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USGS1997

Level II scour analysis for Bridge 32 (HUNTTH00220032) on Town Highway 22, crossing Brush Brook, Huntington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure HUNTTH00220032 on Town Highway 22 crossing Brush Brook, Huntington, 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 5.7-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest except on the downstream right overbank which is pasture. In the study area, Brush Brook has an incised, straight channel with a slope of approximately 0.05 ft/ft, an average channel top width of 58 ft and an average bank height of 6 ft. The channel bed material ranges from gravel to boulder with a median grain size (D 50 ) of 127 mm (0.416 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 25, 1996, indicated that the reach was stable. The Town Highway 22 crossing of Brush Brook is a 36-ft-long, one-lane bridge consisting of one 34-foot steel-beam span and a timber deck (Vermont Agency of Transportation, written communication, December 12, 1995). The opening length of the structure parallel to the bridge face is 35.7 ft. The bridge is supported by vertical, concrete abutments with wingwalls on the left. The channel is skewed approximately 50 degrees to the opening while the measured opening-skew-to-roadway is 15 degrees. A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the left abutment and downstream left wingwall during the Level I assessment. The only scour protection measure at the site was type-2 stone fill (less than 36 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). 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 0.2 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.4 to 10.2 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 31 (HUNTTH00220031) on Town Highway 22, crossing Brush Brook, Huntington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure HUNTTH00220031 on Town Highway 22 crossing Brush Brook, Huntington, 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, obtained 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 5.01-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover consists of trees and brush. In the study area, Brush Brook has an incised, straight channel with a slope of approximately 0.06 ft/ft, an average channel top width of 44 ft and an average bank height of 4 ft. The channel bed material ranges from boulder to gravel with a median grain size (D 50 ) of 107.0 mm (0.352 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 25, 1996, indicated that the reach was stable. The Town Highway 22 crossing of Brush Brook is a 34-ft-long, one-lane bridge consisting of one 30-foot steel I-beam span (Vermont Agency of Transportation, written communication, November 30, 1995). The opening length of the structure parallel to the bridge face is 31.2 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 15 degrees to the opening while the computed opening-skew-to-roadway is 10 degrees. The VTAOT computed opening-skewto-roadway is 2 degrees. A scour hole 1.0 ft deeper than the mean thalweg depth was observed at the downstream end of the left abutment during the Level I assessment. The only scour protection measure at the site was type-2 stone fill (less than 36 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 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 for all modelled flows was computed to be zero ft. Abutment scour ranged from 7.0 to 10.5 ft. The worst-case abutment scour occurred at the 500-year discharge for the left abutment and at the incipient-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 40 (ANDOVT00110040) on State Route 11, crossing Lyman Brook, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ANDOVT00110040 on State Route 11 crossing Lyman Brook, 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 4.18-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture while the immediate banks have dense woody vegetation. In the study area, Lyman Brook has an incised, straight channel with a slope of approximately 0.03 ft/ft, an average channel top width of 42 ft and an average bank height of 8 ft. The channel bed material ranges from gravel to boulder with a median grain size (D 50 ) of 86.0 mm (0.282 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 9, 1996, indicated that the reach was stable. The State Route 11 crossing of Lyman Brook is a 28-ft-long, two-lane bridge consisting of one 27-foot concrete tee-beam span (Vermont Agency of Transportation, written communication, March 29, 1995). The opening length of the structure parallel to the bridge face is 24.8 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 0 degrees to the opening while the opening-skew-to-roadway is 30 degrees. The scour protection measures at the site included type-2 stone fill (less than 36 inches diameter) at the upstream end of the upstream right wingwall and the downstream ends of the downstream left and right wingwalls. There was also a stone wall along the top of the left bank from 36 to 76 feet upstream. 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 0.0 to 0.7 ft. The worst-case contraction scour occurred at the incipient-overtopping discharge which was more than the 100-year discharge. Left abutment scour ranged from 1.2 to 7.5 ft. The worst-case left abutment scour occurred at the 500-year discharge. Right abutment scour ranged from 5.2 to 6.7 ft. The worst-case right abutment scour occurred at the 100-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↗
USGS1996

Level II scour analysis for Bridge 43 (BETHTH00070043) on Town Highway 07, crossing Gilead Brook, Bethel, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure BETHTH00070043 on town highway 7 crossing Gilead Brook, Bethel, 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 province of central Vermont in the town of Bethel. The 6.81-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the banks have dense woody vegetation coverage except for the downstream right bank near the bridge, which is grass covered. In the study area, Gilead Brook has an incised, slightly sinuous channel with a slope of approximately 0.0181 ft/ft, an average channel top width of 36 ft and an average channel depth of 4.0 ft. The predominant channel bed material is cobble (D 50 is 79.6 mm or 0.261 ft). The geomorphic assessment at the time of the Level I and Level II site visit on October 19, 1994, indicated that the reach was stable. The town highway 7 crossing of Gilead Brook is a 31-ft-long, two-lane bridge consisting of one 27-foot concrete slab type superstructure (Vermont Agency of Transportation, written commun., August 24, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 30 degrees to the opening while the opening-skew-to-roadway is 15 degrees. A scour hole 0.5 ft deeper than the mean thalweg depth was observed at the right side of the downstream bridge face during the Level I assessment. The scour protection measures in place at the site were type-1 stone fill (less than 12 inches diameter) along the right abutment and both downstream banks, type-2 stone fill (less than 36 inches diameter) on all of the road approach embankments, both upstream banks, and along the entire base length of the wingwalls. 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 1.4 ft. The worst-case contraction scour occurred at the incipient overtopping discharge, which was between the 100- and 500-year discharges. Abutment scour ranged from 6.6 to 11.0 ft. with the worst-case scenario occurring 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). Many factors, including historical performance during flood events, the geomorphic assessment, scour protection measures, and the results of the hydraulic analyses, must be considered to properly assess the validity of abutment scour results. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein, based on the consideration of additional contributing factors and experienced engineering judgement.

Vermont↗
USGS1996

Level II scour analysis for Bridge 16 (BRNATH00800016) on Town Highway 80, crossing Locust Creek, Barnard, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure BRNATH00800016 on town highway 80 crossing Locust Creek, Barnard, 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, gleaned from Vermont Agency of Transportation (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 province of central Vermont in the town of Barnard. The 22.0-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the left banks are forested and the right banks are covered with shrub and brush. Vermont Route 12 is adjacent to the right bank. In the study area, Locust Creek has an incised channel with a slope of approximately 0.02 ft/ft, an average channel top width of 60 ft and an average channel depth of 4 ft. The predominant channel bed materials are gravel and cobble with a median grain size (D 50 ) of 102 mm (0.336 ft). The geomorphic assessment at the time of the Level I and Level II site visits on September 22, 1994 and October 12, 1994, indicated that the reach was stable. The town highway 80 crossing of Locust Creek is a 36-ft-long, one-lane bridge consisting of one 33-foot steel-beam span with timber deck (Vermont Agency of Transportation, written communication, August 23, 1994). The bridge is supported by vertical, log crib abutments with wingwalls. Type-2 stone fill (less than 36 inches diameter) protects the upstream and downstream left wingwalls and the downstream left road embankment. Type- 3 stone fill (less than 48 inches diameter) protects the upstream and downstream right wingwalls. The upstream left and downstream right road embankments are not protected and road wash is eroding these areas. The channel approach to the bridge is straight with the bridge skewed zero degrees to flow; the opening-skew-to-roadway is also zero degrees. Additional details describing conditions at the site are included in the Level II Summary, Appendix D, and Appendix E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1993). 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 3.7 ft. The worst-case contraction scour occurred at the incipient-overtopping discharge, which was between the 100- and 500-year discharge. Abutment scour ranged from 17.5 to 23.2 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, 1993, p. 48). 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 36 (RICHVT01050036) on State Route 105, crossing Stanhope Brook, Richford, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure RICHVT01050036 on State Route 105 crossing Stanhope Brook, Richford, 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 7.03-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is short grass except for the upstream left overbank area which is forested. In the study area, Stanhope Brook has a steep, sinuous channel with a slope of approximately 0.03 ft/ft, an average channel top width of 47 ft and an average channel depth of 5 ft. The predominant channel bed material is cobble with a median grain size (D 50 ) of 132 mm (0.432 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 28, 1995, indicated that the reach was laterally unstable. The State Route 105 crossing of Stanhope Brook is a 42-ft-long, two-lane bridge consisting of one 38-foot concrete T-beam 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 15 degrees to the opening while the opening-skew-to-roadway is 20 degrees. A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the downstream end of the right abutment wall during the Level I assessment. The scour protection measures at this site were type-2 stone fill (less than 36 inches diameter) along the entire lengths of the upstream wingwalls, at the corner of the downstream left abutment and downstream left wingwall and the downstream end of the downstream right 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 0.0 to 0.3 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.6 to 9.4 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↗
USGS1996

Level II scour analysis for Bridge 4 (MNTGTH00020004) on Town Highway 2, crossing Wade Brook, Montgomery, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure MNTGTH00020004 on town highway 2 crossing Wade Brook, Montgomery, 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, gleaned 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 province of north-central Vermont in the town of Montgomery. The 1.68-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. In the study area, Wade Brook has an incised, sinuous channel with a slope of approximately 0.0454 ft/ft, an average channel top width of 30 ft and an average channel depth of 2 ft. The predominant channel bed materials are gravel and cobbles (D 50 is 77.7 mm or 0.255 ft). The geomorphic assessment at the time of the Level I and Level II site visit on November 8, 1994, indicated that the reach was degraded. There were no scour holes observed during the Level I assessment. However, general streambed lowering was evident as both abutments were undermined equally with no localized scour on one abutment over the other. The town highway 2 crossing of Wade Brook is a 23-ft-long, two-lane bridge consisting of one 20-foot concrete slab span (Vermont Agency of Transportation, written communication, August 3, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 30 degrees to the opening while the computed opening-skew-to-roadway is 25 degrees. The scour protection measures at the site were type-1 stone fill (less than 12 inches diameter) on the upstream right wingwall and all road approach embankments, type-2 stone fill (less than 36 inches diameter) on the left abutment, and a “laid-up” stone wall at the upstream end of the upstream left wingwall and in front of 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 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 was 0.1 ft. The worst-case contraction scour occurred at the 100-year and 500-year discharges. Abutment scour ranged from 3.9 to 5.2 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). Many factors, including historical performance during flood events, the geomorphic assessment, scour protection measures, and the results of the hydraulic analyses, must be considered to properly assess the validity of abutment scour results. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein, based on the consideration of additional contributing factors and experienced engineering judgement.

Vermont↗
USGS1997

Level II scour analysis for Bridge 37 (DUXBTH00120037) on Town Highway 12, crossing Ridley Brook, Duxbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure DUXBTH00120037 on Town Highway 12 crossing Ridley Brook, Duxbury, 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 10.1-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest upstream and downstream of the bridge. In the study area, Ridley Brook has an incised, straight channel with a slope of approximately 0.04 ft/ft, an average channel top width of 67 ft and an average bank height of 9 ft. The channel bed material ranges from gravel to boulders with a median grain size (D 50 ) of 123 mm (0.404 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 1, 1996, indicated that the reach was stable. The Town Highway 12 crossing of Ridley Brook is a 33-ft-long, two-lane bridge consisting of five 30-ft steel rolled beams (Vermont Agency of Transportation, written communication, October 13, 1995). The opening length of the structure parallel to the bridge face is 30 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 50 degrees to the opening while the measured opening-skew-to-roadway is 20 degrees. A scour hole 2 ft deeper than the mean thalweg depth was observed along the right abutment and downstream right wingwall during the Level I assessment. Scour countermeasures at the site include type-2 stone fill (less than 3 feet diameter) along the upstream and downstream left road embankments, and type-3 stone fill (less than 4 feet diameter) along the upstream right bank and upstream right 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 0.6 to 1.7 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 5.0 to 8.3 ft, with the worst-case occurring at the incipient-overtopping discharge. Right abutment scour ranged from 13.1 to 16.7 ft, with the worst-case occurring 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 33 (CONCTH00580033) on Town Highway 58, crossing Miles Stream, Concord, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure CONCTH00580033 on Town Highway 58 crossing Miles Stream, Concord, 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 17.9-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 of the bridge while the immediate banks have dense woody vegetation. Downstream of the bridge, the right bank is forested and the left bank has shrubs and brush. In the study area, Miles Stream has an incised, sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 91 ft and an average bank height of 7 ft. The channel bed material ranges from gravel to boulder with a median grain size (D 50 ) of 61.6 mm (0.188 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 15, 1995, indicated that the reach was stable. The Town Highway 58 crossing of Miles Stream is a 44-ft-long, two-lane bridge consisting of one 39-foot steel-beam span (Vermont Agency of Transportation, written communication, March 24, 1995). The opening length of the structure parallel to the bridge face is 37.4 ft. The bridge is supported by vertical, concrete abutments with stone fill in front creating spillthrough embankments. The channel is skewed approximately 20 degrees to the opening while the opening-skew-to-roadway is zero degrees. The only scour countermeasure at the site was type-3 stone fill (less than 48 inches diameter) along the left and right banks upstream, in front of the abutments forming spill through embankments, and extending along the banks downstream. 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 for all modelled flows ranged from 0.0 to 1.8 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.0 to 9.7 ft. The worst-case abutment scour occurred at the 500-year discharge for the right abutment and at the incipient roadway-overtopping discharge for 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↗
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↗
USGS1996

Level II scour analysis for Bridge 42 (BETHTH00860042) on Town Highway 86, crossing Gilead Brook, Bethel, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure BETHTH00860042 on town highway 86 crossing Gilead Brook, Bethel, 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 were 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 province of central Vermont in the town of Bethel. The 11.4-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the upstream banks are tree covered and the downstream banks are covered with shrubs and brush. In the study area, Gilead Brook is probably incised, has a sinuous channel with a slope of approximately 0.012 ft/ft, an average channel top width of 53 ft, and an average channel depth of 5 ft. The predominant channel bed material is gravel to cobbles (D 50 is 85.6 mm or 0.281 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 30, 1994, indicated that the reach was stable. The town highway 86 crossing of Gilead Brook is a 28-ft-long, one-lane bridge consisting of one 25-foot clear-span structure with a concrete deck (Vermont Agency of Transportation, written commun., August 24, 1994). The bridge is supported by concrete abutments with wingwalls. The bridge skew is approximately 5 degrees and there is no opening-skew-to-roadway. A scour hole approximately 1 ft deeper than the mean thalweg depth was observed along the left bank, near the upstream bridge face during the Level I assessment. There is also approximately 1 ft of scour along the left abutment of the bridge, near the upstream wing wall, exposing the footing. There is type-one (less than 12 in diameter) protection on the US left wingwall and type-two (less than 36 in diameter) along the US and DS right wingwalls. There is no protection along the abutments. 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 to 1.9 ft. The worst-case contraction scour occurred at the incipient-overtopping discharge and the 100-year discharge. Abutment scour ranged from 8.6 to 15.7 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). Many factors, including historical performance during flood events, the geomorphic assessment, scour protection, and the results of the hydraulic analyses, must be considered to properly assess the validity of abutment scour results. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein, based on the consideration of additional contributing factors and engineering judgement.

Vermont↗
USGS1997

Level II scour analysis for Bridge 17 (LYNDTH00020017) on Town Highway 2, crossing Hawkins Brook, Lyndon, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure LYNDTH00020017 on Town Highway 2 crossing Hawkins Brook, Lyndon, 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 northeastern Vermont. The 7.7-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 left and right upstream overbanks. The downstream left and right overbanks are brushland. In the study area, Hawkins Brook has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 78 ft and an average bank height of 7.3 ft. The channel bed material ranges from sand to boulder with a median grain size (D 50 ) of 46.6 mm (0.153 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 4, 1995, indicated that the reach was laterally unstable with the presence of point bars and side bars. The Town Highway 2 crossing of Hawkins Brook is a 49-ft-long, two-lane bridge consisting of a 46-foot steel-stringer span (Vermont Agency of Transportation, written communication, March 27, 1995). The opening length of the structure parallel to the bridge face is 43 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 45 degrees to the opening while the computed opening-skew-to-roadway is zero degrees. A scour hole 0.75 ft deeper than the mean thalweg depth was observed along the downstream left abutment during the Level I assessment. The only scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) at the upstream 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 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. 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 0.1 to 0.9 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 3.8 to 6.6 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 8, (MANCTH00060008) on Town Highway 6, crossing Bourn Brook, Manchester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure MANCTH00060008 on Town Highway 6 crossing Bourn Brook, Manchester, 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 15.5-mi 2 drainage area is in a predominantly rural and forested basin. The bridge site is located within a suburban setting in the Town of Manchester with houses and lawns on the overbanks. In the study area, Bourn Brook has an incised, straight channel with a slope of approximately 0.01 ft/ft, an average channel top width of 61 ft and an average bank height of 7 ft. The channel bed material ranges from sand to cobbles with a median grain size (D 50 ) of 87.2 mm (0.286 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 6, 1996, indicated that the reach was stable. The Town Highway 6 crossing of Bourn Brook is a 44-ft-long, two-lane bridge consisting of one 41-foot concrete T-beam span (Vermont Agency of Transportation, written communication, September 28, 1995). The opening length of the structure parallel to the bridge face is 40.0 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately zero degrees to the opening while the opening-skew-to-roadway is 15 degrees. A scour hole 3.5 ft deeper than the mean thalweg depth was observed along the upstream right wingwall and right abutment during the Level I assessment. The scour countermeasures at the site were stone walls in front of the upstream left wingwall and bank, along the upstream right bank extending from the end of the upstream right wingwall, and in front of the downstream right wingwall and 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 was zero ft. The left abutment scour ranged from 3.6 to 9.2 ft. The worst-case left abutment scour occurred at the 500-year discharge. The right abutment scour ranged from 9.8 to 12.6 ft. 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↗
USGS1996

Level II scour analysis for Bridge 57 (BRIDTH00650057) on Town Highway 65, crossing Broad Brook, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure BRIDTH00650057 on Town Highway 65 crossing Broad Brook, 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 province of central Vermont in the town of Bridgewater. The 26.9-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the US left bank is forested; DS left bank is shrub and brushland; US right bank is lawn and the DS right bank has very little vegetation and is largely coincident with the Ottauquechee River. In the study area, Broad Brook has an incised channel with a slope of approximately 0.0067 ft/ft, an average channel top width of 60 ft and an average channel depth of 3 ft. The predominant channel bed material is gravel (D 50 is 46.2 mm or 0.151 ft). The geomorphic assessment at the time of the Level I and Level II site visit on October 25, 1994, indicated that the reach was stable. The Town Highway 65 crossing of Broad Brook is a 47-ft-long, one-lane bridge consisting of one 44-ft steel-beam span with a timber deck, supported by vertical concrete abutments with wingwalls on the upstream and downstream sides (Vermont Agency of Transportation, written commun., August, 1994). The US right wingwall and road approach is protected by stone fill. The US and DS right wingwalls and the right abutment are reported as having 1.0 to 1.5 ft of scour at the time of the Level one assessment on 10/25/94. Erosion from road wash affects all road approach embankments. Broad Brook flows into the Ottauquechee River approximately 30 ft downstream of the bridge. The channel approach to the bridge is straight with the bridge skewed 20 degrees to flow and 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, 1993). Total scour at a highway crossing is comprised of three components: 1) long-term aggradation or degradation; 2) contraction scour (due to reduction in flow area caused by 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 scour depths for contraction and local scour and a summary of the results follows. Contraction scour for all modelled flows ranged from 0.4 ft to 1.5 ft and the worst-case contraction scour occurred at the incipient overtopping discharge. Abutment scour ranged from 6.0 ft to 14.6 ft and the worst-case abutment scour occurred at the 100-year discharge. Scour depths and depths to armoring are summarized on p. 14 in the section titled “Scour Results”. Scour elevations, based on the calculated depths are presented in tables 1 and 2; a graph of the scour elevations is presented in figure 8 Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. For all scour presented in this report, “the scour depths adopted [by VTAOT] may differ from the equation values based on engineering judgement” (Richardson and others, 1993, p. 21, 27). It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1993, p. 48). Many factors, including historical performance during flood events, the geomorphic assessment, and the results of the hydraulic analyses, must be considered to properly assess the validity of abutment scour results.

Vermont↗
USGS1996

Level II scour analysis for Bridge 8 (BARTTH00020008) on Town Highway 2, crossing Roaring Brook, Barton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure BARTTH00020008 on town highway 2 crossing Roaring Brook, Barton, 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 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 section of the New England physiographic province of North-central Vermont in the town of Barton. The 9.89-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 downstream left bank, which has a few trees and grass and brush coverage. In the study area, Roaring Brook has an incised, sinuous channel with a slope of approximately 0.019 ft/ft, an average channel top width of 35 ft and an average channel depth of 3 ft. The predominant channel bed material is gravel/cobble (D 50 is 49.1 mm or 0.161 ft). The geomorphic assessment at the time of the Level I and Level II site visit on October 18, 1994 indicated that the reach was laterally unstable. A cut-bank on the downstream right bank and overall channel configuration in the valley are indications of the lateral instability at this site. The town highway 2 crossing of Roaring Brook is a 30-ft-long, two-lane bridge consisting of one 26-foot span concrete T-beam type superstructure (Vermont Agency of Transportation, written communication, August 4, 1994). 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 2.5 ft deeper than the mean thalweg depth was observed near mid-channel 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 left upstream and downstream roadway embankments. 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 aggradation or degradation; 2) contraction scour (due to reduction in flow area caused by 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 scour depths for contraction and local scour and a summary of the results follows. Contraction scour for all modelled flows ranged from 1.4 to 2.8 feet and the worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.5 to 16.5 feet and 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 7 (ANDOTH00010007) on Town Highway 1, crossing Andover Brook, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ANDOTH00010007 on Town Highway 1 crossing 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 southern Vermont. The 7.21-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 of the bridge while the immediate banks have dense woody vegetation. Downstream of the bridge, the banks and overbanks are forested. In the study area, Andover Branch has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 45 ft and an average bank height of 5 ft. The channel bed material ranges from gravel to boulder with a median grain size (D 50 ) of 58.0 mm (0.19 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 28, 1996, indicated that the reach was laterally unstable due to evidence of lateral movement of the channel 200 feet upstream along the left bank and near the bridge along the right bank. The Town Highway 1 crossing of Andover Branch is a 32-ft-long, two-lane bridge consisting of one 29-foot concrete slab span (Vermont Agency of Transportation, written communication, March 28, 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 0 degrees. The scour protection measures at the site included type-2 stone fill (less than 36 inches diameter) along the entire base length of the upstream wingwalls and type-3 stone fill (less than 48 inches diameter) along the entire base length of the downstream wingwalls and the right abutment. 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 1.6 ft. The worst-case contraction scour occurred at the 100-year discharge. Abutment scour ranged from 7.1 to 10.7 ft at the right abutment with the worst-case abutment scour occurring at the 500-year discharge. Abutment scour ranged from 7.5 to 8.3 ft at the left abutment with the worst-case abutment scour occurring at the incipient road 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. 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↗
USGS1997

Level II scour analysis for Bridge 5 (DUMMVT00300005) on State Route 30, crossing Stickney Brook, Dummerston, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure DUMMVT00300005 on State Route 30 crossing Stickney Brook, Dummerston, 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 6.31-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest and brush. In the study area, Stickney Brook has an incised, straight channel with a slope of approximately 0.04 ft/ft, an average channel top width of 80 ft and an average bank height of 7 ft. The channel bed material is predominantly cobble with a median grain size (D 50 ) of 80.3 mm (0.264 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 12, 1996, indicated that the reach was aggrading. The State Route 30 crossing of Stickney Brook is a 84-ft-long, two-lane bridge consisting of one 82-foot steel-beam span (Vermont Agency of Transportation, written communication, March 30, 1995). The opening length of the structure parallel to the bridge face is 79.7 ft. The bridge is supported by vertical, concrete abutments with spill-through embankments. The channel is skewed approximately 5 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 toe of the right spill-through slope during the Level I assessment. The scour protection measures at the site were type-2 stone fill (less than 36 inches diameter) along the left and right bank under the bridge forming a spill-through slope and type-2 stone fill from approximately 20 ft to 64 ft upstream on the 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). 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 0.2 ft. The worst-case contraction scour occurred at the 100-year discharge. Left abutment scour ranged from 5.5 to 6.3 ft. Right abutment scour ranged from 2.0 to 3.8 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 (LONDTH00410029) on Town Highway 41, crossing Cook Brook, Londonderry, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure LONDTH00410029 on Town Highway 41 crossing Cook Brook, Londonderry, 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 6.48-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is shrub on the left bank upstream and downstream of the bridge while the right bank is forest upstream and downstream of the bridge. In the study area, Cook Brook has a straight incised channel with a slope of approximately 0.02 ft/ft, an average channel top width of 80 ft and an average bank height of 8 ft. The channel bed material ranges from sand to cobble with a median grain size (D 50 ) of 70.9 mm (0.233 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 7, 1996, indicated that the reach was stable. The Town Highway 41 crossing of Cook Brook is a 25-ft-long, one-lane bridge consisting of one 22-foot steel-beam span (Vermont Agency of Transportation, written communication, April 6, 1995). The opening length of the structure parallel to the bridge face is 20.4 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 5 degrees to the opening while the opening-skew-toroadway is 0 degrees. A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the upstream left wingwall and left abutment during the Level I assessment. The only scour protection measure at the site was type-2 stone fill (less than 36 inches) along the downstream end of the left abutment and upstream end of the downstream right 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 0.0 to 1.5. Abutment scour ranged from 8.4 to 15.1 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↗