Level II scour analysis for Bridge 5 (CHELTH00030005) on Town Highway 3, crossing Jenkins Brook, Chelsea, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHELTH00030005 on town highway 3 crossing Jenkins Brook, Chelsea, 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
study 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
of central Vermont in the town of Chelsea. The 6.97-mi²
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the surface cover is
forest.
In the study area, Jenkins Brook has an incised, sinuous channel with a slope of
approximately 0.04 ft/ft, an average channel top width of 48 ft and an average channel
depth of 3 ft. The predominant channel bed material is cobble with a median grain size
(D₅₀) of 154 mm (0.506 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on November 17, 1994, indicated that the reach was stable.
The town highway 3 crossing of Jenkins Brook is a 23-ft-long bridge consisting of one 20-
foot concrete span (Vermont Agency of Transportation, written communication, August 25,
1994). The bridge is supported by vertical, concrete abutments with wingwalls. The
upstream wingwalls are protected by type-3 stone fill (less than 48 inches diameter) and the
downstream wingwalls have type-2 stone fill (less than 36 inches diameter). The footings of
both abutments are exposed. The channel is skewed approximately 25 degrees to the
opening while the opening-skew-to-roadway is 15 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, 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 was 0.0 ft. Abutment scour ranged from 7.6 to
12.4 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.