Level II scour analysis for Bridge 43 (HARDTH00CU0043) on Church Street, crossing the Lamoille River, Hardwick, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HARDTH00CU0043 on Church Street crossing the Lamoille River, Hardwick, 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 north-central Vermont. The 87.6-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is best characterized as
suburban except for the downstream right surface cover which is pasture.
In the study area, the Lamoille River has an incised, straight channel with a slope of
approximately 0.004 ft/ft, an average channel top width of 90 ft and an average channel
depth of 8 ft. The predominant channel bed materials are cobble and gravel with a median
grain size (DM₅₀) of 99.5 mm (0.327 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on July 26, 1995, indicated that the reach was stable.
The Church Street crossing of the Lamoille River is a 100-ft-long, two-lane bridge
consisting of one 97-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 17, 1995). The bridge is supported by a vertical, stone abutment
with wingwalls on the left and a vertical concrete abutment with a stone spill-through slope
on the right. The channel is skewed approximately 5 degrees to the opening while the
opening-skew-to-roadway is 0 degrees. 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 0.8 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge. Abutment scour ranged
from 6.2 to 10.9 ft at the left abutment with worst-case occurring at the incipientovertopping discharge. Abutment scour ranged from 8.5 to 12.3 ft at the right abutment with
worst-case occurring 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 crosssection 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.