Level II scour analysis for Bridge 42 (BRIDTH00040042) on Town Highway 04, crossing Dailey Hollow Brook, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00040042 on town highway 4 crossing Dailey Hollow Brook, 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 division of central Vermont in the town of
Bridgewater. The 2.20-mi²
drainage area is in a predominantly rural and forested basin. In
the vicinity of the study site, the overbanks are covered by shrubs and trees except for the
upstream right overbank where there is a house. Dailey Hollow Brook enters Dailey Hollow
Branch at the downstream face of the bridge.

In the study area, Dailey Hollow Brook has an incised, sinuous channel with a slope of
approximately 0.035 ft/ft. The channel top width and channel depth upstream of the bridge
is 19 ft and 3 ft, respectively. Downstream of the bridge and the confluence the channel top
width and channel depth is 39 ft and 2 ft respectively. The predominant channel bed
material is cobble and gravel (D₅₀ is 64.7 mm or 0.212 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 4 crossing of Dailey Hollow Brook is a 25-ft-long, one-lane bridge
consisting of one 23-foot concrete span (Vermont Agency of Transportation, written
communication, August 25, 1994). The bridge is supported by vertical, concrete abutments
with wingwalls. Type-2 stone fill (less than 36 inches) exists along all four wingwalls, the
downstream right road approach, and the channel banks in the immediate vicinity of the
bridge. The channel is skewed approximately 20 degrees to the opening; the opening-skewto-roadway is also 20 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 modelled flows was 0.0 ft. Abutment scour ranged from 3.9 to 5.4 ft.
with the worst-case abutment scour occurring 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. 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.