Tsunami Induced Coherent Structures and their impact on our Coastal
Infrastructure
The aim of the proposal is to unravel the complexities involved in how tsunami-induced coherent structures
affect our coastal sediments and infrastructure. We recognize that the coherent structures generated in both
the horizontal and vertical planes have a significant impact on the energy cascade as well as on the dynamics
affecting the neighboring boundaries. What distinguishes this effort is that we seek to examine the transfer
of momentum and energy through the long wave generation, to the coherent structure generation, to the force
applied to boundaries (e.g. bed and harbor sidewalls), and to the response of mobile sediment bed by asking
general questions involving the cascade of energy through multiple scales. This endeavor is transformative
in its coupling of innovative instrumentation and computational tools towards securing understanding of
tsunami induced multi-scale transport. In the end, we seek to significantly advance our ability to resolve and
ultimately constrain the role of coherent structures induced by tsunamis. Insight gained from how coherent
structures affect the neighboring boundaries can be used to mitigate practical concerns regarding mooring
designs, scour of structures, liquefaction of submerged free structures (e.g. underground tanks, previously
buried munitions, etc.), and catastrophic erosion of coastal communities.
Co-PI Patrick J. Lynett will have the primary role of directing the experiments in the Tsunami Wave Basin
(TWB) at Oregon State University, measuring the properties of shallow coherent structures (SCS). The
primary goals of the SCS experiments in the TWB are to:
 Understand the conditions under which SCS can be generated by and subsequently spin-down during
transient wave forcing
 Construct a complete kinematic picture of the SCS free surface, with additional internal point
measurements
 Visualize the 3D flow patterns inside the SCS
 Provide a dataset that can be used to validate numerical models used to predict currents in ports and
harbors
To achieve these goals, a physical configuration will be created in the image of a port entrance - in essence a
detached breakwater. A small-amplitude, long period wave will be generated, and the flow will be
accelerated through the gap in the breakwater. If the flow rate through the gap is strong enough, separated
regions will form, which when coupled with the near-boundary shear layers (along the breakwater) and
transient flow, should lead to the formation of SCS. Numerical simulations using the model of Kim &
Lynett (2011) have been used to approximately design the experimental layout, shown in Figure XX.
This particular application will have the rare property, at least in tsunami experimentation, of using both an
undistorted model spatial scale with a reasonable time scale. A typical port will have channel depths on the
order of 15 m, implying a 1/30 scale with the proposed 50 cm depth in the TWB. The wavemaker will be
retracted slowly through its entire 2 m stroke length over 40 seconds, generating a "drawdown pulse" with a
length of 88 m, or twice the length of the TWB. The amplitude of the 40 second "pulse" will be about 2 cm.
In the prototype scale, this is a "pulse" with length of ~2.5 km, period of about 3.6 minutes, and amplitude of
0.6 m.
Wave gages will be placed in a grid layout, with a grid spacing of 3 m. This will provide an overview of the
long wave motion throughout the entire TWB. Boundaries around SCS generation and drift region will be
outlined with ADVs to provide external forcing information, to provide data on coupling between "external"
tsunami forcing and SCS. Inside the SCS area, shown as “Primary Measurement Area” in Figure XX, a
dense array of measurements will be used to characterize the flow as thoroughly as possible. A grid of
bottom mounted pressure sensors will be constructed, and will provide the data necessary to calculate
horizontal pressure gradients.
Surface and submerged PIV data will be captured. The submerged 2D PIV system will be identical to that
described in the LWF experiments, with the camera mounted at a few positions along the breakwater and
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basin side wall, specifically where the initial SCS is generated and where it encroaches upon the basin side
wall. This data will yield small-scale and detailed information about the near bottom flow in these critical
locations, and with the ADV data and 3D LES numerical simulations, will be used to reconstruct the nearbottom flow field. The surface PIV, however, will comprise the largest measured data component. In the
co-PI Lynett’s previous experimental work in the TWB, a pilot test was completed using a 2D surface PIV
method to measure surface velocities. Ten overlapping field-of-views (FOVs), each approximately 2m by
2m, were recorded by mounting a high resolution camera on the basin bridge. It was found that, while the
data and resulting animations are very effective at visually presenting the flow in a qualitative manner, the
errors introduced by the vertically moving free surface were large enough to make the 2D surface PIV results
difficult to present with precision. However, recently the TWB has obtained a stereo PIV system. This will
allow for the vertical motion of the free surface to be quantified and included in the particle velocity
calculation, thus removing the problems found previously. Here, we proposed to recorded stereo-surface
PIV at the 11 overlapping FOV’s shown in Figure XX. This will provide a truly unique dataset from which
gradients and turbulent characteristics might be calculated with order millimeter resolution across a surface
area of approximately 40 m2.
With this extensive data measurement plan, and the need to extract turbulence information from the data, it is
necessary to run numerous trials of the same experiment (approximately 10-15 from the previous work in the
TWB). Therefore, even with a four month window available to use the TWB, minus build, teardown, and
outreach time, it will likely only be possible to investigate a single incident wave condition. This wave
condition, and the layout of the breakwater, will be refined during the preliminary testing phase in year 2. It
is expected that, even though the physical configuration will be limited in scope, the resulting very detailed
dataset when combined with the proposed 3D LES modeling will yield a unique platform to both understand
the physics of tsunami-induced SCS and provide a dataset for benchmarking the next generation of smallscale 3D models.
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