Level II scour analysis for Bridge 10 (WNDHTH00020010) on Town Highway 2, crossing the Middle Branch of the Williams River, Windham, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WNDHTH00020010 on Town Highway 2 crossing the Middle Branch Williams River,
Windham, 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 Green Mountain section of the New England physiographic province in
south central Vermont. The 1.44-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the predominate surface cover upstream of the bridge
is pasture on the left bank and forest on the right bank. Downstream of the bridge the
surface cover consists of forest on the right bank and grass on the left bank.

In the study area, the Middle Branch Williams River has an incised, sinuous channel with a
slope of approximately 0.03 ft/ft, an average channel top width of 28 ft and an average bank
height of 5 ft. The channel bed material ranges from gravel to boulder with a median grain
size (D₅₀) of 61.4 mm (0.201 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 22, 1996, indicated that the reach was stable.

The Town Highway 2 crossing of the Middle Branch Williams River is a
25-ft-long, two-lane bridge consisting of one 22-foot concrete slab span (Vermont Agency
of Transportation, written communication, March 31, 1995). The bridge is supported by
vertical, concrete abutments with wingwalls. The channel is skewed approximately 60
degrees to the opening while the opening-skew-to-roadway is 50 degrees.

The scour protection measures at the site included type-2 stone fill (less than 36 inches
diameter) along both upstream banks. The scour protection measures downstream were type
-1 stone fill (less than 12 inches diameter) on the left bank and type-3 stone fill (less than 48
inches diameter) on the right bank. Scour protection measures do not exist underneath the
bridge. 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.

Contraction scour for modelled flows ranged from 0.9 to 2.2 ft. The worst-case contraction
scour occurred at the 500-year discharge. Abutment scour ranged from 8.5 to 8.8 ft along
the right abutment and from 8.7 to 10.1 ft along the left abutment. The worst-case abutment
scour at the right abutment occurred at the 100-year discharge and at the left abutment 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.