Estuarine Beaches in Tairua Harbour

This study investigates two eroding estuarine enclosed beaches in Tairua Harbour to better understand the geomorphic mechanisms that influence erosion and sediment transport. From these findings, the role of artificial beach nourishment can be assessed as a management tool. Abstract from thesis: The Tairua estuary is one of a number of estuaries situated on the northeast coast of the North Island, New Zealand. The shoreline of the estuary is largely comprised of intertidal beaches, termed estuarine enclosed beaches. The study of this type of beach has to date been limited in comparison with other coastal environments, with research being more often concentrated on ocean beaches and tidal inlets. However, with an increased concern for the recreational, commercial and aesthetic values of estuaries, appreciation of the value of estuarine enclosed beaches has increased. This study investigates two examples of eroding estuarine enclosed beaches within the Tairua Estuary to identify and measure those geomorphic mechanisms that influence erosion and sediment transport at these beaches. From these findings the role of artificial beach nourishment as a management tool is assessed. The beaches of the Tairua estuary are typical of those found within sheltered estuarine environments being narrow in width, having a shallow beach slope and "well to moderate sorting" of sediment on the beach face. These characteristics are a result of the low energy environment that exists within the confinements of the estuary. From this study it has been found that there are three geomorphic mechnisms governing the morphology of the estuary enclosed beaches of theTairua estuary. These are wind waves generated within the estuary, tidal currents and swell waves. Of these mechanisms, it is wind waves that perform the greatest amount of geomorphic work. Wind waves within the Tairua estuary, with a significant wave height greater than 0.1 m, were found to be rare, thereby limiting the role that these waves play in the shaping of the estuarine enclosed beaches. Tidal currents measured at the beachface of the two beaches proved to be of insufficient velocities for the transport of the sediment. However, the combination of longshore currents generated from wind waves, with tidal currents can promote the transport of sediment. Swell wave propagation through the tidal inlet, so as to cause erosion of the estuary enclosed beaches, is rare, requiring large swells from the easterly quarter. Sediment erosion at both beaches was found to be minimal. The exception to this was during storm events when either large swell waves, propagating through the tidal inlet, or winds from the south, southwest or west were of a sufficient strength as to create waves large enough to entrain and transport the sediments of these beaches. In considering the nourishment of these beaches it is believed that, due to the limited extent of the horizontal and vertical reworking of sediments on the beachface, sand placed on the beaches should be evenly spread with the existing beach slope retained. The removal of new sand by natural processes should not be a large problem and therefore the stability of a renourished beach is viewed as long lasting.

See above. Tidal currents measured nearshore. Sediment samples. Depth of disturbance rods.

Tairua Estuary

1993-1994

Tidal currents measured 30 m from the beachface with S4 current meters (13 sites on Pauanui side of estuary and 4 sites on Tairua side). Sediment samples taken at shoreline - 4 samples from each location, analysed using RSA (25 samples total).

Short term study.

Data presented in hard copy in Appendices of thesis.

Thesis available in University of Auckland library. Copy also held in Waikato Regional Council Library. Public information - no confidentiality.

Project completed.