Level II scour analysis for Bridge 12 (NEWFVT00300012) on State Route 30, crossing Smith Brook, Newfane, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NEWFVT00300012 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 and Green Mountain sections of the New England
physiographic province in southeastern Vermont. The 9.55-mi²
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the surface cover is
primarily pasture with the exception of the downstream right bank which is forested. The
immediate banks have dense woody vegetation except for the downstream right bank which
has cut grass.
In the study area, Smith Brook has a sinuous channel with a slope of approximately 0.01 ft/
ft, an average channel top width of 63 ft and an average bank height of 10 ft. The
predominant channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 75.4 mm (0.247 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 21, 1996, indicated that the reach was stable.
The State Route 30 crossing of Smith Brook is a 43-ft-long, two-lane bridge consisting of a
40-foot concrete T-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 35 degrees to the opening while the measured
opening-skew-to-roadway is 30 degrees.
The scour protection measure at the site included type-2 stone fill (less than 36 inches
diameter) from the upstream end of the left abutment to 35 ft. upstream, along the
downstream end of the downstream left wingwall, along the right bank from 7 ft. to 90 ft.
downstream, and along the left bank from 15 ft. to 40 ft. downstream. Also, there was type-
1 stone fill (less than 24 inches diameter) at the left abutment and downstream left
wingwall. 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 modelled flows ranged from 1.2 to 1.8 ft. The worst-case contraction
scour occurred at the 500-year discharge. Abutment scour ranged from 7.6 to 14.1 ft. The
worst-case abutment scour also 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.