Level II scour analysis for Bridge 80 (JAMAVT01000080) on State Highway 100, crossing the West River, Jamaica, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
JAMAVT01000080 on State Route 100 crossing the West River, Jamaica, 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
southern Vermont. The 227-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 upstream left bank
and downstream of the bridge while the immediate banks have dense woody vegetation.
The upstream right bank of the bridge is forested.
In the study area, the West River has an incised, straight channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 309 ft and an average bank height
of 10 ft. The channel bed material is predominantly cobble with a median grain size (D₅₀)
of 109 mm (0.359 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on August 13, 1996, indicated that the reach was stable.
The State Route 100 crossing of the West River is a 246-ft-long, one-lane steel thru-truss
bridge consisting of three spans, the longest is 161-feet (Vermont Agency of
Transportation, written communication, March 30, 1995). The bridge is supported by
vertical, concrete abutments and two piers. The channel is skewed approximately 5 degrees
to the opening while there is no opening-skew-to-roadway.
A scour hole 3 ft deeper than the mean thalweg depth was observed along the streamward
(right) side of the left pier during the Level I assessment. A scour hole 5 ft deeper than the
mean thalweg depth was observed along the streamward (left) side of the right pier during
the Level I assessment. The only scour protection measure at the site was type-2 stone fill
(less than 36 inches diameter) along the left and right bank below the abutments forming a
“spill-through” slope at each abutment. 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.
There was no computed contraction scour. Abutment scour ranged from 15.8 to 23.9 ft.
The worst-case abutment scour occurred at the 500-year discharge. Pier scour ranged from
9.5 to 22.8 ft. The worst-case pier 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.