Level II scour analysis for brigde 5 (STOCTH00360005) on Town Highway 36, crossing Stony Brook, Stockridge, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
STOCTH00360005 on Town Highway 36 crossing Stony Brook, Stockbridge, 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
central Vermont. The 23.0-mi2
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is forest on the left and right banks
downstream and left bank upstream, while the right bank upstream is pasture with some
shrubs and brush.
In the study area, Stony Brook has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 109 ft and an average bank height
of 11 ft. The channel bed material is predominantly gravel with a median grain size (D50) of
71.7 mm (0.235 ft). The geomorphic assessment at the time of the Level I site visit on April
12, 1995, and Level II site visit on July 9, 1996, indicated that the reach was stable.
The Town Highway 36 crossing of Stony Brook is a 50-ft-long, one-lane bridge consisting
of one 48-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 23, 1995). The opening length of the structure parallel to the bridge
face is 46.3 ft. The bridge is supported by a vertical, concrete abutment on the left and a
vertical, concrete abutment with wingwalls on the right. The channel is skewed
approximately 5 degrees to the opening while the opening-skew-to-roadway is 0 degrees.
A scour hole 2.0 ft deeper than the mean thalweg depth was observed during the Level I
assessment along the left side of the channel at the downstream bridge face where the flow
impacts a bedrock outcrop. Scour protection measures at the site included type-1 stone fill
(less than 12 inches diameter) along the right bank upstream and at the upstream and
downstream ends of the left abutment, type-2 stone fill (less than 36 inches diameter) at the
upstream end of the upstream right wingwall, and type-3 stone fill (less than 48 inches
diameter) at the downstream end of the downstream right 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 Davis, 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 2.0 to 3.2 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 9.7 to
22.2 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 Davis, 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.