Level II scour analysis for Bridge 59 (NWPCVT01050059) on State Route 105 crossing an unnamed Mud Creek Tributary, Newport, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NWPCVT01050059 on state route 105 crossing an unnamed Mud Creek tributary,
Newport, 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
of north-central Vermont in the town of Newport. The 9.18-mi²
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the surface cover is
pasture while the immediate banks are brush covered with some trees.
In the study area, this unnamed Mud Creek tributary has an incised, sinuous channel with a
slope of approximately 0.005 ft/ft, an average channel top width of 42 ft and an average
channel depth of 5 ft. The predominant channel bed materials are gravel and cobbles with a
median grain size (D₅₀) of 54.1 mm (0.178 ft). The geomorphic assessment at the time of
the Level I and Level II site visit on October 21, 1994, indicated that the reach was stable.
The state route 105 crossing of an unnamed Mud Creek tributary is a 33-ft-long, two-lane
bridge consisting of one 29-foot concrete-slab span (Vermont Agency of Transportation,
written communication, August 5, 1994). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 15 degrees to the opening
and the opening-skew-to-roadway is 15 degrees.
Scour protection measures at the site were type-2 stone fill (less than 36 inches diameter) on
each bank upstream and both upstream wingwalls. The downstream wingwalls are
protected by remnant abutment walls from a previous structure. 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, 1993). 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 0.2 to 0.5 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 5.2 to
16.6 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.