Level II scour analysis for Bridge 30 (NEWFVT00300013) on Vermont Highway 30, crossing Smith Brook, Newfane, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NEWFVT00300013 on State Route 30 crossing Smith Brook, Newfane, 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 southeastern Vermont. The 9.38-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is grass and shrubs except
for the upstream right bank which is forested.

In the study area, Smith Brook has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 53 ft and an average bank height
of 5 ft. The channel bed material is predominantly cobbles with a median grain size (D₅₀) of
79.5 mm (0.261 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on August 20, 1996, indicated that the reach was stable.

The State Route 30 crossing of Smith Brook is a 69-ft-long, two-lane bridge consisting of
one 66-foot steel-beam span (Vermont Agency of Transportation, written communication,
March 30, 1995). The bridge is supported by vertical, concrete abutments with wingwalls.
The channel is skewed approximately 45 degrees to the opening while the opening-skew-to-
roadway is 55 degrees.

The scour protection measures at the site were type-1 stone fill (less than 12 inches
diameter) along the upstream right bank. There was also type-2 stone fill (less than 36
inches diameter) along the upstream left bank. A stone wall extends to 72 feet upstream
from the end of the upstream left wingwall. There is another stone wall along the upstream
right bank. 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 0.0 to 0.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 14.4 to
18.2 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.