Level II scour analysis for Bridge 49 (WODSTH00990049) on Town Highway 99, crossing Gulf Brook, Woodstock, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WODSTH00990049 on Town Highway 99 crossing the Gulf Brook, Woodstock, 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 east-central Vermont. The 16.8-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the primary surface cover is pasture except for
upstream right of the bridge which is cover by trees and brush. The immediate banks
throughout the reach have scattered woody vegetation.
In the study area, the Gulf Brook has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 91 ft and an average channel
depth of 6 ft. The channel bed materials range from sand to cobble with a median grain size
(D₅₀) of 85.3 mm (0.280 ft). The geomorphic assessment at the time of the Level I site visits
on September 15, 1994 and December 14, 1994, indicated that the reach was stable.
The Town Highway 99 crossing of the Gulf Brook is a 56-ft-long, one-lane bridge
consisting of one 55-foot steel-beam span (Vermont Agency of Transportation, written
communication, April 4, 1995). The bridge is supported by vertical, concrete abutments
with a spill-through slope constructed of large quarried stone. The channel is skewed
approximately 20 degrees to the opening while the opening-skew-to-roadway is 0 degrees.
Erosion at the right abutment has undermined the toe of the spill-through slope by nearly a
foot. Material has been removed from under the stone spill-through slope so that 0.5 feet of
horizontal penetration was possible at the time of the visits. 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 0.9 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour at the left abutment
ranged from 3.1 to 10.3 ft. with the worst-case occurring at the 500-year discharge.
Abutment scour at the right abutment ranged from 6.4 to 10.4 ft. with the worst-case
occurring at the 100-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.