Level II scour analysis for Bridge 30 (MNTGTH00410030) on Town Highway 41, crossing the Trout River, Montgomery, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MNTGTH00410030 on Town Highway 41 crossing the Trout River, Montgomery,
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
northern Vermont. The 46.1-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover on the left bank is pasture upstream
and downstream of the bridge with dense woody vegetation along the immediate banks.
The upstream and downstream right bank surface cover is brush.
In the study area, the Trout River has an incised, meandering channel with a slope of
approximately 0.005 ft/ft, an average channel top width of 130 ft and an average bank
height of 6 ft. The channel bed material ranges from sand to cobble with a median grain size
(D₅₀) of 68.3 mm (0.224 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on June 27, 1995, indicated that the reach was laterally unstable. At this
site there is visible lateral channel movement upstream and downstream of the bridge with
meanders and cut banks.
The Town Highway 41 crossing of the Trout River is a 90-ft-long, one-lane bridge
consisting of one 87-foot steel-beam span (Vermont Agency of Transportation, written
communication, August 3, 1994). The opening length of the structure parallel to the bridge
face is 86.7 ft.The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 10 degrees to the opening while the opening-skew-toroadway is 0 degrees.
A scour hole 4.5 ft deeper than the mean thalweg depth, was observed 35 ft downstream of
the bridge during the Level I assessment. The scour counter-measures at the site included
type-1 stone fill (less than 12 inches diameter) at the upstream left wingwall, at the left
abutment, along the upstream right bank, and at the upstream end of the downstream left
wingwall. There was also type-2 stone fill (less than 36 inches diameter) along the
downstream right bank. 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).
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 was 0.0 ft. Abutment scour ranged from 2.5 to 8.9
ft. The worst-case abutment scour occurred at the 500-year discharge. The computed scour
depths are well above the pile depths set in bedrock. Additional information on scour depths
and depths to armoring are included in the section titled “Scour Results”. Scouredstreambed 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 particlesize 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.