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Sediment dynamics of a shallow exposed surfing headland

A study of the wave, current and sediment dynamics was conducted at a shallow and exposed world-class surfing headland at Raglan, on the west coast of New Zealand to determine the mechanism for sustaining the sandy seabed. The location features a large-scale headland with titano-magnetite iron sand dominant on the seabed, adjacent to a rocky reef and boulder shoreline. Field experiments were conducted at the headland involving (i) wave, current and suspended sediment measurements, (ii) side-scan sonar surveys, (iii) hydrographic surveys and (iv) seabed sampling and ground truth observations by divers. Waves and currents were numerically modelled using the 3DD model suite (Black, 2001), with model outputs showing a strong comparison to the measured field data. The results show that the mechanism responsible for maintaining the sandy seabed at the Raglan headland is a combination of sediment transport around the headland to the east (i.e. west coast littoral drift system), and a local re-circulating sediment pathway transporting sediment in the west direction. Sediment moves continually around the headland supplying the littoral system at the headland, and not in “slugs’ as reported for other large protruding headlands. The net sediment flux was approximately 275000 m³/yr flowing east inshore and 100000 m³/yr to the west offshore. The difference of 175000 m³/yr flows from the headland littoral cell into the northwards flowing longshore transport system of the west coast. This compares well to the previous estimate of 175000 m³/yr of net transport along the coast. The alignment of the Raglan headland provides an environment where waves of a high oblique angle generate strong wave-driven currents in the surf-zone, flowing inshore along the headland in an easterly direction. Alternatively, a westerly flowing current is generated by the interaction of wave-driven forces and topographic steering where alignment of the headland changes. Numerical model outputs show the presence of recirculating eddy flows in four cells, which compare very closely to the compartmentalisation of the headland characterised by the main surf breaks, with the width and velocity of the current flows depending on both the size of the swell and the level of the tide. Large wave events at low tide generate the strongest currents flowing east along the headland inshore, whilst at high tide the westerly current also reaches its greatest velocity, and the eddy flows are at their most prevalent. The large inshore currents flowing along the headland decrease the surfability of a headland and make the break difficult if not impossible to surf. However, the large scale and variation in topographic alignment of the Raglan headland can allow access for surfers in the zones of lower wave height and decreased current velocity (e.g. Whale Bay reef). It is recommended that the affect of strong currents on surfability be considered in the design of artificial surfing reefs if amenity of the structure is to be maximised. The wave-orbital interaction with the hydrodynamically ‘rough’ substrate on the rocky boulder reef creates a greater turbulent flow over these beds. Sediment is unable to settle in this zone and is scoured or winnowed away to leave the rocky bed exposed. The regions further shoreward are influenced by the easterly flowing currents more often eroding sand away in the absence of sufficient upstream inputs to maintain the sediment flux. The location of the interface between the sand and rocky seafloor corresponds to the point where the long-term mean currents change from flowing easterly to flowing westerly along the headland. Side scan sonar surveys showed the interface generally followed the shape of the headland and remained fairly stable in position. This is confirmed by the bathymetric surveys where despite significant volume changes in the quantity of sand being transported or deposited at the headland, the seabed level fluctuates about a stable long-term equilibrium position of the bed (quasi permanent position). This is supported by the long-term habitation and zonation of marine organisms on the sub-tidal reef edge, demonstrating an upper level of seabed movement. The characteristics of the seabed in the study area reflect the presence of the surf zone and strong unidirectional currents to the east, and show re-circulating sediment pathways flowing to the west.
Type of thesis
Phillips, D. (2004). Sediment dynamics of a shallow exposed surfing headland (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12339
The University of Waikato
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