Level II scour analysis for Bridge 46 (BRIDTH00050046) on Town Highway 05, crossing North Branch Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00050046 on town highway 5 crossing the North Branch 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, gleaned from Vermont Agency of Transportation (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 5.61-mi²
drainage area is a predominantly rural and forested basin. In the
vicinity of the study site, the banks are forested. Town highway 5 parallels the upstream left
bank.

In the study area, the North Branch Ottauquechee River has a sinuous channel with a slope
of approximately 0.015 ft/ft, an average channel top width of 48 ft and an average channel
depth of 6 ft. The predominant channel bed materials are gravel and cobble with a median
grain size (D₅₀) of 66.2 mm (0.217 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on November 2 and 3, 1994, indicated that the reach was stable.

The town highway 5 crossing of North Branch Ottauquechee Riveris a 40-ft-long, one-lane
bridge consisting of a 34-ft steel-beam span, supported by vertical abutments with no
wingwalls (Vermont Agency of Transportation, written communication, August 25, 1994).
The left abutment is stone; the right abutment is log cribwork with type-2 stone fill (less
than 36 inches diameter) along its base. Type-2 stone fill has also been placed on the
upstream and downstream sides of the road embankments, except the upstream left which
has type-3 (less than 48 inches diameter). The channel is skewed approximately 60 degrees;
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
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 these computed results
follow.

Contraction scour for all modelled flows was 0.0 ft. Abutment scour ranged from 5.7 ft to
7.7 ft. with the worst-case abutment scour occurring 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 depths, are presented in
tables 1 and 2. A cross-section of the computed scour 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. 22). Many factors,
including historical performance during flood events, the geomorphic assessment, scour
protection, 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.