Level II scour analysis for Bridge 65 (NEWBTH00500065) on Town Highway 50, crossing Peach Brook, Newbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NEWBTH00500065 on Town Highway 50 crossing Peach Brook, Newbury, 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 15.3-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest upstream of the bridge and
shrub and brushland downstream of the bridge.
In the study area, Peach Brook has an incised, sinuous channel with a slope of
approximately 0.005 ft/ft, an average channel top width of 40 ft and an average bank height
of 8 ft. The channel bed material ranges from cobble to boulder with a median grain size
(D50) of 83.1 mm (0.273 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 29, 1995, indicated that the reach was stable.
The Town Highway 50 crossing of the Peach Brook is a 29-ft-long, two-lane bridge
consisting of one 25-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 24.9 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 50 degrees to the opening while the computed openingskew-to-roadway is 20 degrees.
A channel scour hole 0.75 ft deeper than the mean thalweg depth was observed under the
bridge during the Level I assessment. Also observed was channel scour 0.75 ft deeper than
the mean thalweg at the upstream face of the bridge and channel scour 0.25 ft deeper than
the mean thalweg along the right bank downstream. The scour protection measures at the
site included type-1 stone fill (less than 12 inches diameter) along the upstream and
downstream right wingwalls and type-2 stone fill (less than 36 inches diameter) along the
upstream right bank and along the downstream left wingwall and bank. In addition, there
are four 3 ft square concrete blocks at the corner where the upstream right wingwall joins
the right abutment. The upstream left wingwall and upstream half of the left abutment were
constructed on top of a bedrock outcrop. 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 1.3 ft. The worst-case
contraction scour occurred at the incipient roadway-overtopping discharge, which was less
than the 100-year discharge. The right abutment scour ranged from 6.1 to 7.2 ft. The worstcase right abutment scour occurred at the incipient roadway-overtopping discharge. The left
abutment scour ranged from 7.1 to 10.3 ft. The worst-case left 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 he