auSEABED Seabed Stability under Storms  - Benthic 'Creature Weather' in Bass Strait, Australia Animated maps show the modelled benthic disturbance  under a week-long severe winter storm

Tanya Schindler, Coastal Studies Unit, University of Sydney Chris Jenkins Ocean Sciences Institute, University of Sydney (Now: INSTAAR, University of Colorado) Alfons Lemm, Dave Duncalf WNI Science & Engineering (WNISE), Perth, WA Cameron Buchanan Australian Geological Survey Organization (AGSO), Canberra, ACT

Wave data was provided by WNISE, where wave heights had been modelled on an hourly basis for one week over an 11km grid covering Bass Strait. That modelling was based on wind data from the National Center for Environmental Protection (USA) with validation of results from a monitoring station used by the Bass Strait oil/gas industry. Other inputs included point sediment-attributes from the auSEABED database at Sydney University, 1 nm gridded bathymetry from AGSO and oceanographic inputs from the RAN Australian Oceanographic Data Centre (AODC). The data sets were re-gridded to the wave grid dimensions and sediment types were transferred to motion thresholds using the Shields Criterion with appropriate viscosities and grain densities.

Bottom orbital velocities (in the absence of currents) were computed using Airy Linear Wave Theory and Stokes Second Order Wave Theory (for shallow waters).  Three types of GIF animation were then constructed drawing on both methods:

Maximum sediment disturbance occurred in shallow water areas of Bass Strait, primarily in the lee-sides of islands and elsewhere in the more sheltered waters. Elsewhere, sediment textures and consolidations are apparently adjusted to peak storm conditions in the strait or the water depths were beyond the influence of wave induced motions.

The unusually high degrees of disturbance in the most sheltered waters at first appeared to be a strange result but was checked and verified. The explanation appears to be that in those areas, the sediments deposit during late waning wave conditions and also are not transported away at peak roughness. Having been deposited under late and weak wave conditions they are then very prone to re-mobilization from early in the next storm wave build-up, especially if the deposits create shallow drifts. There are certain similarities to beach sediment behaviour.

The project's successful outcome will be used to refine the modelling, particularly in treatment of shallow water wave behaviour, erosion thresholds, seabed roughness and wave-current interactions.

	<< Water Depths Compiled and gridded by  Cameron Buchanan of AGSO (10m intervals 0..100m,  then at 200,500,1000,2000,4000m) Sediment Texture >> Compiled and gridded by  Chris Jenkins of OSI (2 phi intervals  from dk red gravel -6..-4 phi  to dk blue mud 6..8phi)
	<< Wave Heights (Hs) Modelled by Alfons Lemm of WNI 1 week starting 20 May 1998,  1 hourly frames, 11km gridding	Also available were weekly  data on wave periods and  directions as modelled by  Alfons Lemm of WNI

<< Bottom Orbital Velocity (Umax) - the speed of water flows  across bottom Motion Excess (Uexcess) >> - excess in bottom water velocity  above the threshold  of sediment movement (an index of disturbance)

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Project done at School of Geosciences,
The University of Sydney