Level II scour analysis for Bridge 37 (TOWNTH00290037) on Town Highway 29, crossing Mill Brook, Townshend, Vermont

Abstract

This report provides the results of a detailed Level II analysis of scour potential at structure TOWNTH00290037 on Town Highway 29 crossing Mill Brook, Townshend, 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 13.9-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest upstream of the bridge. Downstream of the bridge the surface cover is pasture on the left bank and shrub and brushland on the right bank. In the study area, Mill Brook has an incised, sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 53 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 70.0 mm (0.230 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 14, 1996, indicated that the reach was laterally unstable. There are large cutbanks and point bars upstream and downstream of the bridge. There is also moderate fluvial erosion on the upstream left bank and downstream right bank. The Town Highway 29 crossing of Mill Brook is a 33-ft-long, one-lane bridge consisting of one 30-foot steel-girder span (Vermont Agency of Transportation, written communication, April 7, 1995). The opening length of the structure parallel to the bridge face is 24.8 ft. The bridge is supported by vertical, concrete abutments with wingwalls, the downstream left wingwall, however, is “laid-up” stone. The channel is skewed approximately 45 degrees to the opening while the computed opening-skew-to-roadway is 25 degrees. 2 A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the right abutment during the Level I assessment. This scour hole continues downstream along the right bank and deepens to 1.5 ft deeper than the mean thalweg. The scour protection measures at the site included type-2 stone fill (less than 36 inches diameter) along the upstream left and right banks and along the upstream right wingwall. Type-3 stone fill (less than 48 inches diameter) was along the downstream right wingwall and downstream right bank and a short stone wall is on the downstream 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 Davis, 1995) for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping discharge was determined and analyzed as another potential worst-case scour scenario. Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows. Contraction scour for all modelled flows ranged from 0.0 to 2.1 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 6.7 to 8.7 ft. The worst-case left abutment scour occurred at the incipient roadway-overtopping discharge. Right abutment scour ranged from 7.8 to 9.5 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 crosssection of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and Davis, 1995, p. 46). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.