Level II scour analysis for Bridge 32 (BRNATH00470032) 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
BRNATH00470032 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). 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 Green Mountain section of the New England physiographic province of
central Vermont in the town of Barnard. The 6.26-mi2
drainage area is in a predominantly
rural and forested basin. In the vicinity of the study site, the banks have dense woody
vegetation coverage except for the upstream right bank, which is grass and brush covered.

In the study area, Locust Creek has an incised, sinuous channel with a slope of
approximately 0.029 ft/ft, an average channel top width of 44 ft., and an average channel
depth of 5 ft. The predominant channel bed material is gravel and cobbles (D50 is 91.7 mm
or 0.301 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 10 degrees to the
opening while the opening-skew-to-roadway is 35 degrees.

The scour protection measures at the site were type-2 stone fill (less than 36 inches
diameter) on the banks upstream, the upstream wingwalls, the downstream right wingwall,
and the downstream right bank. The downstream left wingwall and left bank are protected
with type-3 stone fill (less than 48 inches diameter). 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 these computed results
follow.

Contraction scour for all modelled flows ranged from 1.4 to 2.2 feet. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 10.3 to
15.0 feet. 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 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. 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.