Evaluation of pier-scour measurement methods and pier-scour predictions with observed scour measurements at selected bridge sites in New Hampshire, 1995-98

In a previous study, 44 of 48 bridge sites
examined in New Hampshire were categorized as
scour critical. In this study, the U.S. Geological
Survey (USGS) evaluated pier-scour measurement
methods and predictions at many of these
sites. This evaluation included measurement of
pier-scour depths at 20 bridge sites using Ground-
Penetrating Radar (GPR) surveys. Pier scour was
also measured during floods by teams at 5 of
these 20 sites. At 4 of the 20 sites, fixed instruments
were installed to monitor scour.
At only one bridge site investigated by a
team was any pier scour measurable during a
flood event. A scour depth of 0.7 foot (0.21 m)
was measured at a pier in the channel at the State
Route 18 bridge over the Connecticut River in
Littleton. Measurements made using GPR and
(or) fixed instruments indicated pier scour for six
sites. The GPR surveys indicated scour along the
side of a pier and further upstream from the nose
of a pier that was not detected by flood-team
measurements at two sites.
Most pier-scour equations selected for this
examination were reviewed and published in
previous scour investigations. Graphical comparison
of residual pier-scour depths indicate that the
Shen equation yielded pier-scour depth predictions
closest to those measured, without underestimating.
Measured depths of scour, however,
were zero feet for 14 of the 20 sites. For the
Blench-Inglis II equation and the Simplified
Chinese equation, most differences between
measured and predicted scour depths were within
5 feet. These two equations underpredicted scour
for one of six sites with measurable scour. The
underprediction, however, was within the resolution
of the depth measurements.
The Simplified Chinese equation is less
sensitive than other equations to velocity and
depth input variables, and is one of the few
empirical equations to integrate the influence of
flow competence, or a measure of the maximum
streambed particle size that a stream is capable of
transporting, in the computation of pier scour.
Absence of a flow-competence component could
explain some of the overprediction by other
equations, but was not investigated in this study.
Measurements of scour during large floods at
additional sites are necessary to strengthen and
substantiate the application of alternatives to the
HEC-18 equation to estimate pier scour at
waterway crossings in New Hampshire.