Level II scour analysis for Bridge 22 (JAY-TH00400022) on Town Highway 40, crossing Jay Branch, Jay, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
JAY-TH00400022 on Town Highway 40 crossing Jay Tributary, Jay, 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). Results of
a Level I scour investigation also are included in Appendix E of this report. A Level I
investigation 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 is found in
Appendix D.

The site is in the Green Mountain section of the New England physiographic province in
northern Vermont. The 2.15-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is primarily pasture on the upstream
and downstream left overbank while the immediate banks have dense woody vegetation.
The downstream right overbank of the bridge is forested.

In the study area, Jay Branch Tributary has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 26 ft and an average bank height
of 3 ft. The channel bed material ranges from gravel to cobble with a median grain size
(D₅₀) of 40.5 mm (0.133 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on June 7, 1995, indicated that the reach was stable.

The Town Highway 40 crossing of Jay Branch Tributary is a 27-ft-long, two-lane bridge
consisting of one 25-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 6, 1995). The opening length of the structure parallel to the bridge
face is 23.5 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel skew and the opening-skew-to-roadway are zero degrees.

The scour counter-measures at the site included type-2 stone fill (less than 36 inches
diameter) at the upstream end of the left and right abutments, at the upstream right
wingwall, and at the downstream left wingwall. There was also type-3 stone fill (less than
48 inches diameter) at the upstream left and downstream right wingwall. Additional details
describing conditions at the site are included in the Level II Summary and Appendices D
and E.

Scour depths and recommended 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 0.7 to 1.1 ft. The worst-case
contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 4.6
to 4.9 ft. The worst-case left abutment scour occurred at the 100-year discharge. Right
abutment scour ranged from 4.0 to 5.0 ft. The worst-case right 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, 1995, p. 47). Usually,
computed scour depths are evaluated in combination with other information including (but
not limited to) historical performance during flood events, the geomorphic stability
assessment, existing scour protection measures, and the results of the hydraulic analyses.
Therefore, scour depths adopted by VTAOT may differ from the computed values
documented herein.