Level II scour analysis for Bridge 6 (FAYSTH00010006) on Town Highway 1, crossing Shepard Brook, Fayston, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
FAYSTH00010006 on Town Highway 1 crossing Shepard Brook,
Fayston, 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 in
central Vermont. The 16.6-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is forest.

In the study area, Shepard Brook has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 56 ft and an average bank height
of 3 ft. The channel bed material ranges from sand to boulder with a median grain size (D₅₀)
of 72.6 mm (0.238 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on July 2, 1996, indicated that the reach was stable.

The Town Highway 1 crossing of the Shepard Brook is a 42-ft-long, two-lane bridge
consisting of one 40-foot concrete T-beam span (Vermont Agency of Transportation,
written communication, October 13, 1995). The opening length of the structure parallel to
the bridge face is 39.6 ft. The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 15 degrees to the opening while the
calculated opening-skew-to-roadway is 30 degrees.

Scour, 2.0 ft deeper than the mean thalweg depth, was observed along the right abutment
during the Level I assessment. The left abutment is undermined along the base of the
footing. In addition, 1.5 ft of scour was observed along the left abutment during the Level I
assessment. The only scour protection measure at the site was type-1 stone fill (less than 12
inches diameter) along the left bank upstream and type-2 stone fill (less than 36 inches
diameter) along the upstream end of the upstream right wingwall. 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)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. 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 3.9 ft. The worst-case
contraction scour occurred at the 500-year. Abutment scour ranged from 11.1 to 17.2 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.