This report provides the results of a detailed Level II analysis of scour potential at structure
BRNATH00470030 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 in
central Vermont. The 4.18-mi2
drainage area is a predominantly rural and forested basin. In
the vicinity of the study site, the surface cover consists of trees, shrubs, and brush.
In the study area, Locust Creek has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 32 ft and an average bank height
of 4 ft. The predominant channel bed material is gravel with a median grain size (D50) of
49.5 mm (0.162 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on October 13, 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 concrete span (Vermont Agency of Transportation, written communication,
August 23, 1994). The bridge is supported by vertical, concrete abutments with wingwalls.
The channel is skewed approximately 40 degrees to the opening. Historical bridge data
indicates that the opening-skew-to-roadway is 45 degrees, but 35 degrees was computed by
use of survey data from this study.
A minor scour hole, 0.5 ft deeper than the mean thalweg depth was observed along the left
abutment wall during the Level I assessment. The scour protection measures at the site were
type-2 stone fill (less than 36 inches diameter) on the upstream wingwalls. There also is
type-3 stone fill on the downstream right wingwall. Additional details describing conditions
at the site are included in the Level II Summary and Appendices D and E.
Scour depths and recommended 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
Contraction scour for all modelled flows ranged from 0.0 to 1.4 feet. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 2.3 to
8.9 feet. 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
Additional publication details
USGS Numbered Series
Level II scour analysis for Bridge 30 (BRNATH00470030) on Town Highway 47, crossing Locust Creek, Barnard, Vermont