Ice Force and Scour Instrumentation
for the White River, Vermont
LEONARD J. ZABILANSKY
nor does it allow correlation of scour with the
INTRODUCTION
hydrograph.
Design criteria for bridge piers subjected to
The combination of bed scour undermining
ice forces that result from ice jams are very con-
the pier stability and ice forces can be respon-
servative. There are currently few alternatives to
sible for bridge collapse, as happened on two
such a conservative approach, since we lack full-
occasions in White River Junction, Vermont. This
scale field data as well as a clear understanding
report describes research efforts currently un-
of icestructure interaction. In addition the place-
derway to develop instrumentation for in-situ
ment of any obstruction such as a bridge pier in a
measurements of ice forces on structures and
movable riverbed may induce bed scour adja-
monitoring of bed scour during extreme events,
cent to that structure, and this may be acceler-
especially during ice breakup.
ated during an ice jamming or flooding event.
Local scour occurs in movable-bed or gravel-
bed rivers when the near-bed water velocity in-
BACKGROUND
creases either because of an increase in discharge
during open water flood events or when an ice
The White River in Vermont drains a rela-
sheet or ice jam causes the velocity profile to
tively small watershed, which, combined with
shift toward the bed. As the near-bed velocity
the steep bed slope, results in dramatic ice and
increases, the bed starts eroding around bridge
flooding events. The town of White River Junc-
pier footings, buried utility crossings, etc., un-
tion is located at the confluence of the White
dermining the structure or utility. Once flow con-
River with the Connecticut River (Fig. 1). On
ditions start to subside and velocities decrease,
March 6, 1964, during spring breakup, two spans
fine-grained, non-structured sediment is rede-
of the Bridge Street bridge were pushed off the
posited in the scour hole, camouflaging its depth.
pier by ice forces acting on the superstructure as
During the next event, this fill is quickly removed
well as shearing off the upper portion of the
and scour resumes.
stone masonry pier. Since the bridge is a vital
Because of the redeposition of sediment in the
transportation link, it was quickly repaired us-
scour hole, sounding rods which are typically
ing recycled bridge beams. The beams were ap-
used to profile scour do not define the total depth
proximately 50 ft long, requiring four spans and
of scour. Irregular geophysical density profiling
three piers to fill the gap. Each temporary pier
can define the overall scour boundary but does
was designed using nine 45-ft Douglas fir timber
not allow the correlation of scour depth with
piles driven in line along the centerline of the
particular past events. In particular, it remains
pier. The piers were oriented parallel to the flow.
unknown which of open water events or ice
From the construction drawings, the tops of the
breakup and ice jam events cause the most se-
piles appeared to be 29 ft above the riverbed. At
vere scour. Documenting the depth of scour dur-
the bridge alignment the bedrock is overburdened
ing an event has been nearly impossible, and
by 90 ft of gravel, and the 45-ft piles penetrated
typical post-event documentation usually does
16 ft into the gravel bottom. To protect the ex-
not measure the actual maximum depth of scour,
posed piles from abrasion by ice and debris, the
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