Macrochannels and their significance for flood-risk minimisation: examples from southeast Queensland and New South Wales, Australia

Understanding the frequency and causes of extreme events is crucial for environmental, social and economic protection and planning. In Australia this was never more apparent than January 2011 when widespread flooding across Queensland, New South Wales (NSW), and Victoria resulted in the loss of human lives and devastating impacts to infrastructure and local economies. However, understanding the interplay between the geomorphology of catchments and their hydrology remains poorly developed in floodplain planning guidelines. This paper seeks to explain spatial patterns of flood inundation in terms of downstream variations in channel morphometry; and to discuss the significance of these findings within the context of improving flood risk avoidance strategies and environmental outcomes for urban streams. A prominent characteristic of streams draining catchments in the Lockyer Valley south east Queensland and the Illawarra region of NSW, for example, are well developed macrochannels that have formed in mid-catchment zones. Detailed hydraulic modeling using HEC-RAS, HEC-GeoRAS and ArcGIS indicates that these macrochannels are scaled to accommodate high magnitude floods by operating as \u27bankfull\u27 channels during such events. In south east Queensland, locations where macrochannels debouch onto unconfined low gradient floodplains appear especially vulnerable to catastrophic flooding because of the efficient delivery and minimal attenuation of flood peaks generated in headwater catchments. Macrochannels and associated landforms can be clearly distinguished and mapped on fine-scale digital elevation models, offering the opportunity to integrate analyses of fluvial landforms and channel processes into hydraulic modeling studies, and ultimately, flood-risk avoidance strategies. Such an approach has the potential to improve on traditional flood risk avoidance methods that are focused primarily on design-flood heights by enabling the interpretation of hydraulic modeling outputs in the context of fluvial landforms that exert a significant control on flood behaviour

Magnitude, frequency and duration of instream flows to stimulate and facilitate catadromous fish migrations: Australian bass (macquaria novemaculeata perciformes, percichthyidae)

The migratory response and behaviour of catadromous Australian bass with regard to hourly mean river flows and water temperatures was assessed over 15 months. Fish movement was assessed using a 75-km passive acoustic telemetry array in the regulated Shoalhaven River below Tallowa Dam, NSW, Australia. The majority (62%) of downstream pre-spawning migrations from freshwater to estuarine habitats were stimulated by a series of flow pulses from April to September, but a proportion of fish (38%) commenced downstream migrations under regulated baseflow conditions after a sustained decrease in water temperature to below 15°C in late autumn. Equal numbers of fish undertook post-spawning upstream return migrations during flow pulses and during regulated baseflow conditions, with regulated baseflow migrants exhibiting a preference for dusk–dawn passage through freshwater pool–riffle sequences. The median magnitude of flow pulses at the time of commencement of downstream and upstream freshwater migrations by Australian bass was not large, equivalent to natural (in the absence of river regulation) flows equalled or exceeded for 56% and 48% of time, respectively. There was no evidence for increased numbers of migrants with increasing flow pulse magnitude, with individual fish ignoring some flow pulses but responding to subsequent events. In regulated rivers, the release of more frequent flow pulses with peak magnitudes approximating the natural 50th flow duration percentile may be more effective in stimulating greater numbers of Australian bass to undertake pre-spawning and post-spawning migrations between freshwater and estuarine habitats than the release of a single, larger event. The propensity of Australian bass to also undertake spawning migrations under regulated baseflow conditions emphasizes the need for provision of baseflow regimes in regulated rivers that can facilitate migrations by large bodied fishes

Alluvium of antiquity: Polycyclic terraces in a confined bedrock valley

Confined river valleys are not the localities where long term preservation of alluvium would be expected. The 25 km long low gradient (0.0014 m.m- 1) confined valley setting of the Shoalhaven River has archived alluvium of middle Pleistocene age to maintain a relatively uniform channel as an efficient conduit for a wide range of flows in a confined bedrock valley of variable morphology. Single-grain optically stimulated luminescence (OSL) dating has identified polycyclic terraces up to 193 ka in age (marine isotope stage [MISI7) with lower terrace remnants dating from 173-140 ka (MIS 6) and 106 ka (MIS 5). Holocene alluvium 2-3.5 ka in age caps these old Pleistocene units and a well-constrained combination of one-dimensional and two-dimensional hydraulic modelling demonstrates that these polycyclic terraces are clearly within reach of the modem hydrological regime. The 106 ka terrace at 17 m above low flow is inundated by floods recurring on average every -20 years, and the 140-193 ka terraces at 20-22 m are overtopped every 50-100 years. These ancient diachronous landforms exhibit complex depositional histories and are on-lapped by longitudinal benches of modern sand and gravel. Their polycyclic nature appears to be a response to flow reduction, using alluvium to adjust the boundary of the otherwise inflexible morphology of a bedrock gorge

Combining otolith chemistry and telemetry to assess diadromous migration in pinkeye mullet, Trachystoma petardi (Actinopterygii, Mugiliformes)

© 2017, Springer International Publishing AG, part of Springer Nature. This study examines the complex diadromous movements in pinkeye mullet (Trachystoma petardi) of south-eastern Australia. The techniques used to study these movements included LA-ICP-MS single line ablation transects and microchemical imaging as well as preliminary acoustic telemetry results which were used to aid in interpretation of chemical signatures related to complex movement patterns across salinity gradients. Ba:Ca and Sr:Ca from single ablation transects and microchemical images revealed differences between the otolith core and outer regions. Otolith Ba:Ca and Sr:Ca patterns were more easily distinguished in images compared to transects and these revealed that T. petardi spend their early life in saline waters. Movement patterns for adults varied, with a range of movements identified between fresh and saline waters. Telemetry data assisted in explaining the likely cause of the ambiguity in otolith microchemistry data, including identification of multiple rapid movements across salinity gradients. However, many movements through salinity gradients appear too brief to result in any clear Sr:Ca or Ba:Ca saline or mesohaline signature within the chemistry of the otolith. The combination of otolith chemistry and telemetry proved useful in providing information on this poorly understood species suggesting that T. petardi display a catadromous life history