Level II scour analysis for Bridge 34 (CONCTH00110034) on Town Highway 11, crossing Miles Stream, Concord, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CONCTH00110034 on Town Highway 11 crossing Miles Stream, Concord, 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 24.9-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is shrub and brush on the
left bank upstream and downstream of the bridge. The surface cover on the right bank
upstream is pasture while downstream it is forest.
In the study area, Miles Stream has an incised, sinuous channel with a slope of
approximately 0.005 ft/ft, an average channel top width of 48 ft and an average bank height
of 4 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 102 mm (0.335 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on August 15, 1995, indicated that the reach was stable.
The Town Highway 11 crossing of Miles Stream is a 38-ft-long, two-lane bridge consisting
of one 36-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 33.9 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 20 degrees to the opening. The calculated opening-skewto-roadway is 25 degrees while the VTAOT determined opening-skew-to-roadway is 22
degrees.
The scour countermeasures at the site included type-2 stone fill (less than 36 inches
diameter) along the entire base length of all four wingwalls, scattered in front of the left and
right abutments, and along the downstream left and right banks. Also, there is type-3 stone
fill (less than 48 inches diameter) along the upstream left and right banks. 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 7.4 to 9.6 ft while the
left abutment scour ranged from 12.8 to 14.4 ft. The worst-case abutment scour for the left
and right abutments occurred at the 500-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 exposed bedrock in the channel upstream and
downstream of the bridge.
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.