This report provides the results of a detailed Level II analysis of scour potential at structure
ROCHTH00210034 on Town Highway 21 crossing the White River, Rochester, 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, obtained 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 74.8-mi2
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is suburban on the upstream and
downstream left overbanks, though brush prevails along the immediate banks. On the
upstream and downstream right overbanks, the surface cover is pasture with brush and trees
along the immediate banks.
In the study area, the White River has an incised, straight channel with a slope of
approximately 0.002 ft/ft, an average channel top width of 102 ft and an average bank
height of 5 ft. The channel bed material ranges from sand to cobble with a median grain size
(D50) of 74.4 mm (0.244 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 23, 1996, indicated that the reach was stable.
The Town Highway 21 crossing of the White River is a 72-ft-long, two-lane bridge
consisting of 70-foot steel stringer span (Vermont Agency of Transportation, written
communication, March 22, 1995). The opening length of the structure parallel to the bridge
face is 67.0 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 15 degrees to the opening while the opening-skew-to-roadway is zero degrees.
Channel scour, 1.5 ft deeper than the mean thalweg depth was observed along the left
abutment and wingwalls during the Level I assessment. Scour countermeasures at the site
includes type-1 stone fill (less than 12 inches diameter) along the upstream left bank and the
upstream and downstream left road embankments, type-2 (less than 36 inches diameter)
along the upstream end of the upstream left wingwall and downstream left bank, and type-3
(less than 48 inches diameter) along 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 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).
In addition, the incipient roadway-overtopping discharge is analyzed since it has the
potential of being the worst-case scour scenario. 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 discharges was zero. Left abutment scour ranged from
6.8 to 21.2 ft. Right abutment scour ranged from 13.9 to 18.4 ft. The worst-case abutment
scour occurred at the 500-year discharge at the left and right abutments. 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