This report provides the results of a detailed Level II analysis of scour potential at structure BLOOVT01050096 on Vermont Route 105 crossing the Nulhegan River, Bloomfield, 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 White Mountain section of the New England physiographic province of north-east Vermont in the town of Bloomfield. The 103-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 upstream. Downstream of the bridge, the surface cover is forest.
In the study area, the Nulhegan River has an incised, sinuous channel with a slope of approximately 0.015 ft/ft, an average channel top width of 78 ft and an average channel depth of 5 ft. The predominant channel bed material is cobble with a median grain size (D50) of 133 mm (0.435 ft). About 100 feet upstream, the streambed and bank materials abruptly change predominantly to sand. The geomorphic assessment at the time of the Level I and Level II site visit on July 6, 1995, indicated that the upstream reach, which is experiencing channel scour and severe bank cutting into the alluvial channel boundaries, is not stable. The downstream reach is semi- to non-alluvial and is assessed as stable.
The Vermont Route 105 crossing of the Nulhegan River is a 74-ft-long, two-lane bridge consisting of one 71-foot steel stringer type superstructure with a concrete deck (Vermont Agency of Transportation, written communication, August 5, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 10 degrees to the opening while the opening-skew-to-roadway is 25 degrees.
A scour hole 4.0 ft deeper than the mean thalweg depth was observed along the upstream channel during the Level I assessment. Scour protection measures at the site consist of type-2 stone fill (less than 24 inches diameter) along the entire base length of both abutments and all wingwalls. Additional details describing conditions at the site are included in the Level II Summary and Appendices D
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 0.5 to 1.1 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 10.5 to 16.2 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.