This report provides the results of a detailed Level II analysis of scour potential at structure
MORRTH00020007 on Town Highway 2 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 18.5-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture and forest.
In the study area, Ryder Brook generally is straight and incised with a slope of
approximately 0.002 ft/ft, an average channel top width of 48 ft and an average channel
depth of 3 ft. The channel bed is bedrock with pockets of sand and gravel in several
locations through the reach. The gravel has a median grain size (D50) of 17.7 mm (0.0581
ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 16,
1996 indicated that the reach was stable.
The Town Highway 2 crossing of Ryder Brook is a 84-ft-long, two-lane bridge consisting
of one 84-foot steel-beam span (Vermont Agency of Transportation, written
communication, January 31, 1996). The bridge is supported by vertical, concrete abutment
walls with spill-through embankments. The channel is skewed approximately 10 degrees to
the opening while the opening-skew-to-roadway is 15 degrees.
The only scour protection measure at the site was type-3 stone fill (less than 48 inches
diameter) on the spill-through embankments of each abutment. Additional details
describing conditions at the site are included in the Level II Summary and Appendices D
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 predictions for all modelled flows at this site were zero. Abutment scour
predictions ranged from 5.6 to 8.1 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. However, historical bridge
records and field notes indicate the abutment footings may be set on bedrock.
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
Additional publication details
USGS Numbered Series
Level II scour analysis for Bridge 7 (MORRTH00020007) on Town Highway 2 (FAS 239), crossing Ryder Brook, Morristown, Vermont