Level II scour analysis for Bridge 15 (BRIDTH00220015) on Town Highway 22, crossing Dailey Hollow Branch, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00220015 on town highway 22 crossing Dailey Hollow Branch, Bridgewater,
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 of
central Vermont in the town of Bridgewater. The 1.73-mi2
drainage area is a predominantly
rural and forested basin. In the vicinity of the study site, the left and right banks have dense
tree cover. The upstream right bank of Dailey Hollow Branch is adjacent to town highway
22.

In the study area, Dailey Hollow Branch has a sinuous channel with a slope of
approximately 0.035 ft/ft, an average channel top width of 30 ft and an average channel
depth of 4 ft. The predominant channel bed material is cobble with a median grain size
(D50) of 108 mm (0.354 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on November 1 and 2, 1994, indicates that the reach is stable.

The town highway 22 crossing of Dailey Hollow Branch is a 22-ft-long, one-lane bridge
consisting of one 22-ft. steel-beam span (Vermont Agency of Transportation, written
communication, August 24, 1994). The bridge is supported by vertical, concrete abutments
with wingwalls. Type-1 stone fill (less than 12 inches diameter) protects the left abutment,
but it’s condition was reported as eroded. Type-2 stone fill (less than 36 inches diameter)
protects the upstream left wingwall; it’s condition was reported as slumping.The channel is
skewed approximately 40 degrees to the opening while the opening-skew-to-roadway is 0
degrees. 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, 1993). 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 0.2 ft. with the worst-case
contraction scour occurring at the 500-year discharge. Abutment scour ranged from 4.2 to
6.4 ft. The worst-case abutment scour also 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, 1993, 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.