Level II scour analysis for Bridge 81 (JAMAVT01000081) on State Route 100, crossing the Winhall River, Jamaica, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
JAMAVT01000081 on State Route 100 crossing the Winhall 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
south-central Vermont. The 30.6-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture with trees on the
immediate banks. The upstream left bank of the bridge is forested.
In the study area, the Winhall River has an incised, straight channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 124 ft and an average bank height
of 9 ft. The channel bed material ranges from gravel to bedrock with a median grain size
(D₅₀) of 86.7 mm (0.284 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 8, 1996, indicated that the reach was stable.
The State Route 100 crossing of the Winhall River is an 84-ft-long, two-lane bridge
consisting of one 82-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 30, 1995). The bridge is supported by vertical, concrete abutments
with no wingwalls. The channel is skewed approximately 10 degrees to the opening while
the opening-skew-to-roadway is 15 degrees.
The scour protection measures at the site included type-2 stone fill (less than 36 inches
diameter) along the base of the left abutment. There was also type-4 stone fill (less than 60
inches diameter) along both downstream banks. In addition, there are stone walls placed on
both upstream banks. 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.
The contraction scour ranged from 0.0 to 2.6 ft. The worst-case contraction scour occurred
at the incipient road-overtopping discharge. Abutment scour ranged from 7.9 to 21.9 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.
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.