Level II scour analysis for Bridge 41 (ROCKTH00390041) on Town Highway 39, crossing the Saxtons River, Rockingham, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ROCKTH00390041 on Town Highway 39 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 57.4-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover consists of forest on the left
bank and pasture with some trees on the right bank.

In the study area, the Saxtons River has an sinuous channel with a slope of approximately
0.009 ft/ft, an average channel top width of 112 ft and an average bank height of 10 ft. The
channel bed material ranges from sand to cobbles with a median grain size (D₅₀) of 103 mm
(0.339 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
August 15, 1996, indicated that the reach was laterally unstable. There are wide point bars,
cut-banks with fallen trees, and areas of localized channel scour along the left bank, where
there is bedrock exposure at the surface.

The Town Highway 39 crossing of the Saxtons River is an 85-ft-long, one-lane bridge
consisting of one 82-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 31, 1995). The bridge is supported by vertical, concrete abutments
without wingwalls. The channel is skewed approximately 30 degrees to the opening while
the opening-skew-to-roadway is zero degrees.

A scour hole 3 ft deeper than the mean thalweg depth was observed during the Level I
assessment along the left side of the channel under the bridge exposing the left abutment
footing 5.5 feet. The only scour protection measure at the site was type-2 stone fill (less
than 36 inches diameter) on the left banks upstream and downstream and the left abutment
wall. 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 2.2 to 3.8 feet. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 21.4 to
23.2 feet and 26.2 to 32.4 feet at the left and right abutments respectively. The worst-case
abutment scour occurred for the right abutment at the incipient overtopping 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. Bedrock was exposed at the surface in some areas of the channel and
potentially is located at a shallower depth than the scour depths indicated above.
Nevertheless, 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.