Level II scour analysis for Bridge 46 (CHELTH00680046) on Town Highway 68, crossing the First Branch of the White River, Chelsea, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHELTH00680046 on town highway 68 crossing the First Branch of the White River,
Chelsea, 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 in the town of Chelsea. The 58.2-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, the First Branch of the White River has a sinuous channel with a slope of
approximately 0.0054 ft/ft, an average channel top width of 92 ft and an average channel
depth of 4 ft. The predominant channel bed material is gravel and cobble (D₅₀ is 52.7 mm
or 0.173 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
November 16, 1994, indicated that the reach was stable.

The town highway 68 crossing of the First Branch of the White River is a 61-ft-long, onelane covered bridge with a 52-foot clear-span (Vermont Agency of Transportation, written
commun., August 26, 1994). The bridge is supported by vertical, stone abutments with a
concrete wingwall on the downstream right. The left abutment is laid-up stone supported by
concrete at the upstream and downstream ends of the laid-up stone abutment. The channel
is skewed approximately 40 degrees to the opening while the opening-skew-to-roadway is
15 degrees.

A scour hole 1.5 ft deeper than the mean thalweg depth was observed under the bridge
during the Level I assessment. The scour protection measures in place at the site were type-
2 stone fill (less than 36 inches diameter) at the road approach embankments except the
downstream left embankment which had no protection. The upstream right road
embankment, impacted by the channel bend, has an extensive covering of stone fill for
erosion protection. Type-3 stone fill (less than 48 inches diameter) was noted along the
right abutment. 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 0.9 to 2.6 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 14.3 to
24.0 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. The left abutment sits atop a bedrock outcrop. The
results of the calculated scour depths will be limited by the bedrock.

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.