Level II scour analysis for Bridge 3 (EASTTH00010003) on Town Highway 1, crossing the East Branch Passumpsic River, East Haven, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
EASTTH00010003 on Town Highway 1 crossing the East Branch Passumpsic River, East
Haven, 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 White Mountain section of the New England physiographic province in
northeastern Vermont. The 50.4-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover on the left bank upstream is forest.
On the remaining three banks the surface cover is pasture while the immediate banks have
dense woody vegetation.

In the study area, the East Branch Passumpsic River has an incised, sinuous channel with a
slope of approximately 0.003 ft/ft, an average channel top width of 62 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 61.5 mm (0.187 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on August 14, 1995, indicated that the reach was stable.

The Town Highway 1 crossing of the East Branch Passumpsic River is a 89-ft-long, two-lane bridge consisting of one 87-foot steel-beam span (Vermont Agency of Transportation,
written communication, March 17, 1995). The opening length of the structure parallel to the
bridge face is 84.7 ft. The bridge is supported by vertical, concrete abutments with sloped
stone fill in front that creates a spill through embankment. The channel is skewed
approximately zero degrees to the opening and the opening-skew-to-roadway is also zero
degrees.

Channel scour 0.5 ft deeper than the mean thalweg depth was observed to the left of the
center of the channel under the bridge during the Level I assessment. The scour
countermeasures at the site are type-2 stone fill (less than 36 inches diameter) along the
downstream left bank and type-4 stone fill (less than 60 inches diameter) in front of the
abutments creating spill through slopes. 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. 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 0 to 1.8 ft. The worst-case contraction
scour occurred at the 500-year discharge. Abutment scour ranged from 6.4 to 11.7 ft. The
worst-case abutment scour 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). 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.