Level II scour analysis for Bridge 12 (CHESVT01030012) on State Highway 103, crossing the Williams River, Chester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHESVT01030012 on State Route 103 crossing the Williams River, Chester, 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 eastern Vermont. The 23.9-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture on the downstream right
and upstream left overbank areas and short grass on the downstream left and upstream right
overbank areas. The surface cover along the upstream and downstream immediate banks
consists of trees and brush.

In the study area, the the Williams River has an incised, sinuous channel with a slope of
approximately 0.0054 ft/ft, an average channel top width of 75 ft and an average bank
height of 4 ft. The predominant channel bed material is gravel with a median grain size
(D₅₀) of 52.4 mm (0.172 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on September 18, 1996, indicated that the reach was laterally unstable.

The State Route 103 crossing of the Williams River is a 99-ft-long, two-lane bridge
consisting of three concrete T-beam spans (Vermont Agency of Transportation, written
communication, March 29, 1995). The bridge is supported by two piers and vertical,
concrete abutments with wingwalls and spill-through slopes. The channel is skewed
approximately 20 degrees to the opening while the opening-skew-to-roadway is 0 degrees.
Downstream of the bridge are the remains of a dam which is acting as a drop structure.

A scour hole, approximately 3 ft deeper than the mean thalweg depth, was observed along
the upstream left bank extending from 78 ft upstream of the upstream bridge face to 25 ft
downstream of the downstream bridge face during the Level I assessment. Lateral migration
of the channel has resulted in flow being directed at an angle to the piers, which has resulted
in increased local scour at the bridge. The scour protection measures at the site included
type-2 stone fill (less than 36 inches diameter) under the bridge along the entire base length
of the left and right spill-through slopes and extending up to the abutments. Type-2 stone
fill (less than 36 inches diameter) scour protection was also found along the upstream left
bank from the bridge to 46 ft upstream and along the downstream right bank from the
bridge to 70 ft downstream. Rock walls were found along the left bank from 88 ft to 200 ft
downstream and along the right bank from 124 ft to 224 ft downstream. There are two wood
pile drop structures located at 47 ft and 61 ft 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 0.2 ft. The worst-case
contraction scour occurred at
the 500-year discharge. Abutment scour ranged from 4.0 to
12.4 ft along the right spill-through abutment and from 8.4 to 10.7 ft along the left spill-
through abutment. The worst-case abutment scour
occurred at the 500-year discharge. Pier
scour ranged from 7.1 to 8.9 ft along Pier 1 (
northerly pier) and from 13.5 to 17.1 ft along
Pier 2 (southerly pier). The worst case pier
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.