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USGS2024

Improved computational methods for probabilistic liquefaction hazard analysis

Current procedures for analysis of and design against liquefaction hazards focus primarily on the use of probabilistic ground motions at a single ground-shaking hazard level, with the cyclic loading represented by a peak ground acceleration ( PGA ) corresponding to a target return period and a single representative moment magnitude M w . These parameters are typically used in conjunction with deterministic simplified procedures for estimating liquefaction triggering and consequences that largely neglect the considerable uncertainties inherent to liquefaction problems. Motivated by these limitations and the resulting inconsistencies in liquefaction design levels, early methods for probabilistic liquefaction hazard analysis (PLHA) were proposed that incorporate the full ground motion hazard space, integrated with probabilistic liquefaction triggering models. Such methods provide liquefaction factor of safety ( FS L ) hazard curves for standard penetration test (SPT) data. Recognizing the increased use of higher-resolution cone penetrometer test (CPT) data in engineering analysis and design and the potential computational challenges it presents, an expanded suite of probabilistic triggering models, and wider availability of more detailed seismic hazard data, an improved PLHA computational methodology is presented in this study. The methods described utilize the U.S. Geological Survey National Seismic Hazard Model directly through web services to obtain PGA hazard and disaggregation calculations for any site and average shear wave velocity in the upper 30 m of the site ( V s 30 ) in the conterminous United States, to reconstruct the PGA hazard space for use in PLHA calculations, and to employ array calculations for efficient liquefaction hazard curve estimates for the thousands of CPT measurements in a given profile. The framework presented here is modular in nature, and discussion on the use of alternative models and extension to probabilistic liquefaction consequence evaluation is presented. The importance of appropriate representation of probabilistic liquefaction model uncertainties is also highlighted, along with the impacts of different levels of uncertainty on the PLHA calculation. Finally, a potential roadmap for incorporation of the PLHA framework in seismic provisions is presented, with an illustration of how it can address current limitations and impacts in liquefaction hazard analysis and design.

Soil Dynamics and Earthquake Engineering↗
USGS1996

Level II scour analysis for Bridge 49 (BRIDTH00530049) on Town Highway 53, crossing North Branch Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure BRIDTH00530049 on town highway 53 crossing the North Branch of the Ottauquechee River, Bridgewater, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). 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 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 26.6-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the immediate banks have woody vegetation coverage with grass on the overbanks. In the study area, the North Branch Ottauquechee River has a sinuous channel with a slope of approximately 0.0075 ft/ft, an average channel top width of 66 ft and an average channel depth of 6 ft. The predominant channel bed material is cobble and gravel (D 50 is 68.4 mm or 0.224 ft). The geomorphic assessment at the time of the Level I and Level II site visit on October 27, 1994, indicated that the reach was stable. The town highway 53 crossing of the North Branch of the Ottauquechee Riveris a 51-ftlong, one-lane bridge consisting of one 49-foot steel-beam span (Vermont Agency of Transportation, written communication, August 25, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is not skewed to the opening and the opening-skew-to-roadway is zero degrees. The scour protection measures in place at the site are type-1 stone fill (less than 12 inches diameter) along the upstream left wingwall and type-2 stone fill (less than 36 inches diameter) along 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 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 was 0 feet. Abutment scour ranged from 2.3 to 12.0 feet and 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, 1993, 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 6 (RICHTH00030006) on Town Highway 3, crossing an unnamed tributary to the Missisquoi River, Richford, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure RICHTH00030006 on Town Highway 3 crossing an unnamed tributary to the Missisquoi River, 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 of northern Vermont. The 4.5-mi 2 drainage area is in a predominantly rural basin. In the vicinity of the study site, the surface cover is pasture upstream and downstream of the bridge. In the study area, the unnamed tributary to the Missisquoi River is a sinuous channel with a slope of approximately 0.008 ft/ft, an average channel top width of 39 ft and an average channel depth of 2 ft. The channel slope was obtained from a topographic map (USGS, 1986). The predominant channel bed material is gravel with a median grain size (D 50 ) of 26.2 mm (0.0861 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 stable. The Town Highway 3 crossing of an unnamed tributary to the Missisquoi River is a 26-ftlong, two-lane bridge consisting of one 24-foot concrete T-beam span (Vermont Agency of Transportation, written communication, March 9, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 40 degrees to the opening while the opening-skew-to-roadway is 0.0 degrees. The only scour protection measures at the site were type-2 stone fill (less than 36 inches diameter) along the upstream right wingwall and at the upstream end of 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 1.7 to 1.8 ft. The worst-case contraction scour occurred at the 500-year discharge. Scour at the left abutment ranged from 7.6 to 12.6 ft with the worst case occurring at the 100-year event. Scour at the right abutment ranged from 1.6 to 5.6 ft with the worst case occurring at the 500-year event. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.

Vermont↗
USGS1997

Level II scour analysis for Bridge 6 (STAMVT01000006) on State Route 100, crossing the North Branch of the Hoosic River, Stamford, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure STAMVT01000006 on Vermont Highway 100 crossing the North Branch of the Hoosic River, Stamford, 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 8.61-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is grass with forest along the immediate banks throughout the reach. In the study area, the North Branch of the Hoosic River has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 44 ft and an average bank height of 4 ft. The channel bed material ranges from sand to boulder with a median grain size (D 50 ) of 97.4 mm (0.320 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 31, 1996, indicated that the reach was stable. The Vermont Highway 100 crossing of the North Branch of the Hoosic River is a 99-ft-long, two-lane bridge consisting of one 97-foot steel-beam span (Vermont Agency of Transportation, written communication, September 28, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 60 degrees to the opening and the opening-skew-to-roadway is also 60 degrees. The scour protection measures at the site included type-1 stone fill (less than 12 inches diameter) along the downstream left bank. Type-2 (less than 36 inches diameter) stone fill was found along the upstream left bank and the downsteam right bank. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.7 to 2.1 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 18.8 to 19.7 ft along the right abutment and from 2.8 to 6.8 ft along the left abutment. 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 15 (GRNVTH00230015) on Town Highway 23, crossing the Third Branch of the White River, Granville, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure GRNVTH00230015 on town highway 23 crossing the Third Branch of the White River, Granville, 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 Granville. The 23.6-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 banks, which are residential. In the study area, the Third Branch of the White River has an incised, sinuous channel with a slope of approximately 0.0128 ft/ft, an average channel top width of 42 ft and an average channel depth of 4 ft. The predominant channel bed material is cobble (D 50 is 108 mm or 0.353 ft). The geomorphic assessment at the time of the Level I and Level II site visit on October 21, 1994, indicated that the reach was laterally unstable. The town highway 23 crossing of the Third Branch of the White River is a 35-ft-long, one-lane bridge consisting of one 31-foot steel beam span (Vermont Agency of Transportation, written communication, August 26, 1994). 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 10 degrees. The only scour protection measures in place at the site were type-1 stone fill (less than 12 inches diameter) along the upstream right bank, upstream right wingwall, and right abutment. Retaining walls are in place along the upstream left bank up to the upstream end of the upstream left wingwall and both downstream banks with the left bank wall extending from the downstream left wingwall. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute scour 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 0.4 ft. The worst-case contraction scour occurred at the incipient overtopping discharge. Abutment scour ranged from 9.8 to 13.9 ft. 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 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 145 (HANCVT01000145) on Vermont Highway 100, crossing the Hancock Branch of the White River, Hancock, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure HANCVT01000145 on State Route 100 crossing the Hancock Branch of the White River, Hancock, 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 22.0-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is urban on left bank and forested on the right bank upstream of the bridge while the immediate banks have woody vegetation. Downstream of the bridge surface cover on both banks is pasture while the immediate banks have woody vegetation. In the study area, the Hancock Branch of the White River has an incised, sinuous channel with a slope of approximately 0.006 ft/ft, an average channel top width of 48 ft and an average channel depth of 3 ft. The predominant channel bed materials are cobble and gravel with a median grain size (D 50 ) of 71.9 mm (0.236 ft). The geomorphic assessment at the time of the Level I and Level II site visit on November 16, 1994, indicated that the reach was stable. State Route 100 crossing the Hancock Branch of the White River is a 55-ft-long, two-lane bridge consisting of one 53-foot steel-beam span (Vermont Agency of Transportation, written communication, August 26, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is not skewed to the opening and the opening-skewto-roadway is 0 degrees. The only scour protection measures at the site were type-3 stone fill (less than 48 inches diameter) at the upstream right wingwall, both downstream wingwalls and the downstream ends of both abutments. Also there was type-2 stone fill (less than 36 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 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 3.4 to 4.3 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.2 to 11.1 ft. 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↗
USGS1996

Level II scour analysis for Bridge 38 (RANDTH00640038) on Town Highway 64, crossing the Second Branch of the White River, Randolph, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure RANDTH00640038 on town highway 64 crossing the Second Branch of the White River, Randolph, 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 of central Vermont. The 46.5-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the The upstream left bank is forested, the upstream right bank is covered primarily by brush, the surface cover is pasture on the downstream left and row crops on the downstream right. In the study area, the Second Branch of the White River has an incised, sinuous channel with a slope of approximately 0.0015 ft/ft, an average channel top width of 71 ft and an average channel depth of 8 ft. The predominant channel bed material is gravel with a median grain size (D 50 ) of 32.0 mm (0.105 ft). The geomorphic assessment at the time of the Level I site visits on August 10, 1994 and December 5, 1994 indicated that the reach was laterally unstable. The town highway 64 crossing of the Second Branch of the White Riveris a 43-ft-long, one-lane covered bridge consisting of one 35-foot steel-beam span (Vermont Agency of Transportation, written communication, August 1, 1994). The bridge is supported by vertical, stone abutments with upstream wingwalls. The channel bends sharply at it’s approach to the bridge, however, at the bridge face, the channel is skewed approximately 0 degrees to the opening. The opening-skew-to-roadway is also 0 degrees. A scour hole 2 ft deeper than the mean thalweg depth was observed upstream of the bridge along the outside of the channel bend. Other scour problems at this site included undermining of the right abutment at it’s upstream and downstream ends. 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 1.7 to 2.6 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.2 to 24.2 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). 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 43 (CHESVT00110043) on State Highway 11, crossing the Middle Branch Williams River, Chester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure CHESVT00110043 on State Route 11 crossing the Middle Branch of the Williams River, Chester, 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 principally in the Green Mountain section of the New England physiographic province in southeastern Vermont. The 13.7-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is predominantly forest except for the area downstream and right of the bridge which is pasture. In the study area, the Middle Branch of the Williams River has a sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 77 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 76.6 mm (0.251 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 11, 1996, indicated that the reach was laterally unstable. Lateral instability was evident from the several point bars and cut banks located through the study reach. The State Route 11 crossing of the Middle Branch of the Williams River is a 76-ft-long, two-lane bridge consisting of two 37-foot concrete Tee-beam spans (Vermont Agency of Transportation, written communication, March 29, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 35 degrees to the opening. The computed opening-skew-to-roadway was 30 degrees but the historical records indicate this angle is 25 degrees. Scour protection measures at the site consist of type-1 stone fill (less than 12 inches diameter) along the downstream banks and the upstream right wing wall. Type-2 (less than 36 inches diameter) stone fill protection is noted on the upstream and downstream left wingwalls and upstream along the 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 0.0 to 1.5 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.2 to 10.7 ft. The worst-case abutment scour occurred at the 500-year discharge for the right abutment. Pier scour ranged from 7.3 to 8.6 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 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 (SUNDFLR0030012) on Forest Land Road 3, crossing Roaring Branch Brook, Sunderland, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure SUNDFLR0030012 on Forest Land Road (FLR) 3 (FAS 114) crossing Roaring Branch Brook, Sunderland, 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 southwestern Vermont. The 4.93-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is dense forest along the left bank and primarily shrubs and trees along the right bank, both upstream and downstream of the bridge. In the study area, Roaring Branch Brook has an incised, sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 33 ft and an average bank height of 4 ft. The channel bed material ranges from cobble to bedrock with a median grain size (D 50 ) of 139 mm (0.457 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 30, 1996, indicated that the reach was stable. Forest Land Road 3 (FAS 114) crossing of Roaring Branch Brook is a 37-ft-long, two-lane bridge consisting of one 35-foot steel-beam span (Vermont Agency of Transportation, written communication, December 14, 1995). 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. The scour protection measures at the site included type-3 stone fill (less than 48 inches diameter) along the left and right abutments, along the upstream left and downstream right wing walls and along the downstream right bank. Type-4 (less than 60 inches diameter) stone fill was found along the upstream right and downstream left wingwalls and along the downstream left bank. Type-2 (less than 36 inches diameter) stone fill scour protection was found along the upstream left and right banks. 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 calculated to be 0.0 ft. Abutment scour ranged from 4.3 to 10.4 ft. The worst-case abutment scour occurred at the 500-year discharge along 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 22 (WALDTH00180022) on Town Highway 18, crossing Coles Brook, Walden, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure WALDTH00180022 on Town Highway 18 crossing Coles Brook also known as Joes Brook, Walden, 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 12.5-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is predominantly forested while the downstream left bank is shrub and brushland. In the study area, the Coles Brook has an incised, sinuous channel with a slope of approximately 0.004 ft/ft, an average channel top width of 54 ft and an average bank height of 4 ft. The channel bed material ranges from gravel to bedrock with a median grain size (D50) of 124.1 mm (0.407 ft). The D 50 was taken from a pebble count in the downstream channel, because the upstream channel is primarily bedrock. The geomorphic assessment at the time of the Level I and Level II site visit on August 8, 1995, indicated that the reach was stable. The Town Highway 18 crossing of the Coles Brook is a 46-ft-long, one-lane bridge consisting of one 44-foot steel-beam span with a wooden deck (Vermont Agency of Transportation, written communication, April 5, 1995). The opening length of the structure parallel to the bridge face is 41.4 ft. The bridge is supported by a vertical, concrete abutment with wingwalls on the left and by a vertical, stone abutment with stone wingwalls with a concrete cap on the right. The channel is skewed approximately 5 degrees to the opening while the computed opening-skew-to-roadway is 10 degrees. The only scour protection measure at the site was a stone wall along the upstream left bank and type-2 stone fill (less than 36 inches diameter) along the entire base length of the upstream left wingwall, left abutment, and downstream left wing 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-year 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 was 0.0 ft. Abutment scour ranged from 6.4 to 7.9 ft at the left abutment and from 11.8 to 14.9 ft at the right abutment. 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”. Scouredstreambed 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↗
USGS1996

Level II scour analysis for Bridge 46 (CHELTH00680046) on Town Highway 68, crossing the First Branch of the White River, Chelsea, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure CHELTH00680046 on town highway 68 crossing the First Branch of the White River, Chelsea, 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 of central Vermont in the town of Chelsea. The 58.2-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. In the study area, the First Branch of the White River has a sinuous channel with a slope of approximately 0.0054 ft/ft, an average channel top width of 92 ft and an average channel depth of 4 ft. The predominant channel bed material is gravel and cobble (D 50 is 52.7 mm or 0.173 ft). The geomorphic assessment at the time of the Level I and Level II site visit on November 16, 1994, indicated that the reach was stable. The town highway 68 crossing of the First Branch of the White River is a 61-ft-long, onelane covered bridge with a 52-foot clear-span (Vermont Agency of Transportation, written commun., August 26, 1994). The bridge is supported by vertical, stone abutments with a concrete wingwall on the downstream right. The left abutment is laid-up stone supported by concrete at the upstream and downstream ends of the laid-up stone abutment. The channel is skewed approximately 40 degrees to the opening while the opening-skew-to-roadway is 15 degrees. A scour hole 1.5 ft deeper than the mean thalweg depth was observed under the bridge during the Level I assessment. The scour protection measures in place at the site were type- 2 stone fill (less than 36 inches diameter) at the road approach embankments except the downstream left embankment which had no protection. The upstream right road embankment, impacted by the channel bend, has an extensive covering of stone fill for erosion protection. Type-3 stone fill (less than 48 inches diameter) was noted along 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 0.9 to 2.6 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 14.3 to 24.0 ft. The worst-case abutment scour occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. The left abutment sits atop a bedrock outcrop. The results of the calculated scour depths will be limited by the bedrock. 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 (STAMVT01000008) on State Highway 100, crossing the North Branch of the Hoosic River, Stamford, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure STAMVT01000008 on Vermont Highway 100 crossing the North Branch of the Hoosic River, Stamford, 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 6.8-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the upstream right bank surface cover is short grass and the upstream left bank is a sand/gravel lot while the immediate banks are covered by shrubs and trees. Downstream of the bridge banks are forested. In the study area, the North Branch of the Hoosic River has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 37 ft and an average bank height of 3 ft. The channel bed material is predominantly cobble with a median grain size (D 50 ) of 88.0 mm (0.289 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 1, 1996, indicated that the reach was laterally unstable. The Vermont Highway 100 crossing of the North Branch of the Hoosic River is a 39-ft-long, two-lane bridge consisting of one 37-foot steel-beam span (Vermont Agency of Transportation, written communication, November 1, 1995). The bridge is supported by vertical, concrete abutments. The channel is skewed approximately 20 degrees to the opening while the opening-skew-to-roadway is 15 degrees. A scour hole 2.5 ft deeper than the mean thalweg depth was observed along the upstream end of the right abutment during the Level I assessment. The only scour protection measure at the site was type-3 stone fill (less than 48 inches diameter) at the downstream ends of the left and right abutments extending downstream along the left bank for 13 feet and along the right bank for 16 feet. The plans show stone fill placed at the upstream ends of the abutments. The protection at the upstream end of the right abutment has failed due to stream migration towards the right bank. The protection at the upstream end of the left abutment was not detected due to the sand/gravel pile, for District 1 maintenance, migrating into the channel (Figure 3). 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 0.6 to 3.0 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 14.4 to 17.8 ft. Right abutment scour ranged from 8.1 to 11.1 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↗
USGS1998

Level II scour analysis for Bridge 32 (FERRTH00190032) on Town Highway 19, crossing the South Slang Little Otter Creek, Ferrisburgh, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure FERRTH00190032 on Town Highway 19 crossing the South Slang Little Otter Creek (Hawkins Slang Brook), Ferrisburg, 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 Champlain section of the St. Lawrence Valley physiographic province in west-central Vermont. The 8.00-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover consists of wetlands upstream and downstream of the bridge with trees and pasture on the wide flood plains. In the study area, the South Slang Little Otter Creek has a meandering channel with essentially no channel slope, an average channel top width of 932 ft and an average bank height of 6 ft. The channel bed material ranges from clay to sand. Sieve analysis indicates that greater than 50% of the sample is coarse silt and clay and thus a medium grain size by use of sieve analysis was indeterminate. The median grain size was assumed to be a course silt with a size (D 50 ) of 0.061mm (0.0002 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 2, 1996, indicated that the reach was stable. The Town Highway 19 crossing of the South Slang Little Otter Creek is a 45-ft-long, twolane bridge consisting of one 42-foot concrete box-beam span (Vermont Agency of Transportation, written communication, December 11, 1995). The opening length of the structure parallel to the bridge face is 41.8 ft. The bridge is supported by vertical, concrete abutments. The channel is skewed approximately 5 degrees to the opening while the opening-skew-to-roadway is zero degrees. A scour hole 3.5 ft deeper than the mean thalweg depth was observed in the upstream channel. Also a scour hole 2.0 ft deeper than the mean thalweg depth was observed along the right abutment during the Level I assessment. The scour protection measures at the site are type-1 stone fill (less than 12 inches diameter) around the left and right abutments, along the upstream and downstream road embankments, and across the entire upstream and downstream bridge face. 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 14.0 to 20.2 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 3.2 to 8.3 ft. The worst-case abutment scour occurred at the 500-year discharge. The predicted scour is well above the pile bottom elevations. 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. 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 10 (BENNUS00070010) on U.S. Route 7, crossing the Walloomsac River, Bennington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure BENNUS00070010 on U.S. Route 7, also known as North Street, crossing of the Walloomsac River, Bennington, 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 southwestern Vermont. The 30.1-mi 2 drainage area is a predominantly rural and forested basin. The bridge site is located within an urban setting in the Town of Bennington with buildings, parking lots, lawns, and a playground on the overbank areas. In the study area, the Walloomsac River has a straight channel with constructed channel banks through much of the reach. The channel is located on a delta and has a slope of approximately 0.02 ft/ft, an average channel top width of 37 ft and an average bank height of 5 ft. The predominant channel bed material is cobble with a median grain size (D 50 ) of 96.0 mm (0.315 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 5, 1996, indicated that the constructed reach was stable. The U.S. Route 7 crossing of the Walloomsac River is a 53-ft-long, two-lane bridge consisting of one 50-foot steel span (Vermont Agency of Transportation, written communication, September 27, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The wingwalls are angled in toward the channel because the widths of the upstream and downstream constructed channel banks are narrower than the bridge opening. The channel is skewed approximately 5 degrees to the opening and the opening-skew-to-roadway is 10 degrees. Scour countermeasures at the site include masonry and stone walls on both the upstream and downstream banks. 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 computed for all modelled flows ranged from 0.0 to 0.1 ft. The worstcase contraction scour occurred at the 500-year discharge. Computed left abutment scour ranged from 5.9 to 6.8 ft. with the worst-case scour occurring at the 500-year discharge. Computed right abutment scour for all modelled flows was 6.8 ft. Total scour depths for all modelled flows did not exceed the depth of the abutment footings. 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 9 (JAYVT02420009) on Vermont Highway 242, crossing the Jay Branch of the Missisquoi River, Jay, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure JAY-VT02420009 on Vermont highway 242 crossing the the Jay Branch of the Missisquoi River, Jay, 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 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 is found in Appendix D. The site is in the Green Mountain section of the New England physiographic province of northern Vermont in the town of Jay. The 4.36-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is primarily forest and brush except for the downstream left overbank which is grass. In the study area, the the Jay Branch of the Missisquoi River has an incised, sinuous channel with a slope of approximately 0.021 ft/ft, an average channel top width of 38 ft and an average channel depth of 5 ft. A Level I visual inspection at the site indicates that the predominant channel bed material is cobble and boulder with gravel. Results of a pebble count indicate that the predominant channnel bed material is a very coarse gravel with a median grain size (D 50 ) of 41.7 mm (0.1369 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 6, 1995, indicated that the reach was stable. The Vermont highway 242 crossing of the the Jay Branch of the Missisquoi River is a 60- ft-long, two-lane bridge consisting of one 55-foot steel-beam span (Vermont Agency of Transportation, written communication, March 6, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 60 degrees to the opening while the opening-skew-to-roadway is 45 degrees. The scour protection measures at the site included type-2 stone fill (less than 36 inches diameter) at the upstream right wingwall, the downstream left and right wingwalls and the upstream end of the left abutment. Type-1 stone fill (less than 12 inches) was along the upstream end of the right abutment. Type-4 stone fill (less than 64 inches) was along 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 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 0.6 ft. The worst-case contraction scour occurred at the 100-year discharge. Abutment scour ranged from 0.8 to 5.6 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 7 (CHESTH00030007) on Town Highway 3, crossing the South Branch Williams River, Chester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure CHESTH00030007 on Town Highway 3 which is also State Route 35 crossing the South Branch Williams River, Chester, 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 southern Vermont. The 10.4-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture on the upstream right bank while the immediate bank has some trees. Downstream of the bridge and the upstream left bank are forested. In the study area, the South Branch Williams River has an incised, sinuous channel with a slope of approximately 0.03 ft/ft, an average channel top width of 65 ft and an average bank height of 4 ft. The channel bed material ranges from gravel to boulder with a median grain size (D 50 ) of 70.5 mm (0.231 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 26, 1996, indicated that the reach was laterally unstable. There are cutbanks on both the left and right banks alternating with point bars in the upstream reach. The Town Highway 3 (VT 35) crossing of the South Branch Williams River is a 74-ft-long, two-lane bridge consisting of one 72-foot steel-beam span (Vermont Agency of Transportation, written communication, March 30, 1995). The bridge is supported by spill- through abutments. The channel is skewed approximately 5 degrees to the opening and the opening-skew-to-roadway is also 5 degrees. Three channel scour holes 1.0 ft deeper than the mean thalweg depth were observed during the Level I assessment in the upstream reach. There are no scour protection measures at the site. 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 0.4 ft. The worst-case contraction scour occurred at the 100-year. Abutment scour ranged from 4.1 to 15.5 ft. The worst-case abutment scour occurred at the 500-year. 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↗
USGS2017

PRISM software—Processing and review interface for strong-motion data

Rapidly available and accurate ground-motion acceleration time series (seismic recordings) and derived data products are essential to quickly providing scientific and engineering analysis and advice after an earthquake. To meet this need, the U.S. Geological Survey National Strong Motion Project has developed a software package called PRISM (Processing and Review Interface for Strong-Motion data). PRISM automatically processes strong-motion acceleration records, producing compatible acceleration, velocity, and displacement time series; acceleration, velocity, and displacement response spectra; Fourier amplitude spectra; and standard earthquake-intensity measures. PRISM is intended to be used by strong-motion seismic networks, as well as by earthquake engineers and seismologists.

Techniques and Methods↗
USGS2016

Community exposure to potential climate-driven changes to coastal-inundation hazards for six communities in Essex County, Massachusetts

Introduction Understanding if and how community exposure to coastal hazards may change over time is crucial information for coastal managers tasked with developing climate adaptation plans. This report summarizes estimates of population and asset exposure to coastal-inundation hazards associated with sea-level-rise and storm scenarios in six coastal communities of the Great Marsh region of Essex County, Massachusetts. This U.S. Geological Survey (USGS) analysis was conducted in collaboration with National Wildlife Federation (NWF) representatives, who are working with local stakeholders to develop local climate adaptation plans for the Towns of Salisbury, Newbury, Rowley, Ipswich, and Essex and the City of Newburyport (hereafter referred to as communities). Community exposure was characterized by integrating various community indicators (land cover and land use, population, economic assets, critical facilities, and infrastructure) with coastal-hazard zones that estimate inundation extents and water depth for three time periods. Estimates of community exposure are based on the presence of people, businesses, and assets in hazard zones that are calculated from geospatial datasets using geographic-information-system (GIS) tools. Results are based on current distributions of people and assets in hazard zones and do not take into account projections of human population, asset, or land-use changes over time. Results are not loss estimates based on engineering analysis or field surveys for any particular facility and do not take into account aspects of individual and household preparedness before an extreme event, adaptive capacity of a community during an event, or long-term resilience of individuals and communities after an event. Potential losses would match reported inventories only if all residents, business owners, public managers, and elected officials were unaware of what to do if warned of an imminent threat, failed to take protective measures during an extreme event, or failed to implement any long-term strategies to mitigate potential impacts. This analysis is intended to serve as a foundation for additional risk-related studies, plans, and mitigation efforts that are tailored to local needs. After a summary of the geospatial methods used in the analysis, results are organized by community so that local officials can easily use them in their local adaptation planning efforts.

Massachusetts↗