Level II scour analysis for Bridge 45 (NFIETH00250045) on Town Highway 25, crossing Union Brook, Northfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NFIETH00250045 on Town Highway 25 crossing Union Brook, Northfield, 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 4.04-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, surface cover consists of shrubs and brush on all of the
banks except the upstream right bank which is forested.
In the study area, Union Brook has an incised, meandering channel with a slope of
approximately 0.018 ft/ft, an average channel top width of 41 ft and an average bank height
of 2 ft. The channel bed material ranges from sand to cobble with a median grain size (D₅₀)
of 65.8 mm (0.216 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on July 24, 1996, indicated that the reach was unstable. The stream meanders and
there is a cut bank on the upstream right bank and trees are falling into the channel.
The Town Highway 25 crossing of Union Brook is a 28-ft-long, two-lane bridge consisting
of one 26-foot concrete slab span (Vermont Agency of Transportation, written
communication, October 13, 1995). The opening length of the structure parallel to the
bridge face is 23.8 ft. The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 50 degrees to the opening while the
opening-skew-to-roadway is 0 degrees.
During the Level I assessment, a scour hole 3.0 ft deeper than the mean thalweg depth was
observed at the upstream face of the bridge that extended from the center of the channel to
the front of the upstream left wingwall. An additional scour hole 1.5 ft deeper than the mean
thalweg depth was observed along the downstream right bank near the bridge. The scour
counter measures at the site were a laid-up wall of concrete slabs along the upstream right
bank beginning at the end of the upstream right wingwall and type-1 stone fill (less than 12
inches diameter) along the downstream right wingwall and bank, and type-2 stone fill (less
than 36 inches diameter) along the downstream left wingwall and bank. 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).
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 0.4 to 0.9 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.5 to
9.1 ft. The worst-case abutment scour occurred 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 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.