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The long series (>30 year) of measurement data of erosion events at Narrabeen beach (NWS, Australia) provides insight in erosion volumes and their return periods in this area. The aim of this study was to replicate these data using XBeach in order to assess the validity of both the Joint Probability Model (JPM) and XBeach on beaches such as Narrabeen. In this study, a large number of different storms were simulated using XBeach. The probability and thus return period of the resulting erosion volumes were determined using the JPM. XBeach was calibrated against two individual erosion events, one at Narrabeen beach and one at Hasaki beach (Japan). The best fit for the Narrabeen beach, obtained using a stationary mode, led to an overestimation of erosion volumes at lower return periods (< 3 year) but fell within the boundaries implied by a 95% confidence interval of the measurement data for higher return periods. When calibrated against the erosion volumes with low return periods (<2 year), XBeach slightly underpredicted the erosion volumes at higher return periods. Depending on the method of determining confidence levels, the results were outside or well within the confidence interval of the measurements. This could suggests that this method is a valid way to predict erosion volumes and their return periods, in cases where long term erosion volumes measurements are absent.

Nearshore rhythmicity is often initiated in the period just after a storm where the subtidal bar is turned alongshore uniform. The initiation time as well as the length scales of the created rhythmicity varies from one storm period to another. Here we show that the post-storm wave conditions are related to the initiation of the morphological rhythmicity. Narrow-banded and long wave period, both proxies for swell waves, are often found to be present prior to the initiation of rhythmicity. Furthermore, numerical model computations illustrate that swell waves induce significantly larger wave group induced velocities on the bar. These findings imply that the arrival of swell waves can initiate and stimulate the nearshore morphological rhythmicity

The coastal system consists of a sub-aqueous and a sub-aerial zone which can be separated by a border. At this border sediment exchange takes place from one zone to the other. This paper hypothesizes that conditions and therefore sediment exchange at this border are dependent on active profile characteristics. To analyze morphological developments due to combined marine and aeolian processes data acquired at a measurement site located along the southwest Holland coast is exploited. Based on the monthly morphological profile measurements the border between the marine and aeolian zone is determined. It is found that during accretive conditions the vertical location of this border, and with that the conditions near the border, is dependent not on the foreshore slope of the surf/swash zone as expected but on the slope of the entire active profile.

Coastal lagoons are shallow coastal water bodies separated from the ocean by a barrier and they support a range of natural services that are highly valued by society, including but not limited to fisheries productivity, storm protection, and tourism. Songkhla lagoon is the largest lagoonal water resource in Thailand and Southeast Asia. The lagoon is a combined freshwater and estuarine complex of high productivity which represents an extraordinary combination of environmental resources believed to be unique in the region. Climate change, as a response to increased greenhouse gases in the Earth’s atmosphere, is now a widely accepted phenomenon. Sea level rise, temperature, precipitation, and storminess are expected to change significantly with global climate change and to impact coastal lagoons. The nature and magnitude of these impacts are still not very clear. The general objective of the research is to determine the climate change impacts on mixing and circulation at Songkhla lagoon, Thailand. To archive this objective the lagoon was be modeled with Delft 3D, a model developed by Deltares. After the verification with the available data for the region, different scenarios were created to represent the possible changes in mean sea level and riverflow due to global warming. Then, these results were compared to the current conditions to determine the main changes in mixing and circulation in this coastal lagoon. The results suggest an increase in water velocities of the inlet in future scenarios and a decrease of flushing time. Salinity and stratification showed more complex changes in futures scenarios.

Research done by the IPCC working groups and other organizations has sparked global concern over the possible impacts of climate change and corresponding sea level rise upon coastal communities. In reaction global studies were done (Nicholls et al., 2008, Hanson et al., 2011) to assess the vulnerability of coastal regions. However, most of these publications did not address the development of climate adaptation designs to protect the coastline. In this study it is demonstrated how a localized coastal vulnerability assessment could guide the development of conceptual designs in an African context. The overall aim of this thesis is the appraisal of climate adaptation measures and coastal management strategies for Durban, South Africa. The main focus is on an illustrative case study, for a coastal section along Durban´s central beaches. The case study is an example of how the vulnerability to coastal hazards could be assessed, for different SLR scenarios, to provide guidance for developing conceptual coastal protection designs. The March 2007 storm event (estimated to have a one in 50 year return period) indicated that significant damage can be sustained from coastal hazards in Durban at the current conditions. A one in 100 year storm is shown to already affect the operations of critical infrastructure in current conditions such as the beach road in the vulnerability assessment. The vulnerability increases significantly for future SLR scenarios. Softer, sediment based protection solutions were preferred for the case study due to the environmental, recreational and touristic requirements of the beachfront. Protection solutions developed for the two SLR scenarios give an idea of the footprint of different options and how easily the can be adapted to higher water levels. The vulnerability assessment and proposed protection solutions could be used as a basic tool for budgeting and long term spatial planning as it gives indicative costs and an idea about the areas that could potentially be at risk to coastal hazards. Developing a generic vulnerability assessment methodology could be beneficial for local municipalities. Completing similar vulnerability assessment studies (or more detailed studies) at other vulnerable coastal locations is a recommended starting point for the climate adaptation process and to inform global vulnerability and adaptation studies. Local governments are recommended to gather local data, assess vulnerability, propose a strategy to deal with future SLR scenarios and develop protection solutions for critical areas.

The Morphological Acceleration Factor (MORFAC) approach for morphodynamic upscaling enables the simulation of long term coastal evolution. However the general validity of the MORFAC concept for coastal applications has not yet been comprehensively investigated. Furthermore, a robust and objective method for the a priori determination of the highest MORFAC that is suitable for a given simulation (i.e. critical MORFAC) does not currently exist. This paper presents some initial results of an ongoing, long-term study that attempts to rigorously and methodically investigate the limitations and strengths of the MORFAC approach. Based on the results of a numerical modelling exercise using the morphodynamic model Delft3D, the main dependencies and sensitivities of the MORFAC approach are investigated. Also, a criterion is proposed for the a priori determination of the critical MORFAC, based on the CFL condition for bed form migration.

The coastal system consists of a sub-aqueous and a sub-aerial zone which can be separated by a border. At this border sediment exchange takes place from one zone to the other. This paper hypothesizes that conditions and therefore sediment exchange at this border are dependent on active profile characteristics. To analyze morphological developments due to combined marine and aeolian processes data acquired at a measurement site located along the southwest Holland coast is exploited. Based on the monthly morphological profile measurements the border between the marine and aeolian zone is determined. It is found that during accretive conditions the vertical location of this border, and with that the conditions near the border, is dependent not on the foreshore slope of the surf/swash zone as expected but on the slope of the entire active profile.

Field observations and numerical model simulations are examined to investigate the magnitude of vortical very low frequency (VLF) velocity fluctuations (i.e. large scale surfzone eddies) under different offshore wave forcing. Observations of vortical VLF motions under shore -normal wave incidence at Duck, NC, USA are re-analyzed and compared with the characteristics of the incident wave spectrum. Long wave periods and narrow frequency spread incident waves were found to coincide with stronger vortical VLF motions. Numerical model simulations investigating the effect of the incident wave parameters in a more isolated way confirm the observed effect of frequency spread and wave period on the magnitude of VLF motions. Variations in incident wave spectrum resulted in changes in the vortical VLF magnitude of the same order as the magnitude of the vortical VLF velocity fluctuations themselves. These results imply that under shore-normal incident waves strong vortical VLF velocity fluctuations in the surfzone are more likely under swell conditions and at swell dominated coasts.

Longshore sediment transport (LST) is one of the main drivers of beach morphology. Bulk LST formulas are routinely used in coastal management/engineering studies to assess LST rates and gradients. Over 50 years of research has resulted in several bulk LST formulas that have been tested with varying levels of rigor. In this study, the predictive skill of one of the most recent bulk LST formulas (Bayram et al., 2007) is evaluated. The calibration coefficients in the formula are improved using a least-squares optimization algorithm, resulting in a significant improvement in predictive skill. The generality of the improved formula is verified via the statistical methods of bootstrapping and cross-validation.

Typically a beach is out of equilibrium after a nourishment is installed. To observe how a nourished beach behaves on the timescale of storms a monitoring campaign was set up at Vlugtenburg beach after a nourishment in the spring of 2009. Here we show a sediment budget analysis of the first 2.5 years for a coastal domain spanning 1750 m alongshore from -9 to +5 m NAP. To investigate the redistribution of nourished sand different sections of the profile are examined. Observations show that the initial response (first 6 to 12 months after construction) is large where the sediment eroded from the beach is transported offshore to form a subtidal bar. In the following period (until present) the losses in the domain are on the order of 40 m3 per m alongshore per year. These losses are concentrated in the profile around the waterline.

Reef systems have been estimated to exist along approximately 80% of the world’s coastlines with living coral reefs, relic limestone platforms and submerged rock formations being the most common types observed. The processes of wave breaking on a reef crest, setup on a reef and flow over and within a lagoon, have been the primary focus of research to date, while wave transformation shoreward of the reef crest and surf zone have also been studied. The propagation of low frequency waves has been shown to have a large influence on flow, sediment transport and morphology. Furthermore, it has been demonstrated that these waves may possess periods that, if closely correlated with the reef width and depth, may enter a standing wave type form and possibly resonate. Aim: The aim of this study was to determine the indicators of low frequency resonance in field, laboratory or numerical model data, and to identify the influence of different geometric parameters on the generation of low frequency wave resonance on a fringing reef. Methods: The indicators were tested by the use of the numerical model XBeach, which was demonstrated to consist of a numerical basis suitable for the analysis of reef systems. The model was calibrated with high-resolution field data obtained at the Ningaloo Reef (Western Australia). The tested indicators were then applied to the Ningaloo Reef field data to determine if a resonance signal could be identified at the site. Finally, a geometric parameter sensitivity analysis was conducted with an idealised reef profile based upon the Ningaloo Reef. The wave boundary of the model was forced with a JONSWAP-type spectrum that characterised the peak of a storm at the site. The influence of different geometric parameters (in both non-frictional and frictional cases) was investigated and compared to an analytical model. Results: For two time-series that are spatially lagged across a reef, three indicators need to be satisfied to demonstrate the presence of resonance. They are: the surface elevation variance across the basin must be coherent, a phase relationship associated with the mode of resonance considered must exist, and an amplification of the wave between two points considered at the frequency of resonance must occur. The results of the indicator tests showed strong agreement with a simple basin analytical model that was adapted to include the effect of a lagoon. Strong amplification (resonant) peaks were observed for the first two standing waveforms. The frequency of these peaks was affected by the setup on a reef while the amplitude was affected by the influence of friction. It was shown that for frictional values consistent with Ningaloo Reef, the amplification peaks ‘flatten’ to magnitudes similar to the progressive waves in the spectrum. The geometric sensitivity analysis indicated that the resonant frequency was more sensitive to the reef and lagoon length than the reef and lagoon depth. The amplification was greatest for the zero and first-mode of resonance. However this amplification was dampened with the introduction of friction. It was determined that resonance is not likely to occur on reef systems with the geometry, frictional characteristics and wave forcing similar to the studied section of Ningaloo Reef. Resonance may occur for reef systems with shorter reef and lagoon widths, lower frequency forcing and/or less frictional dissipation. The latter may occur for reefs that have a different roughness to Ningaloo Reef as well as for reef systems that are damaged or dying in which coral assemblages degrade into coral rubble.

Time scales of post-storm nearshore morphological recovery and physical processes governing these time scales are poorly understood at present. The ability to predict nearshore morphological recovery time scales based on pre-, during- or post-resetting storm conditions is an essential requirement for building and validating scale aggregated models that operate at macro- and higher spatio-temporal scales. In this study, quality controlled ARGUS video derived beach states at Palm Beach, Sydney (4 years) and Duck, NC (2 years) and concurrent wave data are analysed to quantify the nearshore morphological recovery time scales (Tmr) and to determine the physical processes that may govern Tmr. The results show that Tmr is of the order of 5-10 days at these two beaches. Tmr is moderately positively correlated with the averaged longshore current over the 3 days immediately after the resetting storm, indicating that it might be possible to develop a predictor for Tmr based on wave conditions immediately after the resetting storm. Weak correlations are present between Tmr and several pre-storm, during-storm and post-storm parameters at the two sites. However, these correlations are inconsistent between the two sites. A thorough analysis employing long-term beach state and wave data at several different study sites is required to fully understand this phenomenon.

The well-known empirical relationship between the equilibrium cross-sectional area of tidal inlet entrances (A) and the tidal prism (P), first developed by O’Brien (1931), has been extensively reviewed. Our theoretical investigations indicate that a unique A-P relationship should only be expected for clusters of inlets that are phenomenological similar (i.e. fairly similar hydrodynamic and morphological conditions), and that the exponent q in the A-P relation should be larger than 1. However, relevant published data available to date do not clearly support this theoretical finding. A re analysis of the available data sets by Stive et al. (2009) indicated that they may not be sufficiently reliable to verify our theoretical finding with regard to q>1 due to the violation of the condition of phenomenological similarity, and possibly also due to violating the initial definitions given by O’Brien (1931) in estimating the tidal prism. The resolution of this issue is important because slightly different values of q result in significantly variable values for the equilibrium cross-sectional area of the tidal entrance. This may have significant implications in determining the true stable equilibrium entrance cross-sectional area. Here we present a re-analysis of the available data with a focus on determining the phenomenological dependencies of the A-P relationship. The available A-P data from the US Pacific, Atlantic and Gulf coasts (Jarrett, 1976 and Powell, 2003) have been re-scrutinized and categorized following the above mentioned phenomenological similarity criteria, viz. similar tidal range, similar sediment size, similar littoral transport and similar hydraulic radius. All together, some 20 different categories were considered and A-P relationships were obtained for each category. Generally, high correlations were found between the stable inlet predicted by each A-P relationship and the corresponding data. However, only in a limited number of categories were they significantly better than the correlations for the complete datasets. Finally, we point out that only in a number of categories the q value associated with the A-P relationship exceeded unity as suggested by the theoretical derivations. In the majority of categories the q value associated with the A-P relationship does not exceed unity. This is truly disappointing, and we have no physical explanation for this and consider this issue unresolved.

Economic and population growth have led to an unprecedented increase in the value at risk in coastal zones over the last century. To avoid excessive future losses, particularly in the light of projected climate change impacts, coastal zone managers have various instruments at their disposal. These primarily concern land-use planning (establishing buffer zones) and engineering solutions (beach nourishment and coastal protection). In this paper, we focus on risk mitigation through the implementation of buffer zones (setback lines). Foregoing land-use opportunities in coastal regions and protecting coasts is costly, but so is damage caused by inundation and storm erosion. Defining appropriate setback lines for land-use planning purposes is a balancing act. It is however unclear what level of protection is facilitated by current approaches for defining setback lines, and whether this is, at least from an economic perspective, sufficient. In this paper, we present an economic model to determine which setback lines would be optimal from an economic perspective. The results provide a useful reference point in the political debate about the acceptability of risk in coastal zones. The main conclusions are (i) that it is useful to define setback lines on the basis of their exceedance probabilities, (ii) that the exceedance probability of an economically efficient setback line will typically be in the order of magnitude of 1/100 per year, (iii) that it is important to distinguish between situations in which morphological conditions are stationary and non-stationary, and (iv) that long-term uncertainties (e.g. due to climate change) influence the exceedance probability of efficient setback lines but only to a limited extent.

The hydrodynamics and sediment transport in the swash zone is currently outside the domain of coastal-area models, which is a significant limitation in obtaining littoral sediment-transport estimates, especially on steep reflective beaches where the waves practically break on the beachface. In this study, an existing process-based coastal model (MIKE 21) is combined with a theoretical derivation of swash processes, resulting in an innovative hybrid modelling approach that is capable of estimating longshore sediment transport in the swash zone. The method relies upon estimation of swash hydrodynamics from an extended ballistic swash model with friction included. The terminal bore and other incident wave properties were computed from the output of a spectral-wave model (MIKE 21 SW). The Bagnold-type equation was applied to estimate gross transport volumes and the longshore component was computed for the sand volume displaced during the up-rush. The newly developed hybrid modelling approach was applied to Jimmys beach, a steep reflective beach (D50 = 0.3 mm, gradient=0.1) along the northern shoreline of Port Stephens, Australia. The model results yield the alongshore swash transport pathways and the indicative transport volumes. A point of divergence is identified at the beach erosion area, which is of critical importance in terms of shoreline erosion and management. The preliminary results suggest that swash-zone transport can account for a large percentage of the total littoral drift for such beaches. However, further field or laboratory data are required to test model utility, as well as to tune calibration parameters based on the site-specific conditions

Recent research done the IPCC (2007) working groups and other organizations has sparked global concern over the possible impacts of climate change and corresponding sea level rise (SLR) upon coastal communities. In reaction studies were done (for example by Nicholls et al., 2008) to assess the vulnerability of coastal regions and get an indication of the magnitude of the potential global impacts. However, most of these studies did not address the development of climate change adaptation designs to protect the coastline. In this paper it is demonstrated how a localised coastal vulnerability assessment could guide the development of conceptual coastal protection designs in an African context. The overall aim of this paper is the appraisal of climate adaptation measures and coastal management strategies for Durban, South Africa. This is illustrated using a case study, for a coastal section along Durban´s central beaches. The case study is an example of how the vulnerability to coastal hazards could be assessed, for different SLR scenarios and should provide guidance for developing conceptual coastal protection designs. A recent extreme storm event indicated that significant damage can be sustained from coastal hazards in Durban under the current conditions. A 1-in-100 year storm is shown to already affect the operations of critical infrastructure under current conditions at the case study site. The projected vulnerability increases significantly for future SLR scenarios.