Level II scour analysis for Bridge 4 (CRAFTH00040004) on Town Highway 4, crossing Whitney Brook, Craftsbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CRAFTH00040004 on town highway 4 crossing Whitney Brook, Craftsbury, 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 secton of the New England physiographic province
of north-central Vermont in the town of Craftsbury. The 13.3-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, Whitney Brook has an incised, sinuous channel with a slope of
approximately 0.014 ft/ft, an average channel top width of 40 ft and an average channel
depth of 3 ft. The observed predominant channel bed material is cobble and boulder while
results from the pebble count provided a D₅₀ of 78.5 mm or 0.258 ft. The geomorphic
assessment at the time of the Level I and Level II site visit on June 7, 1995 indicated that the
reach was stable.

The town highway 4 crossing of Whitney Brook is a 41-ft-long, one-lane bridge consisting
of one 39-foot span steel-beam and concrete superstructure (Vermont Agency of
Transportation, written commun., August 3, 1994). The bridge is supported by slightly
sloping, mortared stone block abutments with wingwalls. The channel is skewed
approximately 20 degrees to the opening while the opening-skew-to-roadway is 20 degrees.

A scour hole 2 ft deeper than the mean thalweg depth was observed along the upstream
right wingwall and right abutment during the Level I assessment. There were no scour
protection measures evident at the site. Additional details describing conditions at the site
are included in the Level II Summary and Appendices D and E.

Scour depths and rock riprap 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
degradation; 2) contraction scour (due to accelerated flow caused by 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 computed scour results follow.

Contraction scour for all modelled flows ranged from 0.7 to 1.7 feet. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 10.7 to
15.3 feet. 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.