Level II scour analysis for Bridge 11 (HINETH00040011) on Town Highway 4, crossing Lewis Creek, Hinesburg, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HINETH00040011 on Town Highway 4 crossing Lewis Creek, Hinesburg, 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). Results of a Level I scour investigation also are included in Appendix E of this
report. A Level I investigation provides a qualitative geomorphic characterization of the
study site. Information on the bridge, gleaned from Vermont Agency of Transportation
(VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is
found in Appendix D.

The site is in the Green Mountain section of the New England physiographic province in
northwestern Vermont. The 38.4-mi² drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture.

In the study area, Lewis Creek has an incised, straight channel with a slope of
approximately 0.001 ft/ft, an average channel top width of 60 ft and an average channel
depth of 7 ft. The channel bed material ranges from gravel to boulder with a median grain
size (D₅₀) of 47.0 mm (0.154 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 3, 1996, indicated that the reach was stable.

The Town Highway 4 crossing of Lewis Creek is an 84-foot-long, two-lane bridge
consisting of one 82-foot steel-beam span (Vermont Agency of Transportation, written
communication, December 15, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls and spill-through embankments at each abutment. The channel is
skewed approximately 40 degrees to the opening while the opening-skew-to-roadway is 15
degrees.

The scour protection measures at the site were type-2 stone fill (less than 36 inches
diameter) at the downstream left and right wingwalls and the downstream right bank. Scour
protection also included type-3 stone fill (less than 48 inches diameter) at the left and right
upstream wingwalls, both abutments, both upstream banks, and the left bank downstream.
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 1.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Left abutment scour ranged from
14.1 to 18.2 ft. Right abutment scour ranged from 9.9 to 13.4 ft. The worst-case abutment
scour occurred at left abutment for 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). 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.