This report provides the results of a detailed Level II analysis of scour potential at structure
SHARTH00040013 on Town Highway 4 crossing Broad Brook, Sharon, 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
The site is in the New England Upland section of the New England physiographic province
in central Vermont. The 16.6-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is brushland on the downstream left
overbank and row crops on the right overbank, while the immediate banks have dense
woody vegetation. Upstream of the bridge, the overbanks are forested.
In the study area, Broad Brook has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 69 ft and an average bank height
of 5 ft. The channel bed material ranges from sand to boulder with a median grain size (D50)
of 112 mm (0.369 ft). The geomorphic assessment at the time of the Level I site visit on
April 11, 1995 and Level II site visit on July 23, 1996, indicated that the reach was stable.
The Town Highway 4 crossing of Broad Brook is a 34-ft-long, two-lane bridge consisting
of one 31-foot concrete tee beam span (Vermont Agency of Transportation, written
communication, March 23, 1995). The opening length of the structure parallel to the bridge
face is 30.1 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 10 degrees to the opening while the opening-skew-to-roadway is 15 degrees.
A scour hole 2.0 ft deeper than the mean thalweg depth was observed along the upstream
end of the right abutment. At the downstream end of the left abutment, a 1.0 foot scour hole
was observed . Scour countermeasures at the site include type-2 stone fill (less than 3 feet
diameter) at each road embankment. 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
Contraction scour for all modelled flows ranged from 0.7 to 1.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 5.6
to 9.4 ft. The worst case left abutment scour occurred at the 500-year discharge. Right
abutment scour ranged from 19.0 to 19.8 ft. The worst-case right abutment scour occurred
at 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
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