Level II scour analysis for Bridge 7 (WARRTH00010007) onTown Highway 1, crossing Freemans Brook, Warren, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WARRTH00010007 on Town Highway 1 crossing Freeman Brook, Warren, 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 Green Mountain section of the New England physiographic province in
central Vermont. The 6.45-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the predominant surface cover is grass and trees with the
exception of the upstream left overbank which is forest. The banks of the channel are tree
covered.
In the study area, Freeman Brook has an incised, straight channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 51 ft and an average channel
depth of 6 ft. The predominant channel bed material ranges from gravel to bedrock with a
median grain size (D₅₀) of 86.8 mm (0.285 ft). The geomorphic assessment at the time of
the Level I and Level II site visit on July 22, 1996 indicated that the reach was stable.
The Town Highway 1 crossing of Freeman Brook is a 64-ft-long, two-lane bridge
consisting of one 62-foot steel-beam span (Vermont Agency of Transportation, written
communication, February 1, 1996). The bridge is supported by vertical, concrete abutments
with spill-through slopes. The channel is skewed approximately 25 degrees to the opening
while the opening-skew-to-roadway is 30 degrees.
The only scour protection measure at the site was type-2 stone fill (less than 36 inches
diameter) along the entire length of the left and right abutments and along the downstream
channel banks. 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.
The computed contraction scour for all modelled flows was 0.0 feet. Abutment scour
ranged from 5.3 to 8.2 ft. The worst-case abutment scour occurred at the right abutment for
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.