Level II scour analysis for Bridge 18 (GROTTH00480018) on Town Highway 48, crossing the Wells River Groton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
GROTTH00480018 on Town Highway 48 crossing the Wells River, Groton, 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 eastern Vermont. The 53.6-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 bank
upstream and the left bank downstream while the surface cover is shrub and brushland
along the left bank upstream and the right bank downstream. The immediate banks are
vegetated with brush and scattered trees.

In the study area, the Wells River has an incised, straight channel with a slope of
approximately 0.003 ft/ft, an average channel top width of 69 ft and an average bank height
of 7 ft. The channel bed material ranges from sand to cobble with a median grain size (D₅₀)
of 66.7 mm (0.219 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on August 28, 1995, indicated that the reach was stable.

The Town Highway 48 crossing of the Wells River is a 38-ft-long, one-lane bridge
consisting of one 36-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 24, 1995). The opening length of the structure parallel to the bridge
face is 33.7 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 0 degrees to the opening and the opening-skew-toroadway is also 0 degrees.

Local scour 3.25 ft deeper than the mean thalweg depth was observed underneath the bridge
along the left and right abutments during the Level I assessment. In addition, a scour hole
extends from 90 ft US to 50 ft DS for a total length of 115 ft with an average scour depth of
2.0 ft. The only scour protection measure at the site was type-2 stone fill (less than 36
inches diameter) along the left bank upstream, along the entire base length of the
downstream right wingwall, and along the left and right banks downstream; and type-1
stone fill (less than 12 inches diameter) along the entire base length of the upstream left
wingwall. 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 was 0.0 ft. Abutment scour ranged from 2.0 to 2.3
ft at the left abutment and 8.8 to 14.6 ft at the right abutment. The worst-case abutment
scour occurred at the 500-year discharge at the right abutment. 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.