This report provides the results of a detailed Level II analysis of scour potential at structure
STOWTH00430036 on Town Highway 43 crossing the Miller Brook, Stowe, 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
north central Vermont. The 5.5-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is predominantly forested.
In the study area, the Miller Brook has an incised, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 43 ft and an average bank height
of 7 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D50) of 70.4 mm (0.231 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 15, 1996, indicated that the reach was stable.
The Town Highway 43 crossing of the Miller Brook is a 24-ft-long, two-lane bridge
consisting of one 21-foot steel-beam span (Vermont Agency of Transportation, written
communication, October 13, 1995). The opening length of the structure parallel to the
bridge face is 21.5 ft. The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 10 degrees to the opening and the
computed opening-skew-to-roadway is also 10 degrees.
The footing on the left abutment was exposed 2.5 ft and the footing on the right abutment
was exposed 3.0 ft during the Level I assessment. Scour protection measures at the site were
type-4 stone fill (less than 60 inches diameter) on the left and right bank upstream, type-3
stone fill (less than 48 inches diameter) along the entire base length of the upstream right
wingwall, right abutment, and type-2 stone fill (less than 36 inches diameter) along the
entire base length of the downstream right wingwall, and left and right banks downstream.
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 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 0.9 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 3.1 to
6.5 ft. The worst-case abutment scour occurred at the 100-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 36 (STOWTH00430036) on Town Highway 43, crossing Miller Brook, Stowe, Vermont