Level II scour analysis for Bridge 4 (MAIDTH00070004) on Town Highway 7, crossing Cutler Mill Brook, Maidstone, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MAIDTH00070004 on Town Highway 7 crossing the Cutler Mill Brook, Maidstone,
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 White Mountain section of the New England physiographic province in
northeastern Vermont. The 18.1-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is predominantly shrub and
brushland.

In the study area, the Cutler Mill Brook has a non-incised, meandering channel with local
braiding and a slope of approximately 0.004 ft/ft, an average channel top width of 43 ft and
an average bank height of 2 ft. The channel bed material ranges from sand to cobble with a
median grain size (D₅₀) of 27.6 mm (0.091 ft). The geomorphic assessment at the time of
the Level I and Level II site visit on July 19, 1995, indicated that the reach was laterally
unstable due to large meanders in the channel.

The Town Highway 7 crossing of the Cutler Mill Brook is a 25-ft-long, one-lane bridge
consisting of one 22-foot concrete span (Vermont Agency of Transportation, written
communication, August 5, 1994). The opening length of the structure parallel to the bridge
face is 21.7 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 20 degrees to the opening while the opening-skew-to-roadway is 0 degrees.

A scour hole 2.0 ft deeper than the mean thalweg depth was observed along the left
abutment during the Level I assessment. The only scour protection measure at the site was
type-2 stone fill (less than 36 inches diameter) along both banks upstream, along the entire
base length of the upstream left wingwall, and along the upstream end of the upstream right
wingwall. 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
computations follows.

Contraction scour for all modelled flows ranged from 2.2 to 4.2 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 5.7 to
12.4 ft. The worst-case 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
documented herein.