Level II scour analysis for Bridge 4 (MNTGTH00020004) on Town Highway 2, crossing Wade Brook, Montgomery, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MNTGTH00020004 on town highway 2 crossing Wade Brook, Montgomery, 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). A Level I study is included in Appendix E of this report. A Level I study provides
a qualitative geomorphic characterization of the study site. Information on the bridge,
gleaned from VTAOT files, was compiled prior to conducting Level I and Level II analyses
and can be found in Appendix D.

The site is in the Green Mountain physiographic province of north-central Vermont in the
town of Montgomery. The 1.68-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the banks have woody vegetation coverage.

In the study area, Wade Brook has an incised, sinuous channel with a slope of
approximately 0.0454 ft/ft, an average channel top width of 30 ft and an average channel
depth of 2 ft. The predominant channel bed materials are gravel and cobbles (D₅₀ is 77.7
mm or 0.255 ft). The geomorphic assessment at the time of the Level I and Level II site visit
on November 8, 1994, indicated that the reach was degraded. There were no scour holes
observed during the Level I assessment. However, general streambed lowering was evident
as both abutments were undermined equally with no localized scour on one abutment over
the other.

The town highway 2 crossing of Wade Brook is a 23-ft-long, two-lane bridge consisting of
one 20-foot concrete slab span (Vermont Agency of Transportation, written
communication, August 3, 1994). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 30 degrees to the opening while the
computed opening-skew-to-roadway is 25 degrees.

The scour protection measures at the site were type-1 stone fill (less than 12 inches
diameter) on the upstream right wingwall and all road approach embankments, type-2 stone
fill (less than 36 inches diameter) on the left abutment, and a “laid-up” stone wall at the
upstream end of the upstream left wingwall and in front of the upstream left bank.
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 all modelled flows was 0.1 ft. The worst-case contraction scour
occurred at the 100-year and 500-year discharges. Abutment scour ranged from 3.9 to 5.2 ft.
The worst-case abutment scour also 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). Many factors,
including historical performance during flood events, the geomorphic assessment, scour
protection measures, and the results of the hydraulic analyses, must be considered to
properly assess the validity of abutment scour results. Therefore, scour depths adopted by
VTAOT may differ from the computed values documented herein, based on the
consideration of additional contributing factors and experienced engineering judgement.