Level II scour analysis for Bridge 12 (NEWFVT00300012) on State Route 30, crossing Smith Brook, Newfane, Vermont

[摘要] This report provides the results of a detailed Level II analysis of scour potential at structure NEWFVT00300012 on State Route 30 crossing Smith Brook, Newfane, Vermont (figures 1�C8). 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 and Green Mountain sections of the New England physiographic province in southeastern Vermont. The 9.55-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is primarily pasture with the exception of the downstream right bank which is forested. The immediate banks have dense woody vegetation except for the downstream right bank which has cut grass.In the study area, Smith Brook has a sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 63 ft and an average bank height of 10 ft. The predominant channel bed material ranges from gravel to boulder with a median grain size (D₅₀) of 75.4 mm (0.247 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 State Route 30 crossing of Smith Brook is a 43-ft-long, two-lane bridge consisting of a 40-foot concrete T-beam span (Vermont Agency of Transportation, written communication, March 30, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 35 degrees to the opening while the measured opening-skew-to-roadway is 30 degrees. The scour protection measure at the site included type-2 stone fill (less than 36 inches diameter) from the upstream end of the left abutment to 35 ft. upstream, along the downstream end of the downstream left wingwall, along the right bank from 7 ft. to 90 ft. downstream, and along the left bank from 15 ft. to 40 ft. downstream. Also, there was type-1 stone fill (less than 24 inches diameter) at the left abutment and downstream left wingwall. Additional details describing conditions at the site are included in the Level II Summary and Appendices Dand E.Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows.Contraction scour for modelled flows ranged from 1.2 to 1.8 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.6 to 14.1 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.

[发布机构] U.S. Geological Survey

[学科分类] 地球科学(综合)