Level II scour analysis for Bridge 35 (BRIDTH00050035) on Town Highway 05, crossing the North Branch Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00050035 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 division of central Vermont. The 6.70-mi²
drainage area is in a predominantly rural and forested basin. In the vicinity of the study site,
surface cover is predominately forest. Town Highway 5 runs parallel to the upstream left
and downstream right banks.

In the study area, the North Branch Ottauquechee River has an incised, sinuous channel
with a slope of approximately 0.015 ft/ft, an average channel top width of 33 ft and an
average channel depth of 3 ft. The predominant channel bed materials are gravel and cobble
(D₅₀ is 74.8 mm or 0.245 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 River is a 30-ft-long, one-lane bridge consisting of one 24-foot steel-beam span with a timber deck (Vermont Agency
of Transportation, written communication, August 25, 1994). The bridge is supported by a
timber cribwork abutment on the right and stone wall abutment on the left. A scour hole 3 ft
deeper than the mean thalweg depth was observed along the left abutment during the Level
I assessment. The channel is skewed approximately 30 degrees to the opening while the
opening-skew-to-roadway is 25 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 all modelled flows ranged from 0.7 to 0.8 ft. The worst-case
contraction scour occurred at the incipient-roadway-overtopping discharge and at the 100-
year discharge. Abutment scour ranged from 8.0 to 15.1 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.