Level II scour analysis for Bridge 26 (JAMATH00010026) on Town Highway 1, crossing Ball Mountain Brook, Jamaica, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
JAMATH00010026 on Town Highway 1 crossing Ball Mountain Brook, Jamaica, 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 Green Mountain section of the New England physiographic province in
southern Vermont. The 29.3-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest.
In the study area, Ball Mountain Brook has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 74 ft and an average bank height
of 6 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 82.6 mm (0.271 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 12, 1996, indicated that the reach was stable.
The Town Highway 1 crossing of Ball Mountain Brook is a 80-ft-long, two-lane bridge
consisting of one 78-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 29, 1995). The opening length of the structure parallel to the bridge
face is 75.7 ft. The bridge is supported by vertical, concrete abutments with wingwalls.
A scour hole 2 ft deeper than the mean thalweg depth was observed along the right
abutment during the Level I assessment. The scour protection measures at the site were
type-4 stone fill (less than 60 inches diameter) along the left bank upstream and extending
underneath the bridge and along the bank downstream and also along the right bank
upstream tapering to type-3 stone fill (less than 48 inches diameter) 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).
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 the modelled flows ranged from 1.0 to 2.7 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge. Abutment scour ranged
from 8.4 to 17.6 ft. The worst-case abutment scour for the right abutment occurred at the
incipient-overtopping discharge. For 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 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.