Marmion Marine Park management Plan - Department of ...

CirculationWater movement on the inner continental shelf off Perth is driven mainly by wind stress; the presenceof a regional current is also evident during calmer periods (Hearn, 1983). Water generally flowsnorthward in summer and southward in winter. In the shallow, nearshore waters of the Park area, thelocal wind driven currents become more dominant relative to the regional water movements. Meancurrent speeds in the range of 0.05-0.1 m/s are typical (Steedman & Associates, 1976).The complex chains of reef within, and adjacent to, the area act as partial barriers, restrictingexchange between inshore and offshore waters. Under stable wind conditions, local circulationpatterns are established as a result of the interaction between the wind stress forces, and sea floortopography. For example, during prolonged easterly winds, shallow water over Lal Bank is drivenwestward, inducing circulation that replaces nearshore water with offshore water (Hearn, 1983).Wind-induced circulation is an important flushing mechanism for the Park area, and may also affectthe water temperature regime experienced by reef-dwelling communities. During prolonged calmperiods, some flushing of the area is still maintained by the regional current.Water replacement time in the Marmion Lagoon is estimated to be of the order of one day (Hearn,1983), although under extended calm conditions, replacement may take up to four or five days.Tide and Sea ChangesThe Wanneroo coast generally experiences one astronomic tide per day, though barometric pressure,prevailing wind direction and seasonal changes are also responsible for changes in water level. Thenormal tidal range is about 0.5 m, though the range in water levels, during a year due to all factors, isof the order of I m. Future planning must consider possible sea level changes as a result of'Greenhouse' predictions.ActionRecognise the likely effects of high winds and associated high energy waves on the coastline, and on any future developments in the Park.13.0 GEOLOGY AND GEOMORPHOLOGYThe Park area is underlain by Tamala limestone, which is covered partially by yellow quartz sand andthe younger carbonate-rich Becher and Safety Bay Sands. Tamala Limestone was deposited duringthe Pleistocene period (1.5 - 1.8 million years ago) as a series of parallel beach and dune sand ridges.Since deposition, these sediments have cemented into a porous limestone that incorporates solution25