The Griffith University Reef Rescue funded a study into the causes, sources and management of sediment runoff to the northern section of the Great Barrier Reef (GBR). In 2012 the study is in the third year of the four year program.

Griffith University’s Australian Rivers Institute researchers have redressed a wide gap in knowledge of erosion sources and causes in the Normanby basin, the catchment area that drains into Princess Charlotte Bay. To date modeling programs indicate  this basin is the third highest contributor of sediment to the GBR, with the Burdekin and Fitzroy River basins, first and second respectively.

However with the exception of two years of sediment monitoring data at DERM’s Kalpower gauge, no measured data had been available to set up or test the modeling that underpins the Reef Plan north of the Daintree River. It was due to this glaring data gap that the Normanby sediment study was established in 2009.

As project leader Andrew Brooks explains:

“If the modelling is inaccurate and this is what the management decisions to reduce sediment pollution are being based on – then the wrong management decisions are likely to be made – and money will be inefficiently utilized. It was for this reason that the Federal Government held off embarking on a major program of on-ground works a few years back, when it was obvious that not 3 enough evidence was available to back up such management decisions,” Dr Brooks said.

Landholder support to manage gully’s on its way

“A key aspect of this project has been testing many of the assumptions that are built into the modeling framework – many of which have been extrapolated from theory and data derived outside the tropical savannah, which dominates Cape York,” he Brooks said.

New results from the Normanby are indicating that a number of assumptions that underpinned earlier modeling studies need to be turned on their head. Previous models assumed that the dominant sediment source in this region was from hillslope sheet erosion, and that this dominated other sources such as gully erosion and river bank erosion by more than double over large areas of the basin. Hillslope erosion rates were predicted to be more than 1000 kg/ha/yr over large parts of the catchment.

The results from the study, while still preliminary, paint a seemingly very different story. Sediment tracing using samples collected from flood waters during the 2009-10 wet season indicate that at many sites 90% or more of the sediment is derived from river bank or gully erosion sources – a complete reversal of the modeling results.

These findings are backed up with data from 11 hillslope sediment traps established on each of the main soil types within the catchment. These have yielded hillslope erosion rates in the range of 1 – 10 kg/ha/yr over two wet seasons - or (100 – 1000 times), less than the modeling predicted.

Detailed mapping of all the active gullies throughout the basin backs up the findings regarding the dominance of gully erosion. In an extremely painstaking process completed by Griffith researcher Dr Leila Eslami-Endargoli, 6540 gullies were mapped, with a total area of 1360 ha.

Sediment yield from these gullies is still being determined from repeat LiDAR topographic surveys and historical air photo analysis – but similar gullies in the adjacent Mitchell catchment typically yielded 10 – 300 t/ha/yr (or around 100, 000 times more than hillslope erosion rates per unit area).

Another gap that is being filled by this study is the collection of data on sediment runoff from the unsealed road network, not addressed or parameterized 4 by previous models. Honours student- Angela Gleeson is highlighting the potential significance of roads as a sediment source. In the Normanby catchment alone Ms Gleeson has mapped three classes of dirt road covering 3060 km with a total surface area of 52.7 km2 (5270ha), and around 1180 stream crossings.

Preliminary data from the 2010-11 wet season showed that sediment concentrations downstream of stream crossings can be elevated by as much as 200 times compared with the concentration upstream of the road crossing. On these figures, roads could be as significant as gullies as a sediment source. All of these field data will be incorporated into a new sediment budget and improved modeling framework by mid-2012.

Concurrently, a new phase of the project is just beginning to bring, what some regard as esoteric science, back down to the grass roots level and translate it into ‘real world’ management actions at the scale of individual properties. To do this the team will be contacting individual land holders and providing them with maps of erosion hotspots and assisting them in the process of developing property management plans aimed at reducing sediment input into the stream network, and ultimately the Reef.

Furthermore, since gully erosion is a major sediment source and there are no agreed ‘best management practices’ to address the types of gullies found in the Cape, Dr Jeff Shellberg is initiating some experimental trials using an array of gully erosion mitigation measures at several properties in the Normanby.

These trials will serve as demonstration sites for others to come and learn which management actions are likely to work in slowing down major gully ‘breakaways’.

For more information contact Dr Andrew Brooks, Australian Rivers Institute, Griffith University, Nathan Qld 4111. Mob 0427 232709; email: Andrew.brooks@griffith.edu.au