Mike Freeman from the WA Department of Mines and Petroleum spoke to us about dune mobility and geo-hazards. In particular, he referred to the mobile sand dunes of our Midwest coast and their impacts on infrastructure. Mike has worked in some very remote and lonely places, in a wide variety of geological fields including stratigraphy, geophysics, groundwater investigation, mineral exploration, predicting geo-hazards, geo-education and even forensic geology.

Mike began with a discussion of sand dunes around the world. Coastal dunes help to protect the hinterland from the effects of storms, feed sand back into eroded beaches and help to reduce coastal flooding. Dunes are classified as crescent-shaped (Barchan), linear, star-shaped, dome-shaped or parabolic.

The dunes that move inland from our Midwest coast are all parabolic. They form as a result of blowouts from the coastal dunes, when disturbance to the vegetation causes a portion of the dune to become mobile. While the middle of a mobile dune moves quickly, the ends are slowed by vegetation, resulting in a parabolic shape. They are moved mainly by the southerly or south-westerly sea breezes in summer when the sand is dry, so their trajectory is mainly to the north or north-east. The coastal fore-dunes and secondary dunes are formed by beach sand blowing and piling up. The beach sand along our Midwest coast is mostly organic in origin—formed mainly from shell fragments—along with some dolomite and quartz. Much of this material comes from the erosion of the offshore reefs, which were once consolidated sand dunes themselves.

Sand grains move in different ways. The tiny grains are blown in suspension, while the bigger grains bounce along and hit others or just creep along. The dune’s slope is 20 to 30 degrees on the windward side, and this can form soil horizons if vegetation has established itself at various times. A good example of those can be seen at Hamelin Bay. Bedding planes occur at the leading edge of the dune, where the sand falls down the slope at a steep angle.

The problem with mobile dunes is that their inexorable movement can cover infrastructure such as roads, railway lines and buildings. Mike was involved in some research carried out by a German geomorphologist who was employed by the WA Department of Mines and Petroleum (DMP) to look at the geo-hazard aspect of the dunes of the Midwest between Lancelin and Geraldton. The research looked at information that spanned the years 1960 to 2010. It used techniques such as aerial photography, Google Earth, an old map from 1909 and digital elevation mapping.

The sand dunes in this area are a little-recognised but important geo-hazard. The whole area, within about five kilometres of the coast, consists of stabilised dunes, with up to about fifty of them still moving. A study has shown that they move at an average of 11 metres per year and there is evidence that some have migrated up to 20 km. (That makes about 2,000 years of movement!) The dunes vary in size, but the biggest cover 19,000 ha and have a height of up to 20 metres. The biggest rate of movement is half a million cubic metres of sand per year.

When dunes of this size advance over a road or a housing estate, the socio-economic costs are very high. A dune is currently encroaching upon Kailis Drive, near Dongara, and it is estimated that Wedge Island will be in danger in 80 years. At Green Head, a road detour has already been made due to dune encroachment, but another will soon be required.

Dunes can be stabilised by spraying Aeromulch or by mulching and establishing suitable plants that will survive the wind and the poor soil. Revegetation really has to be started at the trailing edge of the dune, because that is the source of the sand that feeds the leading edge. Another solution is to mine the sand and sell it to farmers who want the lime-rich sand to counteract soil acidity. This is being done currently just north of the Greenough River, and should save the main road and houses from inundation by sand. Of course dunes cannot be mined in national parks, such as Nambung NP.

Mike says that as well as dealing with the mobile dunes, we need to stop new ones forming by preventing blowout formation in the coastal dunes. He stresses the importance of thinking about and planning for the impact of mobile dunes in WA and deciding how to avoid possible future conflicts.

Mike Gregson