This report provides the results of a detailed Level II analysis of scour potential at structure
IRA-VT01330013 on State Route 133 crossing an Ira Brook Tributary, Ira, 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 Taconic section of the New England physiographic province in westcentral Vermont. The 2.88-mi2
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is pasture while the immediate banks have
dense tree cover.
In the study area, this Ira Brook Tributary has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 40 ft and an average channel
depth of 7 ft. The predominant channel bed material is cobble with a median grain size
(D50) of 71.9 mm (0.236 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on June 14, 1995, indicated that the reach was stable.
The State Route 133 crossing of this Ira Brook Tributary is a 28-ft-long, two-lane bridge
consisting of one 26-foot concrete span (Vermont Agency of Transportation, written
communication, March 13, 1995). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 20 degrees to the opening while the
opening-skew-to-roadway is 5 degrees.
A scour hole, approximately 0.5 ft deeper than the mean thalweg depth, was observed at the
downstream end of the left abutment wall during the Level I assessment. The scour
protection measures at the site were type-1 stone fill (less than 12 inches diameter) on the
upstream left bank and upstream left wingwall and type-2 stone fill (less than 36 inches
diameter) on the upstream right bank, upstream right wingwall, the downstream wingwalls
and downstream banks. 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.
There was no computed contraction scour for any of the modelled flows. Abutment scour
ranged from 3.6 to 4.7 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, 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
Additional Publication Details
USGS Numbered Series
Level II scour analysis for Bridge 13 (IRAVT013300133) on State Route 133, crossing an Ira Brook Tributary, Ira, Vermont