Level II scour analysis for Bridge 8 (BARTTH00020008) on Town Highway 2, crossing Roaring Brook, Barton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BARTTH00020008 on town highway 2 crossing Roaring Brook, Barton, 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 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 New England Upland section of the New England physiographic province
of North-central Vermont in the town of Barton. The 9.89-mi²
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the banks have
woody vegetation coverage except for the downstream left bank, which has a few trees and
grass and brush coverage.

In the study area, Roaring Brook has an incised, sinuous channel with a slope of
approximately 0.019 ft/ft, an average channel top width of 35 ft and an average channel
depth of 3 ft. The predominant channel bed material is gravel/cobble (D₅₀ is 49.1 mm or
0.161 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
October 18, 1994 indicated that the reach was laterally unstable. A cut-bank on the
downstream right bank and overall channel configuration in the valley are indications of the
lateral instability at this site.

The town highway 2 crossing of Roaring Brook is a 30-ft-long, two-lane bridge consisting
of one 26-foot span concrete T-beam type superstructure (Vermont Agency of
Transportation, written communication, August 4, 1994). The bridge is supported by
vertical, concrete abutments. The channel is skewed approximately 15 degrees to the
opening while the opening-skew-to-roadway is zero degrees.

A scour hole 2.5 ft deeper than the mean thalweg depth was observed near mid-channel
downstream of the bridge during the Level I assessment. The only scour protection measure
at the site was type-1 stone fill (less than 12 inches diameter) on the left upstream and
downstream roadway embankments. 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
aggradation or degradation; 2) contraction scour (due to reduction in flow area caused by 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 scour depths
for contraction and local scour and a summary of the results follows.

Contraction scour for all modelled flows ranged from 1.4 to 2.8 feet and the worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.5 to
16.5 feet and 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.