This report provides the results of a detailed Level II analysis of scour potential at structure
BETHTH00310033 on town highway 31 crossing the Lilliesville 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 6.85-mi2 drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the left banks have shrub and brush, and the right banks are
In the study area, Lilliesville Brook has an incised, straight channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 37 ft and an average channel
depth of 4 ft. The predominant channel bed material is gravel and cobble (D50 is 66.8 mm or
0.219 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
October 13 and 14, 1994, indicated that the reach was stable.
The town highway 31 crossing of Lilliesville Brook is a 41-ft-long, one-lane bridge
consisting of one 39-foot steel-beam span with a timber deck (Vermont Agency of
Transportation, written commun., August 24, 1994). The bridge is supported by vertical,
concrete abutments with wingwalls. The channel is skewed approximately 35 degrees to the
opening while the opening-skew-to-roadway is 0 degrees.
Scour protection measures in place at the site were type-1 stone fill (less than 12 inches
diameter) at the downstream left wingwall, left abutment, and upstream and downstream
sides of the left road embankment. Additional details describing conditions at the site are
included in the Level II Summary and Appendices D
Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1993). Scour depths were
calculated assuming an infinite depth of erosive material and a homogeneous particle-size
distribution. The scour analysis results are presented in tables 1 and 2 and a graph of the
scour depths is presented in figure 8.
Additional publication details
USGS Numbered Series
Level II scour analysis for Bridge 33 (BETHTH00310033) on Town Highway 031, crossing Lilliesville Brook, Bethel, Vermont