The Morphology of Barchan-shaped Sand Banks from Western Torres Strait, Northern Australia

Large barchan shaped sand deposits have been observed in the north west of Torres Strait. These deposits share characteristics of both subaerial barchan dunes and subaqueous sand banks. Satellite imagery shows that the deposits migrate in the direction indicated by their horns (10-15 m west per year), and that sediment is shed from their horns, features that are characteristic of subaerial barchan dunes. However the orientation of sandwaves superimposed upon the sand banks indicate the presence of mutually evasive channels and circulation of sediment around the sand bank, a characteristic of subaqueous sand banks. The presence of mutually evasive channels is the criteria used to categorise the deposits as sand banks.

Barchan forms are known to exist in regions with limited sediment supply and unidirectional current or wind regimes. In the Torres Strait both these criteria are met. Previous work has demonstrated the presence of a net westward current through the Torres Strait that is driven by the southeast trade winds. The relatively high displacement of the wind driven currents during the trade wind season relative to the monsoon appears to provide the necessary "unidirectional" regime to form barchans. The low, and typically eastwards, displacement of the residual monsoon season current appears to have a negligible affect on the barchan form.

在托雷斯海峡（Torres Strait）西北部已观测到大型新月形沙质堆积体。这类堆积体兼具陆上新月形沙丘（subaerial barchan dunes）与水下沙坝（subaqueous sand banks）的特征。卫星遥感影像显示，该堆积体沿其翼角指向的方向迁移（每年向西移动10至15米），且沉积物从翼角处剥离，这些特征均为陆上新月形沙丘的典型标志。但叠加于沙坝之上的沙波走向表明，此处存在互斥分流通道及沙坝周边的沉积物环流，这正是水下沙坝的典型特征。互斥分流通道的存在是将该堆积体归类为水下沙坝的判定依据。

已知新月形地貌形成于沉积物供给有限且存在单向水流或风场的区域，托雷斯海峡完全符合这两项形成条件。既往研究已证实，受东南信风驱动，托雷斯海峡存在净向西的洋流。相较于季风期，信风期的风生洋流输运量相对更高，这为新月形沙丘的形成提供了必要的“单向”动力条件。季风期余流通常较弱且向东流动，其对新月形地貌的影响可忽略不计。