Level II scour analysis for Bridge 1 (BLOOTH00020001) on Town Highway 2, crossing Mill Brook, Bloomfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BLOOTH00020001 on town highway 2 crossing Mill Brook, Bloomfield, 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 Upland physiographic
province of north-east Vermont in the town of Bloomfield. The 4.85-mi²
drainage area is in
a predominantly rural and forested basin. In the vicinity of the study site, the banks have
dense woody vegetation coverage.

In the study area, Mill Brook has an incised, sinuous channel with a slope of approximately
0.03 ft/ft, an average channel top width of 28 ft and an average channel depth of 4 ft. The
predominant channel bed materials are gravel and cobbles (D₅₀ is 57.3 mm or 0.188 ft). The
geomorphic assessment at the time of the Level I and Level II site visit on July 6, 1995,
indicated that the reach was stable.

The town highway 2 crossing of Mill Brook is a 26-ft-long, one-lane bridge consisting of
one 24-foot concrete span (Vermont Agency of Transportation, written commun., August 4,
1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel
is skewed approximately 30 degrees to the opening while the opening-skew-to-roadway is
10 degrees.

No scour was observed along the channel or at the bridge during the Level I assessment.
Type-2 stone fill (less than 24 inches diameter) was noted as present along all wingwalls.
Additional details describing conditions at the site are included in the Level II Summary
and Appendices D and E.

Scour depths and 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 aggradation or
degradation; 2) contraction scour (due to reduction in flow area caused by 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 scour depths for contraction
and local scour and a summary of the results follows.

Contraction scour for all modelled flows ranged from 0 to 1.0 feet and the worst-case
contraction scour occurred at the incipient overtopping discharge. Abutment scour ranged
from 7.3 to 10.1 feet and 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.