Level II scour analysis for Bridge 20 (GRAFTH00010020) on Town Highway 1, crossing the Saxtons River, Grafton Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
GRAFTH00010020 on Town Highway 1 crossing the Saxtons River, Grafton, 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 33.9-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is forest upstream of the
bridge and shrub and brush downstream.

In the study area, the Saxtons River has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 97 ft and an average bank height
of 2 ft. The predominant channel bed material is gravel with a median grain size (D₅₀) of
58.6 mm (0.192 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on August 21, 1996, indicated that the reach was laterally unstable due to distinctive
cut bank development on the upstream right bank and point bar development on the
upstream left bank and downstream right bank.

The Town Highway 1 crossing of the Saxtons River is a 191-ft-long, two-lane bridge
consisting of three steel-beam spans (Vermont Agency of Transportation, written
communication, March 29, 1995). The bridge is supported by vertical, concrete abutments
with spill-through embankments and two piers. The channel is skewed approximately 40
degrees to the opening. The opening-skew-to-roadway is 45 degrees in the VTAOT records
but measured 50 degrees from surveyed points.

The scour protection measures at the site were type-1 stone fill (less than 12 inches
diameter) on the left abutment, type-2 stone fill (less than 36 inches diameter) on the right
abutment and downstream right bank, and a stone wall is noted on the left bank
downstream. 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 0.9 feet. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.0 to
14.9 feet. The worst-case abutment scour occurred at the 500-year discharge for the right
abutment. There are two piers for which computed pier scour ranged from 8.7 to 26.0 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.