This report provides the results of a detailed Level II analysis of scour potential at structure
STJOUS00020108 on U.S. Highway 2 crossing the Moose River,
St. Johnsbury, 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 New England Upland/White Mountain sections of the New England
physiographic province in north-east Vermont. The 117-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 right bank, forest on the upstream left bank, shrub and brush on the
downstream left bank, and forest on the downstream right bank.
In the study area, the Moose River has an incised, sinuous channel with a slope of
approximately 0.008 ft/ft, an average channel top width of 96 ft and an average channel
depth of 6 ft. The predominant channel bed material is cobble with a median grain size
(D50) of 94.1 mm (0.309 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 14, 1995, indicated that the reach was stable.
The U.S. Highway 2 crossing of the Moose River is a 103-ft-long, two-lane bridge
consisting of three spans with a maximum 57-foot concrete T-beam span (Vermont Agency
of Transportation, written communication, March 28, 1995). The bridge is supported by
two piers, and vertical, concrete abutments with no wingwalls. The channel is skewed
approximately 10 degrees to the opening while the opening-skew-to-roadway is 0 degrees.
The scour protection measures at the site were type-2 stone fill (less than 36 inches
diameter) at the upstream and downstream channel banks. There is also type-3 stone fill
(less than 48 inches diameter) at both the upstream and downstream ends of the left and
right abutments. 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 0.4 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 9.3 to
12.2 ft. The worst-case abutment scour occurred at the left abutment 500-year discharge.
Pier scour ranged from 8.3 to 15.7 for both piers. The worst case pier scour occurred at the
left pier, for the 100-year discharge analysis. Additional in formation 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
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