Level II scour analysis for Bridge 8 (HANCTH00020008) on Town Highway 2, crossing Hancock Branch White River, Hancock, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HANCTH00020008 on town highway 2 crossing the Hancock Branch White River,
Hancock, 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, gleaned 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
Hancock. The 8.4-mi²
drainage area is predominantly rural and forested. In the vicinity of
this site, the banks have dense woody vegetation coverage.
In the study area, the Hancock Branch White River is an incised, sinuous channel with a
slope of approximately 0.038 ft/ft, an average channel top width of 47.0 ft and an average
channel depth of 3.0 ft. The predominant channel bed material is cobble (D₅₀
is 102 mm or
0.336 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
November 16, 1994, indicated that the reach was stable.
The town highway 2 crossing of the Hancock Branch White Riveris a 33-ft-long, two-lane
bridge consisting of one 30-foot steel-beam span with a concrete deck (Vermont Agency of
Transportation, written commun., August 26, 1994). The bridge is supported by steep
sloping, cement-grouted, cobble-stone abutments with wingwalls. The channel is skewed
approximately ten degrees to the opening while the opening-skew-to-roadway is zero
degrees.
A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the upstream
left wingwall and left abutment during the Level I assessment. The only scour protection
measure at the site was type-2 stone fill (less than 36 inches diameter) 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
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 ranged from 0.6 ft to 1.3 ft and the worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 9.4 ft to
15.2 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.