BOULDER BAY OUTFALL

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Physical Environment

Landscape

Boulder Bay is a small rocky inlet lying on the south-eastern side of the Port Stephens Peninsular between Fingal Bay and Anna Bay. The bedrock surrounding Boulder Bay is part of the Nerong Volcanic formation which consist of toscanite, dacite, andesite, ignimbrite, agglomerate, conglomerate, sandstone and siltstone (Newcastle Geological Series Sheet – SI 56-2). The type of rock on the rock platform, most of the boulders and the offshore reef is rhyodacitic porphyry (CEE 1987). The coastline in this area has a high relief with steep rocky ridges running down to the sea.

Bathymetry & Substrate

At Boulder Bay the seabed is composed of well-rounded boulders which are tumbled by breaking waves. These boulders extend seawards until a depth of 10m is reached. At this depth the boulders are usually set in a matrix of coarse sand. A much dissected and broken rock sheet extends seaward of this point. In 20 m water depth and deeper, isolated patches of rock surrounded by sand form the seabed. The rocky bluffs, which form the sides of the Bay, become near vertical cliffs below the water surface. At the foot of the cliffs there is a smooth rock sheet which slopes towards the centre line of the Bay and borders the boulder zone (Laurie, Montgomery and Pettit 1977a).

Laurie, Montgomery and Pettit (1977a, b) investigated the sea bed contours, geology and stability of the sea bed (including seismic surveys, test borings, soundings and sediment sieve analyses) at Boulder Bay (prior to outfall construction), Burwood Beach and Belmont Beach (see also Burwood Beach and Belmont Beach outfalls).

Currents, Winds and Waves

- waters (less than 50m water depth) are generally thermally stratified during summer and unstratified during winter, although the timing of stratification can vary from year to year.

- warm surface water is derived from the East Australia Current (EAC) and is usually low in soluble nutrients.

- during summer, surface water movement off the central coast of NSW is controlled by the EAC and local wind.

- southerly moving water passing down the NSW coast generally creates a north-flowing counter current in the large embayment between Port Stephens and Jervis Bay.

- north-flowing limb of this inshore counter current in turn causes eddies in the bays between minor headlands.

- on occasions the EAC flows close to the coast and strong southerly currents are experienced in nearshore waters.

- during winter, there is a slow northerly movement of water past the central coast.

- during periods of slow northerly flow and during periods of low flows in the EAC, local wind dominates water movement on the central coast (Laurie, Montgomery and Pettit 1977a).

- Further oceanographic information - CEE (1987), AWACS (1991a, b).

Boulder Bay

(Boul_5) Last updated May 2000