Level II scour analysis for Bridge 12 (SUNDFLR0030012) on Forest Land Road 3, crossing Roaring Branch Brook, Sunderland, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
SUNDFLR0030012 on Forest Land Road (FLR) 3 (FAS 114) crossing Roaring Branch
Brook, Sunderland, 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
southwestern Vermont. The 4.93-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is dense forest along the left bank
and primarily shrubs and trees along the right bank, both upstream and downstream of the
bridge.

In the study area, Roaring Branch Brook has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 33 ft and an average bank height
of 4 ft. The channel bed material ranges from cobble to bedrock with a median grain size
(D₅₀) of 139 mm (0.457 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on July 30, 1996, indicated that the reach was stable.

Forest Land Road 3 (FAS 114) crossing of Roaring Branch Brook is a 37-ft-long, two-lane
bridge consisting of one 35-foot steel-beam span (Vermont Agency of Transportation,
written communication, December 14, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 30 degrees to the opening
while the opening-skew-to-roadway is 15 degrees.

The scour protection measures at the site included type-3 stone fill (less than 48 inches
diameter) along the left and right abutments, along the upstream left and downstream right
wing walls and along the downstream right bank. Type-4 (less than 60 inches diameter)
stone fill was found along the upstream right and downstream left wingwalls and along the
downstream left bank. Type-2 (less than 36 inches diameter) stone fill scour protection was
found along the upstream left and right banks. 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 was calculated to be 0.0 ft. Abutment scour ranged
from 4.3 to 10.4 ft. The worst-case abutment scour occurred at the 500-year discharge along
the right abutment. 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.