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Soil erosion rates after wildfire are strongly controlled by intrinsic properties such as topography, weather, climate, soil, and vegetation. These landscape and hydroclimatic properties are important in determining post-fire erosion rates; however, their influence on post-fire erosion and their interaction with the intensity of a wildfire remains uncertain. A key limitation in resolving this uncertainty is the lack of conceptual models and frameworks for organising data related to the geomorphic sensitivity of landscapes to wildfire. Our aim is to develop a framework for consolidating understanding of post-fire erosion in the context of hydroclimatic conditions which contribute to system states, for example soil and vegetation properties, and wildfire regime. The framework is developed around a simple conceptual model where the change in erosion due to wildfire is a product of change in runoff generation and sediment supply, which is strongly related to landscape net primary productivity (NPP). We hypothesised that geomorphic sensitivity to wildfire should vary as a unimodal humped relationship across a gradient of NPP, peaking at an intermediate level. To develop this framework and to test the hypothesis, we first review intrinsic soil and vegetation properties related to the supply and transport of sediment from burned and unburned hillslopes. Net primary productivity is systematically related to these intrinsic properties because it integrates many processes involved in soil and vegetation development. Empirical data indicate a trend in the change in surface runoff generation with NPP after wildfire, peaking at an NPP of approximately 15 Mg C ha−1 y−1. A simple model of fuel availability and soil heating are correlated with a similar "humped" trend in sediment supply. These results are consistent with our conceptual model, which indicates that sediment supply and runoff contribute towards a distinct peak in wildfire effects on erosion at an intermediate level of NPP. We propose that landscapes of intermediate NPP typically have the highest quantity of fuel available to burn, which cause large changes to the soil surface properties. Landscapes at intermediate NPP also tend to produce intrinsic soil and vegetation properties that promote erosion after wildfire. The interplay between these short and long-term landscape characteristics is strongest at intermediate levels of NPP. Our proposed biogeographic model of geomorphic sensitivity to wildfire was supported by erosion data from burned hillslope and zero-order catchments studies from a range fire-prone landscapes in Australia and North America. Our proposed conceptual model will help identify areas most vulnerable to post-fire erosion changes. [ABSTRACT FROM AUTHOR]

Multi-purpose Artificial Reefs (MPARs) are structures that may provide aesthetically acceptable coastal protection and improve recreational outcomes. Twenty years after construction of the first MPAR, Narrowneck Reef on the Gold Coast of Australia, most of the available literature is still focused on the planning, design and construction of such structures and peer-reviewed publications on their post-construction monitoring, interaction with sediment transport and impacts on coastal morphology are lacking. The aim of this paper is to evaluate how does Narrowneck Reef influence the sediment transport, and morphological changes around the anthropogenic structure, two decades after construction. To do so, a combination of ten high spatial resolution topo-bathymetric surveys from the top of the dune to the 10 m depth captured over 21 months and a series of 60 simulations using a calibrated numerical model were used. Our results demonstrate that: although not expected during design or reported in similar structures, sand can bypass the MPAR around its offshore end; under oblique waves, the longshore currents are deflected as they pass the reef, resulting in a shadow zone on the downdrift side where sand deposits; the bar crest tends to be higher on the reef's updrift side compared to downdrift, indicating that the MPAR can act as a store for sediments, as initially designed. Furthermore, the MPAR can act to stabilise the bar as it moves onshore with a downdrift offset of the inner bar as a result of low oblique wave incidence. The results presented here demonstrate that the short-term response to the MPAR twenty years after construction is more closely related to the updrift build up of sand and the deflection of longshore currents as they encounter the reef than to the dissipation of wave energy. This is because MPARs are designed to dissipate just enough wave energy so that they can achieve their recreational goal (surfing). • Beach morphodynamic state and tide level influence sediment transport pathways. • Sand bypassing occurs mainly inshore of the structure. • Under large oblique waves, sand bypass can occur offshore of the structure. • Deflection of longshore currents influence morphological response to the reef. • Bar crest is usually higher on the reef's updrift side compared to downdrift. [ABSTRACT FROM AUTHOR]

Abstract: Coastal dunes can act as a method of soft coastal protection against inundation and direct impact of waves during storms if they are substantially large enough in volume to withstand erosion without breaching. However, the time evolution of sand dunes under direct wave impact is not well understood and many available models require site specific calibration and have had limited verification at field scales. Here we test three models of varying complexity in their ability to predict both dry beach erosion volumes and dune to a retreat for an East Coast Low storm event that occurred on the Gold Coast, Australia. The process-based model, XBeach, which models the entire profile was able to reproduce both dune toe retreat and dry beach volume, however, was sensitive to calibration parameters. The two parametric models that only modeled erosion above the initial dune toe position were capable of accurately predicting dune toe retreat, however, under-estimated dry beach erosion volumes. With no calibration, the parametric model proposed by Palmsten and Holman (2012) produced the smallest errors of dune toe retreat with mean error in final dune position of 6.6m, or 18% of the total measured dune retreat. With minimal calibration estimated absolute error in average dune toe retreat was less than 13% of observed retreat for all three models. [Copyright &y& Elsevier]

Abstract: The tropics of northern Australia have received relatively little attention with regard to the impact of soil erosion on the many large river systems that are an important part of Australia''s water resource, especially given the high potential for erosion when long dry seasons are followed by intense wet season rain. Here we use 137Cs concentrations to determine the erosion processes supplying sediment to two major northern Australian Rivers; the Daly River (Northern Territory), and the Mitchell River (Queensland). We also present data from five sediment samples collected from a 100km reach of the Cloncurry River, a major tributary of the Flinders River (Queensland). Concentrations of 137Cs in the surface soil and subsurface (channel banks and gully) samples were used to derive ‘best fit’ probability density functions describing their distributions. These modelled distributions are then used to estimate the relative contribution of these two components to the river sediments. Our results are consistent with channel and gully erosion being the dominant source of sediment, with more than 90% of sediment transported along the main stem of these rivers originating from subsoil. We summarize the findings of similar studies on tropical Australian rivers and conclude that the primary source of sediment delivered to these systems is gully and channel bank erosion. Previously, as a result of catchment scale modelling, sheet-wash and rill erosion was considered to be the major sediment source in these rivers. Identifying the relative importance of sediment sources, as shown in this paper, will provide valuable information for land management planning in the region. This study also reinforces the importance of testing model predictions before they are used to target investment in remedial action. [Copyright &y& Elsevier]

Abstract: We present the most complete and new high-resolution multibeam bathymetry datasets from the shelf to the basin in the Ribbon Reef region, northern Great Barrier Reef (GBR). Analysis of these data, combined with existing side-scan sonar data provides a detailed morphologic framework of the submarine canyon system and other related features, their spatial distribution, controlling factors on their evolution, and evidence of recent sedimentary activity. Two morphologically different submarine canyon types are recognised: 1) shelf-incised canyons whose heads are indented into the shelf-break at shallow-water depths (about 60 to 80m). These canyons can be single-fed or multi-fed depending on the number of tributaries that merge into the main canyon valley. According to the degree of connection with the shelf, these canyons can be reef-blocked, partly reef-blocked or shelf-connected; 2) slope-confined canyons, which are located at deeper waters in the slope and show a different canyon head morphology. Canyon formation and development of the different canyon types is explained according to a four-phase model. In the first phase, localised slope failures and/or sediment gravity flows may lead to the formation of an initial canyon by retrogressive headward or downslope erosion respectively. These processes continue during the second transitional phase, leading to upslope canyon progression. Finally, canyons breach the shelf-break during a mature stage that represents the more active phase in the canyon evolution. The development of an extensive shelf-edge barrier reef (Ribbon Reefs) represents a fourth phase that conditioned the sedimentary dynamics of the canyons. The location and morphology of this reef barrier determine the type and amount of sediment supply by controlling the connection of the canyon head with the shelf drainage system. Recent canyon activity is evidenced by the presence of erosive and depositional features that include submarine landslides, gullies and rills on the canyon walls and slopes, sandwaves migrating along the canyons floors, and sediment gravity flows deposited on the lower canyons and adjacent basin floor. [Copyright &y& Elsevier]

Summary: Empirical power law sediment rating curves of the form C = aQb (where C is the sediment concentration, Q is the discharge rate, and the coefficient a and exponent b are fitted parameters), or the alternative form given by log(Qs )=log(a)+(b +1)log(Q) (where Qs = CQ is the sediment delivery rate, and the natural logarithm is used) are widely used for characterising sediment transport across a broad range of spatial and temporal scales. Fire frequently has a large effect on erosion rates and sediment delivery. We investigate the temporal changes in the coefficient a and exponent b for 3years following a wildfire between February 2003 and April 2006, for two South Eastern Australian Eucalyptus forested catchments (136 and 244ha) with mean annual rainfall of 1900mm. Storm-event integrated sediment loads and discharges were calculated for each of 596 storm events using stage-discharge control structures and in situ turbidity measurements at 15min intervals. Measurements were converted to sediment concentrations using regression relationships developed from storm activated water auto-samplers. The analysis identified: (i) strong negative linear relationships between the rating coefficient log(a), and the rating exponent b, reflecting sediment rating curves that “pivot” around a common fulcrum point in log–log space, (ii) a systematic shift in this linear relationship between log(a) and b as a function of time since fire, (iii) maximum values of b of ca. 2.5 (i.e. maximum non-linearity in the relationship between discharge and sediment delivery, and therefore maximum sensitivity to high peak-discharge events) immediately following the fire, which decline rapidly and monotonically by a factor of 10 to ca. 0.25 in the first 8months, attributed to a dominance of hillslope erosion processes and declining rill and interill erodibility, (iv) irregular patterns in the value of b during the vegetation recovery period, probably reflecting a shift from hillslope dominated erosion processes to a greater contribution from channel processes (8–24months after the fire) and (v) annual cycles in the value of b in the recovered state (24–38months after the fire) ranging from a minimum of ca. 0.5 in the dry season to a maximum of ca. 1.5 in the wet season. The physical cause of these cycles could not be isolated in this study. The results provide a robust quantitative perspective on the magnitude and temporal variability of the sensitivity of catchments recovering from wildfire. [Copyright &y& Elsevier]

Identifying the erosion processes contributing to increased basin fine sediment yield is important for reducing downstream impacts on aquatic ecosystems. However, erosion rates are spatially variable, and much eroded sediment is stored within river basins and not delivered downstream. A spatially distributed sediment budget model is described that assesses the primary sources (hillslope soil erosion, gully and riverbank erosion) and sinks (floodplain and reservoir deposition) of fine sediment for each link in a river network. The model performance is evaluated in a 17,000-km2 basin in south-east Australia using measured suspended sediment yields from eight catchments within the basin, each 100–700km2 in area. Spatial variations within the basin in yield and area-specific yield were reliably predicted. Observed yields and area-specific yields varied by 17-fold and 15-fold respectively between the catchments, while predictions were generally within a factor of 2 of observations. Model efficiency at predicting variations in area-specific yield was good outside forested areas (0.58), and performance was weakly sensitive to parameter values. Yields from forested areas were under-predicted, and reducing the predicted influence of riparian vegetation on bank erosion improved model performance in those areas. The model provided more accurate and higher resolution predictions than catchment area interpolation of measured yields from neighbouring river basins. The model is suitable for guiding the targeting of remediation measures within river basins to reduce downstream sediment yields. [Copyright &y& Elsevier]

Abstract: Keppel Bay is a macrotidal embayment on a tectonically stable, tropical coast, which links the Fitzroy River with the Great Barrier Reef continental shelf. Estuaries and deltas act as conduits between catchments and inner shelf environments. Therefore, understanding sediment transport pathways in these complex systems is essential for the management of ecosystems such as coral-reefs that are potentially vulnerable to enhanced river sediment loads. Furthermore, the morphology and sediment dynamics of subtidal sand ridges and dunes are relatively poorly characterised in macrotidal estuaries, particularly in turbid, episodic systems such as the Fitzroy River and Keppel Bay. Our sedimentological analysis of seabed samples, shear-stress modelling and three-dimensional acoustic imaging reveals that Keppel Bay is a mixed wave- and tide-dominated estuarine system. Areas of sediment starvation and shoreward transport characterise the offshore zone, whereas a complex of both active and relict tidal sand ridges, and associated subaqueous dunes, dominate the relatively protected southern Keppel Bay. Transport within this region is highly dynamic and variable, with ebb-dominated sediment transport through tidal channels into the outer bay where there is a switch to wave-dominated shoreward transport. Ultimately, bedload sediments appear to be reworked back inshore and to the north, and are gradually infilling the bedrock-defined embayment. Our characterisation of the Keppel Bay system provides a detailed example of the physiography of the seaward portion of a tide-dominated system, and shows that sediment transport in these areas is influenced by a variable hydrodynamic regime as well as relict channels and bedrock topography. [Copyright &y& Elsevier]