Level II scour analysis for Bridge 30 (BRIDTH00330030) on Town Highway 33, crossing Dailey Hollow Branch, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00330030 on town highway 33 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). A Level I study is included in Appendix E of this report. A Level I study provides
a qualitative geomorphic characterization of the study site. Information on the bridge
available from VTAOT files was compiled prior to conducting Level I and Level II
analyses and can be found in Appendix D.

The site is in the Green Mountain physiographic province of central Vermont in the town of
Bridgewater. The 7.51-mi²
drainage area is in a predominantly rural and forested basin. In
the vicinity of the study site, the surface cover is forest.

In the study area, Dailey Hollow Branch has an incised, sinuous channel with a slope of
approximately 0.013 ft/ft, an average channel top width of 45 ft and an average channel
depth of 5 ft. The channel bed material ranges from sand to boulder with a median grain
size (D₅₀) of 60.7 mm (0.199 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on November 1, 1994, indicated that the reach was stable.

The town highway 33 crossing of Dailey Hollow Branch is a 31-ft-long, one-lane bridge
consisting of one 25-foot steel-beam span with a timber deck (Vermont Agency of
Transportation, written communication, August 25, 1994). 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. Type-2 stone-fill
(less than 36 inches diameter) protection was found at all four 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, 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 0.5 to 3.1 ft. The worst-case
contraction scour occurred at the incipient-roadway-overtopping discharge, which is
between the 100- and 500-year discharge. Abutment scour ranged from 6.9 to 14.6 ft. with
the worst-case scenario also occurring at the incipient-roadway-overtopping 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. 48). Many factors,
including historical performance during flood events, the geomorphic assessment, scour
protection measures, and the results of the hydraulic analyses, must be considered to
properly assess the validity of abutment scour results. Therefore, scour depths adopted by
VTAOT may differ from the computed values documented herein, based on the
consideration of additional contributing factors and experienced engineering judgement.