Level II scour analysis for Bridge 48 (FFIETH00300048) on Town Highway 30, crossing Wanzer Brook, Fairfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
FFIETH00300048 on Town Highway 30 crossing Wanzer Brook, Fairfield, 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 Green Mountain section of the New England physiographic province in
northwestern Vermont. The 6.78-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover upstream of the bridge and on the
downstream right bank is primarily pasture. The downstream left bank is forested.

In the study area, Wanzer Brook has an incised, straight channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 65 ft and an average bank height
of 5 ft. The channel bed material is cobble with a median grain size (D₅₀) of 111 mm (0.364
ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 11,
1995, indicated that the reach was stable.

The Town Highway 30 crossing of Wanzer Brook is a 31-ft-long, two-lane bridge
consisting of one 28-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 8, 1995). The opening length of the structure parallel to the bridge
face is 26 ft.The bridge is supported by vertical stone wall abutments with concrete caps and
“kneewall” footings. The channel is skewed approximately 25 degrees to the opening while
the measured opening-skew-to-roadway is 20 degrees.

A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the downstream
left retaining wall (extended concrete footing) during the Level I assessment. It was also
observed that the right abutment is undermined with a scour depth of 0.5 ft. The scour
protection at the site was limited to four large boulders (type-4, less than 60 inches
diameter) along the downstream right retaining wall. The channel under the bridge is a
“corduroy” log mat floor composed of 13 logs which are parallel to the bridge face and
extend from 5 ft under the bridge to the downstream bridge face. The most downstream log
is approximately 0.3 to 0.4 ft higher than the other logs and controls flow at lower flows.
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 0.3 to 0.6 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 14.1 to
16.0 ft at the left abutment and from 6.8 to 7.6 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.