Level II scour analysis for Bridge 4 (RYEGTH00050004) on Town Highway 5, crossing the Wells River, Ryegate, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RYEGTH00050004 on Town Highway 5 crossing the Wells River, Ryegate, 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 eastern Vermont. The 84.7-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover includes shrubs and brush on the
upstream left bank and downstream right bank of the bridge. The upstream right bank and
downstream left bank of the bridge is forested.
In the study area, the Wells River has an incised, sinuous channel with a slope of
approximately 0.008 ft/ft, an average channel top width of 107 ft and an average bank
height of 11 ft. The channel bed material ranges from gravel to boulder with a median grain
size (D₅₀) of 67.4 mm (0.221 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 21, 1995, indicated that the reach was laterally unstable with
mass wasting along the upstream right bank.
The Town Highway 5 crossing of the Wells River is a 108-ft-long, two-lane bridge
consisting of a 100-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 27, 1995). The opening length of the structure parallel to the bridge
face is 93.4 ft. The bridge is supported by vertical, stone block abutments with wingwalls.
The channel is skewed approximately 50 degrees to the opening while the opening-skew-toroadway is 45 degrees.
The scour protection counter-measures at the site included type-1 stone fill (less than 12
inches diameter) along the downstream left road embankment. Also, type-2 stone fill (less
than 36 inches diameter) along the upstream right wingwall, extending 30 feet upstream
along the right bank, along the downstream end of the downstream right wingwall, along
the downstream right road embankment, and along the downstream left bank below the old
railroad bed. 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)
for the 100- and 500-year discharges. 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 1.8 to 2.6 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 10.2 to
22.6 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
documented herein.