Level II scour analysis for Bridge 57 (BRIDTH00650057) on Town Highway 65, crossing Broad Brook, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00650057 on Town Highway 65 crossing Broad 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 province of central Vermont in the town of
Bridgewater. The 26.9-mi²
drainage area is in a predominantly rural and forested basin. In
the vicinity of the study site, the US left bank is forested; DS left bank is shrub and
brushland; US right bank is lawn and the DS right bank has very little vegetation and is
largely coincident with the Ottauquechee River.

In the study area, Broad Brook has an incised channel with a slope of approximately 0.0067
ft/ft, an average channel top width of 60 ft and an average channel depth of 3 ft. The
predominant channel bed material is gravel (D₅₀ is 46.2 mm or 0.151 ft). The geomorphic
assessment at the time of the Level I and Level II site visit on October 25, 1994, indicated
that the reach was stable.

The Town Highway 65 crossing of Broad Brook is a 47-ft-long, one-lane bridge consisting
of one 44-ft steel-beam span with a timber deck, supported by vertical concrete abutments
with wingwalls on the upstream and downstream sides (Vermont Agency of Transportation,
written commun., August, 1994). The US right wingwall and road approach is protected by
stone fill. The US and DS right wingwalls and the right abutment are reported as having 1.0
to 1.5 ft of scour at the time of the Level one assessment on 10/25/94. Erosion from road
wash affects all road approach embankments. Broad Brook flows into the Ottauquechee
River approximately 30 ft downstream of the bridge. The channel approach to the bridge is
straight with the bridge skewed 20 degrees to flow and 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
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 0.4 ft to 1.5 ft and the worst-case
contraction scour occurred at the incipient overtopping discharge. Abutment scour ranged
from 6.0 ft to 14.6 ft and the worst-case abutment scour occurred at the 100-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.