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Emily C. Wild

Publications and source records attributed to Emily C. Wild.

32 records · Page 2Linked to original sources
USGS1998

Level II scour analysis for Bridge 22 (BRADTH00270022) on Town Highway 27, crossing the Waits River, Bradford, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure BRADTH00270022 on Town Highway 27 crossing the Waits River, Bradford, 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, 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 New England Upland section of the New England physiographic province in east-central Vermont. The 153-mi 2 drainage area is in a predominantly rural and forested basin. However, in the vicinity of the study site, the upstream and downstream left banks are suburban and the upstream and downstream right banks are shrub and brushland. In the study area, the Waits River has an incised, sinuous channel with a slope of approximately 0.0002 ft/ft, an average channel top width of 125 ft and an average bank height of 4 ft. The channel bed material ranges from silt and clay to bedrock with a median grain size (D 50 ) of 0.393 mm (0.00129 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 7, 1995, indicated that the reach was stable. The Town Highway 27 crossing of the Waits River is a 109-ft-long, one-lane bridge consisting of a 104-ft steel-truss span (Vermont Agency of Transportation, written communication, March 16, 1995). The opening length of the structure parallel to the bridge face is 99.2 ft. The bridge is supported by vertical, laid-up stone abutments. The channel is skewed approximately 30 degrees to the opening while the opening-skew-to-roadway is zero degrees. No evidence of scour was observed during the Level I assessment. Scour protection measures at the site included type-2 stone fill (less than 36 inches diameter) along the upstream right and downstream left banks and type-3 stone fill (less than 48 inches diameter) along the left and right abutments. 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 1.5 to 2.0 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 11.8 to 18.8 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 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↗
USGS1998

Level II scour analysis for Bridge 34 (WWINTH00370034) on Town Highway 37, crossing Mill Brook, West Windsor, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure WWINTH00370034 on Town Highway 37 crossing Mill Brook, West Windsor, 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 New England Upland section of the New England physiographic province in east-central Vermont. The 16.6-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture except for the upstream left bank where there is mostly shrubs and brush. In the study area, Mill Brook has a sinuous channel with a slope of approximately 0.003 ft/ ft, an average channel top width of 52 ft and an average bank height of 5 ft. The channel bed material ranges from sand to cobbles with a median grain size (D 50 ) of 43.4 mm (0.142 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 5, 1996, indicated that the reach was laterally unstable. Point bars were observed upstream and downstream of this site. Furthermore, slip failure of the bank material was noted downstream at a cut-bank on the left side of the channel across from a point bar. The Town Highway 37 crossing of Mill Brook is a 37-ft-long, one-lane covered bridge consisting of one 32-foot wood thru-truss span (Vermont Agency of Transportation, written communication, March 23, 1995). The opening length of the structure parallel to the bridge face is 29.6 ft. The bridge is supported by vertical, laid-up stone abutment walls with concrete facing and laid-up stone wingwalls. The channel is skewed approximately 10 degrees to the opening while the opening-skew-to-roadway is zero 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 protection measures at the site included type-3 (less than 48 inches diameter) and type-4 (less than 60 inches diameter) stone fill. Type-3 stone fill was observed along the upstream right bank and along the right abutments. Type-4 stone fill was observed at the upstream end of the 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 Davis, 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. There was no contraction scour predicted for any of the modeled flows. Abutment scour at the left abutment ranged from 5.7 to 7.3 ft, while that at the right abutment ranged from 11.6 to 17.7 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 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↗
USGS1998

Level II scour analysis for Bridge 30 (NEWHTH00050030) on Town Highway 5, crossing the New Haven River, New Haven, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure NEWHTH00050030 on Town Highway 5 crossing the New Haven River, New Haven, 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 (Federal Highway Administration, 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 Champlain section of the St. Lawrence Valley physiographic province in west-central Vermont. The 115-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 bank upstream and downstream of the bridge while the immediate banks have dense woody vegetation. The upstream left bank is also pasture. The downstream left bank is forested. In the study area, the New Haven River has an incised, sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 127 ft and an average bank height of 5 ft. The channel bed material ranges from silt to cobble with a median grain size (D 50 ) of 20.4 mm (0.067 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 19, 1996, indicated that the reach was laterally unstable. The stream bends through the bridge and impacts the left bank where there is a cut bank and scour hole. The Town Highway 5 crossing of the New Haven River is a 181-ft-long, two-lane bridge consisting of four 45-ft concrete tee-beam spans (Vermont Agency of Transportation, written communication, December 15, 1995). The opening length of the structure parallel to the bridge face is 175.9 ft. The bridge is supported by vertical, concrete abutments with stone fill spill-through embankments and three concrete piers. The channel is skewed approximately 15 degrees to the opening while the computed opening-skew-to-roadway is 10 degrees. A scour hole 4.5 ft deeper than the mean thalweg depth was observed along the downstream left bank during the Level I assessment. Also observed was a scour hole 1.5 ft deeper than the mean thalweg depth at the upstream end of the middle pier. The only scour protection measure at the site was type-3 stone fill (less than 48 inches diameter) in front of the left and right abutments creating 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 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 0.7 to 2.1 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 6.8 to 8.4 ft. The worst-case left abutment scour occurred at the 500-year discharge. Right abutment scour ranged from 11.2 to 14.0 ft. The worst-case right abutment scour occurred at the 500-year discharge. Pier scour ranged from 12.9 to 19.3 ft. The worst-case pier 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↗
USGS1998

Level II scour analysis for Bridge 8 (NEWFTH00010008) on Town Highway 1, crossing Wardsboro Brook, Newfane, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure NEWFTH00010008 on Town Highway 1 crossing Wardsboro Brook, Newfane, 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 (Federal Highway Administration, 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 southestern Vermont. The 6.91-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 overbank and downstream left and right overbanks. The surface cover on the upstream left overbank is pasture. In the study area, Wardsboro Brook has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 63 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 95.4 mm (0.313 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 21, 1996, indicated that the reach was stable. The Town Highway 1 crossing of the Wardsboro Brook is a 32-ft-long, two-lane bridge consisting of a 26-foot concrete tee-beam span (Vermont Agency of Transportation, written communication, April 6, 1995). The opening length of the structure parallel to the bridge face is 26.7 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 45 degrees to the computed opening while the openingskew-to-roadway is 45 degrees. A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the right abutment during the Level I assessment. 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 upstream left bank and the upstream ends of the upstream left and right wingwalls. A stone wall extends along the downstream right bank from the 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 Davis, 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 0.1 to 3.9 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 11.1 to 12.9 ft. Right abutment scour ranged from 4.3 to 4.8 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 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↗
USGS1997

Level II scour analysis for Bridge 27 (ANDOTH00290027) on Town Highway 29, crossing Middle Branch Williams River, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ANDOTH00290027 on Town Highway 29 crossing the Middle Branch Williams River, 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 central Vermont. The 12.7-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 left bank upstream of the bridge while the immediate bank has woody vegetation. The surface cover on the upstream right bank is forest. Downstream of the bridge the left bank is pasture and the right bank has woody vegetation. In the study area, the Middle Branch Williams River has an incised, straight channel with a slope of approximately 0.009 ft/ft, an average channel top width of 63 ft and an average bank height of 5 ft. The channel bed material ranges from sand to cobble with a median grain size (D 50 ) of 64.7 mm (0.212 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 10, 1996, indicated that the reach was stable. The Town Highway 29 crossing of the Middle Branch Williams River is a 34-ft-long, two- lane bridge consisting of one 32-foot steel-beam span (Vermont Agency of Transportation, written communication, April 5, 1995). 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 0 degrees. A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the left abutment during the Level I assessment. Scour protection measures at the site include type- 2 stone fill (less than 36 inches diameter) along the upstream right bank and downstream left bank and around the upstream left and right wingwalls. Type- 3 stone fill (less than 48 inches diameter) is located along the base of the left abutment in the scour hole, at the end of the downstream left wingwall and 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 all modelled flows ranged from 0.4 to 0.9 ft. The worst-case contraction scour occurred at the incipient-overtopping discharge and the 100-year discharge. Abutment scour ranged from 10.7 to 13.6 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 12 (HUNTTH00010012) on Town Highway 001, crossing Brush Brook, Huntington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure HUNTTH00010012 on Town Highway 1 crossing Brush Brook, Huntington, Vermont (figures 1–9). 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. In August 1976, Hurricane Belle caused flooding at this site which resulted in road and bridge damage (figures 7-8). This was approximately a 25-year flood event based on flood- frequency data contained in the Flood Insurance Study for the Town of Huntington (U.S. Department of Housing and Urban Development, 1978). The site is in the Green Mountain section of the New England physiographic province in central Vermont. The 9.19-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 some woody vegetation. In the study area, the Brush Brook has a sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 62 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 100.0 mm (0.328 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 1 crossing of Brush Brook is a 64-ft-long, two-lane bridge consisting of one 62-foot steel-stringer span (Vermont Agency of Transportation, written communication, November 30, 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 6 degrees. Channel scour 2.2 ft deeper than the mean thalweg depth was observed along the upstream right bank and along the base of the spill-through protection for the right abutment during the Level I assessment. Scour protection measured at the site was type-2 stone fill (less than 36 inches diameter) along the upstream left and right banks and in front of all four wingwalls. In front of the abutments, there was type-3 stone fill (less than 48 inches diameter) forming a spill-through slope. 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 flow. Abutment scour ranged from 1.4 to 2.8 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 9. 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 7H (HUNTTH0001007H) on Town Highway 1, crossing Cobb Brook, Huntington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure HUNTTH001007H on Town Highway 1 crossing the Cobb Brook, Huntington, Vermont (figures 1–10). 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. In August 1976, Hurricane Belle caused flooding at this site which resulted in road and bridge damage (figures 7-8). This was approximately a 25-year flood event (U.S. Department of Housing and Urban Development, 1978). The site is in the Green Mountain section of the New England physiographic province in central Vermont. The 4.20-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 of the bridge. Downstream of the bridge is brushland and pasture. In the study area, the Cobb Brook has an incised, straight channel with a slope of approximately 0.03 ft/ft, an average channel top width of 43 ft and an average bank height of 6 ft. The channel bed material ranges from sand to boulders with a median grain size (D 50 ) of 65.5 mm (0.215 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 24, 1996, indicated that the reach was stable. The Town Highway 1 crossing of the Cobb Brook is a 23-ft-long, two-lane bridge consisting of one 20-foot concrete slab span (Vermont Agency of Transportation, written communication, June 21, 1996). 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 zero degrees. A scour hole 2.8 ft deeper than the mean thalweg depth was observed along the left abutment during the Level I assessment. Protection measures at the site include type-1 stone fill (less than 12 inches diameter) at the downstream right wingwall, type-2 stone fill (less than 36 inches diameter) at the upstream right wingwall and the downstream end of the downstream left wingwall, and type-3 stone fill (less than 48 inches diameter) at the upstream 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 0.2 to 1.3 ft. The worst-case contraction scour occurred at the incipient-overtopping discharge. Abutment scour ranged from 4.0 to 8.7 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 10. 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 34 (ROCHTH00210034) on Town Highway 21, crossing the White River, Rochester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ROCHTH00210034 on Town Highway 21 crossing the White River, Rochester, 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 central Vermont. The 74.8-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is suburban on the upstream and downstream left overbanks, though brush prevails along the immediate banks. On the upstream and downstream right overbanks, the surface cover is pasture with brush and trees along the immediate banks. In the study area, the White River has an incised, straight channel with a slope of approximately 0.002 ft/ft, an average channel top width of 102 ft and an average bank height of 5 ft. The channel bed material ranges from sand to cobble with a median grain size (D 50 ) of 74.4 mm (0.244 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 23, 1996, indicated that the reach was stable. The Town Highway 21 crossing of the White River is a 72-ft-long, two-lane bridge consisting of 70-foot steel stringer span (Vermont Agency of Transportation, written communication, March 22, 1995). The opening length of the structure parallel to the bridge face is 67.0 ft. 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 zero degrees. Channel scour, 1.5 ft deeper than the mean thalweg depth was observed along the left abutment and wingwalls during the Level I assessment. Scour countermeasures at the site includes type-1 stone fill (less than 12 inches diameter) along the upstream left bank and the upstream and downstream left road embankments, type-2 (less than 36 inches diameter) along the upstream end of the upstream left wingwall and downstream left bank, and type-3 (less than 48 inches diameter) along 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 recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). In addition, the incipient roadway-overtopping discharge is analyzed since it has the potential of being the 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 discharges was zero. Left abutment scour ranged from 6.8 to 21.2 ft. Right abutment scour ranged from 13.9 to 18.4 ft. The worst-case abutment scour occurred at the 500-year discharge at the left and right 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. 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 (JERITH00350031) on Town Highway 35, crossing Mill Brook, Jericho, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure JERITH00350031 on Town Highway 35 crossing Mill Brook, Jericho, 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, gathered 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 and the Champlain section of the St. Lawrence physiographic province in northwestern Vermont. The 15.7-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest upstream of the bridge. The downstream left overbank is pasture. The downstream right overbank is brushland. In the study area, the Mill Brook has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 117 ft and an average bank height of 11 ft. The channel bed material ranges from gravel to boulders with a median grain size (D 50 ) of 81.1 mm (0.266 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 3, 1996, indicated that the reach was laterally unstable. The Town Highway 35 crossing of the Mill Brook is a 53-ft-long, one-lane bridge consisting of a 50-foot steel-beam span with a wooden deck (Vermont Agency of Transportation, written communication, November 30, 1995). The opening length of the structure parallel to the bridge face is 48 ft. The bridge is supported by a vertical, concrete abutment with wingwalls on the left. On the right, the abutment and wingwalls are laid-up stone with a concrete cap. The channel is not skewed to the opening. The roadway is skewed 10 degrees to the opening. A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the left abutment during the Level I assessment. Scour countermeasures at the site were type-2 stone fill (less than 36 inches diameter) at the upstream and downstream left wingwalls, the upstream and downsteam left channel banks, and the downstream 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 recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). In addition, the incipient roadway-overtopping discharge is analyzed since it has the potential of being the 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 0.4 to 1.3 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 9.9 to 12.4 ft. Right abutment scour ranged from 13.8 to 17.8 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 41 (ANDOVT00110041) on State Route 11, crossing the Middle Branch Williams River, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ANDOVT00110041 on State Route 11 crossing the Middle Branch Williams River, 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 southeastern Vermont. The 12.1-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is grass on the upstream right overbank while the immediate banks have dense woody vegetation. The upstream left overbank and downstream right overbank are brushland. The downstream left overbank is forested. In the study area, the Middle Branch Williams River has an incised, sinuous channel with a slope of approximately 0.018 ft/ft, an average channel top width of 71 ft and an average bank height of 4 ft. The channel bed material ranges from gravel to boulders with a median grain size (D 50 ) of 85.0 mm (0.279 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 10, 1996, indicated that the reach was laterally unstable due to a cut-bank present on the upstream right bank and a wide channel bar with vegetation in the upstream reach. The State Route 11 crossing of the Middle Branch Williams River is a 46-ft-long, two-lane bridge consisting of a concrete 44-foot tee-beam span (Vermont Agency of Transportation, written communication, March 29, 1995). The opening length of the structure parallel to the bridge face is 42 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 35 degrees to the opening while the opening-skew-toroadway is zero degrees. A scour hole 0.8 ft deeper than the mean thalweg depth was observed along the downstream end of the left abutment and downstream left wingwall during the Level I assessment. Type- 2 stone fill (less than 36 inches diameter) protects the upstream end of the upstream left wingwall, the downstream ends of the downstream left and right wingwalls and the downstream right road embankment. Type-3 stone fill protects the upstream end of the upstream right wingwall and 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). 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 2.1 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 11.1 to 18.7 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). 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 18 (SHEFTH00410018) on Town Highway 41, crossing Millers Run, Sheffield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure SHEFTH00410018 on Town Highway 41 crossing Millers Run, Sheffield, 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 White Mountain section of the New England physiographic province in northeastern Vermont. The 16.2-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is grass upstream and downstream of the bridge while the immediate banks have dense woody vegetation. In the study area, Millers Run has an incised, straight channel with a slope of approximately 0.01 ft/ft, an average channel top width of 50 ft and an average bank height of 6 ft. The channel bed material ranges from sand to boulder with a median grain size (D 50 ) of 50.9 mm (0.167 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 1, 1995, indicated that the reach was laterally unstable, which is evident in the moderate to severe fluvial erosion in the upstream reach. The Town Highway 41 crossing of the Millers Run is a 30-ft-long, one-lane bridge consisting of a 28-foot steel-stringer span (Vermont Agency of Transportation, written communication, March 28, 1995). The opening length of the structure parallel to the bridge face is 22.2 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 20 degrees to the opening. The computed opening-skewto-roadway is 5 degrees, while it is zero degrees in the historical form. A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the left abutment during the Level I assessment. The scour protection measure at the site includes type-1 stone fill (less than 12 inches diameter) along the upstream right wingwall and the upstream left wingwall. Type-2 stone fill (less than 36 inches diameter) extends along the downstream end of the downstream left wingwall, the upstream right bank and the downstream left bank. The downstream right bank is protected by type-2 stone fill and a stone masonry wall. 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 0.2 to 1.8 ft. The worst-case contraction scour occurred at the 100-year and 500-year discharges. Left abutment scour ranged from 14.1 to 16.4 ft. The worst-case left abutment scour occurred at the 500-year discharge. Right abutment scour ranged from 6.9 to 9.3 ft. The worst-case right abutment scour occurred at the incipient roadway-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↗
USGS1997

Level II scour analysis for Bridge 29 (DORSTH00100029) on Town Highway 10, crossing the Mettawee River, Dorset, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure DORSTH00100029 on Town Highway 10 crossing the Mettawee River, Dorset, 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 9.5-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 left overbank and the upstream and downstream right overbanks. The downstream left overbank is pasture and brushland. In the study area, the Mettawee River has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 66 ft and an average bank height of 8 ft. The channel bed material ranges from gravel to boulders with a median grain size (D 50 ) of 79.0 mm (0.259 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 5, 1996, indicated that the reach was stable. The Town Highway 10 crossing of the Mettawee River is a 26-ft-long, two-lane bridge consisting of a 24-ft steel-stringer span (Vermont Agency of Transportation, written communication, September 28, 1995). The opening length of the structure parallel to the bridge face is 24.1 ft. 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. At the upstream end of the right abutment, there is a scour hole 1.0 ft deeper than the mean thalweg depth. Scour counter-measures at the site include type-1 stone fill (less than 12 inches diameter) along the downstream right wingwall. Type-2 stone fill (less than 36 inches diameter) is present along the downstream left and right banks. Type-3 stone fill (less than 48 inches diameter) is present along the upstream left bank and sparsely in front of the right abutment. A concrete wall (old abutment) extends 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 ranged from 0.4 to 1.9 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 10.5 to 10.8 ft. The worst-case left abutment scour occurred at the 500-year discharge. Right abutment scour ranged from 11.4 to 11.9 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↗
USGS1997

Level II scour analysis for Bridge 19 (SHEFTH00440019) on Town Highway 44, crossing Trout Brook, Sheffield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure SHEFTH00440019 on Town Highway 44 crossing Trout Brook, Sheffield, 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 White Mountain section of the New England physiographic province in northeastern Vermont. The 3.0-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is grass on the upstream and downstream right overbanks, while the immediate banks have dense woody vegetation. The surface cover of the upstream and downstream left overbanks is shrub and brushland. In the study area, Trout Brook 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 bank height of 6 ft. The channel bed material ranges from sand to boulder with a median grain size (D 50 ) of 116 mm (0.381 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 31, 1995, indicated that the reach was stable. The Town Highway 44 crossing of Trout Brook is a 24-ft-long, one-lane bridge consisting of a 22-foot steel-stringer span (Vermont Agency of Transportation, written communication, March 28, 1994). The opening length of the structure parallel to the bridge face is 19.8 ft. 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. 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 analyzed since it has the potential of being the 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 resulted in zero ft. Left abutment scour ranged from 4.4 to 5.6 ft. The worst-case left abutment scour occurred at the 500-year discharge. Right abutment scour ranged from 3.6 to 4.8 ft. The worst-case right abutment scour occurred at the incipient roadway-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 particlesize 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 (ROYATH00920029) on Town Highway 92, crossing the First Branch White River, Royalton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ROYATH00920029 on Town Highway 92 crossing the First Branch White River, Royalton, 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 101-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. In the study area, the First Branch White River has an incised, sinuous channel with a slope of approximately 0.001 ft/ft, an average channel top width of 81 ft and an average bank height of 9 ft. The channel bed material ranges from sand to bedrock with a median grain size (D 50 ) of 1.18 mm (0.00347 ft). The geomorphic assessment at the time of the Level I site visit on July 23, 1996 and Level II site visit on June 2, 1995, indicated that the reach was stable. The Town Highway 92 crossing of the First Branch White River is a 59-ft-long, one-lane bridge consisting of a 57-foot steel-stringer span (Vermont Agency of Transportation, written communication, March 23, 1995). The opening length of the structure parallel to the bridge face is 52.2 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 20 degrees to the opening while the opening-skew-to-roadway is zero degrees. A scour hole 4.0 ft deeper than the mean thalweg depth was observed in the upstream channel 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 left and right wingwalls, the left abutment and 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. 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 4.1 ft. The worst-case contraction scour occurred at the incipient roadway-overtopping discharge, which was less than the 100-year discharge. Left abutment scour ranged from 12.9 to 15.4 ft, where the worst-case abutment scour occurred at the 500-year discharge. Right abutment scour ranged from 14.5 to 15.0 ft, where the worst-case 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↗