Level II scour analysis for Bridge 11R (ROCKTH0001011R) on Town Highway 1 (VT 121 & FAS 125), crossing the Saxtons River, Rockingham, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ROCKTH0001011R on Town Highway 1 crossing the Saxtons River, Rockingham,
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 68.3-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover consists of houses, short
grass, and scattered trees except along the immediate river banks, which are tree covered.

In the study area, the Saxtons River has a sinuous channel with a slope of approximately
0.005 ft/ft, an average channel top width of 121 ft and an average bank height of 8 ft. The
predominant channel bed materials are gravel and cobbles with a median grain size (D₅₀) of
109 mm (0.359 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on September 3, 1996, indicated that the reach was laterally unstable. Lateral
instability was evident with respect to a cut-bank on the left bank upstream with slip failure
of bank material. Furthermore, there is a wide point bar along the right bank upstream
opposite the cut-bank.

The Town Highway 1 crossing of the Saxtons River is a 184-ft-long, two-lane bridge
consisting of three steel-beam spans (Vermont Agency of Transportation, written communication, March 30, 1995).
The bridge is supported by vertical, concrete, skeletal-style abutment walls with spill-through embankments adjacent to
each wall. The channel is skewed approximately 35 degrees to the opening while the opening-skew-to-roadway is 30
degrees.

The only scour protection measure at the site was type-3 stone fill (less than 48 inches
diameter) on the spill-through embankments. 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 all modelled flows at this site. Abutment
scour ranged from 9.0 to 13.4 feet. The worst-case abutment scour occurred at the 500-year
discharge for the left abutment. There are two piers for which computed pier scour ranged
from 9.0 to 18.4 feet. The left and right piers in this report are presented as pier 1 and pier 2,
respectively. The worst-case pier scour occurred at pier 2 for 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.