Level II scour analysis for Bridge 42 (BETHTH00860042) on Town Highway 86, crossing Gilead Brook, Bethel, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BETHTH00860042 on town highway 86 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 were 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 11.4-mi²
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the upstream banks are tree covered and the downstream banks are
covered with shrubs and brush.

In the study area, Gilead Brook is probably incised, has a sinuous channel with a slope of
approximately 0.012 ft/ft, an average channel top width of 53 ft, and an average channel
depth of 5 ft. The predominant channel bed material is gravel to cobbles (D₅₀ is 85.6 mm or
0.281 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
September 30, 1994, indicated that the reach was stable.

The town highway 86 crossing of Gilead Brook is a 28-ft-long, one-lane bridge consisting
of one 25-foot clear-span structure with a concrete deck (Vermont Agency of
Transportation, written commun., August 24, 1994). The bridge is supported by concrete
abutments with wingwalls. The bridge skew is approximately 5 degrees and there is no
opening-skew-to-roadway.

A scour hole approximately 1 ft deeper than the mean thalweg depth was observed along
the left bank, near the upstream bridge face during the Level I assessment. There is also
approximately 1 ft of scour along the left abutment of the bridge, near the upstream wing
wall, exposing the footing. There is type-one (less than 12 in diameter) protection on the
US left wingwall and type-two (less than 36 in diameter) along the US and DS right
wingwalls. There is no protection along the abutments. 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 to 1.9 ft. The worst-case contraction
scour occurred at the incipient-overtopping discharge and the 100-year discharge.
Abutment scour ranged from 8.6 to 15.7 ft. 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 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, 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 engineering judgement.