Level II scour analysis for Bridge 37 (BARTTH00080037) on Town Highway 8, crossing Willoughby River, Barton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BARTTH00080037 on town highway 8 crossing the Willoughby River, Barton, 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
of north-central Vermont in the town of Barton. The 60.4-mi²
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the banks have
sparse to moderate woody vegetation coverage.

In the study area, the Willoughby River is probably incised, has a sinuous channel with a
slope of approximately 0.009 ft/ft, an average channel top width of 108 ft and an average
channel depth of 6 ft. The predominant channel bed material is cobble (D₅₀ is 95.1 mm or
0.312 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
October 20, 1994, indicated that the reach was stable.

The town highway 8 crossing of the Willoughby River is a 96-ft-long, two-lane bridge
consisting of one 94-foot steel-beam span (Vermont Agency of Transportation, written
communication, August 4, 1994). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 15 degrees to the opening while the
opening-skew-to-roadway is 10 degrees.

No scour was reported in the channel or along abutments or wingwalls during the Level I
assessment. Type-2 stone fill (less than 24 inches diameter) was reported at each abutment
and all four wingwalls. 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). Scour depths were
calculated assuming an infinite depth of erosive material and a homogeneous particle-size
distribution. Data in appendix D (Vermont Agency of Transportation, written
communication, August 4, 1994) indicate that the right abutment may be founded on or near
marble bedrock which may limit scour depths. Bedrock was not detected by borings in the
vicinity of the left abutment. The scour analysis results are presented in tables 1 and 2 and a
graph of the scour depths is presented in figure 8.

Contraction scour for all modelled flows was 0 ft. Abutment scour ranged from 7.3 to 10.7
ft and 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, 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.