Level II scour analysis for Bridge 26 (PUTNTH00010026) on Town Highway 1, crossing Sacketts Brook, Putney, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
PUTNTH00010026 on Town Highway 1 crossing Sacketts Brook, Putney, 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 southern Vermont. The 10.1-mi² drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture upstream and downstream
of the bridge while the immediate banks have dense woody vegetation.

In the study area, Sacketts Brook has an incised, straight channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 35 ft and an average bank height
of 5 ft. The channel bed material is predominantly cobble with a median grain size (D₅₀) of
68.3 mm (0.224 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on August 20, 1996, indicated that the reach was stable.

The Town Highway 1 crossing of Sacketts Brook is a 49-ft-long, two-lane bridge consisting
of one 46-foot concrete T-beam span (Vermont Agency of Transportation, written
communication, March 30, 1995). The bridge is supported by vertical, concrete abutments.
The channel is skewed approximately 35 degrees to the opening while the opening-skew-to-
roadway is 45 degrees.

The scour protection measure at the site was type-2 stone fill (less than 36 inches diameter)
at the upstream end of the left abutment and along the entire base length of the right
abutment. There was also a vertical stone wall along the upstream 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 modelled flows ranged from 0.0 to 2.7 ft. The worst-case contraction
scour occurred at the 500-year discharge. Left abutment scour ranged from 7.9 to 9.9 ft.
with the worst-case occurring at the 100-year discharge. Right abutment scour ranged from
12.6 to 17.0 ft. with the worst-case occurring 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.