Level II scour analysis for Bridge 24 (HARDTH00490024) on Town Highway 49, crossing Nichols Brook at Mackville Pond Outlet, Hardwick, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HARDTH00490024 on Town Highway 49 crossing Nichols Brook at Mackville Pond
Outlet, Hardwick, 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 New England Upland section of the New England physiographic province
in north-central Vermont. The 10.7-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is best described as
suburban with residences, lawns, trees and roadways.
There is a dam 54 feet downstream of the bridge which controls Mackville Pond upstream
of the bridge. The vertical drop over the dam is 15 feet. Immediately upstream of the bridge
the width of the waterway is 146 feet. The predominant channel bed material is sand with a
median grain size (D₅₀) of 0.576 mm (0.00189 ft). The geomorphic assessment at the time
of the Level I and Level II site visit on July 25, 1995, indicated that the reach was stable.
The Town Highway 49 crossing of Nichols Brook at Mackville Pond Outlet is a 42-ft-long,
two-lane bridge consisting of one 38-foot steel-beam span (Vermont Agency of
Transportation, written communication, April 3, 1995). The bridge is supported by vertical,
concrete abutments with wingwalls on the downstream end of the left abutment and
upstream and downstream ends of the right abutment. The channel is not skewed to the
opening, but the opening-skew-to-roadway is 5 degrees.
Scour protection measures at the site include type-3 stone fill (less than 48 inches diameter)
on the upstream side of the left roadway embankment and at the upstream end of the left
abutment. Type-2 stone fill (less than 36 inches diameter) was on the upstream right
roadway 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 4.7 to 21.0 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour at the left abutment
ranged from 13.3 to 15.8 ft. with the worst-case occurring at the 500-year discharge.
Abutment scour at the right abutment ranged from 8.1 to 9.8 ft. with the worst-case
occurring at the incipient roadway-overtopping discharge. Additional information on scour
depths and depths to armoring are included in the section titled “Scour Results”. Scouredstreambed 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 particlesize 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.