Level II scour analysis for Bridge 8 (ATHETH00090008) on Town Highway 9, crossing Bull Creek, Athens, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ATHETH00090008 on Town Highway 9 crossing Bull Creek in Athens, 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 New England Upland section of the New England physiographic province
in southeastern Vermont. The 9.04-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the left overbank surface cover is shrub and
brushland and the right overbank surface cover is pasture.
In the study area, Bull Creek has an sinuous channel with a slope of approximately 0.01 ft/
ft, an average channel top width of 41 ft and an average bank height of 4 ft. The
predominant channel bed materials are cobbles and gravel with a median grain size (D₅₀) of
72.1 mm (0.236 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on August 14, 1996, indicated that the reach was laterally unstable. There are several
point bars and cut banks along the reach in the vicinity of this site.
The Town Highway 9 crossing of Bull Creek is a 32-ft-long, one-lane bridge consisting of
one 28-foot steel-beam span (Vermont Agency of Transportation, written communication,
April 5, 1995). The bridge is supported by vertical, “laid-up” stone abutments with concrete
caps and no wingwalls. The channel is skewed approximately 15 degrees to the opening.
The VTAOT bridge records indicate the opening-skew-to-roadway is 9 degrees while that
computed from surveyed points is 5 degrees.
A scour hole 1.75 feet deeper than the mean thalweg depth was observed under the bridge
during the Level I assessment. The scour hole has lowered the streambed along the entire
length of the left abutment and the upstream end of the right abutment. The scour depth at
each abutment wall is 0.75 feet deeper than the mean thalweg depth elsewhere in the reach.
The only scour protection measure at the site was type-2 stone fill (less than 36 inches
diameter) on the upstream banks and downstream left bank. 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
computations follows.
Contraction scour for all modelled flows ranged from 0.0 to 1.4 feet. The worst-case
contraction scour occurred at the incipient-overtopping discharge of 1730 cubic feet per
second, which was less than the 100-year discharge. Abutment scour ranged from 7.6 to
11.4 feet. 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
documented herein.