Geology topics

Research about Rockingham, Vermont

Source-linked reports with geographic coverage including Rockingham, Vermont.

3 recordsLinked to original sources

USGS1998

Level II scour analysis for Bridge 40 (ROCKTH00140040) on Town Highway 14, crossing the Williams River, Rockingham, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ROCKTH00140040 on Town Highway 14 crossing the Williams River, Rockingham, 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 southeastern Vermont. The 99.2-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture downstream of the bridge. Upstream of the bridge, the left bank is forested and the right bank is suburban. In the study area, the Williams River has an incised, sinuous channel with a slope of approximately 0.005 ft/ft, an average channel top width of 154 ft and an average bank height of 11 ft. The channel bed material ranges from silt and clay to cobble with a median grain size (D50) of 45.4 mm (0.149 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 4, 1996, indicated that the reach was stable. The Town Highway 14 crossing of the Williams River is a 106-ft-long, one-lane covered bridge consisting of two steel-beam spans with a maximum span length of 73 ft (Vermont Agency of Transportation, written communication, April 6, 1995). The opening length of the structure parallel to the bridge face is 94.5 ft. The bridge is supported by a vertical, concrete abutment with wingwalls on the left, a vertical, laid-up stone abutment on the right and a concrete pier. The channel is skewed approximately 10 degrees to the opening while the opening-skew-to-roadway is zero degrees. A scour hole 2.1 ft deeper than the mean thalweg depth was observed towards the left side of the channel under and just downstream of the bridge during the Level I assessment. Scour protection measures at the site included type-1 stone fill (less than 12 inches diameter) at the upstream end of the upstream left wingwall and type-2 stone fill (less than 36 inches diameter) along the upstream left bank and the left abutment. 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 was zero ft. Left abutment scour ranged from 13.9 to 19.2 ft. Right abutment scour ranged from 7.0 to 11.7 ft. The worst-case abutment scour occurred at the 500-year discharge. Pier scour ranged from 18.7 to 24.7 ft and the worst case 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 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 11R (ROCKTH0001011R) on Town Highway 1 (VT 121 & FAS 125), crossing the Saxtons River, Rockingham, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ROCKTH0001011R on Town Highway 1 crossing the Saxtons River, Rockingham, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the New England Upland section of the New England physiographic province in southeastern Vermont. The 68.3-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover consists of houses, short grass, and scattered trees except along the immediate river banks, which are tree covered. In the study area, the Saxtons River has a sinuous channel with a slope of approximately 0.005 ft/ft, an average channel top width of 121 ft and an average bank height of 8 ft. The predominant channel bed materials are gravel and cobbles with a median grain size (D 50 ) of 109 mm (0.359 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 3, 1996, indicated that the reach was laterally unstable. Lateral instability was evident with respect to a cut-bank on the left bank upstream with slip failure of bank material. Furthermore, there is a wide point bar along the right bank upstream opposite the cut-bank. The Town Highway 1 crossing of the Saxtons River is a 184-ft-long, two-lane bridge consisting of three steel-beam spans (Vermont Agency of Transportation, written communication, March 30, 1995). The bridge is supported by vertical, concrete, skeletal-style abutment walls with spill-through embankments adjacent to each wall. The channel is skewed approximately 35 degrees to the opening while the opening-skew-to-roadway is 30 degrees. The only scour protection measure at the site was type-3 stone fill (less than 48 inches diameter) on the spill-through embankments. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term 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 all modelled flows at this site. Abutment scour ranged from 9.0 to 13.4 feet. The worst-case abutment scour occurred at the 500-year discharge for the left abutment. There are two piers for which computed pier scour ranged from 9.0 to 18.4 feet. The left and right piers in this report are presented as pier 1 and pier 2, respectively. The worst-case pier scour occurred at pier 2 for 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 (ROCKTH00390041) on Town Highway 39, crossing the Saxtons River, Rockingham, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure ROCKTH00390041 on Town Highway 39 crossing the Saxtons River, Rockingham, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D. The site is in the New England Upland section of the New England physiographic province in southeastern Vermont. The 57.4-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover consists of forest on the left bank and pasture with some trees on the right bank. In the study area, the Saxtons River has an sinuous channel with a slope of approximately 0.009 ft/ft, an average channel top width of 112 ft and an average bank height of 10 ft. The channel bed material ranges from sand to cobbles with a median grain size (D 50 ) of 103 mm (0.339 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 15, 1996, indicated that the reach was laterally unstable. There are wide point bars, cut-banks with fallen trees, and areas of localized channel scour along the left bank, where there is bedrock exposure at the surface. The Town Highway 39 crossing of the Saxtons River is an 85-ft-long, one-lane bridge consisting of one 82-foot steel-beam span (Vermont Agency of Transportation, written communication, March 31, 1995). The bridge is supported by vertical, concrete abutments without wingwalls. The channel is skewed approximately 30 degrees to the opening while the opening-skew-to-roadway is zero degrees. A scour hole 3 ft deeper than the mean thalweg depth was observed during the Level I assessment along the left side of the channel under the bridge exposing the left abutment footing 5.5 feet. The only scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) on the left banks upstream and downstream and the left abutment 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). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 2.2 to 3.8 feet. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 21.4 to 23.2 feet and 26.2 to 32.4 feet at the left and right abutments respectively. The worst-case abutment scour occurred for the right abutment at the incipient overtopping discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Bedrock was exposed at the surface in some areas of the channel and potentially is located at a shallower depth than the scour depths indicated above. Nevertheless, 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↗