Level II scour analysis for Bridge 29 (PUTNTH00210029) on Town Highway 21, crossing East Putney Brook, Putney, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
PUTNTH00210029 on Town Highway 21 crossing East Putney Brook, Putney, 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 10.3-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover consists of pasture and
forest.
In the study area, East Putney Brook has an incised, sinuous channel with a slope of
approximately 0.009 ft/ft, an average channel top width of 33 ft and an average bank height
(channel depth) of 3 ft. The channel bed material is cobbles predominantly with a median
grain size (D₅₀) of 80.7 mm (0.265 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on August 19, 1996, indicated that the reach was stable.
The Town Highway 21 crossing of East Putney Brook is a 35-ft-long, one-lane bridge
consisting of one 29-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 30, 1995). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 40 degrees to the opening. Historical
records show an opening-skew-to-roadway of 10 degrees but 20 degrees was computed
using field survey data and used in this study.
The scour protection measures at the site were type-2 stone fill (less than 36 inches
diameter) on each abutment wall, the upstream right wingwall and the upstream right bank,
and type-3 stone fill (less than 48 inches diameter) on the left bank upstream, the upstream
left wingwall, and the downstream right 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 0.0 to 0.9 feet. The worst-case
contraction scour occurred at the incipient-overtopping discharge, which was less than the
100-year discharge. Abutment scour ranged from 6.1 to 18.4 feet. The worst-case abutment
scour occurred at the 500-year discharge for the right abutment and the incipient
overtopping discharge for the left abutment. 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.