Level II scour analysis for Bridge 39 (PEACTH00620039) on Town Highway 62, crossing South Peacham Brook, Peacham, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
PEACTH00620039 on Town Highway 62 crossing South Peacham Brook, Peacham,
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 9.1-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest on the left bank upstream
and downstream of the bridge. The surface cover on the right bank upstream and
downstream is shrubs and brush.
In the study area, South Peacham Brook has an incised, straight channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 43 ft and an average bank height
of 8 ft. The channel bed material ranges from sand to boulder with a median grain size (D₅₀)
of 51.4 mm (0.168 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 stable.
The Town Highway 62 crossing of South Peacham Brook is a 23-ft-long, one-lane bridge
consisting of one 22-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 27, 1995). The opening length of the structure parallel to the bridge
face is 20.1 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 15 degrees to the opening while the computed openingskew-to-roadway is 10 degrees.
The footing on the right abutment and the footing on the upstream left wingwall were
exposed during the Level I assessment. The scour countermeasures at the site included type-
2 stone fill (less than 36 inches diameter) along the upstream and downstream right
wingwalls and at the upstream end of the upstream left wingwall and at the downstream end
of the downstream left wingwall. Type-3 stone fill (less than 48 inches diameter) was along
the upstream left and right banks and the downstream right bank. On the downstream left
bank, the scour countermeasure was a stone wall. 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 1.0 to 1.6 ft. The worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 5.9 to
7.4 ft. The worst-case abutment scour occurred at the incipient roadway-overtopping
discharge, which is less than the 100-year discharge. Additional information on scour
depths and depths to armoring are included in the section titled “Scour Results”. Scouredstreambed 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 particlesize distribution. However, there is a bedrock outcrop across the channel just upstream of
the bridge.
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.