Level II scour analysis for Bridge 28 (MNTGTH00190028) on Town Highway 19, crossing Wade Brook, Montgomery, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MNTGTH00190028 on town highway 19 crossing Wade Brook, Montgomery, 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 north-central Vermont in the
town of Montgomery. The 6.51-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the banks have dense woody vegetation coverage.

In the study area, Wade Brook has an incised, sinuous channel with a slope of
approximately 0.0253 ft/ft, an average channel top width of 58 ft and an average channel
depth of 4 ft. The predominant channel bed material is gravel and cobbles (D₅₀ is 81.8 mm
or 0.269 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
November 9, 1994, indicated that the reach was stable.

The town highway 19 crossing of Wade Brook is a 24-ft-wide corrugated steel, multi-plate
pipe-arch (Vermont Agency of Transportation, written communication, August 3, 1994).
The culvert is supported by vertical, concrete abutments with wingwalls. The channel is
skewed approximately 30 degrees to the opening while the opening-skew-to-roadway is 26
degrees.

There was no localized scour evident during the Level I assessment. The scour protection
measures at the site were type-2 stone fill (less than 36 inches diameter) on all of the
roadway embankments, the upstream left bank, and each 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.

Contraction scour for all modelled flows ranged from 0.5 to 1.0 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.6 to
16.1 ft. 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, 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.