Level II scour analysis for Bridge 46 (BRNETH00610046) on Town Highway 61, crossing East Peacham Brook, Barnet, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRNETH00610046 on Town Highway 61 crossing East Peacham Brook, Barnet, 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 15.8-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest.
In the study area, East Peacham Brook has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 59 ft and an average bank height
of 5 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 121 mm (0.397 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on August 23, 1995, indicated that the reach was laterally unstable with cut
banks both upstream and downstream of the bridge.
The Town Highway 61 crossing of East Peacham Brook is a 28-ft-long, one-lane bridge
consisting of one 26-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 24, 1995). The opening length of the structure parallel to the bridge
face is 24.5 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-toroadway is zero degrees.
A scour hole 0.7 ft deeper than the mean thalweg depth was observed along the upstream
left wingwall extending along the left abutment during the Level I assessment. The only
scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) at
the upstream end of the upstream left wingwall extending along the upstream left bank and
along the entire base of the downstream left wingwall. 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 to 1.2 ft. The worst-case contraction
scour occurred at the 500-year discharge. Abutment scour ranged from 10.4 to 13.9 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.