Level II scour analysis for Bridge 50 (STARTH00250050) on Town Highway 25, crossing Lewis Creek, Starksboro, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
STARTH00250050 on Town Highway 25 crossing Lewis Creek, Starksboro, 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
west-central Vermont. The 10.9-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture on the left bank
downstream and upstream of the bridge. On the right bank upstream and downstream of the
bridge the surface cover is forest.
In the study area, Lewis Creek has an incised, straight channel with a slope of
approximately 0.007 ft/ft, an average channel top width of 64 ft and an average bank height
of 7 ft. The channel bed material ranges from sand to boulder with a median grain size (D₅₀)
of 35.4 mm (0.116 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on June 12, 1996, indicated that the reach was stable.
The Town Highway 25 crossing of Lewis Creek is a 28-ft-long, one-lane bridge consisting
of one 25-foot steel-beam span (Vermont Agency of Transportation, written
communication, December 15, 1995). The opening length of the structure parallel to the
bridge face is 23.8 ft. The bridge is supported by vertical, concrete abutments with
wingwalls on all corners except the downstream left. The channel is skewed approximately
zero degrees to the opening and the opening-skew-to-roadway is also zero degrees.
A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the right
abutment during the Level I assessment. Also, the footing is exposed along the left and right
abutments and all three wingwalls. The scour countermeasures at the site included type-1
stone fill (less than 12 inches diameter) along the left abutment and type-2 stone fill (less
than 36 inches diameter) along the right abutment and the upstream and downstream right
wingwalls. 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 was 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 5.2 to 9.1 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 13.1 to
18.2 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.