Level II scour analysis for Bridge 25 (CRAFTH00220025) on Town Highway 22, crossing the Wild Branch Lamoille River, Craftsbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CRAFTH00220025 on town highway 22 crossing the Wild Branch Lamoille River,
Craftsbury, 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, gleaned from Vermont Agency of Transportation (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 New England Upland physiographic province of north-central Vermont in
the town of Bridgewater. The 9.52-mi²
drainage area is in a predominantly rural basin with
some pasture on the valley bottom. In the vicinity of the study site, the banks have less than
25% woody vegetation coverage.

In the study area, the Wild Branch Lamoille River has a meandering channel in a low relief
valley setting with wide flood plains and a slope of approximately 0.0044 ft/ft, an average
channel top width of 35 ft and an average channel depth of 4 ft. The predominant channel
bed material is gravel (D₅₀ is 38.6 mm or 0.127 ft). The geomorphic assessment at the time
of the Level I and Level II site visit on November 9, 1994, indicated that the reach was
laterally unstable.

The town highway 22 crossing of the Wild Branch Lamoille Riveris a 31-ft-long, two-lane
bridge consisting of one 29-foot span concrete slab superstructure (Vermont Agency of
Transportation, written commun., August 4, 1994). The bridge is supported by vertical,
concrete abutments with wingwalls. The channel is skewed approximately 20 degrees to the
opening and the opening-skew-to-roadway is 20 degrees.

A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the left bank
side of the channel upstream during the Level I assessment. There are tall, steep stone fill
embankments (artificial levees) that make up both banks between 50 feet upstream and the
upstream face of the bridge, which straighten and constrict the channel. Type-2 stone fill
(less than 36 inches diameter) is reported on the banks upstream, the upstream wingwalls,
the abutments, the downstream left wingwall, and the downstream left bank. 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, 1995). 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 0.0 to 2.5 ft. The worst-case
contraction scour occurred at the incipient overtopping discharge, which was less than the
100-year discharge. Abutment scour ranged from 4.7 to 8.6 ft. The worst-case abutment
scour also occurred at the incipient overtopping 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, 1995, p. 47). Many factors,
including historical performance during flood events, the geomorphic assessment, scour
protection, 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.