Level II scour analysis for Bridge 31 (BRNATH00470031) on Town Highway 47, crossing Locust Creek, Barnard, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRNATH00470031 on town highway 47 crossing Locust Creek, Barnard, 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). A Level I study is 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 VTAOT files, was compiled prior to conducting Level I and Level II analyses
and can be found in Appendix D.

The site is in the Green Mountain physiographic province of central Vermont in the town of
Barnard. The 4.47-mi²
drainage area is a predominantly rural and forested basin. In the
vicinity of the study site, the banks have dense woody vegetation coverage except for areas
of grass and brush on the upstream banks.

In the study area, Locust Creek has an incised, sinuous channel with a slope of
approximately 0.006 ft/ft, an average channel top width of 34 ft and an average channel
depth of 3 ft. The predominant channel bed materials are gravel and cobble (D₅₀ is 55.2 mm
or 0.181 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
October 12, 1994, indicated that the reach was stable.

The town highway 47 crossing of Locust Creek is a 28-ft-long, one-lane bridge consisting
of one 25-foot span concrete slab superstructure (Vermont Agency of Transportation,
written commun., August 23, 1994). The bridge is supported by vertical, concrete
abutments with concrete wingwalls. The channel is skewed approximately 20 degrees to the
opening while the opening-skew-to-roadway is 15 degrees.

The scour protection measures at the site were type-2 stone fill (less than 36 inches
diameter) on the right and left abutments and all wingwalls. The banks upstream and
downstream are not protected. 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, 1993). 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 1.5 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.6 to
9.2 ft. The worst-case abutment scour also 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, 1993, p. 48). 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.