Level II scour analysis for Bridge 38 (CONCTH00060038) on Town Highway 6, crossing the Moose River, Concord, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CONCTH00060038 on Town Highway 6 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 98.4-mi²
drainage area is in a predominantly rural
and forested basin. In the vicinity of the study site, the surface cover of the banks is
primarily forest and brush.
In the study area, the Moose River has an incised, sinuous channel with a slope of
approximately 0.009 ft/ft, an average channel top width of 110 ft and an average channel
depth of 6 ft. The channel bed material ranged from gravel to boulder and had a median
grain size (D₅₀) of 74.4 mm (0.244 ft). 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 6 crossing of the Moose River is a 59-ft-long, two-lane bridge
consisting of one 55-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 24, 1995). The bridge is supported by a vertical, laid-up stone
abutment with wingwalls on the left and a vertical, concrete abutment with wingwalls on
the right. The channel is skewed approximately 10 degrees to the opening while the
opening-skew-to-roadway is 0 degrees.
The footing of the left abutment is exposed as much as 2.8 feet. The footing of the right
abutment is undermined vertically by as much as 0.3 feet. Type-2 stone-fill (less than 36
inches diameter) protection can be found along the left abutment. Type-3 stone-fill (less
than 48 inches diameter) protection can be found along the right abutment. 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 0.1 to 3.1 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge. Abutment scour at the
left abutment ranged from 10.4 to 12.5 ft with the worst-case occurring at the 500-year
discharge. Abutment scour at the right abutment ranged from 25.3 to
27.3 ft with the worst-case occurring at the incipient-overtopping discharge. The worst-case
total scour also occurred at the incipient-overtopping discharge. The incipient-overtopping
discharge was in between the 100- and 500-year discharges. 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.