Level II scour analysis for Bridge 43 (SPRICYBRIG0043) on Bridge Street, crossing the Black River, Springfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
SPRICYBRIG0043 on Bridge Street crossing the Black River, Springfield, 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 New England Upland section of the New England physiographic province
in southeastern Vermont. The 191-mi²
drainage area is a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover consist of some grass, buildings,
and pavement. The immediate banks are covered with trees, shrubs and brush.

In the study area, the Black River has an incised channel with a slope of approximately
0.001 ft/ft, an average channel top width of 156 ft and an average bank height of 14 ft. The
channel bed material is predominantly cobbles with a median grain size (D₅₀) of 90.7 mm
(0.298 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
September 19, 1996, indicated that the reach was stable.

The Bridge Street crossing of the Black River is a 123-foot-long, two-lane bridge consisting
of one 119-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 30, 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 20 degrees.

The scour protection measures at the site were type-2 stone fill (less than 36 inches
diameter) along the downstream left bank and the downstream left wingwall. There was
also type-1 stone fill (less than 12 inches diameter) along right abutment and the
downstream right wingwall. There is a nine foot tall concrete wall along the downstream
right bank to 89 feet 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.

There was no computed contraction scour. Left abutment scour ranged from 9.9 to 11 ft.
The worst-case left abutment scour occurred at the 100-year discharge. Right abutment
scour ranged from 6.5 to 11.2 ft. The worst-case right 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
documented herein.