Factors Controlling Tidal Inlet Characteristics on Low Drift Coasts

Terry M. Hume, Charles E. Herdendorf

Abstract


New Zealand's northeast coast is characterised by low littoral drift and the numerous barrier enclosed estuaries along the shore present an excellent opportunity to examine tidal inlet processes under low littoral drift conditions. The estuaries range in area from c.1 to 98 km2, are micro-mesotidal and have tidal prisms ranging from c.0.8 x 106 to 155 X 106 m3. Tidal inlets mostly occur where Holocene sand barriers have built out towards rock headlands. The inlets generally have positional stability as a result of shelter from wave energy in the lee of rock headlands, however, morphological instability is common and temporal. Ebb tidal delta planform is controlled by the tidal discharge, degree of headland shelter and littoral supply. In situations where a rock headland provides shelter from waves, the ebb tidal delta comprises an elongate or triangular swash platform built normal to the shore in the lee of the headland. Where there is little headland shelter and the littoral drift is very low, the ebb tidal delta is poorly developed and comprises an extension of the beach bar system across the mouth of the inlet. In comparison where there is little headland shelter but the littoral drift and tidal prism are greater, a large and more classical shaped ebb tidal delta bulges out from the coast. In general the inlets can he considered as geometrically stable (i.e. have the ability to return to their initial configuration after a disturbance) because: (1) the generally low width to depth ratio (average 44) indicates that the inlet throat channels are hydraulically efficient, (2) the strong relationship between throat area and tidal prism/discharge indicates that there is a balance between inlet geometry and tidal flow through the gorge, (3) of the similarity of their throat area/tidal prism relationships and width/depth ratios to those inlets stabilized with 2 jetties on the Pacific and particularly the Atlantic coast of the USA, and (4) the high tidal prism/ littoral drift (Ω/Mtot) ratios indicate that entrance conditions are good and that the tidal current is the main mechanism for transporting sediment through the entrance. The rock headlands at the northeast coast inlets appear to act in a similar manner to jetties in directing the ebb tide jet and so forcing tidal dominance of sediment transport, creating an efficient throat profile and a geometrically stable entrance.

 


Keywords


Tidal inlet stability; tidal hydraulics; inlet morphology; tidal deltas; estuary; New Zealand; empirical models

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