Level II scour analysis for Bridge 9 (JAYVT02420009) on Vermont Highway 242, crossing the Jay Branch of the Missisquoi River, Jay, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
JAY-VT02420009 on Vermont highway 242 crossing the the Jay Branch of the Missisquoi
River, Jay, 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 study 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 of
northern Vermont in the town of Jay. The 4.36-mi²
drainage area is in a predominantly rural
and forested basin. In the vicinity of the study site, the surface cover is primarily forest and
brush except for the downstream left overbank which is grass.
In the study area, the the Jay Branch of the Missisquoi River has an incised, sinuous channel
with a slope of approximately 0.021 ft/ft, an average channel top width of 38 ft and an
average channel depth of 5 ft. A Level I visual inspection at the site indicates that the
predominant channel bed material is cobble and boulder with gravel. Results of a pebble
count indicate that the predominant channnel bed material is a very coarse gravel with a
median grain size (D₅₀) of 41.7 mm (0.1369 ft). The geomorphic assessment at the time of
the Level I and Level II site visit on June 6, 1995, indicated that the reach was stable.
The Vermont highway 242 crossing of the the Jay Branch of the Missisquoi River is a 60-
ft-long, two-lane bridge consisting of one 55-foot steel-beam span (Vermont Agency of
Transportation, written communication, March 6, 1995). The bridge is supported by
vertical, concrete abutments with wingwalls. The channel is skewed approximately 60
degrees to the opening while the opening-skew-to-roadway is 45 degrees.
The scour protection measures at the site included type-2 stone fill (less than 36 inches
diameter) at the upstream right wingwall, the downstream left and right wingwalls and the
upstream end of the left abutment. Type-1 stone fill (less than 12 inches) was along the
upstream end of the right abutment. Type-4 stone fill (less than 64 inches) was along the
upstream left wingwall. 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 0.0 to 0.6 ft. The worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 0.8 to
5.6 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.