Level II scour analysis for Bridge 7 (CHARTH00010007) on Town Highway 1, crossing Mad Brook, Charleston, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHARTH00010007 on town highway 1 crossing Mad Brook, Charleston, 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 White Mountain section of the New England physiographic province in
north-central Vermont in the town of Charleston. The 6.59-mi²
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the surface cover is
pasture except for the upstream left bank, which is forest. The stream banks are tree covered
upstream and on the downstream left bank side.
In the study area, Mad Brook has an incised, sinuous channel with a slope of approximately
0.01 ft/ft, an average channel top width of 41 ft and an average channel depth of 5 ft. The
predominant channel bed materials range from gravel to boulders with a median grain size
(D₅₀) of 105 mm (0.344 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on October 28, 1994, indicated that the reach was stable.
The town highway 1 crossing of Mad Brook is a 27-ft-long, two-lane bridge consisting of
one 25-foot concrete T-beam span (Vermont Agency of Transportation, written
communication, August 4, 1994). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 10 degrees to the opening. The
opening-skew-to-roadway computed from surveyed data is 5 degrees, but historical bridge
records indicate this angle is closer to 10 degrees.
There was scour evident during the Level I assessment due to the presence of two
subfootings at the base of each abutment wall. Although the subfootings may have been
constructed at the same time as the abutment walls, the subfootings may have been
constructed at a later time in response to streambed degradation under the bridge. The right
abutment was noted as undermined during the Level I assessment. Scour protection
measures at the site were type-1 stone fill (less than 12 inches diameter) on the upstream
right and downstream road embankments and type-2 stone fill on each wingwall and the
downstream left bank. Additional details describing conditions at the site are included in the
Level II Summary and Appendices D and E.
Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1995). 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 0.3 ft. The worst-case
contraction scour occurred at the incipient overtopping discharge, which was less than the
100-year discharge. Abutment scour ranged from 6.2 to 9.4 ft. The worst-case abutment
scour for the right abutment was 9.4 feet at the 100-year discharge. The worst-case
abutment scour for the left abutment was 8.6 feet at the incipient overtopping 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 herein.