Amphibious operations can be very sensitive to weather and oceanographic conditions. The offloading of troops and equipment from specialist amphibious shipping to a beachhead involves transfers from ships to landing craft, and from landing craft to the beach itself. Most navies possess a range of relatively small watercraft for use during amphibious operations. Such activities are sensitive to sea state, swell and surf conditions, tidal streams, longshore currents and rips, which must all be assessed and forecast in order to ensure mission success.
Many navies use sea, swell and surf models to predict oceanic conditions in the littoral environment, and hence assess their impact on amphibious operations. Figure 24.4 shows the output from an experimental implementation of the 'Simulating Waves Nearshore' (SWAN) wave model, and the US Navy's 'Surf' model, which displays model output using a Geographic Information System (GIS). The model has been run over North Beach, Cronulla, which is on the east coast of New South Wales to the south of Sydney (Fig. 24.1). Figure 24.4 shows: significant wave height (grey contours) and direction (vectors); significant wave period (blue rasters); littoral currents (closely spaced arrows along the approach to the beach); wave trains (displayed in grey as representative wave crests); and breaker percentage (displayed as green for <1%, amber for 1-15% and red (surf zone) for >15%). This information can be used in the planning phase of an amphibious assault, to compare the suitability of
various beaches for the operation, or to predict conditions at the beach at the time of the assault. Depending on the nature of the assault, a suitable beach may be required to have negligible surf and manageable longshore currents, although a single line of low, spilling surf may be tolerated. The location of the beach centre and approach lanes can also be chosen, using model output of this type, to avoid rips. A knowledge of the location and strength of longshore currents in the boat lanes, at the time of the assault, will help the landing craft crews to make a successful approach and beaching.
The RAN is developing a high resolution forecasting system, called the 'Littoral Ocean Modelling System' (LOMS), which will provide sea, swell and surf predictions at greater resolution and fidelity, and over larger domains, than the SWAN/ Surf implementation described above. It will provide a three dimensional characterisation of the wave conditions, at resolutions in the order of tens of metres.
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