Observations of nonlinear run-up patterns on plane and rhythmic beach morphology

Application of non-linear forecasting and bispectral analysis to video observations of run-up over cuspate topography shows that these alongshore patterns in the morphology are accompanied by changes to the fundamental behaviour of the run-up timeseries. Nonlinear forecasting indicates that at beach cusp horns, the behaviour of swash flow is more predictable and global (meaning that characteristics of individual swash events are well represented by the behaviour of the timeseries as a whole). Conversely, at beach cusp bays, the behaviour of swash flow is less predictable and more local (meaning that the characteristics of individual swash events are best represented by the behaviour of a small fraction of the timeseries). Bispectral analysis indicates that there is a nonlinear transfer of energy from the incident wave frequency f to infragravity frequency ~f/2 which only occurs in the bay, suggesting that the local behaviour is caused by interactions between successive swash cycles which are magnified by channelling caused by the beach cusp geometry. The local behaviour and the bispectral signatures are not present in offshore measurements, and are not present in runup timeseries collected when the beach was planar. These results provide evidence that interactions between successive run-ups are a fundamental characteristic of beach cusp bays. Ultimately, these interactions could lead to the growth of an infragravity wave with an alongshore wavelength forced by the presence of beach cusps

How the Marsden Fund has failed to achieve its full potential in the ESA panel: evidence of limitations in scope, biased outcomes, and futile applications

We have analysed the scope of proposals funded by the ‘Earth Sciences and Astronomy’ (ESA) panel of the Marsden Fund for the period 2004 to 2013. The scope of proposals funded is very limited and does not reflect the full remit of the panel: the successful projects fail to encompass the quality and quantity of research being undertaken within the Earth sciences community in New Zealand, and a number of sub-disciplines that seek to address fundamental and important problems within the Earth sciences are largely excluded. Moreover, nearly 50% of the funded proposals for the past decade have been made to just two institutions. To address these limitations, we suggest that: (1) a review is undertaken to examine and widen the scope of the panel to encompass sub-disciplines that demonstrably are never or rarely funded; (2) the composition of panel members be examined and modified to reflect a much wider scope of sub-disciplines within the Earth sciences; and (3) a review of the wide discrepancies in funding distributions on an institutional basis be undertaken. We want to ensure that a more representative range of sub-disciplines, in keeping with modern and realistic definitions of the Earth sciences, is funded through this panel, and so we also recommend the formation of a new panel for ‘Environmental and Earth-system Sciences’ that could encompass the research involving modern-day processes so that applications in these sub-disciplines are not pointless. In addition, it is clear that a very substantial increase in funding to the Marsden Fund must be sought

The use of video imagery to analyse groundwater and shoreline dynamics on a dissipative beach

Groundwater seepage is known to influence beach erosion and accretion processes. However, field measurements of the variation of the groundwater seepage line (GWSL) and the vertical elevation difference between the GWSL and the shoreline are limited. We developed a methodology to extract the temporal variability of the shoreline and the wet-dry boundary using video imagery, with the overarching aim to examine elevation differences between the wet-dry boundary and the shoreline position in relation to rainfall and wave characteristics, during a tidal cycle. The wet-dry boundary was detected from 10-minute time-averaged images collected at Ngaranui Beach, Raglan, New Zealand. An algorithm discriminated between the dry and wet cells using a threshold related to the maximum of the red, green and blue intensities in Hue-Saturation-Value. Field measurements showed this corresponded to the location where the watertable was within 2 cm of the beachface surface. Timestacks, time series of pixels extracted from cross-shore transects in the video imagery, were used to determine the location of the shoreline by manually digitizing the maximum run-up and minimum run-down location for each swash cycle, and averaging the result. In our test data set of 14 days covering a range of wave and rainfall conditions, we found 6 days when the elevation difference between the wet-dry boundary and the shoreline remained approximately constant during the tidal cycle. For these days, the wet-dry boundary corresponded to the upper limit of the swash zone. On the other 8 days, the wet-dry boundary and the shoreline decoupled with falling tide, leading to elevation differences of up to 2.5 m at low tide. Elevation differences between the GWSL and the shoreline at low-tide were particularly large when the cumulative rainfall in the preceding month was greater than 200 mm. This research shows that the wet-dry boundary (such as often used in video shoreline-finding algorithms) is related to groundwater seepage on low-sloped, medium to fine sand beaches such as Ngaranui Beach (mean grain size~0.27 mm, beach slope ~1:70) and may not be a good indicator of the position of the shoreline

On the stabilizing influence of silt on sand beds

In marine environments, sediments from different sources are stirred and dispersed, generating beds that are composed of mixed and layered sediments of differing grain sizes. Traditional engineering formulations used to predict erosion thresholds are however, generally for unimodal sediment distributions, and so may be inadequate for commonly occurring coastal sediments. We tested the transport behavior of deposited and mixed sediment beds consisting of a simplified two-grain fraction (silt (D50  =  55 µm) and sand (D50 =  300 µm)) in a laboratory-based annular flume with the objective of investigating the parameters controlling the stability of a sediment bed. To mimic recent deposition of particles following large storm events and the longer-term result of the incorporation of fines in coarse sediment, we designed two suites of experiments: (1) “the layering experiment”: in which a sandy bed was covered by a thin layer of silt of varying thickness (0.2–3 mm; 0.5–3.7 wt %, dry weight in a layer 10 cm deep); and (2) “the mixing experiment” where the bed was composed of sand homogeneously mixed with small amounts of silt (0.07–0.7 wt %, dry weight). To initiate erosion and to detect a possible stabilizing effect in both settings, we increased the flow speeds in increments up to 0.30 m/s. Results showed that the sediment bed (or the underlying sand bed in the case of the layering experiment) stabilized with increasing silt composition. The increasing sediment stability was defined by a shift of the initial threshold conditions towards higher flow speeds, combined with, in the case of the mixed bed, decreasing erosion rates. Our results show that even extremely low concentrations of silt play a stabilizing role (1.4% silt (wt %) on a layered sediment bed of 10 cm thickness). In the case of a mixed sediment bed, 0.18% silt (wt %, in a sample of 10 cm depth) stabilized the bed. Both cases show that the depositional history of the sediment fractions can change the erosion characteristics of the seabed. These observations are summarized in a conceptual model that suggests that, in addition to the effect on surface roughness, silt stabilizes the sand bed by pore-space plugging and reducing the inflow in the bed, and hence increases the bed stability. Measurements of hydraulic conductivity on similar bed assemblages qualitatively supported this conclusion by showing that silt could decrease the permeability by up to 22% in the case of a layered bed and by up to 70% in the case of a mixed bed

The effects of tides on swash statistics on an intermediate beach

Swash hydrodynamics were investigated on an intermediate beach using runup data obtained from video images. Under mild, near-constant, offshore wave conditions, the presence of a sandbar and the tidally controlled water depth over its crest determined whether most of the incoming waves broke before reaching the shoreline. This forced a change in the pattern of wave energy dissipation across the surf zone between low and high tide, which was reflected by changes to swash on time scales of a few hours. Significant runup height (Rs, defined as 4 times the standard deviation of the waterline time series), was found to vary by a factor of 2 between low tide, when most of the waves were breaking over the sandbar (Rs/Hs ≈ 1.5, where Hs is the offshore significant wave height) and high tide, when the waves were barely breaking (Rs/Hs ≈ 2.7). The increase in wave energy dissipation during low tide was also associated with changes in swash maxima distribution, a decrease in mean swash period, and increasing energy at infragravity frequencies. Bispectral analysis suggested that this infragravity modulation might have been connected with the presence of secondary waves

The hydrodynamics of the southern basin of Tauranga Harbour

The circulation of the southern basin of Tauranga Harbour was simulated using a 3-D hydrodynamic model ELCOM. A 9-day field campaign in 1999 provided data on current velocity, temperature and salinity profiles at three stations within the main basin. The tidal wave changed most in amplitude and speed in the constricted entrances to channels, for example the M2 tide attenuated by 10% over 500 m at the main entrance, and only an additional 17% over the 15 km to the top of the southern basin. The modelled temperature was sensitive to wind mixing, particularly in tidal flat regions. Residence times ranged from 3 to 8 days, with higher residence times occurring in sub-estuaries with constricted mouths. The typical annual storm events were predicted to reduce the residence times by 24%–39% depending on season. Model scenarios of storm discharge events in the Wairoa River varying from 41.69 m3/s to 175.9 m3/s show that these events can cause salinity gradients across the harbour of up to 4 PSU

Matlab application for fitting progress curves to the Equilibrium Model

The general procedures for carrying out the necessary rate determinations required for accurate determination of the Equilibrium Model parameters, and fitting this data to the mathematical model to generate the parameters, are described in "Peterson, M.E., Daniel, R.M., Danson, M.J. & Eisenthal, R. (2007) The dependence of enzyme activity on temperature: determination and validation of parameters. Biochemical Journal, 402, 331-337". It should be borne in mind that the Equilibrium Model equation contains exponentials of exponentials – quite small deviations from ideal behaviour, or a failure to obtain true Vmax values, may lead to difficulty in obtaining reliable Equilibrium Model parameters

The use of imaging systems to monitor shoreline dynamics

The development of imaging systems is nowadays established as one of the most powerful and reliable tools for monitoring beach morphodynamics. Two different techniques for shoreline detection are presented here and, in one case, applied to the study of beach width oscillations on a sandy beach (Pauanui Beach, New Zealand). Results indicate that images can provide datasets whose length and sample interval are accurate enough to resolve inter-annual and seasonal oscillations, and long-term trends. Similarly, imaging systems can be extremely useful in determining the statistics of rip current occurrence. Further improvements in accuracy and reliability are expected with the recent introduction of digital systems

Deposition gradients across mangrove fringes

Observations in a mangrove in the Whangapoua Harbour, New Zealand, have shown that deposition rates are greatest in the fringing zone between the tidal flats and the mangrove forest, where the vegetation is dominated by a cover of pneumatophores (i.e. pencil roots). Current speeds and suspended sediment concentrations dropped substantially across this zone. Near-bed turbulence within the fringe was substantially lower where the pneumatophore canopy was denser, facilitating the enhanced deposition in this zone. However, the near-bed conditions were not the primary control on the instantaneous sediment concentrations at this site. The total deposition across the different zones was the combined result of the reduced near-bed turbulence inside the vegetation and the larger-scale dynamics over the spatially variable vegetation cover, along with other confounding factors such as changing sediment inputs

Model versus nature: Hydrodynamics in mangrove pneumatophores

Water flows through submerged and emergent vegetation control the transport and deposition of sediment in coastal wetlands. Many past studies into the hydrodynamics of vegetation fields have used idealized vegetation mimics, mostly rigid dowels of uniform height. In this study, a canopy of real mangrove pneumatophores was reconstructed in a flume to quantify flow and turbulence within and above this canopy. At a constant flow forcing, an increase in pneumatophore density, from 71 m⁻² to 268 m⁻², was found to cause a reduction of the within-canopy flow velocities, whereas the over-canopy flows increased. Within-canopy velocities reduced to 46% and 27% of the free-stream velocities for the lowest and highest pneumatophore densities, respectively, resulting in stronger vertical shear and hence greater turbulence production around the top of the denser pneumatophore canopies. The maximum Reynolds stress was observed at 1.5 times the average pneumatophore height, in contrast to uniform-height canopies, in which the maximum occurs at approximately the height of the vegetation. The ratios of the within-canopy velocity to the free-stream velocity for the pneumatophores were found to be similar to previous observations with uniform-height vegetation mimics for the same vegetation densities. However, maxima of the scaled friction velocity were two times smaller over the real pneumatophore canopies than for idealized dowel canopies, due to the reduced velocity gradients over the variable-height pneumatophores compared to uniform-height dowels. These findings imply that results from previous studies with idealized and uniform vegetation mimics may have limited application when considering sediment transport and deposition in real vegetation, as the observed turbulence characteristics in nonuniform canopies deviate significantly from those in dowel canopies