Karin R. Bryan

Modeling the morphodynamic response of tidal embayments to sea-level rise

Sea-level rise has a strong influence on tidal systems, and a major focus of climate change effect studies is to predict the future state of these environmental systems. Here, we used a model to simulate the morphological evolution of tidal embayments and to explore their response to a rising sea level. The model was first used to reproduce the formation of channels and intertidal flats under a stable mean water level in an idealised and initially unchannelled tidal basin. A gradual rise in sea level was imposed once a well-developed channel network had formed. Simulations were conducted with different sea-level rise rates and tidal ranges. Sea-level rise forced headward erosion of the tidal channels, driving a landward expansion of the channel network and channel development in the previously non-inundated part of the basin. Simultaneously, an increase in channel drainage width in the lower part of the basin occurred and a decrease in the overall fraction of the basin occupied by channels could be observed. Sea-level rise thus altered important characteristics of the tidal channel network. Some intertidal areas were maintained despite a rising sea level. However, the size, shape, and location of the intertidal areas changed. In addition, sea-level rise affected the exchange of sediment between the different morphological elements. A shift from exporting to importing sediment as well as a reinforcement of the existing sediment export was observed for the simulations performed here. Sediment erosion in the inlet and the offshore transport of sediment was enhanced, resulting in the expansion of the ebb-tidal delta. Our model results further emphasise that tidal embayments can exhibit contrasting responses to sea-level rise.

Beach Rotation at Two Adjacent Headland-Enclosed Beaches

ABSTRACT Bryan, K.R. Foster, R., and MacDonald, I. 2013., Beach Rotation at Two Adjacent Headland-Enclosed Beaches Shoreline rotation occurs on headland-enclosed beaches when the wave climate causes sand to accumulate preferentially on one end of the beach. Past research has shown that these rotation events can occur in response to seasonal variations in the wave climate. Here we investigate 3 years of shoreline variations on two adjacent beaches in New Zealand that are exposed to the same wave climate and tidal conditions, but with different morphodynamic classification. Tairua Beach is approximately 1.2 km long, and is composed of medium-fine sand. Pauanui Beach is approximately 3 km long, and is characterised by lower slopes and finer sand grain sizes. The textural differences are caused by the proximity of Pauanui Beach to the entrance of Tairua Estuary. Both beaches are overlooked by video monitoring stations which collect 15-minute averages of video footage every hour between 2002 and 2004. Shorelines were detected by using the ratio of red to blue light to identify the water-sand boundary. Rotation was quantified by fitting a regression line to the alongshore series of shoreline locations. Results showed that both beaches accreted and eroded by comparable amounts during storm events. However, the rotation characteristics were quite different, with the magnitude of rotation being far greater at the shorter Tairua Beach than at Pauanui. Pauanui responded with a slow rotation over the whole dataset rather than the more event-based rotation that occurred at Tairua Beach. In addition, the two beaches rotated in opposite directions during most of the dataset. A SWAN model was set up to understand the wave induced processes that caused the differences in rotation between the two sites. The model grid was generated from a multibeam survey of the site. The model was set up in a stationary mode providing a solution every 3 hours over the entire duration of the dataset. Results showed that the alongshore wave energy flux was largely controlled by the presence of two offshore islands, Shoe and Slipper island, which caused shadowing of the north end of Pauanui Beach and the south end of Tairua Beach. This in turn accounts for the observed differences in the rotation pattern at the two beaches. These results show that the offshore topography was a stronger control on rotation than overall changes to the alongshore wave energy flux driven by incidence angle changes alone.

Variations in nutrient concentrations at different time scales in two shallow tidally dominated estuaries

Water-quality observations in estuaries can be highly variable in time and space, making it difficult to quantify nutrient fluxes and to discriminate patterns. We measured nitrate, phosphate and ammonium concentrations in two shallow tidally dominated estuaries in Tauranga Harbour, New Zealand, during four periods (winter, start of spring, end of spring and summer) within 1 year, to determine the source of variability observed in a 19-year monitoring program. These measurements consisted of high-frequency monitoring during one 24-h period (covering a daytime flood-ebb tide and a night-time flood-ebb tide) at each estuary. Concentrations of nitrate and ammonium had distinctive tidal patterns, with rising values during ebb flows. This tidal asymmetry caused a net seaward flux of dissolved inorganic nitrogen (nitrate and ammonium), with higher exports at night. Net fluxes were 34–358 kg N per tidal cycle for nitrate and 22–93 kg N per tidal cycle for ammonium. Fluxes were large relative to previously published model-based predictions for the region, particularly during winter. Our results showed that estuarine sampling strategies need to account for tidal variability and the role of episodic runoff events, and highlighted the importance of correctly validated mass fluxes from field measurements for comparisons with nutrient-loading models.

Video-Based Detection of Shorelines at Complex Meso-Macro Tidal Beaches

Abstract Almar, R.; Ranasinghe, R.; Sénéchal, N.; Bonneton, P.; Roelvink, D.; Bryan, K.R.; Marieu, V., and Parisot, J-P., 2012. Video-based detection of shorelines at complex meso–macro tidal beaches. Remote video imagery is widely used to acquire measurements of intertidal topography by means of shoreline detection, but, up to now, problems of accuracy were still encountered in the challenging case of energetic waves in nonuniform, meso–macro tidal environments. Unique, simultaneous, video-based and global positioning system (GPS)–based measurements of shoreline were undertaken at Truc Vert (France), a beach with such characteristics. An innovative video method, referred to herein as the Minimum Shoreline Variability (MSV) method, was developed to cope with highly variable spatiotemporal shoreline properties. The comparison of video-based and GPS-derived shoreline data sets showed that using images averaged over short periods (30 s), rather than the traditionally used 10-min averaged images, significantly improved the accuracy of shoreline determination. A local video-derived, swash-based shoreline correction was also developed to correct for the MSV error, which was found to be linearly correlated to local swash length. By combining shorter time-averaged images and video derived local swash correction factors, the horizontal root mean square error associated with MSV shorelines was reduced to 1.2 m, which is equivalent to errors reported at more uniform, microtidal, and less-energetic beaches.

Effect of selection and sequencing of representative wave conditions on process-based predictions of equilibrium embayed beach morphology

In order to decrease the simulation time of morphodynamic models, often-complex wave climates are reduced to a few representative wave conditions (RWC). When applied to embayed beaches, a test of whether a reduced wave climate is representative or not is to see whether it can recreate the observed equilibrium (long-term averaged) bathymetry of the bay. In this study, the wave climate experienced at Milagro Beach, Tarragona, Spain was discretized into ‘average’ and ‘extreme’ RWCs. Process-based morphodynamic simulations were sequenced and merged based on ‘persistent’ and ‘transient’ forcing conditions, the results of which were used to estimate the equilibrium bathymetry of the bay. Results show that the effect of extreme wave events appeared to have less influence on the equilibrium of the bay compared to average conditions of longer overall duration. Additionally, the persistent seasonal variation of the wave climate produces pronounced beach rotation and tends to accumulate sediment at the extremities of the beach, rather than in the central sections. It is, therefore, important to account for directional variability and persistence in the selection and sequencing of representative wave conditions as is it essential for accurately balancing the effects beach rotation events.

Video observations of rip currents on an embayed beach

Rip currents and their interaction with waves and underwater morphology are still poorly understood. This study presents a conceptual model demonstrating how rip channels respond to changes in wave conditions, focusing on wave energy and wave event duration. Past attempts to relate rip channels to wave conditions have not resulted in good relationships between rip characteristics (e.g. rip spacing) and waves. In order to address this problem, a 3.3 year rip channel data set was obtained using an improved computer-based technique to locate rips from video imagery. In this study we show how the scale of rip channels (i.e. cross-shore extent ), previous wave conditions and the duration of high wave events determine how rip channels will evolve. Observations of six events when rip channels changed their spatial configuration are used to create a conceptual model for how rip channels respond to changes in the wave conditions. When rip channels are small in relation to the wave energy, these rips are more likely than larger rips (extending less than ~70m cross-shore) to evolve. Conversely when rip channels are large in relation to the wave energy, these rips are less likely to evolve than smaller rips (extending more than ~80 m cross-shore).

Regional influence of climate patterns on the wave climate of the southwestern Pacific: The New Zealand region

This work investigates how the wave climate around New Zealand and the southwest Pacific is modulated by the Pacific Decadal Oscillation (PDO), El Nino-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Zonal Wave-number-3 Pattern (ZW3), and Southern Annular Mode (SAM) during the period 1958–2001. Their respective climate indices were correlated with modeled mean wave parameters extracted from a 45 year (1957–2002) wave hindcast carried out with the WAVEWATCH III model using the wind and ice fields from the ERA-40 reanalysis project. The correlation was performed using the Pearsons correlation coefficient and the wavelet spectral analysis. Prior to that, mean annual and interannual variabilities and trends in significant wave height (Hs) were computed over 44 years (1958–2001). In general, higher annual and interannual variabilities were found along the coastline, in regions dominated by local winds. An increasing trend in Hs was found around the country, with values varying between 1 and 6 cm/decade at the shoreline. The greatest Hs trends were identified to the south of 48°S, suggesting a relationship with the positive trend in the SAM. Seasonal to decadal time scales of the SAM strongly influenced wave parameters throughout the period analyzed. In addition, larger waves were observed during extreme ENSO and IOD events at interannual time scale, while they were more evident at seasonal and intraseasonal time scales in the correlations with the ZW3. Negative phases of the ZW3 and ENSO and positive phases of the IOD, PDO, and SAM resulted in larger waves around most parts of New Zealand.

The hydrodynamics of the southern basin of Tauranga Harbour

Abstract 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.

A Numerical Model to Simulate the Formation and Subsequent Evolution of Tidal Channel Networks

Abstract We present a numerical model that simulates morphological change as a result of the interactions between hydrodynamics, sediment transport and bed elevation change. Numerical simulations indicate that these morphodynamic interactions can lead to the initiation of tidal channels and potentially give rise to large-scale channel networks. We perform a sensitivity analysis to show how model outcome is sensitive to the numerical scheme adopted, hydrodynamic and morphological time-steps, and initial bathymetry. Furthermore, the formation of tidal channels and intertidal areas affects both the large-scale flow patterns, as well as the asymmetry between flood-and ebb-tidal currents.

Numerical Simulations of Wave Setup over Barred Beach Profiles: Implications for Predictability

Numerical simulations of cross-shore wave transformation and associated wave setup were performed for 100,000 realistic combinations of wave height, period, and beach profile. Results of numerical simulations were compared with a widely used empirical predictor of wave setup, and the outcome was a large variability in setup relative to that calculated by the empirical predictor. The variability, comparable in magnitude to the scatter around the empirical predictor developed with field observations, arose from wave energy dissipation over the cross-shore profile that was not taken into account. The beach-face slope commonly used in empirical predictors can be a poor descriptor of wave transformation across the beach profile, where most of the wave energy dissipation and setup generation occurs. Shoreline setup appears to be far more dependent on accurate characterization of the offshore profile (e.g., bar location and depth) than the beach-face slope.