Level II scour analysis for Bridge 33 (CONCTH00580033) on Town Highway 58, crossing Miles Stream, Concord, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CONCTH00580033 on Town Highway 58 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 17.9-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 while the immediate banks have dense woody vegetation. Downstream of the
bridge, the right bank is forested and the left bank has shrubs and brush.

In the study area, Miles Stream has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 91 ft and an average bank height
of 7 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 61.6 mm (0.188 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 58 crossing of Miles Stream is a 44-ft-long, two-lane bridge consisting
of one 39-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 37.4 ft. The bridge is supported by vertical, concrete abutments with stone fill in
front creating spillthrough embankments. The channel is skewed approximately 20 degrees
to the opening while the opening-skew-to-roadway is zero degrees.

The only scour countermeasure at the site was type-3 stone fill (less than 48 inches
diameter) along the left and right banks upstream, in front of the abutments forming spill
through embankments, and extending along the banks downstream. 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 1.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.0 to
9.7 ft. The worst-case abutment scour occurred at the 500-year discharge for the right
abutment and at the incipient roadway-overtopping discharge for the left abutment.
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.