Level II scour analysis for Bridge 39 (LOWETH00080039) on Town Highway 8, crossing Potter Brook, Lowell, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
LOWETH00080039 on Town Highway 8 crossing Potter Brook, Lowell, 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 4.69-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover consists of shrub and brushland,
except for the left overbank upstream which is forest.

In the study area, Potter Brook has a sinuous channel with a slope of approximately 0.004
ft/ft, an average channel top width of 34 feet and an average bank height of 3 ft. The
predominant channel bed materials are gravel and sand with a median grain size (D₅₀) of
18.7 mm (0.0613 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on June 15, 1995, indicated that the reach was laterally unstable. This assessment of
the reach was primarily due to the meandering of the channel with cut-banks and narrow
point bars and the fine bank and bed material near the site.

The Town Highway 8 crossing of Potter Brook is a 23-ft-long, one-lane bridge consisting of
one 21-foot steel-beam span (Vermont Agency of Transportation, written communication,
March 7, 1995). The bridge is supported by vertical, concrete abutments with wingwalls.
The channel is skewed approximately 20 degrees to the opening while the opening-skew-toroadway is zero degrees.
A scour hole 2.0 feet deeper than the mean thalweg depth was observed along the left
abutment during the Level I assessment. There were no scour protection measures evident
at the site. 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 0.3 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 1.8 to
5.5 feet. 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
documented herein.