Macrochannels and their significance for flood-risk minimisation, West Dapto, New South Wales

A prominent characteristic of streams draining catchments in West Dapto, New South Wales, are well developed macrochannels that have formed within alluvial terraces in mid-catchment zones. A detailed hydraulic modelling study using HEC-RAS, HEC-GeoRAS and Arcview GIS indicates that these macrochannels are scaled to accommodate high magnitude floods. They offer a significant degree of natural protection from flood events up to and in excess of 100 years recurrence interval, essentially by operating as \u27bankfull\u27 channels during such events. Macrochannel landforms can be clearly distinguished and mapped on fine-scale digital elevation models (DEMs) and other GIS data sources such as rectified aerial photography, offering the opportunity to integrate analyses of fluvial landforms and channel processes into hydraulic modelling studies, and ultimately, flood-risk avoidance strategies. Such an approach has the potential to improve on traditional flood risk avoidance methods that are focussed primarily on design-flood heights by enabling the interpretation of hydraulic modelling outputs in the context of fluvial landforms that exert a significant control on flood behaviour

Hydrospatial assessment of streamflow yields and effects of climate change: Snowy Mountains, Australia

Hydrospatial analyses of catchment topographic indices for 112 unregulated (unimpaired) gauging stations show that mean catchment elevation is the primary control on annual precipitation, runoff depth, runoff coefficients and evapotranspiration in the Snowy Mountains. Catchments with mean elevations greater than 1850m show a steep increase in yield over the trend for lower elevation catchments and have runoff coefficients greater than one. Precipitation undercatch because of high winds and winter snowfall is the cause for this unusual situation, with deep accumulations of blown and drifted snow contributing significantly to runoff from small, high elevation catchments. Climate change effects on precipitation, runoff, runoff coefficients and the timing of peak snowmelt discharges vary across an elevational gradient. Annual precipitation shows strongly significant declines of up to 11.0mmyr from 1944 to 2009, with the magnitude of precipitation declines increasing with increasing elevation. Lower elevation catchments show greater sensitivity to drought than higher elevation catchments, exhibiting sharp declines in annual runoff coefficients due to smaller average differences between evapotranspiration and precipitation, and switching from energy (demand) to supply (precipitation) limited water balances. Climate change effects on the timing of peak winter-spring (June to November) snowmelt discharges for the highest elevation gauged catchments in Australia are pronounced with average shifts toward earlier peak discharges of 6.2 and 4.0days per decade for the Snowy and Geehi Rivers, respectively. A lapse rate model using elevation as a substitute for temperature change highlights the sensitivity of mean annual runoff coefficients in the Snowy Mountains to changes in mean annual temperature, declining by 15% and increasing by 17% per degree centigrade rise and fall, respectively. Runoff coefficient sensitivity is driven by elevation (temperature) driven controls on the proportion of precipitation falling as snow vs. rain, combined with decreasing evapotranspiration with increasing elevation. Temperature (elevation) driven decreases in evapotranspiration resulting from changes in rain-snow precipitation balances, widespread snowpack accumulation and largely treeless catchments dominated by alpine vegetation during cool phases of the last glacial cycle offer a simple but comprehensive explanation for the greater runoff volumes in the Murray-Darling basin from the SE Australian highlands preserved by palaeochannels considerably larger than present river systems

Assessment of downstream trends in channel gradient, total and specific stream power: a GIS approach

Geographic Information Systems (GIS) analyses of Digital Elevation Models (DEMs) coupled with catchment area based discharge estimation techniques provide a relatively simple means of modelling contiguous downstream trends in channel gradient, total stream power, and in riverscapes conducive to regime analysis, also specific stream power. For a small, high relief, coastal catchment in SE Australia, good agreement was obtained between channel gradients derived from a 25 m cell-size DEM and field survey equivalents over distances of several kilometres, indicating that channel gradients derived from DEMs can have a reasonable degree of absolute as well as relative accuracy over multi-kilometre reach scales. Assessment of downstream rates of change in channel gradient and specific stream power across four river systems suggests that some of the river reaches most responsive to high magnitude floods occur in zones where these variables rapidly decrease downstream. Modelling of downstream trends in channel gradient, total and specific stream power from catchment-wide DEMs has potential to provide a framework with which to investigate conceptual and empirical models between channel gradient, stream power and the form and dynamics of river systems

Hydraulic assessment of environmental flow regimes to facilitate fish passage through natural riffles : Shoalhaven River below Tallowa Dam, New South Wales, Australia

Proposed environmental flow regime changes downstream of a major water supply dam have been assessed in terms of effects on depth, velocity and fish passage across natural, gravel-bed riffles and rapids. This study focussed on passage requirements for Australian bass, Macquaria novemaculeata (Perciformes, Percichthyidae), a catadromous fish of considerable ecological and recreational fishing importance. Some 23 major riffles and rapids occur between the dam and the tidal limit over a river length of 25 km. Reconnaissance investigations of riffle slope, length, width, depth and morphologic characteristics indicated that wide-shallow, steep-turbulent and bifurcating riffle morphologies were most likely to cause problems for upstream bass passage under low flow conditions. Two approaches were used to investigate riffle depths and velocities over a range of flows. A rapid assessment approach directly measured thalweg depths and velocities under two controlled flow release rates in riffles identified as being potentially problematic to upstream bass passage. Detailed topographic surveys and two dimensional hydraulic modelling with River2D was undertaken for two riffles identified as ‘worst case’ examples of wide-shallow and steep-turbulent morphologies. Results from both approaches were consistent and complementary. Both approaches identified riffles where minimum depths and maximum velocities were likely to be problematic for upstream passage by Australian bass at a flow rate of 130 MLd⁻¹ (the current regulated flow release) but were mitigated at flow rates above 300 MLd⁻¹. Assessment of environmental flow regime transparency and translucency threshold options with regard to a 300 MLd⁻¹ target flow indicated that options where the transparency threshold was set at the 80th flow duration percentile (flows equalled or exceeded for 80% of time), and varied according to the monthly pattern of natural flows, improved hydraulic conditions for upstream bass migration.16 page(s

Modern depositional processes in a confined, flood-prone setting: benches on the Shoalhaven River, NSW, Australia

Past research has suggested that in-channel alluvial landforms, often termed benches, are associated with flow events with specific return periods and that such landforms are inset within terraces, floodplains, or higher bench surfaces. Chronostratigraphic analysis and hydraulic modelling in a bedrock-confined reach of the lower Shoalhaven River show that they have been extensively eroded and at least partially reformed during the historical period. Lower benches are inundated by flood events with average recurrence intervals (ARI) of 2 or less years, middle benches by events ca. 5 years ARI, and upper benches by events ca. 10 years ARI. The lower benches commonly share over- or onlapping stratigraphic units, demonstrating that deposition can occur on multiple bench surfaces simultaneously. This is in contrast to earlier suggestions of discrete bench surfaces being associated with, and formed by, events that have a specific return period. In the Shoalhaven River, Pleistocene and Holocene terraces determine the large alluvial channel geometry for a wide range of discharges up to the 50-year return interval with benches forming temporary sediment storages within this larger channel. The large channel dimensions and associated hydraulics and high annual flood variability (flash flood magnitude index of 0.53) for the Shoalhaven River facilitate the construction of multiple bench surfaces across a range of elevations. Benches are formed of a wide range of facies from decimetre-thick massive sand units through interbedded sands and silts

Session A8: A Multi-Faceted Approach for Evaluating Fishway Performance in South-Eastern Australia

Abstract: In the coastal rivers of south-eastern Australia 70% of fish migrate at some stage between the rivers and estuary to complete their life cycle, predominantly as catadromous and amphidromous migrations. A major threat to diadromous fishes in this region is the construction of artificial barriers, such as dams, weirs and road crossings. Traditionally, fishways in Australia were based on those designed to pass salmonids, thus limiting fish passage for the comparatively poorly swimming Australian native species. However, in the past few decades fishway design has been refined to accommodate native species’ swimming ability and wide range of body sizes (40- 1000mm). Ongoing monitoring programs have facilitated the refinement of modern fishway design to ensure successful passage of the target community. The Hawkesbury-Nepean and Shoalhaven rivers are the second and fifth largest regulated coastal river systems within the state of New South Wales. Barriers in these systems were retrofitted with either vertical slot or high-dam fishways completed over the last six years, together with new environmental flows. Sampling techniques for fishway evaluation on these rivers includes monitoring fish communities above and below the barrier; PIT tagging to assess timing of migration through fishways; and fishway entrance and exit trapping to assess the success of the fishway at passing individual species. Modern sampling techniques such as genetics and stable isotope analysis (SIA) are also being used on historically fragmented habitats to detect changes in population genetic structure and trophic shifts. Finally, analysis of fish movement (derived via acoustic telemetry) in response to environmental variables is providing detailed information on potential migration cues and flow regimes required to stimulate fish movement. This multi-faceted research approach not only provides empirical and real-time data for evaluating the performance of fishways in improving fish passage, but also informs operational managers on their efficiency and assists engineers in refining future fishway design

Hydrospatial assessment of streamflow yields and effects of climate change: Snowy Mountains, Australia

Hydrospatial analyses of catchment topographic indices for 112 unregulated (unimpaired) gauging stations show that mean catchment elevation is the primary control on annual precipitation, runoff depth, runoff coefficients and evapotranspiration in the Snowy Mountains. Catchments with mean elevations greater than 1850 m show a steep increase in yield over the trend for lower elevation catchments and have runoff coefficients greater than one. Precipitation undercatch because of high winds and winter snowfall is the cause for this unusual situation, with deep accumulations of blown and drifted snow contributing significantly to runoff from small, high elevation catchments. Climate change effects on precipitation, runoff, runoff coefficients and the timing of peak snowmelt discharges vary across an elevational gradient. Annual precipitation shows strongly significant declines of up to 11.0 mm yr−1 from 1944 to 2009, with the magnitude of precipitation declines increasing with increasing elevation. Lower elevation catchments show greater sensitivity to drought than higher elevation catchments, exhibiting sharp declines in annual runoff coefficients due to smaller average differences between evapotranspiration and precipitation, and switching from energy (demand) to supply (precipitation) limited water balances. Climate change effects on the timing of peak winter-spring (June to November) snowmelt discharges for the highest elevation gauged catchments in Australia are pronounced with average shifts toward earlier peak discharges of 6.2 and 4.0 days per decade for the Snowy and Geehi Rivers, respectively. A lapse rate model using elevation as a substitute for temperature change highlights the sensitivity of mean annual runoff coefficients in the Snowy Mountains to changes in mean annual temperature, declining by 15% and increasing by 17% per degree centigrade rise and fall, respectively. Runoff coefficient sensitivity is driven by elevation (temperature) driven controls on the proportion of precipitation falling as snow vs. rain, combined with decreasing evapotranspiration with increasing elevation. Temperature (elevation) driven decreases in evapotranspiration resulting from changes in rain-snow precipitation balances, widespread snowpack accumulation and largely treeless catchments dominated by alpine vegetation during cool phases of the last glacial cycle offer a simple but comprehensive explanation for the greater runoff volumes in the Murray-Darling basin from the SE Australian highlands preserved by palaeochannels considerably larger than present river systems

A multi-scale GIS and hydrodynamic modelling approach to fish passage assessment: clarence and Shoalhaven Rivers, NSW Australia

Natural barriers such as waterfalls, cascades, rapids and riffles limit the dispersal and in-stream range of migratory fish, yet little is known of the interplay between these gradient dependent landforms, their hydraulic characteristics and flow rates that facilitate fish passage. The resurgence of dam construction in numerous river basins world-wide provides impetus to the development of robust techniques for assessment of the effects of downstream flow regime changes on natural fish passage barriers and associated consequences as to the length of rivers available to migratory species. This paper outlines a multi-scale technique for quantifying the relative magnitude of natural fish passage barriers in river systems and flow rates that facilitate passage by fish. First, a GIS-based approach is used to quantify channel gradients for the length of river or reach under investigation from a high resolution DEM, setting the magnitude of identified passage barriers in a longer context (tens to hundreds of km). Second, LiDAR, topographic and bathymetric survey-based hydrodynamic modelling is used to assess flow rates that can be regarded as facilitating passage across specific barriers identified by the river to reach scale gradient analysis. Examples of multi-scale approaches to fish passage assessment for flood-flow and low-flow passage issues are provided from the Clarence and Shoalhaven Rivers, NSW, Australia. In these river systems, passive acoustic telemetry data on actual movements and migrations by Australian bass (Macquaria novemaculeata) provide a means of validating modelled assessments of flow rates associated with successful fish passage across natural barriers. Analysis of actual fish movements across passage barriers in these river systems indicates that two dimensional hydraulic modelling can usefully quantify flow rates associated with the facilitation of fish passage across natural barriers by a majority of individual fishes for use in management decisions regarding environmental or instream flows

Revisiting an arid LGM using fluvial archives: a luminescence chronology for palaeochannels of the Murrumbidgee River, south-eastern Australia

The Riverine Plain in south-eastern Australia contains numerous palaeochannels that are much larger than the present rivers and provide evidence about past hydrological conditions. Previous research suggested optima in fluvial activity both before and after the peak of the Last Glacial Maximum (LGM; 21 ± 3 ka), and, in some cases, throughout the LGM. In this study, we revisit palaeochannel remnants of the Gum Creek and Yanco palaeochannel systems along the Murrumbidgee River, which drains the high-elevation catchments of the Australian Alps in south-eastern Australia. We date fluvial and aeolian sediments using single-grain optically stimulated luminescence (OSL) and apply thermoluminescence (TL) dating to a subset of samples. We compare the OSL ages to new and previously published TL ages and investigate reasons for age discrepancies between these methods, possible effects of partial bleaching and other factors that may affect luminescence ages. We propose a new OSL-based chronology for the Gum Creek and Yanco palaeochannel systems and assign periods of enhanced fluvial activity for the Tombullen and Yanco phases to 41-29 and 29-18 ka, respectively. Importantly, we infer that conditions of increased sediment and water discharge persisted for the Murrumbidgee River at the time of the LGM