Level II scour analysis for Bridge 25 (DANVTH00610025) on Town Highway 61, crossing Water Andric Brook, Danville, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
DANVTH00610025 on Town Highway 61 crossing Water Andric 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 9.69-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is pasture on the
downstream left bank while the upstream right bank is grass with trees along the immediate
banks. The downstream right bank and upstream left bank are forested.
In the study area, Water Andric Brook has a straight channel with a slope of approximately
0.007 ft/ft, an average channel top width of 45 ft and an average bank height of 4 ft. The
predominant channel bed material is gravel with a median grain size (D₅₀) of 53.4 mm
(0.175 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 61 crossing of Water Andric Brook is a 24-ft-long, two-lane bridge
consisting of one 22-foot concrete slab span (Vermont Agency of Transportation, written
communication, March 24, 1995). The opening length of the structure parallel to the bridge
face is 22.9 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 5 degrees to the opening and the computed opening-skewto-roadway is 5 degrees. The VTAOT computed opening-skew-to-roadway is zero degrees.
A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the upstream
half of the left abutment during the Level I assessment. The only scour protection measure
at the site was type-2 stone fill (less than 36 inches diameter) at the upstream end of the
upstream left wingwall. 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. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. 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 0.7 to 1.3 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 9.1 to
12.5 ft. 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 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.