This report provides the results of a detailed Level II analysis of scour potential at structure
ANDOTH00010008 on Town Highway 1 crossing the Andover Branch, Andover ,
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
south-central Vermont. The 5.30-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover along the immediate banks, both
upstream and downstream of the bridge, is grass while farther upstream and downstream,
the surface cover is primarily forest.
In the study area, the Andover Branch has an incised, straight channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 35 ft and an average bank height
of 3 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D50) of 63.6 mm (0.209 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 27, 1996, indicated that the reach was stable.
The Town Highway 1 crossing of the Andover Branch is a 54-ft-long, two-lane bridge
consisting of one 51-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 28, 1995). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 45 degrees to the opening while the
opening-skew-to-roadway is 30 degrees.
A scour hole 0.7 ft deeper than the mean thalweg depth was observed approximately 52 feet
downstream of the downstream face of the bridge during the Level I assessment. Scour
countermeasures at the site include type-2 stone fill (less than 36 inches diameter) along the
entire base length of the left and right abutments and along the left bank from 65 ft to 89 ft
upstream. Type-1 stone fill was found along the right bank from the bridge to 47 ft
upstream and along the left bank from 40 ft to 65 ft upstream. 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, 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 0.1 ft. The worst case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 5.0 to
8.1 ft along the left abutment and from 2.1 to 4.6 ft along the right abutment. 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