Level II scour analysis for Bridge 45 (BRNETH00070045) on Town Highway 7, crossing the Stevens River, Barnet, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRNETH00070045 on Town Highway 7 crossing the Stevens River, Barnet, 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 east-central Vermont. The 41.5-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest upstream and pasture
downstream of the bridge while the immediate banks have dense woody vegetation.

In the study area, the Stevens River has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 100 ft and an average bank height
of 17 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 105 mm (0.344 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on August 22, 1995, indicated that the reach was stable.

The Town Highway 7 crossing of the Stevens River is a 37-ft-long, two-lane bridge
consisting of one 34-foot concrete slab span (Vermont Agency of Transportation, written
communication, March 16, 1995). The opening length of the structure parallel to the bridge
face is 33 ft. 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 20 degrees.

The only scour protection measure at the site was type-2 stone fill (less than 36 inches
diameter) along the entire left and right abutments, upstream and downstream wingwalls,
and upstream and 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)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. 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 0.8 to 5.4 ft. The worst-case
contraction scour occurred at the incipient roadway-overtopping discharge, which was
greater than the 100-year discharge. Left abutment scour ranged from 21.8 to 28.6 ft. The
worst-case left abutment scour occurred at the 500-year discharge. Right abutment scour
ranged from 14.6 to 17.4 ft. The worst-case right abutment scour occurred at the incipient
roadway-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. 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.