Level II scour analysis for Bridge 37 (BRIDTH00050037) on Town Highway 5, crossing North Branch Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00050037 on town highway 5 crossing the North Branch 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 province of central Vermont in the town of
Bridgewater. The 10.5-mi²
drainage area is a predominantly rural basin. In the vicinity of
the study site, the left and right banks are forested. Town highway 5 runs parallel to the
upstream left and downstream right banks.

In the study area, the North Branch Ottauquechee River has a sinuous channel with a slope
of approximately 0.013 ft/ft, an average channel top width of 50 ft and an average channel
depth of 5 ft. The predominant channel bed materials are gravel and cobble (D₅₀ is 79.3 mm
or 0.260 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
November 2, 1994, indicated that the reach was stable.

The town highway 5 crossing of the North Branch Ottauquechee Riveris a 38-ft-long, onelane bridge consisting of one 35-foot steel beam span (Vermont Agency of Transportation,
written commun., August 25, 1994). The bridge is supported by vertical, stone abutments
with wingwalls. The right abutment has settled due to scour. Type-3 stone fill (less than 36
inches diameter) provides protection to the upstream end of the upstream left wingwall and
the base of the downstream right wingwall. The channel is skewed approximately 35
degrees; the opening-skew-to-roadway is 20 degrees. Additional details describing
conditions at the site are included in the Level II Summary 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 0.4 to 1.5 ft. The worst-case
contraction scour occurred at the incipient overtopping discharge, which was less than the
100-year discharge. Abutment scour ranged from 11.0 to 14.9 ft. The worst-case abutment
scour occurred 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 crosssection 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.