Level II scour analysis for Bridge 13 (PFRDTH00030013) on Town Highway 3, crossing Furnace Brook, Pittsford, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
PFRDTH00030013 on Town Highway 3 crossing Furnace Brook, Pittsford, 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 Taconic section of the New England physiographic province in western
Vermont. The 17.1-mi²
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the surface cover is grass along the downstream right bank while
the remaining banks are primarily forested.

In the study area, Furnace Brook has an incised, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 49 ft and an average channel
depth of 4 ft. The predominant channel bed material ranges from gravel to bedrock with a
median grain size (D₅₀) of 70.2 mm (0.230 ft). The geomorphic assessment at the time of
the Level I and Level II site visit on June 20, 1995, indicated that the reach was stable.

The Town Highway 3 crossing of Furnace Brook is a 75-ft-long, two-lane bridge consisting
of one 72-ft-long steel stringer span (Vermont Agency of Transportation, written
communication, March 14, 1995). The bridge is supported by vertical, concrete abutments
with spill-through slopes. The channel is skewed approximately 20 degrees to the opening
while the opening-skew-to-roadway is 35 degrees. The opening-skew-to-roadway was
determined from surveyed data collected at the bridge although, information provided from
the VTAOT files, indicates that the opening-skew-to-roadway is 30 degrees (Appendix D).

The scour protection measures at the site included type-2 stone fill (less than 36 inches
diameter) on the spill-through slope along each abutment. Type-2 stone fill scour protection
was also found along the upstream left wingwall and downstream right wingwall. Type-1
(less than 12 inches diameter) stone fill scour protection was found along the upstream right
wingwall and downstream left wingwall. No bank protection was observed downstream or
upstream. 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 ranged from 1.2 to 2.0 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.8 to
13.1 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 although, bedrock outcropping is apparent both
upstream and downstream of this bridge.

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.