The distribution of nearshore bedforms and effects on sand suspension on low-energy, micro-tidal beaches in southwestern Australia
In: Marine Geology. Elsevier: Amsterdam. ISSN 0025-3227; e-ISSN 1872-6151, more
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| Keywords |
Coastal zone
Energy
Sedimentary structures > Bed forms
Sediments > Clastics > Sand
Transport > Sediment transport
ISW, Australia [Marine Regions]
Marine/Coastal |
| Author keywords |
Bedforms; Sand suspension; Low-energy nearshore environment; Sediment transport |
| Abstract |
Field measurements of bed morphologies, hydrodynamics and sediment suspension were made on 15 low energy (Hsig<0.50 m) micro-tidal sheltered sandy beaches in Southwestern Australia. The aim of the study was to differentiate bedform types and cross-shore sequences on low-energy beaches. At each beach, a portable instrument pod capable of measuring steady currents, waves and suspended sediment concentrations, with minimal disturbance of the sandy bed, was deployed at 3–5 cross-shore locations. Cross-shore deployment locations were determined by differences in bedform morphologies and were restricted to the nearshore zone of active sediment suspension. Free divers concurrently measured the bedform wavelengths and heights while noting overall planform pattern at each location.Two general trends in cross-shore bedform patterns were identified among the low-energy beaches. The first trend, found on beaches of finer sand (median grain diameter of 0.24 mm), consisted of a cross-shore sequence from irregular ripples to cross ripples to parallel ephemeral ripples to plane bed in the direction of increasing bottom shear stress. The second sequence, found on beaches of medium sand (median grain diameter 0.39 mm), consisted mainly of parallel ripples with bifurcations that became increasingly organised into parallel ripples as water depth shallowed. The wavelength of the cross ripples was found to increase linearly with increasing near-bed orbital diameter while the wavelength of the ephemeral ripples was found to decrease slightly with increasing near-bed orbital diameter.Suspended sediment concentrations were found to be greater over cross ripples and parallel ripples than over plane bed and ephemeral ripples, even though bottom shear stresses were greater over ephemeral ripples. Co-spectra of suspended sediment and cross-shore currents revealed transport directions to be different over each of the ripple patterns. |
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