Assessing components of the natural inundation regime to restore through infrastructure projects

Alteration to the natural flow regime of rivers and their floodplains has had a variety of impacts, including a reduction in habitat due to change infrequency and duration of inundation events and a corresponding reduction in abundance, distribution and condition of floodplain communities. Furthermore, aquatic habitats with hydraulic complexity, particularly those offering fast flowing habitat, have significantly declined under river regulation. The River Murray is an example where alterations to the flow regime associated with water extractions and regulation have resulted in degradation of ecological function and condition. With the implementation of the Basin Plan, overbank inundation events are expected to become more common compared to the recent past. However, under the Basin Plan the frequency and duration overbank events is still likely to remain lower than that of the natural water regime. Infrastructure projects and engineering solutions can be used to further restore a more natural inundation regime, extending the area of inundation for a given flow. For those biotic communities that are primarily driven by the duration and frequency of inundation events, this would be expected to increase their abundance, distribution or condition. As part of these infrastructure projects, the South Australian Riverland Floodplains Integrated Infrastructure Program (SARFIIP) has been initiated to improve the condition of the biotic communities of Pike Floodplain and Katarapko Floodplain, two important floodplain complexes of the River Murray in South Australia. This paper presents the hydrological and hydraulic investigations undertaken as part of SARFIIP to assess inundation and hydraulic regimes of these two floodplains for a range of conditions: • Without development; without modifications to the floodplains or development influences on flow in the River Murray, • Baseline; current floodplain structures and the flow regime expected based on pre-Basin Plan conditions, • Basin Plan; current floodplain structures with a range of Basin Plan water recovery scenarios considered, • Intervention; proposed floodplain infrastructure of SARFIIP and Basin Plan water recovery scenarios considered. The hydraulic modelling was done using MIKE FLOOD modelling platform and Inundation extents were modelled by simulating a range of flow rates for each Floodplains. The results indicated that the frequency of inundation of a given location on the floodplains was significantly reduced in the current and Basin Plan flow regimes compared to the natural flow regime. The ecological implications of this are that even under the Basin Plan, it is likely that native vegetation condition and extent will continue to decline resulting in the loss of habitat for fauna dependent on that habitat. Comparison of inundation extents, frequencies and durations within the floodplain between natural conditions and Basin Plan conditions indicates that additional flows delivered to the Murray-Darling Basin through the Basin Plan alone cannot be expected to achieve full ecological benefits provided by natural conditions. Through the use of infrastructure solutions, the modelling suggests that inundation events can be created that align with the natural extent for the same frequency and duration of an events, particularly for longer duration events that are more difficult to restore through higher flows. This work has demonstrated the combined benefits of improving inundation regime through the Basin Plan, along with complimentary infrastructure projects.