Level II scour analysis for Bridge 23 (WALDTH00060023) on Town Highway 6, crossing Stannard Brook, Walden, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WALDTH00060023 on Town Highway 6 crossing Stannard Brook, Walden, 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 5.61-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the upstream surface cover is shrub and brushland
with some trees. The downstream surface cover is forest.

In the study area, Stannard Brook has an incised, straight channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 54 ft and an average bank height
of 9 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 64.0 mm (0.210 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 8, 1995, indicated that the reach was stable.

The Town Highway 6 crossing of Stannard Brook is a 59-ft-long (bottom width), two-lane
pipe arch culvert consisting of one 22-foot corrugated plate pipe arch span (Vermont
Agency of Transportation, written communication, March 28, 1995). The opening length of
the structure parallel to the bridge face is 21.9 ft.The pipe arch is supported by vertical,
concrete kneewalls. The channel is skewed approximately 10 degrees to the opening while
the opening-skew-to-roadway is zero degrees.

A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the upstream
end of the right kneewall during the Level I assessment. There was also a scour hole 0.5 ft
deeper than the mean thalweg depth observed along the downstream end of the left
kneewall. The scour counter measures at the site included type-3 stone fill (less than 48
inches diameter) at the upstream and downstream end of the left and right kneewall. There
was also type-2 stone fill (less than 36 inches diameter) along the upstream right bank.
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 determined and analyzed as another potential 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
kneewalls). 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 2.3 ft. The worst-case
contraction scour occurred at the incipient roadway-overtopping discharge, which was
greater than the 100-year discharge. Left kneewall scour ranged from 11.7 to 16.8 ft. The
worst-case left kneewall scour occurred at the 500-year discharge. Right kneewall scour
ranged from 13.7 to 16.7 ft. The worst-case right kneewall scour occurred at the incipient
roadway-overtopping 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.
During the Level I survey ledge was discovered at the upstream end of the right abutment.
The ledge in the channel may limit scour depths.

It is generally accepted that the Froehlich equation (abutment/ kneewall 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.