Level II scour analysis for Bridge 24 (WODSTH00190024) on Town Highway 19, crossing North Bridgewater Brook, Woodstock, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WODSTH00190024 on Town Highway 19 crossing North Bridgewater 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). A Level I study is included in Appendix E of this report. A Level I study provides
a qualitative geomorphic characterization of the study site. Information on the bridge
available from VTAOT files was compiled prior to conducting Level I and Level II
analyses and can be found in Appendix D.
The site is in the Green Mountain physiographic province of east-central Vermont in the
town of Woodstock. The 3.6-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the left and right banks are covered by moderate tree
cover along the immediate banks with some pasture/ grassland beyond.
In the study area, the North Bridgewater Brook has a sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 44 ft and an average channel
depth of 4 ft. The channel bed materials ranges from sand to boulders with a D₅₀ (median
diameter)of 70.1 mm or 0.229 ft. The geomorphic assessment at the time of the Level I and
Level II site visits on August 17, 1994 and December 13, 1994, indicated that the reach was
stable. Localized bank cutting existed at the immediate downstream left bank.
The Town Highway 19 crossing of the North Bridgewater Brook is a 26-ft-long, one-lane
bridge consisting of one 23-ft steel-beam span (Vermont Agency of Transportation, written
commun., August 3, 1994). The bridge is supported by vertical, concrete abutments with
wingwalls. Type-2 (less than 3 ft diameter) stone fill protects the upstream left wingwall
which is impacted by flow. The channel bed under the bridge is constructed of wood. This
construction is preventing channel degradation along the impacted left abutment.The
channel is skewed approximately 40 degrees to the opening; the opening-skew-to-roadway
is 10 degrees. 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, 1993). 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 0.8 ft. Abutment scour ranged
from 6.6 to 14.9 ft. with the worst-case scenario occurring 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, 1993, p. 48). Many factors,
including historical performance during flood events, the geomorphic assessment, scour
protection measures, and the results of the hydraulic analyses, must be considered to
properly assess the validity of abutment scour results. Therefore, scour depths adopted by
VTAOT may differ from the computed values documented herein, based on the
consideration of additional contributing factors and experienced engineering judgement.