Level II scour analysis for Bridge 29 (HARDTH00310029) on Town Highway 31, crossing the Lamoille River, Hardwick, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HARDTH00310029 on town highway 31 crossing the Lamoille River, Hardwick, 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 section of the New England physiographic province
of north-central Vermont. The 64.4-mi²
drainage area is in a predominantly rural basin. In
the vicinity of the study site, the surface cover is pasture except for the immediate
downstream channel banks and the downstream left overbank which are brush covered.

In the study area, the Lamoille River has a sinuous channel with a slope of approximately
0.001 ft/ft, an average channel top width of 84 ft and an average channel depth of 4 ft. The
predominant channel bed materials are cobble and gravel with a median grain size (D₅₀) of
36.1 mm (0.119 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on July 26, 1995, indicated that the reach was stable.

The town highway 31 crossing of the Lamoille River is a 65-ft-long, one-lane bridge
consisting of one 61-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 27, 1995). The bridge is supported by vertical, stone abutments with
wingwalls. The right abutment has a concrete facing and a concrete subfooter. The channel
is skewed approximately 5 degrees to the opening while the opening-skew-to-roadway is 0
degrees. 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 2.7 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 10.3 to
18.6 ft. The worst-case abutment scour occurred at the 100-year discharge at the right
abutment. 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.