Level II scour analysis for Bridge 1 (JAY-TH00040001) on Town Highway 4, crossing Crook Brook, Jay, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
JAY-TH00040001 on Town Highway 4 crossing Crook Brook, 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 20.7-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is thick woody vegetation and/or
forest except for the upstream right bank and overbank which is pasture.

In the study area, Crook Brook has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 86 ft and an average bank height
of 6 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 48.7 mm (0.160 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on June 5, 1995, indicated that the reach was stable.

The Town Highway 4 crossing of Crook Brook is a 49-ft-long, two-lane bridge consisting
of one 45-foot concrete span (Vermont Agency of Transportation, written communication,
March 6, 1995). The opening length of the structure parallel to the bridge face is 42 ft.The
bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed
approximately 5 degrees to the opening. The opening-skew-to-roadway is also 5 degrees.
Channel scour is present along the left abutment. The scoured area was 1.5 ft deeper than
the mean thalweg depth during the Level I assessment. Scour countermeasures include
type-2 stone fill (less than 36 inches diameter) on the upstream and downstream sides of the
left road embankment and at the upstream end of the left abutment. There is type-3 stone fill
(less than 48 inches diameter) along the base of the upstream left 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 2.5 to 3.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour at the left abutment
ranged from 15.4 to 18.5 ft. Abutment scour at the right abutment ranged from 12.3 to 15.3
ft. The worst-case abutment scour occurred at the 500-year discharge for both abutments.
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.