Level II scour analysis for Bridge 94 (FERDVT01050094) on State Route 105, crossing the Nulhegan River, Ferdinand, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
FERDVT01050094 on State Route 105 crossing the Nulhegan River, Ferdinand, 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 White Mountain section of the New England physiographic province in
northeastern Vermont. The 38.4-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is grass and brush with wetlands
immediately adjacent to the stream channel.
In the study area, the Nulhegan River has a meandering channel with a slope of
approximately 0.002 ft/ft, an average channel top width of 60 ft and an average channel
depth of 6 ft. The predominant channel bed material is sand with a median grain size (D₅₀)
of 0.465 mm (0.00153 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on July 5, 1995, indicated that the reach was laterally unstable.
The State Route 105 crossing of the Nulhegan Riveris a 44-ft-long, two-lane bridge
consisting of one 42-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 6, 1995). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 10 degrees to the opening while the
opening-skew-to-roadway is zero degrees.
Scour protection measures at the site were type-2 stone fill (less than 36 inches diameter) on
the upstream right bank, the upstream right wingwall, the right abutment, the downstream
end of the left abutment and the downstream wingwalls. 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 1.9 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.6 to
11.0 ft. The worst-case abutment scour also occurred at the 500-year discharge. Total scour
depths computed for this site were not below the bottom of the footings, except for the 500-
year discharge model at the left 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 crosssection 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.