Level II scour analysis for Bridge 27 (STJOTH00080027) on Town Highway 8, crossing the Sleepers River, St. Johnsbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
STJOTH00080027 on Town Highway 8 crossing the Sleepers River,
St. Johnsbury, 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 northeastern Vermont. The 40.4-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is forest on the upstream
right bank with some pasture on the upstream left bank. The downstream right overbank
cover is comprised of cut grass, trees and shrubs while the immediate banks have dense
woody vegetation. The downstream left bank is forested with some pasture.
In the study area, the Sleepers River has an incised, sinuous channel with a slope of
approximately 0.007 ft/ft, an average channel top width of 72 ft and an average bank height
of 5 ft. The channel bed material ranges from gravel to cobble with a median grain size
(D₅₀) of 48.5 mm (0.159 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 10, 1995, indicated that the reach was stable.
The Town Highway 8 crossing of the Sleepers River is a 74-ft-long, two-lane bridge
consisting of one 71-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 28, 1995). The opening length of the structure parallel to the bridge
face is 68 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 50 degrees to the opening while the computed openingskew-to-roadway is 45 degrees. The VTAOT database states the opening-skew-to-roadway
as 30 degrees.
A scour hole 2.5 ft deeper than the mean thalweg depth was observed along the right
abutment during the Level I assessment. There is also a three to four foot deep scour hole in
the channel adjacent to the upstream right wingwall. The scour protection at the site
included type-3 stone fill (less than 48 inches diameter) at the upstream end of the upstream
left wingwall, at the downstream end of the downstream right wingwall, and along the
downstream left bank. There was also type-2 stone fill (less than 36 inches diameter) at the
downstream end of the downstream left wingwall, along the upstream left bank, and along
the downstream right 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)
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 computed for all modelled flows was zero ft. Abutment scour ranged
from 6.2 to 9.7 ft. The worst-case abutment scour occurred at the 100-year discharge at the
right abutment and at the 500-year discharge at the left 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.