This report provides the results of a detailed Level II analysis of scour potential at structure BOLTTH00150015 on Town Highway 15 crossing Joiner Brook, Bolton, 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 Green Mountain section of the New England physiographic province in north central Vermont. The 9.6-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture (lawn) downstream of the bridge and on the upstream right bank. The surface cover on the upstream left bank is shrub and brushland.

In the study area, Joiner Brook has an incised, straight channel with a slope of approximately 0.01 ft/ft, an average channel top width of 61 ft and an average bank height of 7 ft. The channel bed material ranges from gravel to cobble with a median grain size (D₅₀) of 43.6 mm (0.143 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 27, 1996, indicated that the reach was stable.

The Town Highway 15 crossing of Joiner Brook is a 39-ft-long, two-lane bridge consisting of one 36-foot concrete tee-beam span (Vermont Agency of Transportation, written communication, November 3, 1995). The opening length of the structure parallel to the bridge face is 34.6 ft. The bridge is supported by nearly vertical, concrete abutments with wingwalls. The channel is skewed approximately 10 degrees to the opening while the opening-skew-to-roadway is zero degrees. 

A scour hole 1.5 ft deeper than the mean thalweg depth was observed at the downstream end of the right abutment and along the downstream right wingwall during the Level I assessment. A second scour hole 1.2 ft deeper than the mean thalweg depth was observed at the upstream end of the left abutment and along the upstream left wingwall. The left abutment footing is exposed in the area of the scour hole. Scour protection measures at the site included type-1 stone fill (less than 12 inches diameter) at the upstream end of the upstream left wingwall and at the downstream end of the downstream right wingwall and type-2 stone fill (less than 36 inches diameter) along the downstream left bank. There is also type-3 stone fill (less than 48 inches diameter) along the upstream left and right banks. 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 0.8 to 3.5 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.9 to 15.1 ft. The worst-case abutment scour occurred at the incipient roadway-overtopping 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 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.