Level II scour analysis for Bridge 1 (CANATH00010001) on Town Highway 1, crossing Halls Stream, Canaan, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CANATH00010001 on town highway 1 crossing Halls Stream, Canaan, 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). A Level I
study is included in Appendix E of this report. A Level I study 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 can be found in Appendix D.
The site is in the White Mountain section of the New England physiographic province of
northeastern Vermont in the town of Canaan. The 89.5-mi2
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the banks have tree,
shrub and brush, and grass vegetation coverage.
In the study area, Halls Stream has a sinuous channel with a slope of approx-imately 0.0012
ft/ft, an average channel top width of 109 ft and an average channel depth of 4 ft. The
predominant channel bed materials are sand and gravel (D50 is 5.03 mm or 0.0165 ft). The
geomorphic assessment at the time of the Level I and Level II site visit on October 27,
1994, indicated that the reach was laterally unstable. The lateral instability was evident due
to a wide point-bar and cut-banks with undermining of bank material, slumping, fallen bank
vegetation evident in the upstream channel.
The town highway 1 crossing of Halls Stream is a 99-ft-long, two-lane bridge consisting of
one 33-foot and two 31-foot concrete T-beam spans (Vermont Agency of Transportation,
written communication, August 5, 1994). The bridge is supported by vertical, concrete
abutments with spill-through embankments in front of each abutment wall. The channel is
skewed approximately 10 degrees to the opening while the opening-skew-to-roadway is
zero degrees.
There are two piers in the channel at this site. Field notes and the channel survey at the
bridge indicate that the streambed elevation is higher on the downstream right sides of each
pier and lower on the downstream left sides. This asymmetrical streambed condition
suggests a flow attack angle may influence scour on each pier. Furthermore, field
observations suggest that the flow attack angle is higher for the right pier (pier 2) than the
left pier (pier 1).
The scour protection measures at the site were type-2 stone fill (less than 36 inches
diameter) on both upstream banks and both downstream road embankments. Type-3 stone
fill (less than 48 inches diameter) was found on the spill-through slopes of each abutment
and both downstream banks. The stone fill protection on the spill-through embankment of
the right abutment was noted as slumped with some of the fill material evident in the
channel immediately downstream of the bridge. 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 8.0 to 8.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.9 to
17.3 ft. The worst-case abutment scour occurred at the 500-year discharge. For the two
piers, scour ranged from 11.1 to 15.8. The worst-case pier scour occurred for pier2 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.