Level II scour analysis for Bridge 49 (FFIETH00290049) on Town Highway29, crossing Black Creek, Fairfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
FFIETH00290049 on Town Highway 29 crossing Black Creek, Fairfield, 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 83.5-mi²
drainage area is in a predominantly rural and forested
basin. There is agricultural land in the basin as well, especially along the immediate river
valley. In the vicinity of the study site, the surface cover is pasture except for on the
downstream right bank which has row crops.
In the study area, Black Creek has an incised, meandering channel with a slope of
approximately 0.0005 ft/ft, an average channel top width of 85 ft and an average bank
height of 9 ft. The channel bed material ranges from sand to cobble with a median grain size
(D₅₀) of 3.23 mm (0.0106 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 13, 1995, indicated that the reach was laterally unstable. This
assessment was due to apparent long term lateral movement of the channel in the vicinity of
the bridge.
The Town Highway 29 crossing of Black Creek is a 48-ft-long, one-lane bridge consisting
of one 45-foot steel pony thru-truss type span (Vermont Agency of Transportation, written
communication, March 8, 1995). The opening length of the structure parallel to the bridge
face is 42.5 ft.The bridge is supported by vertical, concrete abutments. The channel is
skewed approximately 30 degrees to the opening while the opening-skew-to-roadway is
zero degrees.
Channel scour, approximately 6 ft deeper than the mean thalweg depth, was observed
through the immediate channel reach including underneath the bridge. Type-2 stone fill
(less than 36 inches diameter) has been placed as a scour countermeasure along both
abutments, on the channel bed under the bridge and along immediate channel banks
upstream and downstream of the bridge. Additional details describing conditions at the site
are included in the Level II Summary and Appendices D and E.
Scour depths and recommended 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 4.4 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.5 to
14.3 ft and 12.2 to 16.3 ft on the left and right abutments respectively. 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”. Scouredstreambed 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 particlesize 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.