This report provides the results of a detailed Level II analysis of scour potential at structure
MANCTH00060008 on Town Highway 6 crossing Bourn Brook, Manchester, 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 Taconic section of the New England physiographic province in
southwestern Vermont. The 15.5-mi2
drainage area is in a predominantly rural and forested
basin. The bridge site is located within a suburban setting in the Town of Manchester with
houses and lawns on the overbanks.
In the study area, Bourn Brook has an incised, straight channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 61 ft and an average bank height
of 7 ft. The channel bed material ranges from sand to cobbles with a median grain size (D50)
of 87.2 mm (0.286 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on August 6, 1996, indicated that the reach was stable.
The Town Highway 6 crossing of Bourn Brook is a 44-ft-long, two-lane bridge consisting
of one 41-foot concrete T-beam span (Vermont Agency of Transportation, written
communication, September 28, 1995). The opening length of the structure parallel to the
bridge face is 40.0 ft. The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately zero degrees to the opening while the
opening-skew-to-roadway is 15 degrees.
A scour hole 3.5 ft deeper than the mean thalweg depth was observed along the upstream
right wingwall and right abutment during the Level I assessment. The scour
countermeasures at the site were stone walls in front of the upstream left wingwall and
bank, along the upstream right bank extending from the end of the upstream right wingwall,
and in front of the downstream right 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)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge was 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 was zero ft. The left abutment scour ranged from
3.6 to 9.2 ft. The worst-case left abutment scour occurred at the 500-year discharge. The
right abutment scour ranged from 9.8 to 12.6 ft. The worst case right 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
Additional Publication Details
USGS Numbered Series
Level II scour analysis for Bridge 8, (MANCTH00060008) on Town Highway 6, crossing Bourn Brook, Manchester, Vermont