Level II scour analysis for Bridge 25 (JAMATH00010025) 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
JAMATH00010025 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 29.5-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 downstream
right bank which is pasture with some trees along the channel.

In the study area, Ball Mountain Brook has an incised, straight channel with a slope of
approximately 0.021 ft/ft, an average channel top width of 86 ft and an average bank height
of 9 ft. The channel bed material ranges from gravel to bedrock with a median grain size
(D₅₀) of 222 mm (0.727 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 13, 1996, indicated that the reach was stable.

The Town Highway 1 crossing of Ball Mountain Brook is a 78-ft-long, two-lane bridge
consisting of one 75-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 73 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 30 degrees to the opening while the opening-skew-to-roadway is 30 degrees.

A scour hole 1.0 ft deeper than the mean thalweg depth was observed at the upstream bridge
face. The scour protection measures at the site were type-2 stone fill (less than 36 inches
diameter) along the upstream banks and along both abutments, and type-3 stone fill (less
than 48 inches diameter) along the downstream banks. 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 only occurred at the 500-year discharge and was 0.1 ft. Abutment scour
ranged from 11.2 to 15.7 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.