Wilson Inlet 6
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Managing the bar and the Inlet
Introduction
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* Mean sea level is roughly 0 m AHD (Australian Height Datum).
† Note 1 GL = 1000 ML, 1 ML = 1000 kL, 1 kL = 1000 litres. |
The swell on the south coast is predominantly south-westerly throughout the year. Winds are from the south-west, south, south-east and east in summer, and are from west and north-westerly directions in winter. Wilson Head provides significant protection from the predominant swell waves and from south-westerly seabreeze and storm waves. The eastern end of Ratcliffe Bay, out of the lee of Wilson Head, is more exposed to the south-west and receives more wave energy than the western end. The headland provides no protection from south-easterly sea-breeze waves for any part of the bay.
The mouth of the Inlet is completely blocked by a sand bar for half of the year (usually about February to July). The bar is about 150 m wide between Inlet and ocean and has a length of 500 m in a north-east to south-west direction. To the west of the mouth are limestone cliffs while to the east of the mouth lies the coastal dunes of the Nullaki Peninsula. The tip of the Nullaki Peninsula appears to be growing westward as the sands at the eastern end of the bar are stabilised by vegetation.
Figure 2: Major features at the mouth of Wilson Inlet.
The sand bar is a dynamic feature that is naturally built and eroded as the balance between processes depositing and scouring sand in the mouth of the Inlet changes over the year. The highly seasonal flows of rivers to the Inlet (approximately 80% of the river flow occurs from July to October) and the small tidal range on the south coast result in limited scouring capacity through the mouth of the Inlet for most of the year. On the other hand, despite the shelter of Wilson Head, the relatively intense wave climate on the south coast results in deposition of sand in the mouth of the Inlet by wave generated currents. The bar builds to about 1.8 m above AHD and is composed of medium to coarse grained marine sands.
Behind the sand bar is a large, shallow, flood tide delta that extends from the mouth back about 2 km into the Inlet. Like the bar, the flood tide delta consists of marine sands that have been washed by waves and blown by winds back into the Inlet over hundreds of years. Flows following bar opening have scoured several channels though this delta. These channels are unstable and fill or scour according to the pattern of flow when the bar is open. The channels at the back of the delta, closer to Poddy Point, are older while those at the front, closer to the bar, are more active and dynamic in their alignment.
The Wilson Inlet sand bar has been artificially opened each winter since the 1920s to limit flooding of low lying lands adjacent to the Inlet. Once the Inlet water level reaches 1.01 m above AHD, the bar is breached by cutting a channel through it with an excavator. This breach water level was originally derived from a point on the old railway bridge that was later determined to be 1.01 m above AHD.
Some effort is made to time the breaching with the lowest water level on the ocean side (so as to maximise the difference between Inlet and ocean water levels and consequently maximise the flow velocities and scour). However circumstances do not always make this possible.
Note that a breach of 200 m or less from the limestone cliffs west of the bar is called a 'western opening' and a breach 300 m or more from the limestone cliffs west of the bar is called an 'eastern opening'.
