This report provides the results of a detailed Level II analysis of scour potential at structure
BURKTH00070016 on Town Highway 7 crossing Dish Mill Brook, Burke, 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 White Mountain section of the New England physiographic province in
northeastern Vermont. The 6.0-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest except on the left bank
upstream which is brushland.
In the study area, Dish Mill Brook has an incised, sinuous channel with a slope of
approximately 0.04 ft/ft, an average channel top width of 40 ft and an average bank height
of 6 ft. The channel bed material ranges from sand to boulder with a median grain size (D50)
of 94.1 mm (0.309 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on August 7, 1995, indicated that the reach was stable.
The Town Highway 7 crossing of Dish Mill Brook is a 28-ft-long, two-lane bridge
consisting of one 24-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 24, 1995). The opening length of the structure parallel to the bridge
face is 24.8 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 35 degrees to the opening while the computed opening-skew-to-roadway is 35 degrees.
A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the left and
right abutments during the Level I assessment. In front of the upstream and downstream left
wingwalls the scour depth was only 0.5 ft, while in front of the downstream right wingwall
it was 0.75 ft and in front of the upstream right wingwall it was 0.3 ft. The scour
countermeasures at the site include type-1 stone fill (less than 12 inches diameter) at the
downstream end of the right abutment and along the downstream right wingwall. Type-2
stone fill (less than 36 inches diameter) is along the upstream left bank, the upstream and
downstream left wingwalls, and at the upstream end of the upstream right 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
Contraction scour for all modelled flows ranged from 0.0 to 0.5 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.7 to
9.3 ft. The worst-case abutment scour occurred at the 500-year discharge for the left
abutment and at the incipient road-overtopping discharge for the right abutment. 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