Engineers Australia

Marine geophysical investigations of palaeo-drainage systems in the Hawkesbury River Estuary, New South Wales, Australia

R. J. Whiteley and S. B. Stewart

Abstract

The Hawkesbury River is a key element in a major river system in eastern Australia. The river and its tributaries virtually encircle Sydney’s metropolitan area, extending northward to the Pittwater and Brisbane Water embayments and entering the Tasman Sea at Broken Bay, some 35 km north of Sydney Harbour. Since the 1960’s marine geophysical techniques, principally seismic reflection, supported by land gravity surveys have revealed extensive and deep palaeodrainage systems incising the underlying sedimentary rocks mainly beneath the River and its tributaries. These are masked by considerable thicknesses of recently deposited sandy sediments.

Case studies from three recent infrastructure and research projects, completed near the mouth of the Hawkesbury River system demonstrate the application of marine seismic and gravity technologies in the mapping parts of this palaeodrainage system. These projects are within the maritime zone of the Hawkesbury River. In this zone the Hawkesbury River estuary is a drowned river valley within steeply incised gorges surrounded by dissected plateaus. The terrain is dominated by the sandstone geology with an extensively dissected and generally rugged landscape.

Installation of a wastewater transfer main beneath the Hawkesbury River between the then unsewered Dangar Island and Brooklyn on the mainland was required. This involved a 1400 m long directional bore beneath tidal mud flats and a deep tidal channel. The marine geophysics mapped the bedrock profile, identified a fault and strong seismic reflectors within the bedrock near the centre of the palaeochannel at about 45 m depth. These were interpreted as regions of stress concentration in the Newport Formation created by valley bulging processes following rapid erosion. The geotechnical model inferred from these investigations was applied in the design of the directional drilling operation that was successfully completed in rock. This upgraded sewer system is now in operation and has removed a significant pollution source from the Hawkesbury River.

Upgrading of the electricity supply from Wagstaffe to Booker Bay required installation of an 11kV power cable across Brisbane Water, a distance of 630 m. Previous regional gravity surveys in this area had identified a deep palaeodrainage system beneath the Woy Woy and Ettalong peninsulas. A marine seismic reflection and refraction survey along the proposed crossing confirmed the presence of a palaeochannel margin extending to about 25 m below the seabed. The conduit was subsequently successfully installed by horizontal directional boring up to 30 m below sea bed.

Development of an airborne electromagnetic system for bathymetric mapping and sea-floor characterisation required independent calibration using marine geophysics within Broken Bay. A broad and deep channel representing a high energy palaeo-fluvial drainage system in the Hawkesbury River outreaches was identified. This extended to approximately 80 m depth below river level and was somewhat shallower than indicated by previous studies suggesting that there may be some uncertainty in seismic bedrock depth possibly due to the dense basal sediments. Also in another nearby area a dendritic fluvial pattern extending to approximately 70 m depth was observed. A moderately narrow palaeochannel extending to 90 m depth either side of the Palm Beach tombolo was also clearly identified.