Level II scour analysis for Bridge 16 (CHESVT01030016) on State Route 103, crossing the Williams River, Chester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHESVT01030016 on State Route 103 crossing the Williams River, Chester, 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 15.1-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is pasture except for the
downstream right overbank which is forested.

In the study area, the Williams River has an incised, straight channel with a slope of
approximately 0.008 ft/ft, an average channel top width of 56 ft and an average bank height
of 6 ft. The channel bed material ranges from gravel to cobbles with a median grain size
(D₅₀) of 67.5 mm (0.222 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on September 16, 1996, indicated that the reach was stable.

The State Route 103 crossing of the Williams River is a 162-ft-long, two-lane bridge
consisting of three steel-beam spans (Vermont Agency of Transportation, written
communication, March 13, 1995). The opening length of the structure parallel to the bridge
face is 157.7 ft.The bridge is supported by vertical, concrete abutments and piers with no
wingwalls. The channel is skewed approximately 55 degrees to the opening while the
opening-skew-to-roadway is also 55 degrees.

The scour protection measures at the site included type-4 stone fill (less than 60 inches
diameter) along the upstream left bank. There was type-3 stone fill (less than 48 inches
diameter) along the upstream right bank and both spill-through embankments and both
downstream banks. There was type-1 stone fill (less than 12 inches diameter) along the
upstream right and downstream left road embankments. 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 was 0.0. Abutment scour ranged from 6.4 to 9.0 ft.
The worst-case abutment scour occurred at the 500-year discharge. Pier scour ranged from
7.9 to 10.1 ft. The worst-case pier scour occurred at the incipient-overtopping discharge for
both piers. 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.