Level II scour analysis for Bridge 6 (VICTTH000110006) on Town Highway 1, crossing the Moose River, Victory, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
VICTTH00010006 on Town Highway 1 crossing the Moose River, Victory, 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 White Mountain section of the New England physiographic province in
northeastern Vermont. The 27.9-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, the Moose River has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 108 ft and an average channel
depth of 14 ft. The channel bed ranges from gravel to boulder with a median grain size
(D₅₀) of 126 mm (0.412 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on July 20, 1995, indicated that the reach was stable.
The Town Highway 1 crossing of the Moose River is a 101-ft-long, two-lane bridge
consisting of one 98-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 28, 1995). The bridge is supported by vertical, concrete abutments
with a spill-through slope at the face of each abutment consisting of type-3 stone fill (less
than 48 inches diameter). The channel is skewed approximately 40 degrees to the opening
while the opening-skew-to-roadway is 45 degrees.
A scour hole 3 ft deeper than the mean thalweg depth was observed under the bridge during
the Level I assessment. 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 0.2 to 0.4 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.3 to
8.2 ft. 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.