Level II scour analysis for Bridge 32 (CONCTH00030032) on Town Highway 3, crossing the Moose River, Concord, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CONCTH00030032 on Town Highway 3 crossing the Moose River, Concord, 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.
Approximately 85 percent of the drainage above the site is in the White Mountain section
and 15 percent is in the New England Upland section of the New England physiographic
province in northeastern Vermont. The 98.7-mi²
drainage area is in a predominantly rural
and forested basin. In the vicinity of the study site, the surface cover is primarily grass with
several houses and other buildings while the immediate channel banks have dense woody
vegetation.
In the study area, the Moose River has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 83 ft and an average channel
depth of 3 ft. The predominant channel bed material is cobble with a median grain size
(D₅₀) of 86.2 mm (0.283 ft). There are bedrock exposures downstream of the bridge. The
geomorphic assessment at the time of the Level I and Level II site visit on August 17, 1995,
indicated that the reach was stable.
The Town Highway 3 crossing of the Moose River is a 96-ft-long, two-lane bridge
consisting of two steel-beam spans (Vermont Agency of Transportation, written
communication, March 24, 1995). The bridge is supported by vertical, concrete abutments
with wingwalls and a concrete pier. The channel is skewed approximately 10 degrees to the
opening while the opening-skew-to-roadway is 0 degrees.
The right upstream end of the pier is undermined by 1.3 feet. The footing of the right
abutment is exposed by as much as 4.0 feet vertically. The footing of the downstream right
wingwall is exposed 3.5 feet and the end of the wingwall has broken and fallen into the
river. Type-3 stone fill (less than 48 inches diameter) has been placed at the end of the
existing wingwall. 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 0.0 to 0.7 ft. Abutment scour ranged
from 9.9 to 16.4 ft. Pier scour ranged from 14.4 to 16.2 ft. The worst-case contraction,
abutment, and pier 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.
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.