Level II scour analysis for Bridge 137 (FERRUS00070137) on U.S. Route 7, crossing Little Otter Creek, Ferrisburg, Vermont
This report provides the results of a detailed Level II analysis of scour potential at structure FERRUS00070137 on U.S. Route 7 crossing Little Otter Creek, 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 (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 Champlain section of the St. Lawrence Valley physiographic province in northwestern Vermont. The 56.7-mi 2 drainage area is in a predominantly rural and forested basin with some pasture on the valley bottom. In the vicinity of the study site, the surface cover consists of pasture upstream of the bridge. Downstream of the bridge the surface cover consists of trees, shrubs, and grass. In the study area, Little Otter Creek has a meandering channel with a slope of approximately 0.007 ft/ft, an average channel top width of 86 ft and an average channel depth of 3 ft. The predominant channel bed materials are cobbles and gravel with a median grain size (D 50 ) of 54.9 mm (0.180 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 1, 1996, indicated that the reach was laterally unstable. The U.S. Route 7 crossing of Little Otter Creek is a 157-ft-long, two-lane bridge consisting of three steel-beam spans (Vermont Agency of Transportation, written communication, December 12, 1995). The bridge is supported by vertical, concrete abutment walls with spill-through embankments in front of each abutment wall and two solid concrete piers. The channel is skewed approximately 15 degrees to the opening while the opening-skew-to-roadway is zero degrees. The scour protection measures at the site consist of type-3 stone fill (less than 48 inches diameter) on the banks upstream and downstream of the bridge and the lower half of the spill-through embankment slopes on each abutment. Type-1 stone fill (less than 12 inches diameter) protects the upper half of the spill-through embankments and each roadway embankment. 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 1.8 to 2.3 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 10.4 to 14.9 ft. The worst-case abutment scour occurred at the 500-year discharge. There are two piers for which computed pier scour ranged from 7.5 to 13.4 ft. 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 1 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.