Water Journal : Water Journal April 2012
catchment management technical features 104 APRIL 2012 water developing a Natural Asset Maintenance Plan (NAMP) for each catchment, which aims to effectively manage catchments through the identification, prioritisation and scheduling of actions that improve or maintain the natural assets function. The process is informed by the tools discussed above and the risk assessment framework Seqwater manages. The key to achieving any of these identified actions will be the development and continuation of the partnership with our research partners, NRM bodies such as South-East Queensland Catchments, Healthy Waterways Limited and local and state agencies. Challenges in a Changing Climate A key consideration for the planning and development of an integrated Catchment Management strategy is forecasting how activities, actions and plans may be affected by a changing climate. Key conclusions from a recent report on climate change in Australia have predicted we are likely to see an increase in climate variability and the occurrence of extreme events (CSIRO and BoM, 2007), both of which can have a significant impact on any catchment management process. While not necessarily related to climate change, the recent floods experienced in South-East Queensland can demonstrate how quickly long-term understanding can change very rapidly and shift the focus of catchment management activities. This is best illustrated by examining the impact the floods have had on water quality in Lake Wivehoe, Seqwater's largest reservoir at a capacity of 1,165,000 ML. Under most circumstances, Lake Wivenhoe has an excellent buffering/ assimilative capacity for flows in the upper catchment, which are typically poor water quality standard. Figure 5 demonstrates that previous significant events such as February 2008, May 2009 and March and October 2010 had a dramatic effect on the turbidity at the upper end of the lake. However, turbidity at the dam wall was only slightly affected during these events, allowing a relatively consistent and stable water source for related water treatment plants. The volume of water delivered during the flood event had a profound and significant effect on the water quality right across the lake. While it has a certain ability to assimilate small to medium events, large events such as these will not be buffered. This is demonstrated by the pronounced and large rise in turbidity seen at the Wivenhoe Dam wall (Figure 5). This impact is compounded by the nature of the flows that Lake Wivenhoe received. During previous events, the impacts seen (particularly in the lower lake) are not prolonged, with water quality quite rapidly returning to baseline conditions. The impact of the January flood event has seen a dramatic shift in water quality in the lake that may take years to recover to previous condition. This change in water quality may have a profound effect on the ecology of the lake and water supplied for treatment. As evidenced by the increased turbidity, the January flood event carried significant sediment load into the Wivenhoe and mid-Brisbane systems. Analysis of the total suspended solids composition revealed over 90% was inorganic sediment, likely as a result of channel erosion pressures mentioned earlier. The size of the sediments present will influence how they settle and respond with time. Analysis of the particle sizes in Lake Wivenhoe, over time post-flood (Figure 6), revealed the dominance of very fine suspended sediment particles. This is contrary to previous events and would explain why the water quality is not improving as rapidly as seen previously. These very fine sediments are not conducive to settling and will remain suspended with very small amounts of energy and water movement. Preliminary investigations would suggest that this elevated turbidity and flow-on effects to other water quality parameters in Lake Wivenhoe could last for years. Some very simple modelling work has been applied to the information gathered so far that would support this assessment. It relies on the principles of Stokes' Law, which assumes no external forces present (i.e. a still body of water) and looks at the predicted settling of particles based on their size, density and form in a water medium. Figure 7 demonstrates this as predicted over the next 12 months, with little expected change in turbidity in the dam for the next 12 months based purely on settling of the suspended particles. These results are preliminary and exclude many external processes that may influence the process (such as natural flocculation/coagulation, biological processes, mixing Figure 5. Inflow of water to Wivenhoe Dam and its effect on turbidity at the upper and lower parts of the lake. Figure 7. Simple modelling of changes in water column turbidity with time based on particle size and settling predictions. Figure 6. Particle size distribution from Lake Wivenhoe Dam wall, showing the large fragments settling out and the small particles persisting well after the event.
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