Level II scour analysis for Bridge 51 (BRIDTH00460051) on Town Highway 46, crossing Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00460051 on town highway 46 crossing the Ottauquechee River, 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 103-mi²
drainage area is a predominantly rural basin. In the vicinity of
the study site, the immediate left and right banks are covered by trees and brush with
residences beyond.

In the study area, the Ottauquechee River has a straight channel with a slope of
approximately 0.008 ft/ft, an average channel top width of 150 ft and an average channel
depth of 6 ft. The predominant channel bed materials are gravel and cobble with a median
grain size (D₅₀) of 81.8 mm (0.268 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on October 24, 1994, indicated that the reach was stable.

The town highway 46 crossing of the Ottauquechee Riveris a 135-ft-long, two-lane bridge
consisting of two 66-ft steel-beam spans, supported by vertical, concrete abutments with
upstream wingwalls and one concrete pier (Vermont Agency of Transportation, written
commun., August 24, 1994). Type-2 stone fill (less than 36 inches diameter) has been
placed along the left abutment and both upstream wingwalls. The upstream side of both
road embankments are also protected by type-2 stone fill. Abutments of a previous bridge
still exist at the downstream side of the present structure’s abutments. The channel is
skewed 10 degrees to the bridge face; the opening-skew-to-roadway is 0 degrees.

There are remains of a breached dam 160 feet upstream of the bridge which deflect flow
toward the left bank. Additional details describing conditions at the site are included in the
Level II Summary, Appendix D, and Appendix 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 2.6 to 5.4 ft. The worst-case
contraction scour occurred at the incipient overtopping discharge, which was between the
100- and 500-year discharges. Pier scour ranged from 9.9 to 10.9 ft with the worst-case
scenario also occurring at the incipient roadway overtopping discharge. Abutment scour
ranged from 25.3 to 33.6 ft. with the worst-case 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.