Level II scour analysis for Bridge 27 (ANDOTH00290027) on Town Highway 29, crossing Middle Branch Williams River, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ANDOTH00290027 on Town Highway 29 crossing the Middle Branch Williams River,
Andover, 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
central Vermont. The 12.7-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is pasture on the left bank upstream of the
bridge while the immediate bank has woody vegetation. The surface cover on the upstream
right bank is forest. Downstream of the bridge the left bank is pasture and the right bank
has woody vegetation.

In the study area, the Middle Branch Williams River has an incised, straight channel with a
slope of approximately 0.009 ft/ft, an average channel top width of 63 ft and an average
bank height of 5 ft. The channel bed material ranges from sand to cobble with a median
grain size (D₅₀) of 64.7 mm (0.212 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on September 10, 1996, indicated that the reach was stable.

The Town Highway 29 crossing of the Middle Branch Williams River is a 34-ft-long, two-
lane bridge consisting of one 32-foot steel-beam span (Vermont Agency of Transportation,
written communication, April 5, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 25 degrees to the opening
while the opening-skew-to-roadway is 0 degrees.

A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the left
abutment during the Level I assessment. Scour protection measures at the site include type-
2 stone fill (less than 36 inches diameter) along the upstream right bank and downstream
left bank and around the upstream left and right wingwalls. Type- 3 stone fill (less than 48
inches diameter) is located along the base of the left abutment in the scour hole, at the end
of the downstream left wingwall and along the upstream left 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 0.4 to 0.9 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge and the 100-year
discharge. Abutment scour ranged from 10.7 to 13.6 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.