Level II scour analysis for Bridge 49 (BRIDTH00530049) on Town Highway 53, crossing North Branch Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00530049 on town highway 53 crossing the North Branch of 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). 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 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 26.6-mi²
drainage area is in a predominantly rural and forested basin. In
the vicinity of the study site, the immediate banks have woody vegetation coverage with
grass on the overbanks.

In the study area, the North Branch Ottauquechee River has a sinuous channel with a slope
of approximately 0.0075 ft/ft, an average channel top width of 66 ft and an average channel
depth of 6 ft. The predominant channel bed material is cobble and gravel (D₅₀ is 68.4 mm or
0.224 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
October 27, 1994, indicated that the reach was stable.

The town highway 53 crossing of the North Branch of the Ottauquechee Riveris a 51-ftlong, one-lane bridge consisting of one 49-foot steel-beam span (Vermont Agency of
Transportation, written communication, August 25, 1994). The bridge is supported by
vertical, concrete abutments with wingwalls. The channel is not skewed to the opening and
the opening-skew-to-roadway is zero degrees.

The scour protection measures in place at the site are type-1 stone fill (less than 12 inches
diameter) along the upstream left wingwall and type-2 stone fill (less than 36 inches
diameter) along the upstream right wingwall. 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, 1993).
Total scour at a highway crossing is comprised of three components: 1) long-term
aggradation or degradation; 2) contraction scour (due to reduction in flow area caused by 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 scour depths
for contraction and local scour and a summary of the results follows.

Contraction scour for all modelled flows was 0 feet. Abutment scour ranged from 2.3 to
12.0 feet and 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, 1993, 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.