Level II scour analysis for Bridge 21 (MONKTH00340021) on Town Highway 34, crossing Little Otter Creek, Monkton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MONKTH00340021 on Town Highway 34 crossing Little Otter Creek, Monkton, 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 D 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 C.

The site is in the Champlain section of the Saint Lawrence Valley physiographic province
in northwestern Vermont. The 34.1-mi²
drainage area is in a predominantly rural and
forested basin with pasture in the valleys. In the vicinity of the study site, the surface cover
consists of pasture. The most significant tree cover is immediately adjacent to the channel
on the right bank downstream.

In the study area, Little Otter Creek has a sinuous channel with a slope of approximately
0.008 ft/ft, an average channel top width of 92 feet and an average bank height of 6 feet.
The predominant channel bed materials are silt and clay. Sieve analysis indicates that
greater than 50% of the sample is silt and clay and thus a median grain size by use of sieve
analysis was indeterminate. Therefore, the median grain size was assumed to be medium
silt with a size (D₅₀) of 0.0310 mm (0.000102 ft). The geomorphic assessment at the time of
the Level I and Level II site visit on June 19 and June 20, 1996, indicated that the reach was
stable.

The Town Highway 34 crossing of Little Otter Creek is a 50-ft-long, one-lane bridge
consisting of one 26-foot concrete span and three “boiler tube” smooth metal pipe culverts
through the left road approach (Vermont Agency of Transportation, written
communication, December 15, 1995). The opening length of the bridge parallel to the
bridge face is 25.1 feet. The bridge is supported by vertical, concrete abutments with
wingwalls on the right abutment only. The channel is skewed approximately 25 degrees to
the opening. The VTAOT records indicate the opening-skew-to-roadway is 20 degrees but
measurement from surveyed data suggests the skew is five degrees.

The scour protection measures at the site were type-1 stone fill (less than 12 inches
diameter) on the upstream and downstream embankments of the left road approach and
type-2 stone fill (less than 36 inches diameter) surrounding the entrance of each culvert.
Additional details describing conditions at the site are included in the Level II Summary
and Appendices C and D.

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
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 10.3 to 12.3 feet. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.6 to
22.5 feet. The worst-case abutment scour occurred at the 500-year discharge for the left
abutment and at the incipient overtopping discharge for the right abutment. 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.