure WOLCTH00130023 on Town Highway 13 crossing the Wild Branch Lamoille River, Wolcott, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, collected from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the Green Mountain section of the New England physiographic province in northcentral Vermont. The 27.7-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture on the upstream right overbank. The upstream left overbank is brushland. Downstream of the bridge, the surface cover is forested on the right overbank. The downstream left overbank is pasture while the immediate bank has dense woody vegetation. In the study area, the Wild Branch Lamoille River has an incised, straight channel with a slope of approximately 0.009 ft/ft, an average channel top width of 65 ft and an average bank height of 7 ft. The channel bed material ranges from sand to boulders with a median grain size (D 50 ) of 85.3 mm (0.280 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 17, 1996 indicated that the reach was laterally unstable. The Town Highway 13 crossing of the Wild Branch Lamoille River is a 41-ft-long, one-lane bridge consisting of a 39-foot steel girder span (Vermont Agency of Transportation, written communication, October 13, 1995). The opening length of the structure parallel to the bridge face is 38 ft. The bridge is supported by vertical, concrete abutments. The right abutment has concrete wingwalls. The channel is skewed approximately 45 degrees to the opening while the opening-skew-to-roadway is zero degrees. A scour hole 3.5 ft deeper than the mean thalweg depth was observed in the channel during the Level I assessment. Scour countermeasures at the site includes type-2 stone fill (less than 3 feet diameter) along the banks, the right wingwalls, the right abutment and the road embankments. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 1.0 to 2.1 ft. The worst-case contraction scour occurred at the 100-year discharge. Left abutment scour ranged from 9.1 to 13.2 ft. Right abutment scour ranged from 15.7 to 22.3 ft. The worst-case abutment scour occurred at the 500- year discharge for both abutments. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. During the August 1995 flood, the Wild Branch Lamoille River overtopped the bridge deck at structure WOLCTH00130023. Debris also was caught in the upstream I-beam of the structure. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 5 (MORRTH00060005) on Town Highway 6, crossing Bedell Brook, Morristown, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure MORRTH00060005 on Town Highway 6 crossing Bedell Brook, Morristown, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the Green Mountain section of the New England physiographic province in north-central Vermont. The 6.28-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover consists of pasture, shrubs, and brushland. In the study area, Bedell Brook has a sinuous channel with a slope of approximately 0.01 ft/ ft, an average channel top width of 56 ft and an average bank height of 4 ft. The predominant channel bed material is gravel with a median grain size (D 50 ) of 35.8 mm (0.117 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 16, 1996, indicated that the reach was laterally unstable. There are wide point bars and cut-banks with slipping bank material noted upstream and downstream of this site. The Town Highway 6 crossing of Bedell Brook is a 44-ft-long, two-lane bridge consisting of one 42-foot concrete T-beam span (Vermont Agency of Transportation, written communication, October 26, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 45 degrees to the opening while the opening-skew-to-roadway is zero degrees. A scour hole up to 1.5 ft deeper than the mean thalweg depth was observed along the left abutment and upstream and downstream left wingwalls during the Level I assessment. The scour protection measure at this site was type-4 stone fill (less than 60 inches diameter) on the left bank and left wingwall upstream, the left abutment and the downstream left wingwall. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 1.1 to 2.0 feet. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 3.9 to 8.6 feet. The worst-case abutment scour occurred at the 500-year event. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 33 (TUNBTH00450033) on Town Highway 45, crossing the First Branch White River, Tunbridge, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure TUNBTH00450033 on Town Highway 45 crossing the First Branch White River, Tunbridge, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the New England Upland section of the New England physiographic province in central Vermont. The 86.4-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture upstream and downstream of the bridge, while woody vegetation sparsely covers the immediate banks. In the study area, the First Branch White River has an incised, sinuous channel with a slope of approximately 0.003 ft/ft, an average channel top width of 68 ft and an average bank height of 7 ft. The channel bed material ranges from sand to gravel with a median grain size (D 50 ) of 27.1 mm (0.089 ft). The geomorphic assessment at the time of the Level I and Level II site visit on October 18, 1995, indicated that the reach was laterally unstable due to a cut-bank present on the upstream right bank and a wide channel bar in the upstream reach. The Town Highway 45 crossing of the First Branch White River is a 67-ft-long, one-lane bridge consisting of one 54-foot timber thru-truss span (Vermont Agency of Transportation, written communication, March 23, 1995). The opening length of the structure parallel to the bridge face is 53.5 ft. The bridge is supported on the right by a vertical, concrete abutment with an upstream wingwall, and on the left by a vertical, stone abutment. The channel is skewed approximately 20 degrees to the opening while the computed opening-skew-to-roadway is 10 degrees. A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the right abutment during the Level I assessment. Scour countermeasures at the site include type-1 stone fill (less than 12 inches diameter) along the upstream right wingwall, type-2 stone fill (less than 36 inches diameter) along the right abutment, and type-3 stone fill (less than 48 inches diameter) along the upstream right bank. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995) for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping discharge was determined and analyzed as another potential worst-case scour scenario. Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.0 to 3.0 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 12.8 to 31.0 ft. Right abutment scour ranged from 9.8 to 19.0 ft. The worst-case left and right abutment scour occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in Tables 1 and 2. A cross-section of the scour computed at the bridge is presented in Figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1998

Level II scour analysis for Bridge 13 (LINCTH00010013) on Town Highway 1, crossing Cota Brook, Lincoln, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure LINCTH00010013 on Town Highway 1 crossing Cota Brook, Lincoln, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (FHWA, 1993). Results of a Level I scour investigation also are included in appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in appendix D. The site is in the Green Mountain section of the New England physiographic province in west-central Vermont. The 3.0-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest along the upstream right bank and brushland along the upstream left bank. Downstream of the bridge, the surface cover is pasture along the left and right banks. In the study area, Cota Brook has an sinuous channel with a slope of approximately 0.01 ft/ ft, an average channel top width of 30 ft and an average bank height of 2 ft. The channel bed material ranges from sand to cobble with a median grain size (D50) of 34.7 mm (0.114 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 10, 1996, indicated that the reach was laterally unstable due to cut-banks and wide, vegetated point bars upstream and downstream of the bridge. The Town Highway 1 crossing of Cota Brook is a 38-ft-long, two-lane bridge consisting of a 36-foot steel-stringer span (Vermont Agency of Transportation, written communication, December 14, 1995). The opening length of the structure parallel to the bridge face is 34.4 ft. The bridge is supported by vertical, concrete abutments. The channel is skewed approximately 15 degrees to the opening while the opening-skew-to-roadway is zero degrees. A scour hole 2.0 ft deeper than the mean thalweg depth was observed along the upstream right bank during the Level I assessment. Along the right abutment, it is 0.25 ft deeper than the mean thalweg depth. Scour protection measures at the site included type-1 stone fill (less than 12 inches diameter) along the upstream right bank and type-2 stone fill (less than 36 inches diameter) along the left and right abutments and along the downstream left bank. Additional details describing conditions at the site are included in the Level II Summary and appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and Davis, 1995) for the 100- and 500-year discharges. Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.0 to 1.7 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 9.1 to 11.3 ft. The worst-case abutment scour occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and Davis, 1995, p. 46). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1996

Level II scour analysis for Bridge 34 (BRIDTH00050034) on Town Highway 005, crossing North Branch Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure BRIDTH00050034 on town highway 5 crossing the North Branch Ottauquechee River, Bridgewater, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). A Level I study is included in Appendix E of this report. A Level I study provides a qualitative geomorphic characterization of the study site. Information on the bridge available from VTAOT files was compiled prior to conducting Level I and Level II analyses and can be found in Appendix D. The site is in the Green Mountain physiographic division of central Vermont in the town of Bridgewater. The 5.45-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, left and right banks are forested. Upstream of bridge 34, Town Highway 5 runs parallel to the right bank and DS of the bridge, parallel to the left bank. In the study area, the North Branch Ottauquechee River has an incised channel with a slope of approximately 0.015 ft/ft, an average channel top width of 32 ft and an average channel depth of 7 ft. The predominant channel bed material is sand and gravel (D 50 is 105 mm or 0.345 ft). The geomorphic assessment at the time of the Level I and Level II site visit on November 3, 1994, indicated that the reach was stable. The town highway 31 crossing of Lilliesville Brook is a 41-ft-long, one-lane bridge consisting of one 39-foot steel-beam span with a timber deck (Vermont Agency of Transportation, written commun., August 24, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 35 degrees to the opening while the opening-skew-to-roadway is 0 degrees. Scour protection measures in place at the site were type-1 stone fill (less than 12 inches diameter) at the downstream left wingwall, left abutment, and upstream and downstream sides of the left road embankment. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1993). Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. The scour analysis results are presented in tables 1 and 2 and a graph of the scour depths is presented in figure 8.

Vermont↗
USGS1997

Level II scour analysis for Bridge 25 (ANDOTH00230025) on Town Highway 23, crossing Andover Branch, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ANDOTH00230025 on Town Highway 23 crossing the Andover Branch, Andover, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the Green Mountain section of the New England physiographic province in south-central Vermont. The 6.74-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture on the right overbank and forest on the left overbank while the immediate banks, both upstream and downstream, are forested. In the study area, the Andover Branch has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 55 ft and an average bank height of 9 ft. The channel bed material ranges from gravel to boulder with a median grain size (D 50 ) of 78.4 mm (0.257 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 27, 1996, indicated that the reach was stable. The Town Highway 23 crossing of the Andover Branch is a 25-ft-long, two-lane structure consisting of a multi-plate corrugated steel arch culvert with concrete footings (Vermont Agency of Transportation, written communication, March 29, 1995). The culvert is mitered at the inlet and outlet. The channel is skewed approximately zero degrees to the opening while the opening-skew-to-roadway is zero degrees. The footings are exposed approximately 1.25 ft, with the exception of the downstream end of the right footing which is exposed approximately 0.5 ft. The only scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) along the upstream left bank. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for modelled flows ranged from 1.6 to 2.8 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 10.0 to 11.7 ft along the left footing and from 11.8 to 16.7 along the right footing. The worst-case abutment scour occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A crosssection of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1996

Level II scour analysis for Bridge 46 (NORWTH00030046) Town Highway 3 (VT132) crossing the Ompompanoosuc River, Norwich, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure NORWTH00030046 on town highway 3, which is also Vermont State Route 132 crossing the Ompompanoosuc River, Norwich, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). A Level I study is included in Appendix E of this report. A Level I study provides a qualitative geomorphic characterization of the study site. Information on the bridge, available from VTAOT files, was compiled prior to conducting Level I and Level II analyses and can be found in Appendix D. The site is in the New England Upland physiographic province of east-central Vermont. The 135-mi 2 drainage area is a predominantly rural basin. A flood-control reservoir located approximately 2 mi upstream has 1.66 billion cubic feet of usable storage. In the vicinity of the study site, the left bank is forested and the right bank is covered by shrubs and brush, adjacent to woods. The Ompompanoosuc River is parallel to Town Highway 3. In the study area, the Ompompanoosuc River has a sinuous channel with a slope of approximately 0.003 ft/ft, an average channel top width of 166 ft and an average channel depth of 6 ft. The predominant channel bed material is sand (D 50 is 0.744 mm or 0.00244 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 19, 1994, indicated that the reach was stable. The town highway 3 crossing of the Ompompanoosuc Riveris a 100-ft-long, two-lane bridge consisting of two steel-beam spans (Vermont Agency of Transportation, written commun., July 29, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 25 degrees to the opening while the opening-skew-to-roadway is 12 degrees. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1993). Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. The scour analysis results are presented in tables 1 and 2 and a graph of the scour depths is presented in figure 8.

Vermont↗
USGS1996

Level II scour analysis for Bridge 29 (CRAFTH00550029) on Town Highway 55, crossing the Black River, Craftsbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure CRAFTH00550029 on town highway 55 crossing the Black River, Craftsbury, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the New England Upland section of the New England physiographic province of north-central Vermont in the town of Craftsbury. The 24.7-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the banks have woody vegetation coverage except for the upstream left bank and the downstream right bank, which have more brush cover than trees. In the study area, the Black River has an incised, sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 41 ft and an average channel depth of 5.5 ft. The predominant channel bed material is sand and gravel (D 50 is 44.7 mm or 0.147 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 7, 1995, indicated that the reach was stable. The town highway 55 crossing of the Black Riveris a 32-ft-long, one-lane bridge consisting of one 28-foot span steel stringer superstructure with a timber deck (Vermont Agency of Transportation, written communication, August 4, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 40 degrees to the opening while the opening-skew-to-roadway is 10 degrees. A scour hole 2 ft deeper than the mean thalweg depth was evident at mid-channel immediately downstream of the bridge during the Level I assessment. The only scour protection measure at the site was type-1 stone fill (less than 12 inches diameter) on the upstream right bank and road approach embankment. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.9 to 1.4 ft. The worst-case contraction scour occurred at the 100-year discharge. Abutment scour ranged from 12.1 to 15.5 ft. The worst-case abutment scour occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 5C (CORITH0003005C) on Town Highway 3, crossing Cooksville Brook, Corinth, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure CORITH0003005C on Town Highway 3 crossing Cooksville Brook, Corinth, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the New England Upland section of the New England physiographic province in east-central Vermont. The 20.2-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture with a residence on the upstream right bank near the bridge. The immediate channel banks have some woody vegetation cover. In the study area, Cooksville Brook has an incised, sinuous channel with a slope of approximately 0.005 ft/ft, an average channel top width of 46 ft and an average channel depth of 8 ft. The channel bed material ranged from sand to cobble and had a median grain size (D 50 ) of 41.0 mm (0.135 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 5, 1995, indicated that the reach was stable. The Town Highway 3 crossing of Cooksville Brook is a 39-ft-long, two-lane bridge consisting of one 37-foot steel-beam span (Vermont Agency of Transportation, written communication, March 17, 1995). The bridge is supported by vertical, concrete abutments with wingwalls on the left abutment. The channel is skewed approximately 30 degrees to the opening while the opening-skew-to-roadway is 0 degrees. A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the right abutment during the Level I assessment. The only scour protection measures at the site were type-2 stone fill (less than 36 inches diameter) at the upstream and downstream ends of the right abutment and type-4 (less than 60 inches diameter) along the upstream right bank below the residence. Also, there is a wall along the upstream right bank. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 2.7 to 3.3 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.0 to 19.0 ft. The worst-case left abutment scour occurred at the incipient overtopping discharge. The worst-case right abutment scour occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 27 (STJOTH00080027) on Town Highway 8, crossing the Sleepers River, St. Johnsbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure STJOTH00080027 on Town Highway 8 crossing the Sleepers River, St. Johnsbury, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the New England Upland section of the New England physiographic province in northeastern Vermont. The 40.4-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest on the upstream right bank with some pasture on the upstream left bank. The downstream right overbank cover is comprised of cut grass, trees and shrubs while the immediate banks have dense woody vegetation. The downstream left bank is forested with some pasture. In the study area, the Sleepers River has an incised, sinuous channel with a slope of approximately 0.007 ft/ft, an average channel top width of 72 ft and an average bank height of 5 ft. The channel bed material ranges from gravel to cobble with a median grain size (D 50 ) of 48.5 mm (0.159 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 10, 1995, indicated that the reach was stable. The Town Highway 8 crossing of the Sleepers River is a 74-ft-long, two-lane bridge consisting of one 71-foot steel-beam span (Vermont Agency of Transportation, written communication, March 28, 1995). The opening length of the structure parallel to the bridge face is 68 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 50 degrees to the opening while the computed openingskew-to-roadway is 45 degrees. The VTAOT database states the opening-skew-to-roadway as 30 degrees. A scour hole 2.5 ft deeper than the mean thalweg depth was observed along the right abutment during the Level I assessment. There is also a three to four foot deep scour hole in the channel adjacent to the upstream right wingwall. The scour protection at the site included type-3 stone fill (less than 48 inches diameter) at the upstream end of the upstream left wingwall, at the downstream end of the downstream right wingwall, and along the downstream left bank. There was also type-2 stone fill (less than 36 inches diameter) at the downstream end of the downstream left wingwall, along the upstream left bank, and along the downstream right bank. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995) for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping discharge is determined and analyzed as another potential worst-case scour scenario. Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour computed for all modelled flows was zero ft. Abutment scour ranged from 6.2 to 9.7 ft. The worst-case abutment scour occurred at the 100-year discharge at the right abutment and at the 500-year discharge at the left abutment. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS2006

Major- and trace-element concentrations in rock samples collected in 2004 from the Taylor Mountains 1:250,000-scale quadrangle, Alaska

The Kuskokwim mineral belt of Bundtzen and Miller (1997) forms an important metallogenic region in southwestern Alaska that has yielded more than 3.22 million ounces of gold and 400,000 ounces of silver. Precious-metal and related deposits in this region associated with Late Cretaceous to early Tertiary igneous complexes extend into the Taylor Mountains 1:250,000-scale quadrangle. The U.S. Geological Survey is conducting geologic mapping and a mineral resource assessment of this area that will provide a better understanding of the geologic framework, regional geochemistry, and may provide targets for mineral exploration and development. During the 2004 field season 137 rock samples were collected for a variety of purposes. The 4 digital files accompanying this report reflect the type of analysis performed and its intended purpose and are available for download as an Excel workbook, comma delimited format (*.csv), dBase 4 files (*.dbf) or as point coverages in ArcInfo interchange format (*.e00). Data values are provided in percent, pct (1gram per 100grams), or parts per million, ppm (1gram per 1,000,000grams) per the column heading in the table. All samples were analyzed for a suite of 42 trace-elements (icp42.*) to provide data for use in geochemical exploration as well as some baseline data. Selected samples were analyzed by additional methods; 104 targeted geochemical exploration samples were analyzed for gold, arsenic, and mercury (auashg.*); 21 of these samples were also analyzed to obtain concentrations of 10 loosely bound metals (icp10.*); 33 rock samples were analyzed for major element oxides to support the regional mapping program (reg.*), of which 28 sedimentary rock samples were also analyzed for total carbon, and carbonate carbon.

Open-File Report↗
USGS1996

Level II scour analysis for Bridge 46 (ENOSVT01080046) on State Route 108, crossing an Unnamed "The Branch" Tributary, Enosburg, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ENOSVT01080046 on State Route 108 crossing an unnamed "The Branch" tributary, Enosburg, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the Green Mountain section of the New England physiographic province in north-central Vermont. The 1.55-mi 2 drainage area is in a predominantly rural, pasture and forested basin. In the vicinity of the study site, the surface cover is pasture. In the study area, this unnamed "The Branch" tributary has an incised, sinuous channel with a slope of approximately 0.03 ft/ft, an average channel top width of 45 ft and an average channel depth of 3 ft. The predominant channel bed material is gravel and cobbles with a median grain size (D 50 ) of 42.4 mm (0.139 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 29, 1995, indicated that the reach was laterally unstable. Block failure slumping of bank material was evident at an upstream cut-bank and another minor cut-bank was noted downstream. The State Route 108 crossing of this unnamed "The Branch" tributary is a 25-ft-long, twolane bridge consisting of one 22-foot concrete span (Vermont Agency of Transportation, written communication, March 8, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 10 degrees to the opening while the opening-skew-to-roadway is zero degrees. The only scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) at the downstream end of the downstream left wingwall. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.3 to 0.5 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.0 to 8.0 ft. The worst-case abutment scour occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 29 (HARDTH00310029) on Town Highway 31, crossing the Lamoille River, Hardwick, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure HARDTH00310029 on town highway 31 crossing the Lamoille River, Hardwick, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the New England Upland section of the New England physiographic province of north-central Vermont. The 64.4-mi 2 drainage area is in a predominantly rural basin. In the vicinity of the study site, the surface cover is pasture except for the immediate downstream channel banks and the downstream left overbank which are brush covered. In the study area, the Lamoille River has a sinuous channel with a slope of approximately 0.001 ft/ft, an average channel top width of 84 ft and an average channel depth of 4 ft. The predominant channel bed materials are cobble and gravel with a median grain size (D 50 ) of 36.1 mm (0.119 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 26, 1995, indicated that the reach was stable. The town highway 31 crossing of the Lamoille River is a 65-ft-long, one-lane bridge consisting of one 61-foot steel-beam span (Vermont Agency of Transportation, written communication, March 27, 1995). The bridge is supported by vertical, stone abutments with wingwalls. The right abutment has a concrete facing and a concrete subfooter. The channel is skewed approximately 5 degrees to the opening while the opening-skew-to-roadway is 0 degrees. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.0 to 2.7 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 10.3 to 18.6 ft. The worst-case abutment scour occurred at the 100-year discharge at the right abutment. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 68 (NFIETH00960068) on Town Highway 96, crossing the Dog River, Northfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure NFIETH00960068 on Town Highway 96 crossing the Dog River, Northfield, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the Green Mountain section of the New England physiographic province in central Vermont. The 30.7-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover on the left bank upstream and downstream is pasture while the immediate banks have dense woody vegetation. The right bank upstream is forested and the downstream right bank is pasture. Vermont state route 12A runs parallel to the river on the right bank. In the study area, the Dog River has an incised, straight channel with a slope of approximately 0.004 ft/ft, an average channel top width of 70 ft and an average bank height of 7 ft. The channel bed material ranges from sand to cobble with a median grain size (D 50 ) of 47.9 mm (0.157 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 25, 1996, indicated that the reach was stable. The Town Highway 96 crossing of the Dog River is a 45-ft-long, one-lane bridge consisting of one 43-foot steel-beam span with a timber deck (Vermont Agency of Transportation, written communication, October 13, 1995). The opening length of the structure parallel to the bridge face is 41.5 ft.The bridge is supported by vertical, concrete abutments with wingwalls. The channel is not skewed to the opening and the opening-skew-to-roadway is zero degrees. Channel scour 0.5 ft deeper than the mean thalweg depth, was observed under the bridge during the Level I assessment. The scour protection measures at the site included type-1 stone fill (less than 12 inches diameter) along the left bank upstream and type-2 stone fill (less than 36 inches diameter) along the upstream and downstream right banks that extends partially in front of the right wingwalls. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.8 to 1.2 ft. The worst-case contraction scour occurred at the 100-year and 500-year discharges. Abutment scour ranged from 8.5 to 12.2 ft. The worst-case abutment scour occurred at the incipient roadway-overtopping discharge for the right abutment. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 86 (VERNVT01420086) on State Route 142, crossing Broad Brook, Vernon, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure VERNVT01420086 on State Route 142 crossing Broad Brook, Vernon, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the New England Upland section of the New England physiographic province in southeastern Vermont. The 23.7-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is primarily forest with the exception of the downstream left bank which is a wetland. In the study area, Broad Brook has an incised, meandering channel with a slope of approximately 0.001 ft/ft, an average channel top width of 132 ft and an average bank height of 3 ft. The channel bed material ranges from silt to cobbles with a median grain size (D 50 ) of 80.0 mm (0.262 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 14, 1996, indicated that the reach was vertically and laterally unstable. The State Route 142 crossing of Broad Brook is a 98-ft-long, two-lane bridge consisting of two steel-beam spans with a maximum span length of 47 feet (Vermont Agency of Transportation, written communication, March 30, 1995). The bridge is supported by vertical, concrete abutments with spill-through slopes and a concrete pier. The channel is skewed approximately 30 degrees to the opening while there is no opening-skew-to- roadway. A scour hole 2 ft deeper than the mean thalweg depth was observed along the left bank side of the pier during the Level I assessment. There was also a scour hole 1 ft deeper than the mean thalweg depth observed along the length of the right abutment. The only scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) along the entire base length of the spill-through slopes. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. There was no computed contraction scour for any modelled flows. Scour at the left abutment ranged from 13.2 to 15.9 ft and at the right abutment ranged from 12.0 to 16.3 ft. The worst-case abutment scour occurred at the 500-year discharge. Pier scour ranged from 12.0 to 16.3 ft. The worst-case pier scour occurred at the incipient-overtopping discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗