Level II scour analysis for Bridge 5 (IRASTH00010005) on Town Highway 1, crossing Lords Creek, Irasburg, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
IRASTH00010005 on town highway 1 crossing Lords Creek, Irasburg, 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 VTAOT files, was compiled prior to conducting
Level I and Level II analyses and can be 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 Irasburg. The 15.1-mi²
drainage area is in a
predominantly rural and forested basin with some pasture land mainly along the valley
bottom. In the vicinity of the study site, the bank vegetation coverage is pasture grasses.
In the study area, Lords Creek has a meandering channel with a slope of approximately
0.0026 ft/ft, an average channel top width of 32 ft and an average channel depth of 3 ft. The
channel bed material ranged from gravel (D50 is 46.6 mm or 0.153 ft) to silt/clay material
(D₅₀ of 1.006 mm or 0.0033 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on October 5 and 6, 1994, indicated that the reach was laterally unstable.
The town highway 1 crossing of Lords Creek is a 65-ft-long, two-lane bridge consisting of
one 61-foot, steel-beam span (Vermont Agency of Transportation, written communication,
August 2, 1994). The bridge is supported by vertical, concrete abutments on wooden piles
driven to bedrock with no wingwalls. Each abutment wall has a spill-through slope
protected with type-2 stone fill (less than 36 inches diameter). The channel is skewed
approximately 25 degrees to the opening while the opening-skew-to-roadway is 15 degrees.
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 2.4 to 4.6 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.2 to
9.8 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.