Level II scour analysis for Bridge 9 (BLOOVT01020009) on State Route 102, crossing the Nulhegan River, Bloomfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BLOOVT01020009 on State Route 102 crossing the Nulhegan River, Bloomfield, 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 144-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest except for the downstream
right bank area which is shrub and brush land. The Nulhegan River flows into the
Connecticut River 210 feet downstream of this bridge.

In the study area, the Nulhegan River has an incised, sinuous channel with a slope of
approximately 0.005 ft/ft, an average channel top width of 164 ft and an average channel
depth of 5 ft. The predominant channel bed material is cobble with a median grain size
(D₅₀) of 152 mm (0.498 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on July 6, 1995, indicated that the reach was laterally unstable. This was due to
numerous point bars and side bars indicating an unstable thalweg.

The State Route 102 crossing of the Nulhegan River is a 134-ft-long, two-lane bridge
consisting of one 130-foot steel-truss span (Vermont Agency of Transportation, written
communication, August 4, 1994). The field measured clear span was 131.6 ft. The bridge is
supported by vertical, concrete abutments with rip-rapped spill-through slopes. The channel
is skewed approximately 25 degrees to the opening while the measured opening-skew-to-roadway is 5 degrees.

A scour hole 3.5 ft deeper than the mean thalweg depth was observed 250 ft upstream
during the Level I assessment. It was noted that the scour was localized on the right bank
side and due to the presence of an old abutment. Scour countermeasures include the type-3
stone-fill (less than 48 inches diameter) which forms the spill-through slopes of the
abutments. Additional details describing conditions at the site are included in the Level II
Summary and Appendices
D and E.

Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1995). 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.

Computed contraction scour for all modelled flows was zero ft. Abutment scour ranged
from 4.5 to 5.0 ft at the left abutment and 9.6 to 11.4 ft at the right abutment. 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.