Reducing sediment loads to the Great Barrier Reef: developing optimal approaches for treating alluvial gully erosion

Andrew Brooks
Led by: Assoc Prof Andrew Brooks, GU

 

Project Summary

Alluvial gullies have been shown to be major sources of fine sediment and nutrients to the GBR lagoon, and yet to date there is no accepted practice as to how these major pollution sources can be stabilised and rehabilitated. Working in collaboration with delivery partners implementing gully rehabilitation works through the Reef Trust phases 2 and 4, and various other gully mitigation programs currently being developed by Greening Australia, WWF and GBRF, this project will develop a series of large field trials that will test the effectiveness of different treatments for large active alluvial gully rehabilitation in different soil types in different catchments. These trials will complement similar research effort currently underway through NESP on hillslope gullies.

Problem Statements

Problem

Alluvial gullies have been shown to be major sources of fine sediment and nutrients to the GBR lagoon, and yet to date there is no accepted practice as to how these major pollution sources can be stabilised and rehabilitated. Working in collaboration with delivery partners implementing gully rehabilitation works through the Reef Trust phases 2 and 4, and various other gully mitigation programs currently being developed by Greening Australia, WWF and GBRF, this project will develop a series of large field trials that will test the effectiveness of different treatments for large active alluvial gully rehabilitation in different soil types in different catchments. These trials will complement similar research effort currently underway through NESP on hillslope gullies.

To date gully erosion rehabilitation efforts undertaken in GBR catchments have tended to focus on the less active smaller scale gullies which can be dealt with using manual labour and low tech solutions (Wilkinson et al., 2013b, 2015). Such strategies, however, can do little to stabilise and reduce sediment yields from the large active alluvial gullies that are prevalent in major sediment producing catchments like the Bowen-Bogie & lower Burdekin Rivers which between them have been shown to contribute on average 70% of the total sediment load at the Burdekin River mouth and around 65% of the silt/clay fraction, representing around 30% of the total silt-clay input to the entire GBR lagoon (GBRL). Preliminary mapping indicates that there are >2000 ha of alluvial gullies within the lower 100 km of the Bowen floodplain/terrace system (Brooks et al., 2016), which likely deliver a significant proportion of fine sediment load to the GBRL. Sediment tracing data (Wilkinson et al 2013) indicates that 86–96% of sub 10 micron sediment sourced from the Bowen is derived from sub-surface sources and it is likely that the majority, and most spatially confined proportion, of this sediment is sourced from large alluvial gullies along the Bowen River.

Similar proportions of sub-surface source dominance have been found in most of the large catchments draining to the GBR (Hughes et al., 2009; Tims et al., 2010; Olley et al., 2013), with extensive gullies being responsible for a significant proportion of fine sediment load in all of the large dry-tropics catchments. Large alluvial gullies are also one of the most connected sources of fine sediment, delivering sediment in many cases directly into the mainstream channels of the largest rivers draining to the reef. A recent pilot study has also demonstrated that alluvial gullies are major sources of particulate nutrients to the GBR (Garzon-Garcia et al., 2016). Hence, significantly reducing the loads derived from these alluvial gullies is critical if Reef 2050 targets are to be met (Anon 2015).

How Research Addresses Problem

A series of plot-scale trials have been undertaken in the Normanby catchment with extremely promising results, which show that erosion rates can be reduced by as much as 80% over a few years with the appropriate treatment of the dispersive alluvial soils (see Shellberg and Brooks., 2013; Brooks et al., 2016a). However, these plot scale trials now need to be upscaled to complete alluvial gully complexes and different treatments tested at the whole of gully scale to ensure that the sort of results achieved at the plot scale can be replicated at the gully complex scale. Recent research (e.g. Brooks., et al., 2009, Shellberg et al., 2013, Rose et al., 2015) has identified a suite of geomorphic and hydrological controls and drivers

of alluvial gully erosion, all of which need to be considered in the management of these highly concentrated sediment sources. Recent reports by Shellberg and Brooks (2013, 2016b) and Carey (2015) have canvassed a range of management strategies that can be employed to reduce sediment yields from gullies, but there have been few studies over the last few decades in the GBR catchments that have rigorously evaluated these approaches at scale and within the specific context of large alluvial gully systems. Furthermore, a range of additional factors come into play when gullies are managed at this scale, such as:

  • preventing re-incision and reactivation of the gullies associated with flow accumulation from runoff within the gully itself
  • intermediate slope drainage
  • dealing with excessively high gully walls (>15m in places)
  • dealing with extensive tunnelling

The specific locations of the trial sites and the nature of the experiments will be developed in collaboration with the various project partners who are delivering on-ground works through the various programs outlined above. All of the sites will involve the establishment of BACI designed trials that will be monitored using a variety of techniques, as outlined in the project methods.

The specific treatments trialled at each location will vary depending on local site conditions, but these will be developed in close collaboration with the delivery partners. The treatments undertaken and the associated research questions may include the following:

  • What are the optimal gypsum application rates to stabilise sodic soils as a function of soil type?
  • How is tunnel erosion best addressed?
  • What are the optimal grass/shrub/tree mixes for cost-effective gully stabilisation?
  • Is there a role for intensive cattle grazing in alluvial gully management?
  • To what extent does revegetation within the gully catchment reduce gully erosion rates?
  • What are the stable slope angles for regraded gullies in differing soil types?
  • What is the most cost-effective methods for preventing gully floor re-incision?
  • Can irrigation of regraded gullies ahead of early wet-season rains improve the likelihood of success?
  • How effective are more passive rehabilitation measures in large alluvial gullies – e.g. hydro-mulch, mulch, gypsum application without regrading?
  • Can large alluvial gullies be rehabilitated without treating the active source areas, relying on sediment trapping alone?

Parallel Research on Bioavailable Particulate Nutrients

Given the recent evidence that alluvial gullies may also represent significant sources of anthropogenically elevated bioavailable particulate nutrients (Garzon-Garcia et al., 2016), a parallel project is being developed by Alex Garcon-Garcia, Jo Burton and others from DSITI that will investigate the extent to which nutrients are delivered from alluvial gullies and to what extent nutrient contributions can be reduced in concert with sediment reductions. While this research will be a separate project funded by OGBR and DSITI, it will necessarily have to work closely with this project, given that the high resolution geomorphic and soils data collected through this project, as well as the pre- and post-treatment sampling, are required to inform the nutrient analyses. Some of the questions addressed through this parallel research would include:

  • What are the current nutrient loads being delivered from alluvial gullies in different soil types and in different catchments?
  • What level of nutrient source reduction can be expected to accompany alluvial gully rehabilitation?
  • What are the risks of nutrient leakage from intensive gully rehabilitation works, particularly in association with soil treatments that involve the addition of compost and mulch?

Alignment with NESP Research Priorities

This is a brokered project focusing on a strategic management problem identified in the round 2 priorities (i.e. reducing water quality impacts: identify and prioritise practical management actions capable of protecting and improving water quality in the Great Barrier Reef region):

a) Local scale identification of priority contaminant export loss (hot spots) for better targeting of on-ground works and extension activity.
b) Determining the source and marine fate of environmentally relevant sediments
c) Develop/evaluate practical on-farm nutrient and sediment loss mitigation and capture and land management practices that will influence behavioural change and improve water quality outcomes – link to field trials.
d) New methods for encouraging behaviour/practice change/improving compliance with BMP.

 

Project Keywords

Alluvial gully management; Erosion; Sediment; Nutrient loads; Rehabilitation.

Project Funding

This project is jointly funded through GU, DSITI and the Australian Government’s National Environmental Science Programme.

Project Publications

 

 

Project Map

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