Level II scour analysis for Bridge 24 (MANCUS00070024) on U.S. Route 7, crossing Lye Brook, Manchester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MANCUS00070024 on U.S. Route 7 crossing Lye Brook, Manchester, 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 Taconic section of the New England physiographic province in
southwestern Vermont. The 8.13-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the primary surface cover consists of brush and trees.

In the study area, Lye Brook has an incised, sinuous channel with a slope of approximately
0.03 ft/ft, an average channel top width of 66 ft and an average bank height of 11 ft. The
channel bed material ranges from gravel to boulder with a median grain size (D₅₀) of 90.0
mm (0.295 ft). The geomorphic assessment at the time of the Level I and Level II site visit
on August 6, 1996, indicated that the reach was stable. Although, the immediate reach is
considered stable, upstream of the bridge the Lye Brook valley is very steep (0.05 ft/ft).
Extreme events in a valley this steep may quickly reveal the instability of the channel. In the
Flood Insurance Study for the Town of Manchester (Federal Emergency Management
Agency, January, 1985), Lye Brook’s overbanks were described as “boulder strewn” after
the August 1976 flood.

The U.S. Route 7 crossing of Lye Brook is a 28-ft-long, two-lane bridge consisting of one
25-foot concrete span (Vermont Agency of Transportation, written communication,
September 28, 1995). The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 45 degrees to the opening while the
opening-skew-to-roadway is 55 degrees.

At the time of construction, the downstream channel was relocated (written communication,
Dan Landry, VTAOT, January 2, 1997). A levee on the downstream right bank was also
constructed and is protected by type-4 stone-fill (less than 60 inches diameter) extending
from the bridge to more than 300 feet downstream. Type-2 stone fill (less than 36 inches
diameter) covers the downstream right bank from the bridge to more than 300 feet
downstream. Type-2 stone-fill also extends from the bridge to 220 feet upstream on both
upstream banks. 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)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge is analyzed since it has the potential of being the worst-case scour scenario. 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 1.0 to 1.6 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour computations for the
left abutment ranged from 14.5 to 16.1 ft. with the worst-case occurring at the 100-year
discharge. Abutment scour computations for the right abutment ranged from 6.9 to 10.4 ft.
with the worst-case occurring 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.