Level II scour analysis for Bridge 29 (BRIDTH00360029) on Town Highway 36, crossing North Branch Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00360029 on town highway 36 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 in the town of
Bridgewater. The 27.1-mi^{2
drainage area is a predominantly} rural basin. In the vicinity of
the study site, the left and right banks are covered by pasture and (or) fields with the
immediate stream banks covered by woody vegetation. The left bank of North Branch
Ottauquechee River is adjacent to Bridgewater town highway 001.

In the study area, the North Branch Ottauquechee River has a sinuous channel with a slope
of approximately 0.008 ft/ft, an average channel top width of 73 ft and an average bank
height of 6 ft. The predominant channel bed materials are gravel and cobble with a median
grain size (D₅₀) of 61.0 mm (0.200 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on October 26, 1994, indicated that the reach was stable.

The town highway 36 crossing of the North Branch Ottauquechee Riveris a 46-ft-long,
one-lane bridge consisting of one 43-foot steel-beam span (Vermont Agency of
Transportation, written communication, August 25, 1994). The bridge is supported by
vertical, concrete abutments with wingwalls. Type-2 (less than 36 inches) stone fill protects
the upstream and downstream wingwalls. Sparse type-2 stone fill was also observed along
the right abutment. The channel approach to the bridge is not skewed, however, the
measured opening skew-to-roadway is five 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 1.4 to 2.8 ft. The worst-case
contraction scour occurred at the incipient overtopping discharge, which was less than the
100-year discharge. Abutment scour ranged from 7.3 to 13.2 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 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.