This report provides the results of a detailed Level II analysis of scour potential at structure BLOOVT01020009 on State Route 102 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 in northeastern Vermont. The 144-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest except for the downstream right bank area which is shrub and brush land. The Nulhegan River flows into the Connecticut River 210 feet downstream of this bridge.

In the study area, the Nulhegan River has an incised, sinuous channel with a slope of approximately 0.005 ft/ft, an average channel top width of 164 ft and an average channel depth of 5 ft. The predominant channel bed material is cobble with a median grain size (D₅₀) of 152 mm (0.498 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 6, 1995, indicated that the reach was laterally unstable. This was due to numerous point bars and side bars indicating an unstable thalweg.

The State Route 102 crossing of the Nulhegan River is a 134-ft-long, two-lane bridge consisting of one 130-foot steel-truss span (Vermont Agency of Transportation, written communication, August 4, 1994). The field measured clear span was 131.6 ft. The bridge is supported by vertical, concrete abutments with rip-rapped spill-through slopes. The channel is skewed approximately 25 degrees to the opening while the measured opening-skew-to-roadway is 5 degrees.

A scour hole 3.5 ft deeper than the mean thalweg depth was observed 250 ft upstream during the Level I assessment. It was noted that the scour was localized on the right bank side and due to the presence of an old abutment. Scour countermeasures include the type-3 stone-fill (less than 48 inches diameter) which forms the spill-through slopes of the abutments. 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.

Computed contraction scour for all modelled flows was zero ft. Abutment scour ranged from 4.5 to 5.0 ft at the left abutment and 9.6 to 11.4 ft at the right abutment. The worst-case 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. 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.