Abstract:
Tsunamis are extreme events triggered by earthquakes, landslides or meteors. A large-scale
tsunami can cause devastating damage to coastal structures and claim massive casualties in
densely populated areas. Intensive research has been conducted to investigate tsunami
propagation and tsunami loads on coastal structures (buildings, bridges, breakwaters).
However, flow characteristics in bays induced by tsunamis still need thorough research. The
stability of composite breakwaters has not been thoroughly researched. The mitigation effect
of barriers lacks in-depth study. The research about loads on bridges due to tsunami bores is
lacking.
This study aims to investigate the tsunami propagation in a bay and the influence of local
macro-roughness (sand bar), the stability of composite breakwaters under tsunami waves,
tsunami loads on bridges and the mitigation of tsunami loads with barriers. One numerical
study and three experimental studies were conducted.
In the numerical study, non-linear shallow-water equations were applied to study the flow in a
V-shaped bay due to the 2016 Kaikoura tsunami, New Zealand. The flow characteristics in the
bay were sensitive to the incident wave angle. Based on the bathymetry of the bay, idealised
models were set-up to investigate the flow amplification and the influence of a sand bar across
the bay. The sand bar with a relatively low crest height was ineffective in reducing the
maximum velocity and the maximum momentum flux near the shore.
To study the stability of composite breakwaters under tsunami attack, a series of experiments
were conducted in a tsunami flume connected to a reservoir. To simulate a tsunami bore, water
was released rapidly from the reservoir to the channel. Scale models of typical breakwaters of
different heights and vulnerable to tsunami attack were constructed in the flume. Higher
breakwaters can resist a stronger bore impact and resist initiation of damage for longer.
Tsunami loads on slab bridges were tested with a range of tsunami bores and pier heights.
Pressures on the front surface of the bridge deck, horizontal and vertical forces, and overturning
moments were analysed. The overturning moments are closely related to the flexural failure of
bridges in tsunamis. Flow interaction with the bridge was assessed, and empirical equations
were proposed to estimate the loads. For the mitigation of tsunami forces, experiments were conducted on the flow impact on a wall
with and without the protection of a barrier. If it is not high enough, the barrier can increase
forces on the wall rather than decrease them. Predictive equations are proposed for reduction
ratios of horizontal forces and overturning moments. Four methods to calculate the loads on a
vertical wall protected by a barrier under tsunami bore impact have been analysed and assessed.
This study provides preliminary design guidance for armour units in composite breakwaters,
slab bridges, and for barriers to mitigate tsunami forces. Design suggestions and
recommendations for future research are summarised at the end of the thesis.