Level II scour analysis for Bridge 43 (CHELTH00460043) on Town Highway 46, crossing Jail Brook, Chelsea, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHELTH00460043 on Town Highway 46 crossing Jail 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
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 central Vermont. The 4.68-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is best described as suburban with
homes, lawns, and a few trees.

In the study area, Jail Brook has an incised, straight channel with a slope of approximately
0.02 ft/ft, an average channel top width of 32 ft and an average bank height of 6 ft. The
channel bed material ranges from coarse sand to boulder with a median grain size (D₅₀) of
43.0 mm (0.141 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on November 18, 1994, indicated that the reach was stable.

The Town Highway 46 crossing of Jail Brook is a 27-ft-long, two-lane bridge consisting of
one 23-foot concrete span (Vermont Agency of Transportation, written communication,
August 25, 1994). The opening length of the structure parallel to the bridge face is 22.8
ft.The bridge is supported by vertical, concrete abutments with wingwalls. The channel is
skewed approximately zero degrees to the opening and the opening-skew-to-roadway is
also zero degrees.

Channel scour was not observed. However, the left abutment footing was exposed one foot.
Scour countermeasures at the site consisted of type-2 stone fill (less than 36 inches
diameter) on the banks and road embankments upstream and downstream of the bridge.
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).
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 1.1 to 1.2 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 5.0 to
6.5 ft at the left abutment and 4.7 to 6.2 ft at the right abutment. 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.