Mobile Sand Dunes

The topic was, in full: “Mobile Sand Dunes – Migration of Lime-sand Dunes on the Midwest Coast and their Impacts on the Infrastructure” and the speaker was Mike Freeman, a geologist with the WA Department of Mines and Petroleum. Mike provided details and results of a study into the migration of dunes in Western Australia. The presentation gave the audience an introduction to sand dunes; details of the study area and methods; dune morphology and mobile sand dunes; migration of dunes in WA; and their impacts and possible options for preventing the migration.

The study area is on the northern Swan Coastal Plain and extended from Geraldton to Lancelin; an area 265 km long by 5 km wide, in which 19 dunes were studied in 12 groups. The goals of the study were to research the background to the dunes and their migration; determine the speed and direction of dune movement; try to understand controlling forces; attempt to foresee the future impacts of the dunes; and provide advice on the need for future monitoring and amelioration of impacts. Dunes were mapped using aerial photographs, Google Earth images, the Campbell map from 1909 and digital elevation modelling. Data from the period 1960 to 2010 was used for the study. The study concentrated on the head of the dunes as the tail is less well defined. Dunes were classified using the system used by the US Geological Survey and using this system most of the dunes in the study area were defined as Parabolic Dunes.

Mike outlined the natural process for the development of shore-parallel dune ridges and the part played by vegetation in trapping sand grains. Mobile sand dunes are initiated when the vegetation is removed (by human activity, grazing rabbits, droughts or fires) and is no longer available to fix the dune. Consequently, strong winds erode a gap in the dune, blowouts occur and the dune migrates inland. The direction of migration is controlled by wind direction with the wind moving sand grains up the windward slope and over the crest. The rate of migration was measured using the nose of the dune. It was found that the direction is always roughly to the north and that the speed of migration has increased over the last 50 years. It was found that there is a correlation between dune size and migration rate – the bigger the dune the faster the dune migrates. The influence of wind on migration was explained, especially the influence of the shore-parallel summer sea breeze from south to north in the afternoon. The sand dries out during the day then in the afternoon the sea breeze pushes the dune inland. Declining rainfall over the last 50 years has also had an impact on migration with the period from 2001 to 2010 having the lowest rainfall and the highest migration rate during the period studied.

Mike then discussed the impacts of dune migration on infrastructure. Mobile sand dunes in Australia are geohazards and can blow over roads, settlements and railways and result in consequences to human welfare such as delays, accidents and economic loss. Currently affected infrastructure is Indian Ocean Drive and Kailis Drive.

Mike then discussed the impacts of dune migration on infrastructure. Mobile sand dunes in Australia are geohazards and can blow over roads, settlements and railways and result in consequences to human welfare such as delays, accidents and economic loss. Currently affected infrastructure is Indian Ocean Drive and Kailis Drive. The scale of migration was highlighted by the following measurements: area—migrating dune areas in the study varied from 21 ha to 19,000 ha; width: from 150 m to 4.2 km across; height—from 2 m to 20 m high; speed—dunes move from 6 m per year to 38 m per year (averaged over a decade); and sand volume—from 6,000 m3 to over 500,000 m3 per year. Control of this volume of sand is an expensive exercise.

The results of the study were used to predict outcomes for some of the dunes that were studied. For example the Green Head North dune was predicted to hit the coastal access road by 2013, the prediction being validated by the subsequent closure of the road. Options for preventing the migration of dunes include Biological Dune Stabilisation – cover the dune surface with stockpiled top soil, mulch material, and straw and then plant appropriate tube-stock in winter (species used must be able to withstand strong hot, dry winds and a salty environment). Another option is to use mechanical dune stabilisation: installing fences to decrease the wind speed and saltation flux and using the straw checkerboard technique. Mining of the dunes has proved effective—mechanical excavation of lime-sand can decrease the size of the dune and cut dunes off from sediment supply. A good example is the Lancelin Dune, which has slowed down its migration rate due to mining of the dune for agricultural use.

To highlight the need for concern regarding dune migration Mike presented two cases from the study area. In the City of Geraldton the Southgate Dune is migrating north towards the Brand Highway and Wandina (50 m and 250 m ahead respectively, to be reached in 12 to 30 years). At the current time mining appears to be holding the head at a private road. The second case is the Wedge Island Dunes, which are migrating out of the defence training area into Wanagarren Nature Reserve. The dunes are 2.4 km across and up to 10 m high and migrating at 11 m/year. The volume of sand being moved is 250,000 m3 per year. They are predicted to reach Indian Ocean Drive in 80 years. Currently, mining is not an option due to the defence training area and the possibility of encountering unexploded ordnance. Mining of the head once it migrates outside the defence training area may be an option in future.

In conclusion Mike stated that mobile lime-sand dunes in WA are slow and pervasive geomorphologic events and are geohazards covering valuable infrastructure and causing major material and socio-economic damage. Solutions to halt or slow down the dunes were developed and proved effective. However, it should be of highest importance to prevent the formation of new mobile dunes by employing protective measures and restoring vegetation. Mike then answered numerous questions from the audience. It was a very interesting and informative presentation and very pertinent to our local coastal dune systems.

Colin Prickett