Level II scour analysis for Bridge 37, (BRNETH00740037) on Town Highway 74, crossing South Peacham Brook, Barnet, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRNETH00740037 on Town Highway 74 crossing South 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 northeastern Vermont. The 12.1-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is pasture upstream of the
bridge and on the downstream left bank while the immediate banks have sparse shrubs and
trees. Downstream of the bridge, the surface cover is shrub and brushland.
In the study area, South Peacham Brook has an incised, sinuous channel with a slope of
approximately 0.004 ft/ft, an average channel top width of 33 ft and an average bank height
of 3 ft. The channel bed material ranges from sand to cobble with a median grain size (D₅₀)
of 0.914 mm (0.003 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on August 24, 1995, indicated that the reach was laterally unstable. There are cutbanks upstream and downstream of the bridge.
The Town Highway 74 crossing of South Peacham Brook is a 30-ft-long, two-lane bridge
consisting of one 28-foot concrete slab span (Vermont Agency of Transportation, written
communication, March 16, 1995). The opening length of the structure parallel to the bridge
face is 25.7 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 30 degrees to the opening while the computed openingskew-to-roadway is 5 degrees.
A channel scour hole 2.0 ft deeper than the mean thalweg depth was observed at the
upstream bridge face, along the upstream right wingwall protection, during the Level I
assessment. The scour protection measures at the site included type-1 stone fill (less than 12
inches diameter) along the downstream left and right wingwalls, downstream banks, and at
the downstream end of the left and right abutments. There is also type-2 stone fill (less than
36 inches diameter) along the upstream right bank and upstream right 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 was 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 15.8 to 22.5 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.7 to
11.1 ft. The worst-case abutment scour also 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.
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.