Level II scour analysis for Bridge 5 (DUMMVT00300005) on State Route 30, crossing Stickney Brook, Dummerston, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
DUMMVT00300005 on State Route 30 crossing Stickney Brook, Dummerston, 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 New England Upland section of the New England physiographic province
in southeastern Vermont. The 6.31-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is forest and brush.

In the study area, Stickney Brook has an incised, straight channel with a slope of
approximately 0.04 ft/ft, an average channel top width of 80 ft and an average bank height
of 7 ft. The channel bed material is predominantly cobble with a median grain size (D₅₀) of
80.3 mm (0.264 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on August 12, 1996, indicated that the reach was aggrading.

The State Route 30 crossing of Stickney Brook is a 84-ft-long, two-lane bridge consisting of
one 82-foot steel-beam span (Vermont Agency of Transportation, written communication,
March 30, 1995). The opening length of the structure parallel to the bridge face is 79.7 ft.
The bridge is supported by vertical, concrete abutments with spill-through embankments.
The channel is skewed approximately 5 degrees to the opening while the opening-skew-to-roadway is 0 degrees.

A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the toe of the
right spill-through slope during the Level I assessment. The scour protection measures at
the site were type-2 stone fill (less than 36 inches diameter) along the left and right bank
under the bridge forming a spill-through slope and type-2 stone fill from approximately 20
ft to 64 ft upstream on the right bank. 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 0.0 to 0.2 ft. The worst-case
contraction scour occurred at the 100-year discharge. Left abutment scour ranged from 5.5
to 6.3 ft. Right abutment scour ranged from 2.0 to 3.8 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, 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.