Level II scour analysis for Bridge 16 (BRNATH00800016) on Town Highway 80, crossing Locust Creek, Barnard, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRNATH00800016 on town highway 80 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 Vermont Agency of Transportation (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 22.0-mi²
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the left banks are forested and the right banks are covered with
shrub and brush. Vermont Route 12 is adjacent to the right bank.

In the study area, Locust Creek has an incised channel with a slope of approximately 0.02
ft/ft, an average channel top width of 60 ft and an average channel depth of 4 ft. The
predominant channel bed materials are gravel and cobble with a median grain size (D₅₀) of
102 mm (0.336 ft). The geomorphic assessment at the time of the Level I and Level II site
visits on September 22, 1994 and October 12, 1994, indicated that the reach was stable.

The town highway 80 crossing of Locust Creek is a 36-ft-long, one-lane bridge consisting
of one 33-foot steel-beam span with timber deck (Vermont Agency of Transportation,
written communication, August 23, 1994). The bridge is supported by vertical, log crib
abutments with wingwalls. Type-2 stone fill (less than 36 inches diameter) protects the
upstream and downstream left wingwalls and the downstream left road embankment. Type-
3 stone fill (less than 48 inches diameter) protects the upstream and downstream right
wingwalls. The upstream left and downstream right road embankments are not protected
and road wash is eroding these areas. The channel approach to the bridge is straight with the
bridge skewed zero degrees to flow; the opening-skew-to-roadway is also zero degrees.
Additional details describing conditions at the site are included in the Level II Summary,
Appendix D, and Appendix 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 3.7 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge, which was between the
100- and 500-year discharge. Abutment scour ranged from 17.5 to 23.2 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, 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.