Level II scour analysis for Bridge 38 (TOPSTH00570038) on Town Highway 57, crossing Waits River, Topsham, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
TOPSTH00570038 on Town Highway 57 crossing the Waits River, Topsham, 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 east central Vermont. The 37.3-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is predominantly pasture while the
left bank upstream is suburban.

In the study area, the Waits River has a sinuous locally anabranched channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 76 ft and an average bank height
of 6 ft. The channel bed material ranges from sand to cobble with a median grain size (D₅₀)
of 57.2 mm (0.188 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on August 28, 1995, indicated that the reach was considered laterally unstable due
to cut-banks upstream, mid-channel bars and lateral migration of the channel towards the
left abutment.

The Town Highway 34 crossing of the Waits River is a 34-ft-long, one-lane bridge
consisting of one 31-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 28, 1995). The opening length of the structure parallel to the bridge
face is 30.4 ft. The bridge is supported by a vertical, stone abutment with concrete facing
and wingwalls on the right and by a vertical, concrete abutment with wingwalls on the left.
The channel is skewed approximately 0 degrees to the opening and the opening-skew-to-roadway is also zero degrees.

A scour hole 2.0 ft deeper than the mean thalweg depth was observed towards the left bank
underneath the bridge. The only scour protection measure at the site was type-2 stone fill
(less than 36 inches diameter) along the left bank upstream, in the upstream left wing wall
area, along the left abutment, at the downstream end of the right abutment, and in the
downstream left wing wall area. There is type-3 stone fill (less than 48 inches diameter) in
the downstream right wing wall area. 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 1.6 to 5.2 ft. The worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 9.8 to
18.5 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.