Level II scour analysis for Bridge 17 (SHEFTH00380017) on Town Highway 38, crossing Miller Run, Sheffield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
SHEFTH00380017 on Town Highway 38 crossing Miller Run, Sheffield, 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 24.2-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture along the right bank while
the immediate banks are covered by trees, shrubs, and brush. The surface cover along the
left bank is grass and Route 122 with shrubs and brush along the immediate banks.
In the study area, Miller Run has a sinuous channel with a slope of approximately 0.01 ft/ft,
an average channel top width of 52 ft and an average bank height of 3 ft. The channel bed
material ranges from sand to bedrock with a median grain size (D₅₀) of 80.5 mm (0.264 ft).
The geomorphic assessment at the time of the Level I and Level II site visit on August 1,
1995, indicated that the reach was stable.
The Town Highway 38 crossing of Miller Run is a 52-ft-long, one-lane bridge consisting of
one 48-foot steel I-beam span with a wooden deck (Vermont Agency of Transportation,
written communication, March 28, 1995). The opening length of the structure parallel to the
bridge face is 42.4 ft. The bridge is supported by vertical, concrete abutments with
wingwalls on the upstream end. The channel is skewed approximately 30 degrees to the
opening while the computed opening-skew-to-roadway is 5 degrees.
A scour hole 3.0 ft deeper than the mean thalweg depth was observed under the bridge
during the Level I assessment. The only scour protection measure at the site was type-4
stone fill (less than 60 inches diameter) at the upstream end of the upstream left wingwall.
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)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. 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 2.4 ft. Abutment scour ranged
from 6.1 to 7.9 ft at the left abutment and 11.4 to 17.4 ft at the right abutment. The worstcase contraction and 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.
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.