Community pressure on managing stormwater in North Queensland Coastal areas

Managing stormwater quantity in North Queensland coastal plain areas is a key challenge for local authorities and has come under recent pressure by local communities. Recent extreme events have resulted in significant increases in insurance premiums in flood prone areas, along with generational changes resulting in changes in property uses. Coupled with the future risk of rising sea water levels, local authorities have been reminded to maintain vigilance in developing strategies and policies in a proactive manner. Effective management of stormwater in North Queensland coastal areas however remains a difficult task due to the comparatively high rainfall and resulting runoff, along with ever increasing development pressures and encroachment on waterway areas. Recent studies developed for two central residential suburbs in Cairns identified a significant number of properties were affected by the 1 % annual exceedance probability event or less depending on the location. A number of factors contributed to the issue, including the impact of elevated tailwater levels, the naturally low ground levels, the runoff responsiveness of catchments, and the constriction of conveyance paths resulting from flat terrain, historic frontal dunes, bank stability, and sediment deposition. In addition, many new properties are built as slab on ground, or existing pole homes have been renovated to include living areas on the ground floor which are now being affected. Traditional flood mitigation techniques have been investigated to improve the current situation. These included detention basins, levees, channel widening/dredging and tidegates. However, due to the magnitude of rainfall and the unique conditions encountered in coastal plain areas, traditional techniques did not prove cost effective for the expected level of improvement. As a result, nontraditional methods were investigated. Flooding has historically been caused by low conveyance in waterways. The encroachment of development near these waterways has also limited the scope for waterway widening, so the provision of alternate outlet arrangements was contemplated. Based on the assessments undertaken, it was concluded that increasing the flow capacity of the existing stormwater systems, rather than reducing the runoff quantity, was deemed to be more effective in improving the upstream flood extent. Both gravity systems and pumped systems have been investigated in Cairns and found to be effective in reducing the flood water levels. These techniques have also been assessed and intended to be implemented in other parts of coastal North Queensland with similar stormwater management issues.