Level II scour analysis for Bridge 25 (ANDOTH00230025) on Town Highway 23, crossing Andover Branch, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ANDOTH00230025 on Town Highway 23 crossing the Andover Branch, 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 6.74-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture on the right overbank and
forest on the left overbank while the immediate banks, both upstream and downstream, are
forested.

In the study area, the Andover Branch has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 55 ft and an average bank height
of 9 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 78.4 mm (0.257 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 27, 1996, indicated that the reach was stable.

The Town Highway 23 crossing of the Andover Branch is a 25-ft-long, two-lane structure
consisting of a multi-plate corrugated steel arch culvert with concrete footings (Vermont
Agency of Transportation, written communication, March 29, 1995). The culvert is mitered
at the inlet and outlet. The channel is skewed approximately zero degrees to the opening
while the opening-skew-to-roadway is zero degrees.

The footings are exposed approximately 1.25 ft, with the exception of the downstream end
of the right footing which is exposed approximately 0.5 ft. The only scour protection
measure at the site was type-2 stone fill (less than 36 inches diameter) along the upstream
left bank. 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 modelled flows ranged from 1.6 to 2.8 ft. The worst-case contraction
scour occurred at the 500-year discharge. Abutment scour ranged from 10.0 to 11.7 ft along
the left footing and from 11.8 to 16.7 along the right footing. 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 crosssection 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.