Geological indicators of seabed mobility – Narrabeen Beach (Sydney, Australia)

Stratigraphic cross section off Narrabeen beach based on vibrocore and drilling data (Hudson and Roy, 1988)

There are many gems hidden in the unpublished historic literature. They reflect our knowledge from decades ago but are just as relevant to our understanding of shoreline evolution and coastal processes today. One such report, prepared for the Australian and Overseas Telecommunications Corporation in 1988, is ”Seabed mobility in south-eastern Australia – the Narrabeen Beach embayment”.

Technical reports prepared for clients serve their immediate purpose and are soon forgotten, sometimes to resurface later when the report authors secure the agreement of a client to publish sought after research material.  The following information is extracted from a report prepared for the Australian and Overseas Telecommunications Corporation over 30 years ago and is ’published’ here for the first time. See full report here.

In the early 1980’s, an international submarine communications cable was to be installed offshore of Sydney. At the time, most existing cables terminated at Bondi but to bring a new cable onshore an alternative site on the northern side of Sydney Harbour was sought and detailed geotechnical investigations required

At the time, marine geological studies of the coast and inner continental shelf in the vicinity of Sydney included beach profiling, surface sediment sampling, detailed bathymetric surveys, marine seismic profiling, side-scan sonar mapping, vibrocoring and drilling. The work had been carried out primarily by the Geological Survey of NSW, the Coastal Engineering Branch of the Department of Public Works, the Department of Geography at the University of Sydney and the School of Applied Geology at the University of New South Wales. A team of researchers from the the Department of Geography at the University of Sydney and Geological Survey of NSW oversaw the vibrocoring program at Narrabeen in the late 1980’s summarised here.

Vibrocoring of a proposed submarine cable route offshore of Narrabeen Beach was completed in the late 1980’s and reported by Hudson and Roy (1988).  The report was prepared for the Australian and Overseas Telecommunications Corporation and remained ’unpublished’ until permission was given for the report’s authors to share the results of the work with other researchers. Amongst other things, the report included detailed core logs and radiocarbon dates of core samples to develop estimates of seabed disturbance based on geological indicators.

Geotechnical Results

Ten vibrocores were obtained from the sea bed along the line of the main bedrock paleo-channel (see Figure) which extended east of Narrabeen beach (see stratigraphic section). A number of vibrocores and auger holes had been drilled in Narrabeen Beach and Narrabeen Lake and included in the stratigraphic reconstruction. The maximum penetration of the offshore cores was 5 m and most cores were between 3 and 4 m long. All cores penetrated a relatively thin layer of ”modern” marine sediments overlying older dune or estuarine deposits. Their generalised lithologies are included in the stratigraphic reconstruction which shows a shore normal cross section through the line of cores and one of the auger holes drilled on Narrabeen Beach.

The inshore sea bed to water depths of about 32m, around 3 km offshore, has oxidised dune sand in the subsurface. This relict unit (which possibly includes some estuarine deposits) extends landwards beneath Narrabeen Lake and is thought to be in excess of 25,000 years old based on the available radiocarbon dates (see radiocarbon dates in stratigraphic reconstruction). Modern sediments of Holocene age – beach, nearshore and inner shelf sands – overlie the relict dunes. In vibrocores N6, 9, 12, 15, 20 and 25 the contact between the modern and relict deposits is erosional. Holocene sediments reach considerable thicknesses beneath Narrabeen beach where they infill a palaeo-river valley incised at lower sea levels. Their mobile, upper-most part is about 5.5 m thick thinning to less than 0.3 m further offshore.

Seabed Mobility

The geological indicators of seabed mobility (ie. a sharp discontinuity between modern shoreface sediments and the underlying older dune and estuary deposits) show a maximum bed erosion of 1.9 m in 6 m water depth, decreasing to 0.3 m or less in water depths greater than 20 m. The zone of greatest bed mobility (> 1 m) corresponds to the beach face and the offshore bar(s).

Modern day assessments of sea bed mobility – Narrabeen Beach

A comparison of the geological indicators of seabed mobility can be made with over four decades of coastal monitoring at Narrabeen-Collaroy Beach (see Harley et al., 2015) which provides a unique insight to beach fluctuations (subaerial beach sand volumes and orientation) and change. The monitoring relies on traditional survey methods, and in the past decade new survey technologies such as RTK-GPS, coastal imaging, LiDAR and UAV technology. Recent investigations (see Harley et al in review) suggest extreme storms can have a positive contribution to beach and nearshore sediment budgets by exchanging sediment between the lower and upper shoreface and/or between adjacent headlands, potentially mitigating adverse SLR impacts. 

Harley et al (in review) comment that ’complementing the continuous subaerial beach measurement records were detailed three-dimensional surveys of the entire upper shoreface at time intervals prior to, immediately following and approximately 12 months after an extreme storm event or extended storm cluster……each three-dimensional survey extended from the upper beach to beyond the theoretical depth of closure over the respective storm sequences, calculated as -11.6 m relative to mean sea level at Narrabeen’. The level of seabed change is inferred to be ’.…at lower depths, profile variability displays the characteristic ‘pinching’ towards the theoretical depth of closure  that is typical of the upper shoreface, although seabed variability outside of the survey error (~?0.14 m) is still evident at these lower depths.’

Linking modern day assessments of seabed mobility with geological indicators

The remote monitoring of seabed elevation changes to be published by Harley et al (in review) appear consistent with the geological indicators of seabed mobility at Narrabeen as determined from geological indicators. Seabed elevation changes are  both at a scale of decimetres.

Merging of the two types of data, geological indicators and contemporary high resolution monitoring, can serve to provide better understanding of long term coastal behaviour which builds confidence in projections of shoreline change in response to climate change drivers (i.e. sea level rise, increased storminess, changing wave direction).

What unpublished reports with high quality data do you have which could be shared and used by today’s researchers?