Level II scour analysis for Bridge 39 (TOPSTH00510039) on Town Highway 51, crossing Tabor Branch Waits River, Topsham, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
TOPSTH00510039 on Town Highway 51 crossing the Tabor Branch 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 17.4-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is predominantly pasture. However,
beyond one bridge length on the right bank upstream the surface cover abruptly changes to
forest.
In the study area, the Tabor Branch Waits River has a sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 53 ft and an average bank height
of 6 ft. The predominant channel bed material is cobbles with a median grain size (D₅₀) of
86.4 mm (0.283 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on August 30, 1995, indicated that the reach was stable.
The Town Highway 51 crossing of the Tabor Branch Waits River is a 34-ft-long, one-lane
bridge consisting of one 32-foot concrete slab span (Vermont Agency of Transportation,
written communication, March 28, 1995). The opening length of the structure parallel to the
bridge face is 31.0 ft. The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 5 degrees to the opening while the
opening-skew-to-roadway is 10 degrees.
The only scour protection measure at the site was type-2 stone fill (less than 36 inches
diameter) along the left and right bank upstream, along the base of the upstream left
wingwall, upstream right wingwall, left abutment, right abutment, downstream left
wingwall, downstream right wingwall, and along the left and right bank downstream.
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)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. 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 0.0 to 0.4 ft. The worst-case
contraction scour occurred at the maximum free surface flow discharge, which was less
than the 100-year discharge. Abutment scour ranged from 4.8 to 8.0 ft. The worst-case
abutment scour occurred at 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 crosssection 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.