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Coastal and sediment dynamics in a high-energy, rocky environment

A study of the wave dynamics, hydrodynamics and sediment dynamics was conducted at an energetic open-coast site at New Plymouth, New Zealand. The location features large, shore-normal reefs of volcanic agglomerate extending from an irregular shoreline, with “black” volcanic sands intermittently overlying a rocky bed. The breakwaters of an artificial harbour (Port Taranaki) interrupt the longshore littoral transport, and dredging and offshore dumping causes “downstream” erosion along the city foreshore. The key study objectives were to investigate the local coastal dynamics, and to examine ways to mitigate the effects of the harbour on the adjacent coast. Two large experimental field programmes were conducted involving: i) wave, current and suspended sediment measurements, ii) side-scan sonar and hydrographic survey, iii) sediment tracing, and iv) experimental dumping of 47,000 m³ of dredged sand in a rocky nearshore zone (6-10 m depth). Sediment trap calibration experiments were also performed. Analysis of the measured wave data has identified wave-induced motion as a significant source of velocity error in p, u, v meters on taut-wire moorings, and wave directions resolved from the cross-spectra of pressure and velocity have frequency dependent errors due to the phase difference between the pressure and velocity data. Accordingly, the spectra of wave direction should be resolved solely from velocity data. Calibration of sediment traps against an automated water sampler has provided the first strong evidence that traps are effective in deriving the time-averaged suspended sediment concentration SSC in a wave-dominated coastal environment. Rigid cylindrical traps of high Aspect Ratio have a 75% sampling efficiency. Wave transformations across the complex nearshore bathymetry are dominated by seabed friction and refraction. This process was simulated using the WBEND wave refraction model, applying a constant friction coefficient for sandy beds, and a Nikuradse roughness for rocky beds. Wind vector was found to influence the spatial distribution of wave energy, with whitecapping conditions under onshore-directed winds inducing a longshore “smoothing” in the wave energy, which appears to act in opposition to wave height reinforcement from refraction. Sediment tracing using an artificial fluorescent material proved to be an effective method for observing the transport of littoral sediments over sandy and rocky beds, over distances of up to 5 km. The tracer exhibited a horizontal diffusivity of the order 0.1 m²s⁻¹, of which approximately 5% was expressed in the cross-shore direction. The time-averaged entrainment (C₀) of “black” volcanic sands under high-energy conditions may be effectively predicted using the excess non-dimensional skin friction (θ₂.₅) raised to the power of 1.5, with the mean grain size used in the sediment mobility term (Ψ) and the median grain size in the friction factor (fw). The scale of turbulent mixing (lₛ) increases linearly with elevation above the bed, with a gradient of 0.46. Suspended sediments show a systematic (and linear) reduction in mean grain size with increasing elevation above bed. SSCs over rocky beds are supply-limited, and inversely proportional to the distance from contiguous sandy beds. Sediments are principally transported in the suspended form, and a system of irregular shore-normal reefs does not significantly obstruct the longshore flux. Rocky beds appear to be resilient to sediment inundation and the sand/rock sediment facies have remained positionally stable over 16 years. Dumping sand in a rocky reef environment does not necessarily result in adjacent beds becoming inundated with sand. In this environment, a nearshore dump mound did not migrate as a contiguous body, but slowly dispersed as suspended sediments. Net sediment transport at New Plymouth is directed alongshore to the northeast, although the sediments respond to local and temporal perturbations, which give rise to reversing and circulating fluxes. Port Taranaki traps 174,000 m³ of coastal sediments per year, of which 142,000 m³ accumulates at the breakwater tip. The latter are derived from the shallow zone seaward of the breakwater wall (i.e. within ∼50 m). Sediments in depths > 6 m (i.e. > 100 m from the breakwater wall) bypass the harbour and traverse the rocky reefs toward the eastern beaches. The historical offshore dumping practice removes sediments from the nearshore littoral budget, while the orientation of the main port breakwater causes the longshore sediment flux to be directed away from the adjacent (Kawaroa) coast, further reducing the supply of littoral sediments to that region. A new dumping ground has been identified for the breakwater-tip sediments, which is projected to retain the dredged sediments within the nearshore littoral system, feeding sand to the central and eastern city foreshore. Sedimentation of the coast immediately adjacent to the port is not expected to result from the long-term use of this ground.
Type of thesis
McComb, P. (2001). Coastal and sediment dynamics in a high-energy, rocky environment (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14354
The University of Waikato
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