Level II scour analysis for Bridge 16 (GROTTH00170016) on Town Highway 17, crossing the Wells River, Groton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
GROTTH00170016 on Town Highway 17 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 43.4-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is predominantly shrub and
brushland, while the left bank downstream is forested.
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 57 ft and an average bank height
of 4 ft. The channel bed material ranges from sand to boulder with a median grain size (D₅₀)
of 77.8 mm (0.255 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on August 29, 1995, indicated that the reach was stable.
The Town Highway 17 crossing of the Wells River is a 43-ft-long, one-lane bridge
consisting of one 41-foot steel-beam span with a concrete deck (Vermont Agency of
Transportation, written communication, March 24, 1995). The opening length of the
structure parallel to the bridge face is 39.4 ft. The bridge is supported by vertical, concrete
abutments. The channel is skewed approximately 0 degrees and the opening-skew-toroadway is also zero degrees.
A scour hole 1.7 ft deeper than the mean thalweg depth was observed from 30 ft upstream
to 70 ft downstream in mid-channel during the Level I assessment. Scour protection
measures at the site included: type-3 stone fill (less than 48 inches diameter) along the left
and right bank upstream, and along the left and right bank downstream. The protection
along the banks begins in the road embankment areas where the wingwalls would be
located. 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 ft. Abutment scour ranged from 7.6 to 8.4 ft
at the left abutment and from 9.9 to 14.8 ft at the right abutment. 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.