Case study - exploring solutions to sediment run-off

Published: 29th October 2019

Unsealed roads represent thousands of square kilometres of exposed earth across Reef catchments, often in environmentally sensitive locations. These roads can be a major source of sediment entering waterways and making its way into the Reef lagoon and have been found to be the source points for gully erosion on nearby properties.

Cassowary Coast Regional Council (CCRC) is one of many Reef catchment councils with unsealed roads and as a case study, demonstrates the issue. The CCRC is in far north Queensland, south of Cairns and centred around the towns of Innisfail, Cardwell and Tully. The local government covers approximately 4, 700km2 and has a population of just under 30,000 people.

Like many councils within the GBR catchment CCRC owns and manages a significant local road network. Of CCRC’s 1,200km of local road network, approximately 45% or around 550km are unsealed. Coupled with 3,134mm annual average rainfall which mostly falls between January and April, it is unsurprising that maintaining unsealed roads for safe use by residents and visitors is a significant pressure. While it must be acknowledged that the township of Tully holds the nation’s annual rainfall record, the pressure of maintaining unsealed roads with significant rainfall events during the wet season is a common story across many Reef councils.

While funding sources such as the Natural Disaster Relief and Recovery Arrangements (NDRRA, to be replaced by the Disaster Funding Arrangements 2018 on 1 November 2018) provide financial assistance to affectedcouncils during declared natural disasters, this does not provide for enhancement works leading to some locations failing regularly. Capacity to address this repeat failure, is limited by ongoing financial pressure from low rates growth and frequent, wide disruption of the rate base by disasters. This means that proactive works are often outside the reach of council’s available funding. Assessment of these types of works under any traditional Cost Benefit Analysis (CBA) would never justify costly works such as sealing or targeted works at high risk, due to very low traffic volumes.

Annual road maintenance, new road construction and legacy road problems contribute to sediment levels in GBR catchments. Repeat road reconstruction and maintenance needs to be assessed with another lens with the protection of the Reef as a key outcome. Local councils such as CCRC are acutely aware of the impacts and are currently investing in exploring solutions from within their own constrained budgets, which means progress is slow. Using binding agents, varying gravel mixes, reviewing cross falls and drainage are all techniques that are being explored.