Level II scour analysis for Bridge 43 (BETHTH00070043) on Town Highway 07, crossing Gilead Brook, Bethel, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BETHTH00070043 on town highway 7 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 6.81-mi²
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the banks have dense woody vegetation coverage except for the
downstream right bank near the bridge, which is grass covered.

In the study area, Gilead Brook has an incised, slightly sinuous channel with a slope of
approximately 0.0181 ft/ft, an average channel top width of 36 ft and an average channel
depth of 4.0 ft. The predominant channel bed material is cobble (D₅₀ is 79.6 mm or 0.261
ft). The geomorphic assessment at the time of the Level I and Level II site visit on October
19, 1994, indicated that the reach was stable.

The town highway 7 crossing of Gilead Brook is a 31-ft-long, two-lane bridge consisting of
one 27-foot concrete slab type superstructure (Vermont Agency of Transportation, written
commun., August 24, 1994). The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 30 degrees to the opening while the
opening-skew-to-roadway is 15 degrees.

A scour hole 0.5 ft deeper than the mean thalweg depth was observed at the right side of the
downstream bridge face during the Level I assessment. The scour protection measures in
place at the site were type-1 stone fill (less than 12 inches diameter) along the right
abutment and both downstream banks, type-2 stone fill (less than 36 inches diameter) on all
of the road approach embankments, both upstream banks, and along the entire base length
of the wingwalls. 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, 1995). 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 0.0 to 1.4 ft. The worst-case
contraction scour occurred at the incipient overtopping discharge, which was between the
100- and 500-year discharges. Abutment scour ranged from 6.6 to 11.0 ft. with the worst-case scenario occurring 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, 1995, p. 47). 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.