Level II scour analysis for Bridge 6 (BRISVT01160006) on State Highway 116, crossing Little Notch Brook, Bristol, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRISVT01160006 on State Route 116 crossing the Little Notch Brook, Bristol, 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 of
West-central Vermont in the town of Bristol. The 8.59-mi²
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the surface cover is
dense forest except for the downstream left side, which is row crops.
In the study area, Little Notch Brook has a sinuous channel with a slope of approximately
0.005 ft/ft, an average channel top width of 32 ft and an average channel depth of 4 ft. The
predominant channel bed material is sand and gravel with a median grain size (D₅₀
) of 17.4
mm (0.0570 ft). The geomorphic assessment at the time of the Level I and Level II site visit
on June 13, 1996, indicated that the reach was laterally unstable. The sinuous configuration
of the channel with fine bed and bank material, a sharp channel bend upstream, and point
bars and cut-banks upstream and downstream of this site are among the primary
characteristics, which suggest lateral instability.
In addition, there is evidence of streambed degradation at this site. A large eddy was noted
at the location where Little Notch Brook enters the New Haven River about 100 feet
downstream. There was a large scour hole noted at the location of the eddy, which is likely
to remove streambed material at least as quickly as supplied from upstream on Little Notch
Brook. Hence, channel degradation may be significant during a flood event.
The state route 116 crossing of Little Notch Brook is a 24-ft-long, two-lane bridge
consisting of one 21-foot concrete span (Vermont Agency of Transportation, written
communication, December 14, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 15 degrees to the opening
while the opening-skew-to-roadway is 25 degrees.
There was one foot of scour evident along the downstream half of the left abutment footing
and some separation of the left abutment wall from the deck above due to settling. The left
abutment footing was undermined up to a foot at the downstream end. The scour protection
measures at the site were type-1 stone fill (less than 12 inches diameter) on the upstream left
bank and type-2 stone fill (less than 36 inches diameter) on the right banks and right
wingwalls upstream and downstream of the structure. 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 3.2 to 4.3 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.0 to
10.0 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.