Level II scour analysis for Bridge 35 (BETHTH00190035) on Town Highway 19, crossing Gilead Brook, Bethel, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BETHTH00190035 on town highway 19 crossing Gilead Brook, Bethel, 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). A Level
I study is included in Appendix E of this report. A Level I study provides a qualitative
geomorphic characterization of the study site. Information on the bridge available from
VTAOT files was compiled prior to conducting Level I and Level II analyses and can be
found in Appendix D.

The site is in the Green Mountain physiographic province of central Vermont in the town of
Bethel. The 6.40-mi²
drainage area is predominantly rural and forested. In the vicinity of
the study site, the immediate banks have woody vegetation coverage with pasture beyond.

In the study area, Gilead Brook is an incised, sinuous channel with a slope of approximately
0.015 ft/ft, an average channel top width of 31 ft and an average channel depth of 2.5 ft. The
predominant channel bed material is gravel and cobble (D₅₀ is 62.5 mm or 0.205 ft). The
geomorphic assessment at the time of the Level I and Level II site visit on October 20,
1994, indicated that the reach was stable.

The town highway 19 crossing of Gilead Brook is a 30-ft-long, one-lane bridge consisting
of one 24-foot steel-beam span with timber deck (Vermont Agency of Transportation,
written commun., August 24, 1994). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 5 degrees to the opening
while the opening-skew-to-roadway is 10 degrees.

The scour protection measures at the site included type-1 stone fill (less than 12 inches
diameter) at the downstream wingwalls, left abutment, and upstream right road
embankment; type-2 stone fill (less than 36 inches diameter) is at the upstream right
wingwall. 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, 1993). 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 0.1 to 2.1 ft. with the worst-case
scenario occurring at the 500-year discharge. Abutment scour ranged from 3.9 to 9.5 ft. The
worst-case abutment scour also 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 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, 1993, p. 48). Many factors,
including historical performance during flood events, the geomorphic assessment, scour
protection measures, and the results of the hydraulic analyses, must be considered to
properly assess the validity of abutment scour results. Therefore, scour depths adopted by
VTAOT may differ from the computed values documented herein, based on the
consideration of additional contributing factors and experienced engineering judgement.