Level II scour analysis for Bridge 44 (CHESVT00110044) on State Route 11, crossing Andover Brook, Chester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHESVT00110044 on State Route 11 crossing Andover Brook, Chester, 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 southeastern Vermont. The 12.6-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is pasture with dense
woody vegetation on the immediate banks except the downstream left bank of the bridge
which is forested.

In the study area, Andover Brook has an incised, meandering channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 74 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 83.6 mm (0.274 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on September 11, 1996, indicated that the reach was stable.

The State Route 11 crossing of Andover Brook is a 58-ft-long, two-lane bridge consisting of
one 56-foot concrete T-beam span (Vermont Agency of Transportation, written
communication, March 29, 1995). The opening length of the structure parallel to the bridge
face is 52.9 ft.The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 35 degrees to the opening while the opening-skew-to-roadway is 45 degrees.

A scour hole 1.8 ft deeper than the mean thalweg depth was observed along the upstream
left wingwall and left abutment during the Level I assessment. The scour protection
measures at the site included type-4 stone fill (less than 60 inches diameter) along the
upstream left bank between the wingwall and a concrete wall. There was type-2 stone fill
(less than 36 inches diameter) along the entire base of the upstream left wingwall, and the
downstream end of the downstream right wingwall. There was type-1 stone fill (less than
12 inches diameter) at the downstream end of the downstream left wingwall. There was
also a concrete wall along the upstream left bank from 18 to 50 ft upstream of the bridge.
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 1.2 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge. The incipientovertopping discharge is 520 cfs less than the 100-year discharge. Left abutment scour
ranged from 16.4 to 20.9 ft. The worst-case left abutment scour occurred at the 500-year
discharge. Right abutment scour ranged from 8.4 to 9.4 ft. The worst-case right abutment
scour occurred at both the 100-year and 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.