Level II scour analysis for Bridge 7 (ANDOTH00010007) on Town Highway 1, crossing Andover Brook, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ANDOTH00010007 on Town Highway 1 crossing 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
southern Vermont. The 7.21-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture upstream of the bridge
while the immediate banks have dense woody vegetation. Downstream of the bridge, the
banks and overbanks are forested.

In the study area, Andover Branch has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 45 ft and an average bank height
of 5 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 58.0 mm (0.19 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on August 28, 1996, indicated that the reach was laterally unstable due to
evidence of lateral movement of the channel 200 feet upstream along the left bank and near
the bridge along the right bank.

The Town Highway 1 crossing of Andover Branch is a 32-ft-long, two-lane bridge
consisting of one 29-foot concrete slab span (Vermont Agency of Transportation, written
communication, March 28, 1995). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 10 degrees to the opening while the
opening-skew-to-roadway is 0 degrees.

The scour protection measures at the site included type-2 stone fill (less than 36 inches
diameter) along the entire base length of the upstream wingwalls and type-3 stone fill (less
than 48 inches diameter) along the entire base length of the downstream wingwalls and the
right abutment. Additional details describing conditions at the site are included in the Level
II Summary and Appendices
D and E.

Scour depths and 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 1.6 ft. The worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 7.1 to
10.7 ft at the right abutment with the worst-case abutment scour occurring at the 500-year
discharge. Abutment scour ranged from 7.5 to 8.3 ft at the left abutment with the worst-case
abutment scour occurring at the incipient road overtopping 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. 46). 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.