This report provides the results of a detailed Level II analysis of scour potential at structure
ENOSVT01080046 on State Route 108 crossing an unnamed "The Branch" tributary,
Enosburg, 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
north-central Vermont. The 1.55-mi2
drainage area is in a predominantly rural, pasture and
forested basin. In the vicinity of the study site, the surface cover is pasture.
In the study area, this unnamed "The Branch" tributary has an incised, sinuous channel with
a slope of approximately 0.03 ft/ft, an average channel top width of 45 ft and an average
channel depth of 3 ft. The predominant channel bed material is gravel and cobbles with a
median grain size (D50) of 42.4 mm (0.139 ft). The geomorphic assessment at the time of
the Level I and Level II site visit on June 29, 1995, indicated that the reach was laterally
unstable. Block failure slumping of bank material was evident at an upstream cut-bank and
another minor cut-bank was noted downstream.
The State Route 108 crossing of this unnamed "The Branch" tributary is a 25-ft-long, twolane bridge consisting of one 22-foot concrete span (Vermont Agency of Transportation,
written communication, March 8, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 10 degrees to the opening
while the opening-skew-to-roadway is zero degrees.
The only scour protection measure at the site was type-2 stone fill (less than 36 inches
diameter) at the downstream end of the downstream left wingwall. Additional details
describing conditions at the site are included in the Level II Summary and Appendices D
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 0.3 to 0.5 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.0 to
8.0 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, 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 46 (ENOSVT01080046) on State Route 108, crossing an Unnamed "The Branch" Tributary, Enosburg, Vermont