Each ANZAC Day I am reminded of links between military exploits and coastal geography. This year my attention was again drawn to the incredible exploits of the US Marines and their landings in the Pacific during WW2. Early in the war these amphibious landings were conducted at a time when the Japanese Navy possessed both naval and air power to counter the efforts of the Americans and their allies including Australia. The one location where I have personal experience is on the west coast of the island of Bougainville, along the shores of Empress Augusta Bay.
The Americans were looking to establish an airbase in late 1942 within striking distance for their bombers to attack the Japanese stationed at Rabaul. They selected Torokina at the north end on Empress Augusta Bay, in what they called Operation Cherry Blossom. Although there was limited information about the site with only some old German bathymetric maps as guidance, the beach site offered what was thought to be a place where there would be limited opposition to landings as the main Japanese force was located elsewhere on Bougainville. On 1 November, 1942, the 1st Battalion, 3rd Marine Division, successfully landed although a small enemy contingent did manage to inflict casualties. With the beachhead secured, an airfield was made operational by 9 December. As island to island engagement continued in the Pacific war, the marines were replaced by an Australian force in 1944 leading to protracted and politically contentious mopping up operation until the abandoned Japanese force surrendered in 1945.
In 1975, I was invited by the mining company, Bougainville Copper (BCL), to investigate the impact of their operations on the coast at the mouth of the Jaba River. This river flowed into the Bay south of Torokina. Of course I got interested in the war history of that section of coast although it was apparent there was little in the way of war relicts to be seen. After an initial clean up, the jungle proved voracious in covering the site. So I started looking at the impact of tailings from a huge pit at Panguna in the hills to the east that were sluiced down the river.
BCL had several interests in the river and the delta it was creating. One was the magnitude of sand and fine sediments that they were transporting and just how that changed the course of flow. Second, they were conscious that any major impacts on land or the coast could lead to compensation claims from local landholders. And three, there was an interest in how they could best remediate new land formed by the tailings. My task was to map changes since mining commenced in 1972 and to provide recommendations on impacts and the possibility of using mangroves to vegetate the new prograding delta. But soon after I left an earthquake with a tsunami destroyed much of the new delta. It did not take long for the delta to re-grow from tailings stored in the river bed.
The company invited myself and Don Wright back to Panguna in 1977 to look in more detail at delta processes. We undertook measurements of topography, salinity, and currents and helped BCL in their assessment of future changes. A model of shoreline growth from mid-Holocene to 1972 formed a base from which we could show how the present cuspate feature would grow with abundant sand supplies from the mine. Two main phases were apparent: (1) an initial shore-parallel beach-ridge system at the inner part of the embayment representing deposition during a rapid phase of progradation; and (2) a smaller area of recurved beach ridges reflecting changes in river-mouth position with limited sand supply either alongshore or from upstream. Discharge from the river since 1972 resulted in a new mouth being formed. A single channel now entered the sea across a well-developed river-mouth bar of a cuspate delta; this rapidly prograded delta could be subject to subsidence with earthquake “shaking”, but would rapidly reform as long as mining continued.
In 1989, mining ceased due to civil unrest. An examination of current photography shows that while there are still abundant sand supplies stored in the Jaba River downstream of the mine site, little appears to be entering the mouth. Shorelines appear to have stabilised with a small village on the north side of the channel amidst a well-vegetated delta surface. A small dunefield is apparent to the south of the channel. Clearly another field trip is needed to verify the changes that have occurred to this coast since 1989. Empress Augusta Bay has experienced since the early 1940s a dramatic history of warfare, mining, civil war, re-settlement and dramatic shoreline changes to the coastal geomorphology that all warrant more study.
Words by Prof Bruce Thom. Please respect Bruce Thom’s thoughts and reference where appropriately: (c) ACS, 2017, posted 26th April 2017, for correspondence about this blog post please email firstname.lastname@example.org