MORPHOLOGY AND SIZE OF EBB TIDAL DELTAS AT NATURAL INLETS ON OPEN-SEAAND POCKET-BAY COASTS, NORTH-ISLAND, NEW-ZEALAND

Sand volumes and morphologies of 17 ebb tidal deltas off natural inlet s on the New Zealand North Island coast, in both open-sea and pocket-b ay settings, were investigated. Four basic ebb-delta forms were identi fied. 'Free form' deltas, typically 'bat-winged' in shape, occur on op en shorelines. 'Constricted' deltas are similarly shaped but occur in shoreline angles lacking space for the free form to fully develop. 'Hi gh-angle half-deltas' are typically shore-normal or L-shaped' and occu r in embayment corners where the ebb jet flows against the rocky headl and, resulting in a significant shoal forming only on the beach side o f the inlet. 'Low-angle half-deltas' are almost shore-parallel sand 'w edges' that form between the ebb jet and the beach where the ebb jet i s forced by rock controls to Bow at a low angle to the beach. Sand sto rage volumes ranged from 3.8 x 10(4) m(3) to 1.2 x 10(16) m(3). The ma in controls on ebb delta sand volume (V) are the tidal prism volume TP ), the angle between the outflow jet and the shoreline (theta), and th e wave climate. The empirical equation V = 1.37 x 10-3P(L32)(sin theta )(1.38) accounts for 83% of the variance in sand volume in the dataset . Deltas on the high-energy west coast tended to be smaller than east coast deltas with similar tidal prisms. The supply of littoral drift a lso appears to influence delta volume in some cases. These results may be used to help assess permissible rates of sand mining from ebb delt as, to estimate ebb-delta sand entrapment associated with changes in t he tidal prism, and to re-design the alignment of inlet channels in or der to control ebb-delta sand volumes.