A major rainfall event occurred in June 2016 across the east coast of Australia and Tasmania as a result of an extensive upper-level trough and the formation of an East Coast Low Complex in the Tasman Sea. In some areas, the rainfall event was more intense than a 1% probability of exceedance (1 in 100-year average recurrence interval) event, for rainfall events of 12 hour to 4 day durations.
There was over 200 mm of rainfall in a 24-hour period over the headwaters of northern Tasmania. This high intensity of rainfall over a long duration led to extensive flooding in the major stream systems of Tasmania. The impacts were extensive, with hundreds of damaged sites reported across the state. The damage to waterways included bank erosion, log jams, floodplain deposition of cobbles, gravels and fine materials, and extensive floodplain scour. At some major sites the waterway had changed course by around 100 m laterally across the floodplain. There was significant Bank erosion through areas of agricultural land.
Alluvium was engaged by the Tasmanian Government (Department of Primary Industries, Parks, Water and Environment, DPIPWE) to assist in understanding the waterway impacts following significant flooding across the state in June 2016. A key part of our work was to understand the relationship between the pre-flood conditions, flood hydrology, physical impacts and alignment with natural disaster investment.
Our investigation included on-ground assessments, hydrologic analysis, desktop assessments of existing conditions, prioritisation of waterways susceptible to flood damage, and flood recovery program cost estimates.
The pre-flood conditions assessed were land use and condition (including fire), waterway type and vegetation condition. Land use is an important factor in the way flood events occur across the landscape, the consequent damages, and the significance of those damages. Conservation areas with remnant vegetation slow down fast moving flood waters and can assist to protect waterways from significant erosion. In large conservation areas, flood effects are generally less, but also the consequent impact on the community are less significant. In productive land areas, the land is more likely to be cleared and the effects of floods are more visible and have a greater impact on the community.
The headwaters of many north-western Tasmania catchments, subject to the 2016 flood event, experienced significant bushfires in January 2016. Bushfires directly affect a range of physical characteristics and processes and cause almost an instantaneous change in the hydrologic and geomorphic response to rainfall, which can see increases in runoff and sediment loads to waterways.
Different waterway types (based on their valley setting, river planform and bed material) respond differently to flood events. In the June 2016 flood event, alluvial gravel bed rivers experienced some of the most significant damage. These are high energy systems that can move significant bed and bank material downstream; they are also have an unconfined valley setting which means that the waterway can move laterally across the floodplain, and in some cases change course.
Native riparian vegetation can reduce the severity and extent of flooding by slowing the flow and providing greater resistance to erosion. A suite of healthy riparian vegetation is an important part of a resilient waterway. Our investigation found that sites without established native vegetation in the riparian zone experienced more significance flood impacts, that those with a robust suite of vegetation.
By combining the susceptibility to flood damage with the assessment of the flood hydrology and recorded damage sites, we provided recommendations and indicative costings for the severely, moderately and marginally impacted catchments.
We recommended that damage sites in severely and moderately impacted catchments are addressed through a flood recovery program. Flood impacted sites can be managed with a combination of debris management, willow management, and river and floodplain erosion management. Implementing a riparian vegetation program, paired with structural interventions as required, will improve the resilience and robustness of waterways to future flood events.