Browne, A 2011, 'Coarse coastal deposits as palaeo-environmental archives for storms and tsunamis', PhD thesis, Southern Cross University, Lismore, NSW.
Copyright A Browne 2011
In coastal environments the history of evolution is stored in landforms and sediments. Therefore, geomorphology and geology (in particular sedimentology) are the most relevant disciplines for analysing this history. From landforms and sediments, scientists draw conclusions about processes, their energy, combinations and sequences. It seems logical, therefore, that the strongest processes and most significant single events are the ones which will be best preserved, or at least that they will dominate the geomorphologic and sedimentologic data.
My hypothesis is that coarse coastal deposits, and in particular large boulders, not only have the greatest potential to be preserved of all coastal sediments, but also deliver the best means of calculating the transport energy of a single wave. The size and weight of boulders, the distance they have moved against gravity, and the horizontal distance they have been moved all give hints about the ways in which they were transported and enable physical equations to be used to calculate the energy that was required to move them. Using observations and modelling, it should be possible to determine the amount of energy required to transport a boulder of a specified mass, and whether the necessary energy could have been delivered by extreme storm waves with a very short impact (about 0.2 sec), a height at the coast of rarely more than 10–12 metres, and a limited velocity (of 8–9 metres/sec at the most), or only by tsunami waves with long-lasting (minutes or tens of minutes) flows of huge water masses with velocities which can be more than 20 metres/sec and with significant flow depths.
As energy arises from a combination of mass and velocity it should be evident that the difference in the transport power of storm and tsunami waves may well be in the order of one to many thousands. Field observations and conclusions, when interpreted using the theoretical framework hitherto developed for the boulder transport problem, can at least identify the sizes of those boulders which could never have been moved by storm waves against gravity but only by strong tsunamis. This will give a field instrument to identify possible palaeo-tsunami deposits. With a lot of objective field evidence in mind (for example the size, weight, position inland and height above sea-level of boulders, the kinds of surfaces on which they have been moved, and the kinds of movement which their form and preservation show they have transported by), I and co-authors of Benner et al. (2010) have developed some general and basic principles of the physics of boulder transport by shallow (tsunami) and deep (storm) water waves. The results are consistent with empirical data from coastal engineers and the observation of boulder movements caused by hundreds of strong storm impacts.