Modelling Morphological Changes and Migration of Large Sand Waves in a Very Energetic Tidal Environment: Banks Strait, Australia

Banks Strait, Tasmania, Australia, has been identified as a potential site for the deployment of tidal turbines. In this study, the characterization of sediment transport and large sand waves for this site is performed. Observations of bed level change collected from surveys in 2018 showed a migration of large sand waves over a period of nine months. Migration rates in an excess of one hundred meters for nine months were found, which are large compared to the rate reported at other coastal sites, by several meters per year. A validated hydrodynamic model is coupled with a morphodynamic model to perform sensitivity tests and identify what parameters influence migration to better understand sediment dynamic in the Banks Strait. Numerical analysis showed a constant shift of the sand waves profile in an eastward direction, consistent with the observations. This migration was strongly linked with tidal asymmetry, with a residual current flowing towards the east. The principal parameters driving the migration of sand waves in the Banks Strait were found to be sediment sorting, bed friction and residual current. This study gives new insights for the seabed of Banks Strait and provides an assessment of the natural variability of sediment for futures tidal farms deployments.

A 4-D reconstruction of post debris-flow sediment dynamic inferred from multi-temporal terrestrial laser scanning and photogrammetry (Roßbichelgraben, Germany)

<p>Debris flows are destructive mass movements in steep alpine torrents. Due to their high magnitudes and impact pressures economic goods and human lives are threatened in inhabited areas. The amount of entrained material depends largely on the mobilisable loose debris available for transport, which in turn controls debris-flow mobility and runout. However, still very limited data exists regarding rates and controls of sediment recharge in debris-flow channels.</p><p>In June 2015 an extraordinary rainfall event triggered a debris flow in the Roßbichelgraben torrent in southern Germany. Twelve terrestrial laser scan campaigns (> 450 scans positions) and nine temporally synchronised UAV surveys were carried out between June 2015 and September 2019. Both TLS and SfM-based photogrammetry reveal the temporal, spatial and seasonal sediment dynamic in the channel. A nearby meteorological station recorded the rainfall intensity in 10 min intervals. The results show that both terrestrial laser scanning and SfM-based photogrammetry provide equivalent erosion and deposition volumes (difference < 5%). Between June 2015 and September 2019 the channel was refilled with material of adjacent slopes and the above lying catchment (≈ 1.2 m³/d), whereby a higher activity was observed in summer than in winter. In addition, the activity decreased with elapsed time since the debris-flow event, as most over-steepened river banks failed shortly after the event and stabilised over time. Short, intense rainstorm events best explain the sediment dynamic in the channel (R² up to 0.9).</p><p>The results contribute to better understand the sediment dynamic in highly active debris-flow channels and allow for a more reliable estimation of potential debris-flow volumes.</p>

SEDIMENT DYNAMICS BEHAVIOR AND BEACH PROFILE TYPES BASED ON SEDIMENT PROPERTIES

Sand samples along the shorelines and in the rivers of Tottori Prefecture were assayed for chemical elements using energy dispersive X-ray spectrometer. Samples contained more than 60 wt. % Si, Al, Ca, Fe, Na and Mg less than 10 wt. %. It was found by EOF analysis that Si, Al, Ca and Fe were the significant chemical elements. The spatial variations of first mode of Si are related to sediment dynamic behavior. The transport regime is net accretion when the first mode value of Si is increasing, and net erosion when being decreasing.