James R. Degnan Ronda L. Burns 1997 This report provides the results of a detailed Level II analysis of scour potential at structure NEWBTH00500016 on Town Highway 50 crossing Halls Brook, Newbury, 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 east-central Vermont. The 23.4-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is shrub and brushland. In the study area, Halls Brook has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 53 ft and an average bank height of 7 ft. The channel bed material ranges from silt to gravel with a median grain size (D₅₀) of 40.4 mm (0.133 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 29, 1995, indicated that the reach was laterally unstable. The channel bed and banks are composed of fine material and show signs of erosion. There is also evidence of beaver activity in the area. The Town Highway 50 crossing of Halls Brook is a 44-ft-long, two-lane bridge consisting of one 38-foot prestressed concrete slab span (Vermont Agency of Transportation, written communication, March 27, 1995). The opening length of the structure parallel to the bridge face is 35.2 ft. The bridge is supported by vertical, stone masonry abutments. The channel is skewed approximately 40 degrees to the opening while the computed opening-skew-toroadway is 5 degrees. A channel scour hole 1.0 ft deeper than the mean thalweg depth was observed just upstream of the bridge behind the remains of a beaver dam during the Level I assessment. An additional channel scour hole 4.5 ft deeper than the mean thalweg depth was observed in the downstream reach. The scour countermeasures at the site included type-1 stone fill (less than 12 inches diameter) along the left abutment and type-2 stone fill (less than 36 inches diameter) along the right abutment and left bank upstream and downstream. Along the downstream right bank is type-3 stone fill (less than 48 inches diameter) and along the upstream right bank is type-4 stone fill (less than 60 inches diameter). Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995) for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping discharge was analyzed since it has the potential of being the worst-case scour scenario. Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 2.6 to 4.6 ft. The worst-case contraction scour occurred at the incipient roadway-overtopping discharge. The left abutment scour ranged from 11.6 to 12.1 ft. The worst-case left abutment scour occurred at the incipient road-overtopping discharge. The right abutment scour ranged from 13.6 to 17.9 ft. The worst-case right abutment scour occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in Tables 1 and 2. A cross-section of the scour computed at the bridge is presented in Figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 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. application/pdf 10.3133/ofr97814 en U.S. Geological Survey Level II scour analysis for Bridge 16, (NEWBTH00500016) on Town Highway 50, crossing Halls Brook, Newbury, Vermont reports