Level II scour analysis for Bridge 3 (BRIDTH000100003) on Town Highway 1, crossing Dailey Hollow Branch, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00010003 on town highway 1 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 division of central Vermont in the town of
Bridgewater. The 9.88-mi²
drainage area is in a predominantly rural, forested basin. In the
vicinity of the study site, the immediate channel banks have moderate tree cover and shrubs
with residential properties on the overbank.

In the study area, Dailey Hollow Branch has an incised, sinuous channel with a slope of
approximately 0.009 ft/ft, an average channel top width of 46 ft and an average channel
depth of 4 ft. The predominant channel bed materials are gravel and cobble with a median
grain size (D₅₀) of 89.7 mm (0.294 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on October 27, 1994, indicated that the reach was vertically
degrading.

The town highway 1 crossing of Dailey Hollow Branch is a 45-ft-long, two-lane bridge
consisting of one 42-foot steel-beam span (Vermont Agency of Transportation, written
communication, August 24, 1994). The bridge is supported by vertical, concrete abutments
with wingwalls. Type-2 stone fill (less than 36 inches diameter) protects the downstream
right and left wingwall. Type-3 stone fill (less than 48 inches diameter) exists on the
downstream right bank. The left abutment is undermined by up to one foot. Horizontal
probing under the abutment resulted in penetration up to 6 feet.

The bridge is misaligned with the channel. Higher discharges may directly impact the left
wingwall. The channel is skewed approximately 20 degrees to the bridge; the opening-skew-to-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
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 0.6 ft to 1.3 ft and the worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.7 ft to
12.2 ft and the worst-case abutment scour occurred at the 500-year discharge. Scour depths
and depths to armoring are summarized on p. 14 in the section titled “Scour Results”.
Scour elevations, based on the calculated depths are presented in tables 1 and 2; a graph of
the scour elevations is presented in figure 8 Scour depths were calculated assuming an
infinite depth of erosive material and a homogeneous particle-size distribution.

For all scour presented in this report, “the scour depths adopted [by VTAOT] may differ
from the equation values based on engineering judgement” (Richardson and others, 1993, p.
21, 27). 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, and the results of the hydraulic analyses, must be considered to properly assess
the validity of abutment scour results.