The Role of Negative Buoyancy in the Morphodynamics of River Dominated Shelves: a Global Synthesis

This study synthesized knowledge regarding waveand current-supported sediment gravity flows and their importance to shelf deposition and morphology. Data and publications from STRATAFORM, EuroSTRATAFORM and several river-nourished shelves in Asia and the Western Pacific were reviewed and models for wave-supported fluid muds and resulting gravity flows were applied. Field and modeling evidence suggests that wave and current supported gravity driven transport is a primary cause of across-shelf transport and emplacement of flood deposits on many muddy shelves and may be a major contributor to and control on the large-scale formation and morphology of subaqueous deltas and shelf clinoforms. River effluents deliver sediment to the inner shelf for subsequent remobilization by high wave and current induced bed stresses but there is little field evidence that hyperpycnal river effluents regularly traverse shelves without interruption. Model results predict that convergence of down-slope sediment transport by wave-supported gravity flows increases with bed slope but decreases with slope gradient, such that greatest deposition occurs where steep slopes first become constant with distance offshore. New field data from the 40 m and 60 m isobaths on the shelf off the mouth of the Waiapu River, New Zealand support these conclusions and highlight the additional importance of currents under some conditions.