Level II scour analysis for Bridge 145 (HANCVT01000145) on Vermont Highway 100, crossing the Hancock Branch of the White River, Hancock, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HANCVT01000145 on State Route 100 crossing the Hancock Branch of the White River,
Hancock, 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 22.0-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is urban on left bank and forested on the
right bank upstream of the bridge while the immediate banks have woody vegetation.
Downstream of the bridge surface cover on both banks is pasture while the immediate
banks have woody vegetation.
In the study area, the Hancock Branch of the White River has an incised, sinuous channel
with a slope of approximately 0.006 ft/ft, an average channel top width of 48 ft and an
average channel depth of 3 ft. The predominant channel bed materials are cobble and gravel
with a median grain size (D₅₀) of 71.9 mm (0.236 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.
State Route 100 crossing the Hancock Branch of the White River is a 55-ft-long, two-lane
bridge consisting of one 53-foot steel-beam span (Vermont Agency of Transportation,
written communication, August 26, 1994). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is not skewed to the opening and the opening-skewto-roadway is 0 degrees.
The only scour protection measures at the site were type-3 stone fill (less than 48 inches
diameter) at the upstream right wingwall, both downstream wingwalls and the downstream
ends of both abutments. Also there was type-2 stone fill (less than 36 inches diameter) at the
upstream left wingwall. 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 3.4 to 4.3 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.2 to
11.1 ft. The worst-case abutment scour occurred at the 100-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.