Level II scour analysis for Bridge 6 (MORRTH00030006) on Town Highway 3, crossing Ryder Brook, Morristown, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MORRTH00030006 on Town Highway 3 crossing Ryder Brook, Morristown, 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
north-central Vermont. The 19.1-mi² drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover also is forested.

In the study area, Ryder Brook has a straight channel with an average channel top width of
450 ft and an average bank height of 7 ft. The predominant channel bed material is silt and
clay with a median grain size (D₅₀) of 0.0719 mm (0.000236 ft). The geomorphic
assessment at the time of the Level I and Level II site visit on July 18, 1996, indicated that
the reach was aggraded, but the channel through the bridge was scoured.

The Town Highway 3 crossing of Ryder Brook is a 72-ft-long, two-lane bridge consisting
of one 70-foot steel-beam span (Vermont Agency of Transportation, written
communication, January 31, 1996). The bridge is supported by vertical, concrete abutments
with spill-through embankments and wingwalls. The channel is not skewed to the opening
and the opening-skew-to-roadway is zero degrees.

Channel scour under the bridge was evident at this site during the Level I assessment. The
depth of the channel increases from 3 feet at the upstream bridge face to 10 feet at the
downstream bridge face. The only scour protection measure at the site was type-2 stone fill
(less than 36 inches diameter) on the spill-through embankments of each abutment, the
upstream road embankments and the downstream left road embankment. 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 20.4 to 25.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.3 to
10.5 ft. The worst-case abutment scour also 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.

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.