Seabed Sediment Transport and Offshore Pipeline Risks in the Australian Southeast

Abstract

The Australian seabed is influenced by extreme weather conditions of various types: cyclones, high tidal ranges, offshore currents and storm waves. In the past two centuries substantial progress in our understanding of the seabed and environmental conditions has been made by studies of the seabed sedimentology, hydrodynamics, and through habitat mapping. As part of the CSIRO Wealth from Oceans Flagship program the authors are involved in a new fiveyear study to investigate and predict the effect of possible climate change scenarios on the seabed in the next 50 years. Those changes will assess undersea infrastructure installations such as pipelines in a changed regime of seafloor stability, burial, erosion and abrasion.As an initial phase of this project the authors have extrapolated the current climatic conditions into the next 50 years. It was found that (under an extension of present day climatic conditions):the majority of terrigenous sediment carried down by major rivers (Murray River, Snowy River, Tamar River, etc) is trapped in the inland lakes or estuaries. Only a marginal fraction of fine grain sediment reaches the continental shelf;a high energy wave climate, significant tidal currents, and the frequent surges of wind-driven currents make the local seabed highly mobile and sensitive to the modern hydrodynamic changes. Both results imply that an exposed pipeline may suffer from local scour and fatigue damage due to oscillatory loads induced by vortex shedding behind the pipelines; andthere are several high risk zones in the region where turbidity current and submarine slope failure post a great threat to offshore pipelines.The next phase of the project will be to provide testable predictions of the changes under a variety of global warming scenarios.

Transformation of dense shelf water cascade into turbidity currents: insights from high-resolution geophysical datasets

“…Therefore, when seawater leaves the Bass Strait on its eastern side, it has a prominent density contrast against the Tasman Sea water (Tomczak, 1985). As a consequence, warm, denser Bass Strait seawater can flow into and sink beneath the cooler, fresher water of the Gippsland shelf, generating the northeast flowing "Bass Cascade Current" which sinks to the 200-400 m isobaths and extends more than tens of kilometres (Figure 2B; Godfrey et al, 1980;Li et al, 2005;Mitchell et al, 2007b). Observations from the ocean bottom stations have revealed that the BCC has transported significant quantities of water and spreads along the shelf edge over a long distance (Boland, 1971).…”

“…The Bass Cascade Current (BCC) is a high-energy, seasonal (especially in winter) phenomenon. When it flows through the Bass Strait, it is further fed by the Leeuwin current (LC), Zeehan current (ZC) and the wind stress within the Bass Strait, jointly transporting Bass Strait water towards the front (Li et al, 2005;Mitchell et al, 2007b).…”

“…In the Gippsland Basin, the central continental shelf is dominated by the Westerly wind throughout the year (see Figure 2B; especially in winter; Li et al, 2005). The eastwardflowing Westerly wind flows at 10-30 km/h with maximum gusts reaching 100 km/h.…”

“…The East Australia Current (EAC) is a western boundary current that carries warm equatorial waters and flows southward adjacent to the Australia's southeast coast (Figure 2B,2D). It is up to 500 m deep and 100 km wide, occasionally extending far enough south to reverse the movement of water in the Gippsland Basin during summer months (Li et al, 2005).…”

Complex seabed geomorphology shaped by various oceanographic processes: a case study from the Gippsland Basin, offshore south-eastern Australia

“…The Southern and Central regions of the Gippsland Basin are influenced by the seasonal Bass Cascade Current (BCC) (Figure 1A, 1C;Mitchell et al, 2007b). The BCC was formed due to the cold, denser Bass Strait seawater flowing into and sinking beneath the warmer, fresher water of the Gippsland shelf, generating a northeast flowing current and sinking to the 200 m isobath (Godfrey et al, 1980;Li et al, 2005;Mitchell et al, 2007b). The BCC is a high-energy current with an average transport rate of 3.6 km 3 /h (Godfrey et al, 1986).…”