This report provides the results of a detailed Level II analysis of scour potential at structure
NEWHTH00050030 on Town Highway 5 crossing the New Haven River, New 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 (Federal Highway Administration,
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
The site is in the Champlain section of the St. Lawrence Valley physiographic province in
west-central Vermont. The 115-mi2 drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture on the right bank
upstream and downstream of the bridge while the immediate banks have dense woody
vegetation. The upstream left bank is also pasture. The downstream left bank is forested.
In the study area, the New Haven River has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 127 ft and an average bank height
of 5 ft. The channel bed material ranges from silt to cobble with a median grain size (D50)
of 20.4 mm (0.067 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on June 19, 1996, indicated that the reach was laterally unstable. The stream bends
through the bridge and impacts the left bank where there is a cut bank and scour hole.
The Town Highway 5 crossing of the New Haven River is a 181-ft-long, two-lane bridge
consisting of four 45-ft concrete tee-beam spans (Vermont Agency of Transportation,
written communication, December 15, 1995). The opening length of the structure parallel to
the bridge face is 175.9 ft. The bridge is supported by vertical, concrete abutments with
stone fill spill-through embankments and three concrete piers. The channel is skewed
approximately 15 degrees to the opening while the computed opening-skew-to-roadway is
A scour hole 4.5 ft deeper than the mean thalweg depth was observed along the downstream
left bank during the Level I assessment. Also observed was a scour hole 1.5 ft deeper than
the mean thalweg depth at the upstream end of the middle pier. The only scour protection
measure at the site was type-3 stone fill (less than 48 inches diameter) in front of the left and
right 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 Davis, 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 0.7 to 2.1 ft. The worst-case
contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 6.8
to 8.4 ft. The worst-case left abutment scour occurred at the 500-year discharge. Right
abutment scour ranged from 11.2 to 14.0 ft. The worst-case right abutment scour occurred
at the 500-year discharge. Pier scour ranged from 12.9 to 19.3 ft. The worst-case pier 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 Davis, 1995, p. 46). 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