Kerry Black

Control of estuarine sediment dynamics by interactions between currents and waves at several scales

Abstract The effect of interactions between continuous (tidal currents) and intermittent (waves) processes on sediment dynamics and transport is addressed by presenting detailed field measurements of waves, boundary-layer currents and suspended sediment from an estuarine channel and an adjacent intertidal sandflat in Manukau Harbour, New Zealand. The aim is to determine in what ways it is necessary to couple waves and currents in numerical models, and thereby put limits on the fundamental structure of process-based estuarine sediment transport models. Waves were important on the intertidal flat: turbidity switched on and off with the appearance and disappearance of waves; wave groups dominated entrainment of bed sediment; a wave-current boundary-layer model explained measured bed shear stress and hydraulic roughness; and the measured near-bed time-averaged suspended-sediment concentration was mostly well predicted by a pure-wave model. Both the waves themselves and wave-related processes varied markedly over the tidal cycle. The variation in the former (principally changes in wave height) was related to changes in fetch caused by the harbour-wide emergence and submergence of intertidal regions. The variation in the latter was related to changes in water depth relative to the wavelength of the waves, which controlled the penetration to the bed of wave-orbital currents. In addition to the variation over the tidal cycle in the ‘intensity’ of wave processes, there was also a change in ‘kind’, which occurred with the arrival at the measurement site of the ‘turbid fringe’, which is the narrow, highly turbid edge of the estuarine water body. The relationship between suspended-sediment concentration and wave-orbital velocity in the turbid fringe was radically different to the relationship in the estuarine water body proper, which suggests a change in dynamics, perhaps related to breaking waves. A ‘hybrid’ modelling approach is required, i.e. one that treats discrete events but resolves tidal-cycle-scale variation within the event. There is a need to resolve the variation in the wave train over the tidal cycle and the penetration to the bed of wave-orbital motions, both of which could only be done adequately within an estuary tidal model. In contrast to the situation on the intertidal flat where waves intermittently entrained sediment, sediment transport in the channel was continuous, driven by tidal currents. To predict sediment flux in the channel we need to know the upstream sediment-transporting capacity of the flow (including that contributed by waves), the character of the bed sediment, and the sediment-settling time scale. These factors confounded even the simplest notion of flood and ebb dominance, which frequently has been applied to understand estuarine morphodynamics.

Cross-shore suspended sediment transport in the surf zone: a field-based parameterization

Existing cross-shore sediment transport models for two-dimensional surf zone bathymetries almost invariably predict offshore-directed sand transports and bar migrations during storm conditions. However, onshore-directed suspended sediment fluxes and associated nearshore bar migration were observed during recent field experiments on a gently sloping beach on the Danish North Sea coast. Field measurements of suspended sediment flux obtained during three experiments on two different beaches are used to parameterize the observed fluxes. This parameterization predicts suspended sediment transport due to incident waves and undertow across bars in two-dimensional surf zones. First, a non-dimensional sediment flux index is formulated which describes the tendency towards net onshore or offshore transport and the strength of that tendency. The non-dimensional formulation circumvents the problem of measurement inconsistencies due to varying elevations of sediment concentration sensors relative to the bed. The index is found to depend upon the undertow velocity, the incident wave skewness and the cross-correlation between orbital velocity and sediment concentration. However, some of these parameters are difficult to predict, particularly in barred surf zones and therefore, the independent variables are recast in terms of a set of more easily obtainable parameters. The sediment flux index depends on a combination of the following: non-dimensional bed shear stress (the Shields parameter), relative water depth, wave orbital velocity, relative wave height and bed slope. Finally, a formulation of suspended sediment transport across bars is obtained by linking the flux index with a parameterization of the sediment concentration/distribution in the water column. These concentrations are found to depend on non-dimensional bed shear stress, relative wave height and water depth. The formulation predicts a tendency for onshore-directed sediment transport due to incident waves on gently sloping beaches and/or with large bed shear stresses. On steeply sloping beaches and/or in the inner part of the surf zone there is a tendency towards offshore sediment transports due to the undertow.

Spectral Estimates of Dissipation Rate within and near the Surf Zone

Abstract Three-dimensional point velocity measurements from within and near the surf zone are used to examine changes in turbulent dissipation rate with location relative to the breakpoint and wave conditions. To separate turbulence from wave orbital currents, dissipation rate is derived using the inertial subrange of the wavenumber spectrum. The measured frequency spectrum is transformed into a wavenumber spectrum by generalizing Taylors hypothesis to advection of turbulence past a sensor by monochromatic water waves in arbitrary water depth. The resulting dissipation rate is compared with that obtained by averaging the dissipation rates calculated from frequency spectra of very short segments of the time series, over which Taylors hypothesis for steady currents can be used. The dissipation rate calculated using the latter compares well, in the averaged sense, to the dissipation rate calculated using the whole time series, even though the whole time series includes segments that violate Taylors assump...

A spectral analysis procedure for dating Quaternary deep-sea cores and its application to a high-resolution Brunhes record from the Southwest Pacific

Abstract Date-32 is a fast and easily used computer program developed to date Quaternary deep-sea cores by associating variations in the earths orbit with recurring oscillations in core properties, such as carbonate content or isotope composition. Starting with known top and bottom dates, distortions in the periodicities of the core properties due to varying sedimentation rates are realigned by fast Fourier analysis so as to maximise the spectral energy density at the orbital frequencies. This allows age interpolation to all parts of the core to an accuracy of 10 kyrs, or about 1.5% of the record duration for a typical Brunhes sequence. The influence of astronomical forcing is examined and the method is applied to provide preliminary dates in a high-resolution Brunhes record from DSDP Site 594 off southeastern New Zealand.

Sediment transport near the break point associated with cross-shore gradients in vertical eddy diffusivity

Sediment clouds entrained and mixed vertically into the water column at the breakpoint were observed to advect offshore with the return current following wave breaking at a natural beach. An advection/diffusion model is applied to this phenomenon, providing for cross-shore gradients in vertical eddy diffusivity within and beyond the surf zone which result in differential entrainment and settlement near the break point. Results indicate that the horizontal gradient in turbulent vertical sediment mixing can provide a significant mechanism for bar generation, even if the surface gravity and infra-gravity waves remain uncoupled. Quantitative estimates of the rate of bar accretion are obtained after calibrating the sediment transport model using field measurements of currents and suspended sediment loads. In the absence of other processes, accretion rates of order 20 mm/hour associated with the horizontal gradient in vertical diffusivity are predicted on the offshore bar.

A Multipurpose, Artificial Reef at Mount Maunganui Beach, New Zealand

A multipurpose, artificial, offshore reef has been designed for construction at Mount Maunganui Beach, New Zealand. The proposed reef will form the basis for research into coastal protection, amenity enhancement (particularly surfing, but also diving, fishing, and beach recreation), biological response, and social and economic impacts. In order to proceed with reef construction, a five-year resource permit is being sought from the regulatory authority, and this application required an assessment of the likely environmental impacts of the proposed reef. The studies undertaken for the assessment included the examination of physical, biological, social, and economic impacts. A comprehensive design process was undertaken to incorporate the amenity of surfing into a submerged reef shape. Programs to monitor physical and biological responses, as well as social and economic impacts, were also established. These studies support the use of multipurpose, artificial, offshore reefs as an environmentally friendly sol...

Bars formed by horizontal diffusion of suspended sediment

Abstract The mechanism responsible for the ubiquitous presence of convex beach profiles and shoreward migration of linear bars is examined using numerical circulation and sediment transport models. The models are validated against laboratory measurements and observed natural beach cross-sections. While not discounting the importance of infragravity and advective horizontal circulation or bed-return flow mechanisms, a robust diffusive process explains the convex profile shape and bar formation. In the presence of concentration gradients across the surf zone, a diffusive sediment flux from high to low concentration results in the transfer of sediment outwards from the breakpoint, both onshore and offshore, and the subsequent formation of a “diffusion bar” and “diffusion profile”. The profiles are characterised by single- and double-convex dome-like shapes, developing during shoreward migration of the bars by the diffusion mechanism. The mechanism explains several phenomena observed on natural beaches, including (i) convex beach profiles; (ii) shoreward migration of the bar with concomitant beach accretion under narrow-band swell; (iii) reduced propensity for bar formation on low-gradient, fine-sand beaches or under wide-band wave spectra (even though multiple bars are common on some low-gradient beaches) and (iv) offshore migration of the bar during periods of increasing wave height. The diffusion mechanism can be dependent on orbital motion alone and, as such, requires no frequency selection or strong correlation between multiple processes for bar formation.

Port redesign and planned beach renourishment in a high wave energy sandy-muddy coastal environment, Port Gisborne, New Zealand

Redesign of Port Gisborne for the 21st century has encompassed a broad interdisciplinary approach. This procedure has taken into account the operational requirements of the port, effects of dredging and construction upon the benthic fauna, and wave activity within the port confines after the proposed development. Added amenity value of the development to the local community is an important ancillary redesign consideration. Initially, a major research project into the environmental impacts of the developments has been undertaken.The project, which commenced in 1996 and is still continuing, involves an iterative approach integrating the initial design and development options with the operational feasibility, construction constraints, environmental constraints, social acceptability, and economic practicality of the port; all of these require in-depth assessment to obtain the necessary planning and development approvals. This requires close liaison between the professional environmental research scientists, port management, port operation staff (pilots), construction engineers, planners, and the community interest groups.Numerical modelling of the hydrodynamics of Poverty Bay, simulating waves and current effects on the various initial designs options, and calibrated against data from a substantial field program, has been a fundamental tool. It was applied experimentally to determine the best option for the port layout, as well as to assess sedimentation impacts. Modelling results indicated a significant increase in maintenance dredging expected as a result of deepening the navigation approach channel. Because this may have an impact on the nearby sandy beach by inducing erosion, the best option for disposal of the sandy dredged material was determined to be disposal in the surf zone for subtidal beach profile renourishment. Textural analysis of the sediments trapped in the navigation channel demonstrated that they were suitable for this purpose. D 2002 Elsevier Science B.V. All rights reserved.

Detailed Observations of Littoral Transport Using Artificial Sediment Tracer, in a High-Energy, Rocky Reef and Iron Sand Environment

Abstract On a high-energy coast, with heterogeneous nearshore bathymetry and sediment dynamics (New Plymouth, New Zealand), two colours of artificial fluorescent tracer were used to i) investigate the sediment entrapment/bypassing of a port entrance, and ii) to monitor sediment movement from an experimental nearshore dredged-sand dump mound (47,000 m3). Two simultaneous releases (one for each colour) were made in 6 to 10 m water depths, near the port entrance and on the dump mound, 1400 m apart. The tracer was tracked over 10 months by collecting and analysing 1179 surficial sediment samples, with concomitant wave/current recording at up to 13 nearshore sites. The results demonstrate trapping in the breakwater tip-shoal of the part, by-passing and movement through the complex nearshore reef system. The dominant transport was alongshore. Tracer released near the port entrance depicted a natural sediment “pathway” past the harbour shipping entrance and over the rocky reefs to the beaches 4.5 km away. Very little tracer was detected within the harbour, indicating that the main transport from the release depths (6–10 m) by-passed the harbour entrance and tip-shoal. The tracer shows that only sediments very close to the harbour breakwater (i.e. within 100 m) are trapped at the harbour entrance. From the dump mound, tracer was observed to migrate mostly longshore away from the port, although a minor “updrift” migration was observed. The tracer results are in accordance with measured and numerically modeled currents, which show a dominant longshore flow away from the port and weak reversing flows in the quiescent lee of the harbour at the dump mound. The tracer data also suggest that the raised shore-normal reefs do not present a significant impediment to longshore transport, but rather that the nearshore circulation patterns are the predominant influence on the sediment transport vectors. Rates of tracer spreading were found to range from 0.095 to 0.288 m2s−1, which is consistent with a high-energy coastal environment. Diffusion and advection of suspended sediments is the primary mechanism of sediment transport, and tracer was found on beaches 4.5 km distant from the release point within 13 days of injection.

The sediment threshold over tidally induced megaripples

Abstract Formulae to predict the initiation of sediment transport over bedforms are derived and field tested. Unlike a Shields-type curve which is valid only for plane beds, both the form drag and skin friction are accounted for. The method relies on successful isolation of the skin friction component of the flow energy losses from the total loss which includes an ineffective form drag component. This is achieved using the Vanoni and Hwang equation for form drag losses while the skin friction is taken from the Yalin sediment threshold curve for plane beds. In conjunction, formulae are derived for the total friction velocity and the speed at 1 m above the bed at the sediment threshold. The theory is tested using thresholds measured in mesotidal estuaries.