This report provides the results of a detailed Level II analysis of scour potential at structure BRIDUS00040045a on U.S.. Route 4 crossing the Ottauquechee River, Bridgewater, 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). A Level I study is included in Appendix E of this report. A Level I study provides a qualitative geomorphic characterization of the study site. Information on the bridge available from VTAOT files was compiled prior to conducting Level I and Level II analyses and can be found in Appendix D.

The site is in the Green Mountain physiographic province of central Vermont in the town of Bridgewater. The 72.1-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the overbank areas are lawn or pasture with a few residences. The immediate channel banks have moderately dense woody vegetation.

In the study area, the Ottauquechee River has a sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 81 ft and an average channel depth of 3 ft. The predominant channel bed materials are gravel and cobble (D₅₀ is 54.9 mm or 0.180 ft). The geomorphic assessment at the time of the Level I and Level II site visit on October 26, 1994, indicated that the reach was stable.

The U.S. Route 4 crossing of the Ottauquechee Riveris a 172-ft-long, two-lane bridge consisting of three steel-beam spans supported by spill-through abutments and two concrete piers (Vermont Agency of Transportation, written commun., August 25, 1994). The abutment and road approaches are protected by type-2 stone fill (less than 36 inches diameter). The North Branch of the Ottauquechee River joins the Ottauquechee River approximately 200 feet upstream of the bridge on the main branch’s left bank. The channel approach to the bridge has a mild bend with the bridge skewed 15 degrees to flow; the opening-skew-to-roadway is 30 degrees. Additional details describing conditions at the site are included in the Level II Summary, Appendix D, and Appendix E.

Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1993). 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 3.1 to 4.0 ft. with the worst-case contraction scour occurring at the 500-year and incipient road-overflow discharges. Abutment scour ranged from 9.3 to 15.2 ft. The worst-case abutment scour also occurred at the 500-year discharge. Pier scour ranged from 11.4 to 12.4 ft. with the worst-case scenario occurring at the incipient roadway overflow discharge. The incipient roadway overflow discharge was between the 100- and 500-year discharges. 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, 1993, p. 48). Many factors, including historical performance during flood events, the geomorphic assessment, scour protection measures, and the results of the hydraulic analyses, must be considered to properly assess the validity of abutment scour results. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein, based on the consideration of additional contributing factors and experienced engineering judgement.