This report provides the results of a detailed Level II analysis of scour potential at structure
BRISTH00270020 on Town Highway 27 crossing 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 in
west-central Vermont. The 8.43-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover consists of pasture with trees,
shrubs, and brush along the road embankments and the stream banks, except for the
downstream left overbank area. Surface cover on the downstream left overbank is forest
with dense undergrowth consisting of vines, shrubs, and brush.
In the study area, Little Notch Brook has a sinuous channel with a slope of approximately
0.006 ft/ft, an average channel top width of 47 feet and an average bank height of 3 feet.
The predominant channel bed materials are gravel and cobbles with a median grain size
(D50) of 66.0 mm (0.216 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on June 19, 1995, indicated that the reach was stable.
The Town Highway 27 crossing of Little Notch Brook is a 48-ft-long, one-lane bridge
consisting of one 45-foot steel pony-truss span (Vermont Agency of Transportation, written
communication, November 30, 1995). The opening length of the structure parallel to the
bridge face is 42.8 feet. 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 zero degrees.
A scour hole 1.0 feet deeper than the mean thalweg depth was observed along the upstream
left wingwall and the upstream end of the left abutment during the Level I assessment. The
only scour protection measure at the site was a crude, block-cut stone wall, which extended
from the upstream end of the upstream left wingwall to 45 feet upstream. Additional details
describing conditions at the site are included in the Level II Summary and Appendices D
Scour depths and recommended rock rip-rap sizes were computed using the general
guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. 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
Contraction scour for all modelled flows ranged from 0.0 to 0.2 feet. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 12.2 to
13.4 feet at the left abutment and from 3.6 to 5.0 feet at the right abutment. 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