Level II scour analysis for Bridge 36 (BRIDTH00050036) on Town Highway 5, crossing Bridgewater Hollow Brook, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00050036 on town highway 5 crossing Bridgewater Hollow Brook, 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, 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 of
central Vermont. The 3.60-mi²
drainage area is in a predominantly forested basin. In the
vicinity of the study site, the banks have dense woody vegetation coverage.

In the study area, Bridgewater Hollow Brook has an incised, sinuous channel with a slope
of approximately 0.028 ft/ft, an average channel top width of 24 ft and an average channel
depth of 4 ft. The predominant channel bed material is cobble (D₅₀ is 196 mm or 0.644 ft).
The geomorphic assessment at the time of the Level I and Level II site visit on November 2,
1994, indicated that the reach was stable.

The town highway 5 crossing of Bridgewater Hollow Brook is a 30-ft-long, one-lane bridge
consisting of one 27-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 skewed approximately 30 degrees to the opening and the
opening-skew-to-roadway is also 30 degrees.

The scour protection measures at this site were sparse type-2 stone fill (less than 36 inches
diameter) along both abutments, upstream wingwalls, and the downstream left wingwall
and type-1 stone fill (less than 12 inches diameter) along the downstream 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, 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.

There was no contraction scour for all modelled flows. Abutment scour ranged from 4.9 to
7.0 ft. 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.