Level II scour analysis for Bridge 4 (DANVTH00010004) on Town Highway 1, crossing Joes Brook, Danville, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
DANVTH00010004 on Town Highway 1 crossing Joes Brook, Danville, 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 northeastern Vermont. The 42.5-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is pasture along the
upstream and downstream left banks with trees and brush along the immediate banks. The
upstream and downstream right banks are forested.

In the study area, Joes Brook has an incised, sinuous channel with a slope of approximately
0.02 ft/ft, an average channel top width of 68 ft and an average bank height of 5 ft. The
channel bed material ranges from gravel to bedrock with a median grain size (D₅₀) of 80.1
mm (0.263 ft). The geomorphic assessment at the time of the Level I and Level II site visit
on August 22, 1995, indicated that the reach was stable.

The Town Highway 1 crossing of Joes Brook is a 49-ft-long, two-lane bridge consisting of
one 45-foot steel-beam span (Vermont Agency of Transportation, written communication,
March 17, 1995). The opening length of the structure parallel to the bridge face is 45 ft.The
bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed
approximately 15 degrees to the opening and the computed opening-skew-to-roadway is 15
degrees.

A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the right
abutment during the Level I assessment. The scour hole also extends upstream and
downstream of the bridge, along the right side of the channel. The scour protection
measures at the site include type-2 stone fill (less than 36 inches diameter) at the upstream
end of the upstream left wingwall and along the entire base length of the downstream right
wingwall. Type-3 stone fill (less than 48 inches diameter) is along the entire base length of
the upstream right wingwall and type-5 protection (stone block wall) is along the upstream
right bank. Additional details describing conditions at the site are included in the Level II
Summary and Appendices D and E.

Scour depths and recommended rock rip-rap sizes were computed using the general
guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995)
for the 100- and 500-year discharges. 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 was computed to be zero ft. Abutment scour
ranged from 11.7 to 13.0 ft along the right abutment and from 6.6 to 9.4 ft along the left
abutment. The worst-case abutment 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 and Hire equations (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.