Final Report Summary - URBANWAVES (Urban Waves: evaluating structure vulnerability to tsunami and earthquakes)
Since the 2004 Boxing Day tsunami in the Indian Ocean (December 2004), and even more so since the Tohoku tsunami (March 2011), governments have shown greater awareness of the threat of coastal flooding and/or erosion by tsunami, and the need for more robust strategies and structures to evacuate and/or protect populations. Essential to these processes are an improved understanding of tsunami from near-shore (where conventional Shallow-water Wave Equation models may give reasonable predictions of behaviour) to onshore where data are very sparse and model reliabilities are highly uncertain. Tsunami are generally characterised by small wave heights in deep water but very long wavelengths. The latter feature provides a major challenge to the simulation of tsunami in a laboratory environment, resulting in a lack of knowledge on the interaction of tsunami with beaches and coastal structures.
The URBANWAVES project arose from the conviction that none of the historical attempts to reproduce tsunamis in the laboratory had been realistic. There had to be a better way! URBAN WAVES brought together expertise at UCL EPICentre and HR Wallingford to develop a unique laboratory device, the Pneumatic Tsunami Simulator (TS). This device can recreate stable depression-led and elevated waves to provide realistic representations of tsunamis. Within URBAN WAVES, experiments were carried out with the TS to measure tsunami impact on coastal defences, forces on single buildings and arrays of structures, and for the first time, scour around simple structures from tsunami inundation. These experiments have led to an improved understanding of tsunami interaction with beaches and coastal structures. Surprising observations include: 1. That tsunami do not shoal (increase in height) over beach slopes that are short compared to their wavelengths, 2. that the scour hole that is observed after a tsunami may not give a good representation of the maximum scour that the structure has experienced during the tsunami inundation and 3. that tsunami inundation imposes a quasi-steady loading on structures it interacts with. In particular, the latter observation has informed the development of simplified relationships for estimating tsunami forces from knowledge of basic tsunami flow characteristics, which are suitable for inclusion in codes of practice. These experiments have also been used to develop simple computation fluid dynamics models for estimating tsunami forces on buildings accounting for complex topography.
In parallel to the experiments, new approaches have been developed for the structural analysis of buildings subjected to tsunami inundation. Fundamental structural mechanics has been used to demonstrate that, due to the quasi-static and long duration of the loads imposed by tsunami flows on buildings, structures must rely only on their strength (and not their ductility, i.e. their ability to deform plastically) for resisting imposed tsunami forces. Essentially, if a structure experiences yield under a tsunami, it will collapse. These findings help set performance objectives for the design of structures. URBANWAVES has also developed a new non-linear static analysis technique, called the variable Depth Pushover (VDPO) for the structural analysis of buildings under tsunami inundation. The VDPO is proven to reproduce well the tsunami response of buildings and is computationally efficient, as it assesses the response structures under tsunami of different sizes (avoiding the need to repeat analyses for different tsunami).
Finally, we have investigated the response of structures subjected to earthquakes followed by tsunami. Following thousands of numerical analyses of structures subjected to different earthquake and tsunami pairs, the first fragility surface (tool for assessing the damage to structures under the sequential hazards) has been developed. The analyses also show that the preceding earthquake damage has a very limited effect on the response of the building under the subsequent tsunami. This is mainly due to the significant difference in loading type and patterns of the two hazards.
An aim of URBANWAVES is to make all the developed codes and tools freely available, so that they can be used to assess coastal structures, and estimate the level of physical damage and resulting economic and life loss to be expected in future tsunami events.
Further information is available at the following website: https://www.ucl.ac.uk/epicentre/research/engineering-for-natural-disasters/Urban_Waves
The URBANWAVES project arose from the conviction that none of the historical attempts to reproduce tsunamis in the laboratory had been realistic. There had to be a better way! URBAN WAVES brought together expertise at UCL EPICentre and HR Wallingford to develop a unique laboratory device, the Pneumatic Tsunami Simulator (TS). This device can recreate stable depression-led and elevated waves to provide realistic representations of tsunamis. Within URBAN WAVES, experiments were carried out with the TS to measure tsunami impact on coastal defences, forces on single buildings and arrays of structures, and for the first time, scour around simple structures from tsunami inundation. These experiments have led to an improved understanding of tsunami interaction with beaches and coastal structures. Surprising observations include: 1. That tsunami do not shoal (increase in height) over beach slopes that are short compared to their wavelengths, 2. that the scour hole that is observed after a tsunami may not give a good representation of the maximum scour that the structure has experienced during the tsunami inundation and 3. that tsunami inundation imposes a quasi-steady loading on structures it interacts with. In particular, the latter observation has informed the development of simplified relationships for estimating tsunami forces from knowledge of basic tsunami flow characteristics, which are suitable for inclusion in codes of practice. These experiments have also been used to develop simple computation fluid dynamics models for estimating tsunami forces on buildings accounting for complex topography.
In parallel to the experiments, new approaches have been developed for the structural analysis of buildings subjected to tsunami inundation. Fundamental structural mechanics has been used to demonstrate that, due to the quasi-static and long duration of the loads imposed by tsunami flows on buildings, structures must rely only on their strength (and not their ductility, i.e. their ability to deform plastically) for resisting imposed tsunami forces. Essentially, if a structure experiences yield under a tsunami, it will collapse. These findings help set performance objectives for the design of structures. URBANWAVES has also developed a new non-linear static analysis technique, called the variable Depth Pushover (VDPO) for the structural analysis of buildings under tsunami inundation. The VDPO is proven to reproduce well the tsunami response of buildings and is computationally efficient, as it assesses the response structures under tsunami of different sizes (avoiding the need to repeat analyses for different tsunami).
Finally, we have investigated the response of structures subjected to earthquakes followed by tsunami. Following thousands of numerical analyses of structures subjected to different earthquake and tsunami pairs, the first fragility surface (tool for assessing the damage to structures under the sequential hazards) has been developed. The analyses also show that the preceding earthquake damage has a very limited effect on the response of the building under the subsequent tsunami. This is mainly due to the significant difference in loading type and patterns of the two hazards.
An aim of URBANWAVES is to make all the developed codes and tools freely available, so that they can be used to assess coastal structures, and estimate the level of physical damage and resulting economic and life loss to be expected in future tsunami events.
Further information is available at the following website: https://www.ucl.ac.uk/epicentre/research/engineering-for-natural-disasters/Urban_Waves