Level II scour analysis for Bridge 40 (ANDOVT00110040) on State Route 11, crossing Lyman Brook, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ANDOVT00110040 on State Route 11 crossing Lyman Brook, Andover, 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
south-central Vermont. The 4.18-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture while the immediate
banks have dense woody vegetation.

In the study area, Lyman Brook has an incised, straight channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 42 ft and an average bank height
of 8 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 86.0 mm (0.282 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on September 9, 1996, indicated that the reach was stable.

The State Route 11 crossing of Lyman Brook is a 28-ft-long, two-lane bridge consisting of
one 27-foot concrete tee-beam span (Vermont Agency of Transportation, written
communication, March 29, 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 0 degrees to the opening while the opening-skew-to-roadway is 30 degrees.

The scour protection measures at the site included type-2 stone fill (less than 36 inches
diameter) at the upstream end of the upstream right wingwall and the downstream ends of
the downstream left and right wingwalls. There was also a stone wall along the top of the
left bank from 36 to 76 feet upstream. 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 all modelled flows ranged from 0.0 to 0.7 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge which was more than the
100-year discharge. Left abutment scour ranged from 1.2 to 7.5 ft. The worst-case left
abutment scour occurred at the 500-year discharge. Right abutment scour ranged from 5.2
to 6.7 ft. The worst-case right abutment scour occurred at the 100-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.