Level II scour analysis for Bridge 25 (HARDTH00420025) on Town Highway 42, crossing Lamoille River, Hardwick, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HARDTH00420025 on town highway 42 crossing the Lamoille River, Hardwick, 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 north-central Vermont in the
town of Hardwick. The 119-mi²
drainage area is in a predominantly rural basin. In the
vicinity of the study site, the left banks are covered by pasture and (or) fields. The right
bank of Lamoille River is adjacent to Vermont Route 15 near the north edge of the Lamoille
River valley.

In the study area, the Lamoille River has a sinuous channel with a slope of approximately
0.0004 ft/ft, an average channel top width of 89.0 ft and an average channel depth of 8.0 ft.
The predominant channel bed material is sand and gravel (D₅₀ is 22.4 mm or 0.0733 ft). In
general, the banks have sparse or no woody vegetative cover and the reach was noted to be
laterally unstable at the time of the Level I site visit on July 25, 1995. The Level II work
was completed on 07/27/95 and the site was revisited on August 16, 1995, just after the
August 5-6, 1995 flood on the Lamoille River. Findings from this follow-up visit are
presented in Appendix G.

The town highway 42 crossing of the Lamoille Riveris a 62-ft-long, two-lane bridge
consisting of one 60-foot steel- beam span with a concrete deck, supported by vertical
abutments with wingwalls on upstream and downstream sides (Vermont Agency of
Transportation, written commun., August 24, 1994). The bridge is supported by vertical
abutments with wingwalls on upstream and downstream sides. The channel is not skewed to
the opening and the opening-skew-to-roadway is 0 degrees.

A scour hole 3.0 ft deeper than the mean thalweg depth was observed 5 feet upstream from
the bridge face at mid-channel during the Level I assessment. 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
aggradation or degradation; 2) contraction scour (due to reduction in flow area caused by 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 scour depths
for contraction and local scour and a summary of the results follows.

Contraction scour for all modelled flows was 0.0 ft. Abutment scour ranged from 6.5 ft to
15.6 ft and the worst-case abutment scour occurred at the 500-year discharge. Scour depths
and depths to armoring are summarized on p. 14 in the section titled “Scour Results”.
Scour elevations, based on the calculated depths are presented in tables 1 and 2; a graph of
the scour elevations is presented in figure 8 Scour depths were calculated assuming an
infinite depth of erosive material and a homogeneous particle-size distribution.

For all scour presented in this report, “the scour depths adopted [by VTAOT] may differ
from the equation values based on engineering judgement” (Richardson and others, 1993, p.
21, 27). It is generally accepted that the Froehlich equation (abutment scour) gives
“excessively conservative estimates of scour depths” (Richardson and others, 1993, p. 48).
Many factors, including historical performance during flood events, the geomorphic
assessment, and the results of the hydraulic analyses, must be considered to properly assess
the validity of abutment scour results.