Level II scour analysis for Bridge 23 (GLOVTH00410023) on Town Highway 41, crossing Sherburne Brook, Glover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
GLOVTH00410023 on Town Highway 41 crossing Sherburne Brook, Glover, 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 New England Upland section of the New England physiographic province
in northern Vermont. The 2.57-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is primarily forest with small areas
of lawn and a home on the right overbank and a gravel roadway along the upstream left
bank.

In the study area, Sherburne Brook has an incised, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 33 ft and an average bank height
of 6 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 57.3 mm (0.188 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on October 24, 1994, indicated that the reach was stable.

The Town Highway 41 crossing of Sherburne Brook is a 24-ft-long, one-lane bridge
consisting of one 21-foot steel-beam span with a timber deck (Vermont Agency of
Transportation, written communication, August 4, 1994). The opening length of the
structure parallel to the bridge face is 20.3 ft. The bridge is supported by vertical, granite
block abutments. The channel is skewed approximately 55 degrees to the opening while the
measured opening-skew-to-roadway is 30 degrees.

One foot of scour below the mean thalweg depth was observed along the right abutment
undermining the abutment by 0.5 feet vertically. Additional details describing conditions at
the site are included in the Level II Summary and Appendices D and E.

Scour depths and recommended 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 0.4 to 0.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.6 to
7.2 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.