Bank erosion and channel width change in a tropical catchment

Catchment sediment budget models are used to predict the location and rates of bank erosion in tropical catchments draining to the Great Barrier Reef lagoon, yet the reliability of these predictions has not been tested due to a lack of measured bank erosion data. This paper presents the results of a 3 year field study examining bank erosion and channel change on the Daintree River, Australia. Three different methods were employed: (1) erosion pins were used to assess the influence of riparian vegetation on bank erosion, (2) bench‐marked cross‐sections were used to evaluate annual changes in channel width and (3) historical aerial photos were used to place the short term data into a longer temporal perspective of channel change (1972–2000). The erosion pin data suggest that the mean erosion rate of banks with riparian vegetation is 6·5 times (or 85%) lower than that of banks without riparian vegetation. The changes measured from cross‐section surveys suggest that channel width has increased by an average of 0·74 (±0·47) m a^?1 over the study period (or ～0·8% yr^?1). The aerial photo results suggest that over the last 30 years the Daintree River has undergone channel contraction of the order of 0·25 m a^?1. The cross‐section data were compared against modelled SedNet bank erosion rates, and it was found that the model underestimated bank erosion and was unable to represent the variable erosion and accretion processes that were observed in the field data. The reach averaged bank erosion rates were improved by the inclusion of locally derived bed slope and discharge estimates; however, the results suggest that it will be difficult for catchment scale sediment budget models to ever accurately predict the location and rate of bank erosion due to the variation in bank erosion rates in both space and time. Copyright © 2008 John Wiley & Sons, Ltd.     

Modelling wetland connectivity during overbank flooding in a tropical floodplain in north Queensland,Australia

Hydrological connectivity between floodplain wetlands and rivers is one of the principal driving mechanisms for the diversity, productivity and interactions of the major biota in river–floodplain systems. This article describes a method of quantifying flood‐induced overbank connectivity using a hydrodynamic model (MIKE 21) to calculate the timing, the duration and the spatial extent of the connections between several floodplain wetlands and rivers in the Tully–Murray catchment, north Queensland, Australia. Areal photogrammetry and field surveyed stream cross data were used to reproduce floodplain topography and rivers in the model. Laser altimetry (LiDAR)–derived fine resolution elevation data, for the central floodplain, were added to the topography model to improve the resolution of key features including wetlands, flow pathways and natural and artificial flow barriers. The hydrodynamic model was calibrated using a combination of in‐stream and floodplain gauge records. A range of off‐stream wetlands including natural and artificial, small and large were investigated for their connectivity with two main rivers (Tully and Murray) flowing over the floodplain for flood events of 1‐, 20‐ and 50‐year recurrence intervals. The duration of the connection of individual wetlands varied from 1 to 12 days, depending on flood magnitude and location in the floodplain, with some wetlands only connected during large floods. All of the wetlands studied were connected to the Tully River for shorter periods than they were to the Murray River because of the higher bank heights and levees on the Tully River and wetland proximity to the Murray River. Other than hydrology, land relief, riverbank elevation and levee banks along the river were found key factors controlling the degree of connectivity. These variations in wetland connectivity could have important implications for aquatic biota that move between rivers and off‐stream habitats during floods. Copyright © 2011 John Wiley & Sons, Ltd.     

Grazing impacts on gully dynamics indicate approaches for gully erosion control in northeast Australia

Drainage network extension in semi‐arid rangelands has contributed to a large increase in the amount of fine sediment delivered to the coastal lagoon of the Great Barrier Reef, but gully erosion rates and dynamics are poorly understood. This study monitored annual erosion, deposition and vegetation cover in six gullies for 13 years, in granite‐derived soils of the tropical Burdekin River basin. We also monitored a further 11 gullies in three nearby catchments for 4 years to investigate the effects of grazing intensity. Under livestock grazing, the long‐term fine sediment yield from the planform area of gullies was 6.1 t ha^‐1 yr^‐1. This was 7.3 times the catchment sediment yield, indicating that gullies were erosion hotspots within the catchment. It was estimated that gully erosion supplied between 29 and 44% of catchment sediment yield from 4.5% of catchment area, of which 85% was derived from gully wall erosion. Under long‐term livestock exclusion gully sediment yields were 77% lower than those of grazed gullies due to smaller gully extent, and lower erosion rates especially on gully walls. Gully wall erosion will continue to be a major landscape sediment source that is sensitive to grazing pressure, long after gully length and depth have stabilised. Wall erosion was generally lower at higher levels of wall vegetation cover, suggesting that yield could be reduced by increasing cover. Annual variations in gully head erosion and net sediment yield were strongly dependent on annual rainfall and runoff, suggesting that sediment yield would also be reduced if surface runoff could be reduced. Deposition occurred in the downstream valley segments of most gullies. This study concludes that reducing livestock grazing pressure within and around gullies in hillslope drainage lines is a primary method of gully erosion control, which could deliver substantial reductions in sediment yield. Copyright © 2018 John Wiley & Sons, Ltd.     

Modelling hydrological connectivity of tropical floodplain wetlands via a combined natural and artificial stream network

The ecological condition and biodiversity values of floodplain wetlands are highly dependent on the hydrological connectivity of wetlands to adjacent rivers. This paper describes a method for quantifying connectivity between floodplain wetlands and the main rivers in a wet tropical catchment of northern Australia. We used a one‐dimensional hydrodynamic model to simulate time‐varying water depths across the stream network (i.e. rivers, streams and man‐made drains). The timing and duration of connectivity of seven wetlands (four natural and three artificial) with the two main rivers in the catchment were then calculated for different hydrological conditions. Location and areal extent of the wetlands and the stream network were identified using high‐resolution laser altimetry, and these data formed key inputs to the hydrodynamic model. The model was calibrated using measured water depths and discharges across the floodplain. An algorithm was developed to identify contiguous water bodies at daily time steps, and this gave the temporal history of connection and disconnection between wetlands and the rivers. Simulation results show that connectivity of individual wetlands to both rivers varies from 26 to 365 days during an average hydrological condition. Location, especially proximity to a main river, and wetland type (natural stream or artificial drain) were identified as key factors influencing these levels of connectivity. Some natural wetlands maintain connection with the river for most or all of the year, whereas the connectivity of some artificial wetlands varies from 26 to 36 days according to their patterns of network connection to adjacent rivers – a result that has important implications for the accessibility of these types of wetland to aquatic biota. Using readily available river gauge data, we also show how connectivity modelling can be used to identify periods when connectivity has fallen below critical thresholds for fish movement. These connectivity patterns within the floodplain network are central to the setting of river flows that will meet environmental requirements for biota that use floodplain wetlands during their life history. Copyright © 2013 John Wiley & Sons, Ltd.     

In Situ Alignment Calibration of Attitude and Doppler Sensors for Precision Underwater Vehicle Navigation: Theory and Experiment

This paper reports the development and experimental evaluation of two in situ least squares techniques for estimating the alignment matrix of Doppler sonars commonly used for precision navigation of oceanographic submersibles. Most previously reported methods addressed the problem of single degree-of-freedom heading alignment using bottom-lock Doppler sonar data and global positioning system (GPS) navigation data. This paper reports and evaluates two techniques for three degree-of-freedom calibration of attitude and Doppler sonar sensors using sensor data available to vehicles at full ocean depth. The first technique provides a general linear least squares estimate of the alignment matrix. The second technique results in a least squares alignment matrix estimate constrained to the group of rotation matrices. The performance of these estimates is evaluated with a laboratory remotely operated vehicle (ROV) and a field-deployed autonomous underwater vehicle (AUV). Experimental results are reported which demonstrate that Doppler navigation employing the reported alignment calibration techniques significantly improves navigation precision. The experiments show that the latter technique provides calibration estimates that improve Doppler navigation precision not only on the calibration data set itself, but also provide improved precision over a wide variety of vehicle trajectories other than the calibration data set.     

Holocene reef growth — One Tree Island,Great Barrier Reef

Data from investigations at One Tree Reef and from other sources suggest that post-glacial coral reefs in the Great Barrier Reef probably date from only about 9,000 years ago. They are likely to have grown on the elevated parts of an irregular karst surface, their general morphology having been largely determined by the shape and depth of this surface. Growth has been dominantly vertical because of the rapid sea-level rise, with lateral modification being quite recent and dominantly in a leeward direction.     

The significance of coral reefs as global carbon sinks— response to Greenhouse

Coral reefs are net sinks for C, principally as CaCO₃ accretion. It is possible to predict quite accurately the rate of production, given adequate information about any particular reef environment. The best data set for an extensive region is that for the Great Barrier Reef (GBR). Careful analysis of this region and the incorporation of previously documented present day system calcification rates suggest net production (G) from G = 1 (kg CaCO₃ m⁻² yr⁻¹) for fringing reefs, to G = 1.9 for planar (infiled platform) reefs, G = 3 for ribbon reefs and lagoonal reefs. The 20,055 km² of reefs in the GBR are thus estimated to average G = 2.4, resulting in a total production of 50 million tonnes yr⁻¹. In a 50–100 year Greenhouse scenario of rising sealevel, we predict that recolonisation of present day reef flats will be extensive and prolific. Production will increase substantially, and this could be by as much as 40% (ranging from 0% for deep shoals to 180% for fringing reefs) to give 70 million tonnes yr⁻¹ if the rate of sealevel rise reaches or exceeds 6–8 mm yr⁻¹We estimate 115,000 km² of oceanic atolls worldwide. Drawing on points equivalence from the detailed analysis of the GBR, we estimate the atolls presently produce 160 million tonnes yr⁻¹. We predict that a similar 40% increase could be possible in the next 100 years or so resulting in a production of 220 million tonnes.Accepting an existing estimate of 617,000 km² for reefs worldwide, drawing from our projections from the GBR and the atolls, and making some assumptions about the remaining reef types (we suggest fringing reefs to dominate) we estimate global reef production at the present time to be 900 million tonnes yr⁻¹. Within the next 100 years or so, we suggest this rate could almost double to 1800 million tonnes. In the long term (several centuries) we predict that the continuing trend of recolonisation, particularly of fringing and planar reefs could result in the production of > 3000 million tonnes yr⁻¹ if rates of sealevel rise approaching or exceeding 6–8 mm yr⁻¹ are achieved. Eventually (> 500 yr), reefs could actually “drown” due to inability to match the rate of sealevel increase if that rate significantly exceeds 6–8 mm yr⁻¹.Thus, coral reefs at present act as a sink for 111 million tonnes C yr⁻¹, the equivalent of 2% of present output of anthropogenic CO₂. In the short term Greenhouse scenario (100 yr) we predict this could increase to the equivalent of 4% of the present CO₂ output. In the much longer term (several centuries), if all trends continue, this could increase to the equivalent of as much as 9% of the present CO₂ output.Unfortunately, we also predict that this considerable sink for C will be most likely of negative value in alleviating Greenhouse because of the immediate effect of CaCO₃ precipitation is to raise the P_CO2 of the surface oceans — ie, ot encourage CO₂ efflux to the atmosphere. We do not attempt to quantify this effect.Other Greenhouse changes such as seawater temperature increase, changes in cloud cover, increased rainfall and runoff, increased storm activity, and changes in dissolved CO₂ concentration and surface ocean circulation may complicate the reef response. However, we suggest that sealevel rise will be the dominant influence, at least during the next 100 years or so.     

The Roosevelt County 079–090 meteorites

Abstract— We have classified 12 new, moderately to severely weathered meteorites from Roosevelt County, New Mexico (RC 079–090) that were recovered between 1969 and 1993. They include nine H chondrites and three L chondrites of petrologic types 4 to 6 and shock classification S1 to S4. Among these are a flight-oriented specimen of an H5 chondrite, an L4 chondrite with a porphyritic impact-melt rock clast, an H5 fragmental breccia with an unusual weathering assemblage (probably a Ca sulfate), and an H4 chondrite with unequilibrated pyroxenes.     

Lifetime of solar flare particles in coronal storage regions

Most discussions of lifetime of flare particles in the solar corona have assumed that collision loss is the dominant means of slowing and stopping these particles. The customary formulas used to estimate the rate of collision loss assume individual fast particles interacting with relatively cold matter. However, it is quite possible that the solar cosmic rays are not imbedded in 10⁶ K coronal material but rather all particles in the storage region are energetic. Collision times are sufficiently short so that the energy spectrum may approach a maxwellian distribution with kT on the order of 30 keV. If this is the case, the rate of collision loss will be greatly reduced. Bremsstrahlung and magnetobremsstrahlung then will be the important energy losses. To account for the presence of appreciable numbers of MeV particles, it is probably necessary to postulate the existence of a non-thermal tail in the stored particle distribution.