Level II scour analysis for Bridge 49 (BETHTH00790049) on Town Highway 79, crossing Locust Creek, Bethel, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BETHTH00790049 on town highway 79 crossing Locust Creek, 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 24.4-mi²
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the banks are forested.

In the study area, Locust Creek has an incised, sinuous channel with a slope of
approximately 0.015 ft/ft, an average channel top width of 74 ft and an average channel
depth of 6 ft. The predominant channel bed material is gravel and cobble (D₅₀ is 124 mm or
0.407 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
September 21 & 26, 1994, respectively, with a check on
12/15/94, indicated that the reach was stable.

The town Highway 79 crossing of Locust Creek is a 55-ft-long, one-lane bridge consisting
of one 50-foot concrete span (Vermont Agency of Transportation, written commun., August
24, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 50 degrees to the opening while the opening-skew-toroadway is 45 degrees.
Scour protection measures in place at the site were type-1 stone fill (less than 12 inches
diameter) at the upstream right and downstream left road embankment, type-2 stone fill
(less than 36 inches diameter) at the upstream left bank, upstream wingwalls, and
downstream left 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
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 these computed results
follow.

Contraction scour for all modelled flows ranged from 0.0 ft to 1.0 ft. The worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 10.3 ft
to 13.3 ft. with the worst-case abutment scour also occurring at the 100-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 depths, are
presented in tables 1 and 2. A cross-section of the computed scour 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. 22). 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 experienced engineering judgement.