Level II scour analysis for Bridge 15 (TROYTH00290015) on Town Highway 29, crossing Beetle Brook, Troy, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
TROYTH00290015 on Town Highway 29 crossing Beetle Brook, Troy, 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 8.97-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest except for the downstream
right bank which is grass.
In the study area, Beetle Brook has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 41 ft and an average bank height
of 4 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 118 mm (0.387 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on June 7, 1995, indicated that the reach was stable.
The Town Highway 29 crossing of Beetle Brook is a 30-ft-long, one-lane bridge consisting
of one 25-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 7, 1994). The opening length of the structure parallel to the bridge
face is 23.4 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 15 degrees to the opening while the opening-skew-toroadway is 0 degrees.
A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the right
abutment during the Level I assessment. Scour counter measures at the site include type-3
stone fill (less than 48 inches diameter) at the downstream end of the downstream right
wingwall, type-2 stone fill (less than 36 inches diameter) along the downstream left
wingwall and the upstream left road embankment, and type-1 stone fill (less than 12 inches
diameter) at the upstream right 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
computations follows.
Contraction scour for all modelled flows ranged from 0.0 to 0.6 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge. Left abutment scour
ranged from 8.0 to 8.9 ft. The worst-case left abutment scour occurred at the 500-year
discharge. Right abutment scour ranged from 15.4 to 16.5 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”. Scouredstreambed 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 particlesize 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.