Level II scour analysis for Bridge 25 (BRNAVT00120025) on State Highway 12, crossing Locust Creek, Barnard, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRNAVT00120025 on State Highway 12 crossing Locust Creek, Barnard, 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 division of central Vermont in the town of
Barnard. The 11.6-mi²
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the banks have woody vegetation coverage.

In the study area, Locust Creek has a sinuous channel with a slope of approximately 0.023
ft/ft, an average channel top width of 49 ft and an average channel depth of 4 ft. The
predominant channel bed material is cobble (D₅₀ is 109 mm or 0.359 ft). The geomorphic
assessment at the time of the Level I and Level II site visits on September 23 and December
16, 1994, indicated that the reach was stable.

The State Highway 12 crossing of Locust Creek is a 41-ft-long, two-lane bridge consisting
of one 39-foot concrete slab type superstructure (Vermont Agency of Transportation,
written communication, August 23, 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 45 degrees.

A scour hole 1 ft deeper than the mean thalweg depth was observed along a bedrock outcrop
near the upstream left wingwall during the Level I assessment. The scour protection
measures in place at the site are type-1 stone fill (less than 12 inches diameter) along the left
abutment, upstream right bank, and both downstream banks; type-2 stone fill (less than 36
inches diameter) at the downstream side of the right road approach and upstream left bank;
type-3 stone fill (less than 48 inches diameter) at the upstream end of the upstream right
wingwall and downstream end of downstream left wingwall; type-5 (wall/ artificial levee)
at the upstream end of the upstream 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 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 100-year discharge. Abutment scour ranged from 8.5 to
20.9 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, 1993, p. 48). 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.