1,720,954 research outputs found
Does a more sophisticated storm erosion model improve probabilistic erosion estimates?
No full textThe dependency between the accuracy/uncertainty of storm erosion exceedance estimates obtained via a probabilistic model and the level of sophistication of the structural function (storm erosion model) embedded in the probabilistic model is assessed via the application of Callaghan et al.'s (2008) Joint Probability Model (JPM) at Narrabeen beach, Australia with three different structural functions: (a) Kriebel and Dean (1993) (analytical); (b) SBEACH (semiempirical); and (c) XBeach (fully process based). Results indicate that the accuracy is greatest for JPM-SBEACH and lowest for JPM-XBeach. The most uncertain results are given by JPM-XBeach while the most robust results are given by JPM-SBEACH. Thus, it appears that increasing the level of sophistication of the structural function beyond the semi-empirical SBEACH model, may not always lead to better results and may even be counter-productive.Hydraulic EngineeringCivil Engineering and Geoscience
Coastal Adaptation to Climate Change: A case study in Durban, South Africa
No full textResearch 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.CoMEM - Coastal and Marine Engineering and ManagementSection Hydraulic EngineeringCivil Engineering and Geoscience
Climate change impacts on mixing anc circulation at Songkhla Lagoon, Thailand
No full textCoastal 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.CoMEM - Coastal and Marine Engineering and ManagementHydraulic EngineeringCivil Engineering and Geoscience
A risk-informed approach to coastal zone management
No full textEconomic 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.Hydraulic EngineeringCivil Engineering and Geoscience
Climate change and the morphological stability of the Tu Hien inlet, Vietnam
No full textHistorically, the coastal zone has always attracted people and across the globe the benefits of inlet systems have made communities settle there. The Tu Hien inlet and the Tam Giang–Cau Hai lagoon system in Vietnam are no exception, as approximately half a million people depend on it for their livelihood. Tidal inlets are also known for their volatile and disruptive nature, because they are dependent on the delicate balances between oceanic processes such as tidal flow, wave action and mean sea level, and riverine processes such as discharges and sediment supply. Disturbances of this balance cause changes in the behaviour of the inlet system, which will have an impact on the surrounding coastal area. The crux is that these processes can all be significantly affected by climate change. The goal of this thesis is to understand the effect of changes in climatic conditions on the morphological stability of the Tu Hien inlet. Achieving this goal was done by studying the present day behaviour of the inlet in order to use this knowledge to validate a process-based model for the present day and subsequently applying climate change scenarios to that model. Newly acquired satellite images show that in recent years (since 1999) the behaviour has changed and the inlet is not closing anymore as was the case in the time before 1999. A conceptual model has been created that captures this behaviour of the inlet and shows that in the present day the inlet is often shifting between being narrowed and open. This cycle is caused by the processes of spit growth and widening due to a certain degree of breaching. A simulation strategy has been drafted which is mainly founded on the yearly seasonal character of the forcing conditions. During a simulation of the northeast monsoon the growth of a spit should be visible; during a simulation of the southwest monsoon a status quo should be maintained. To model this behaviour an existing model (Lam, 2009) has been adapted to include sediment transport and morphology. The adapted model has been calibrated to reproduce the sediment transport up and down the Tu Hien coast and subsequently morphology simulations have been carried out to reproduce the desired morphological behaviour. The results show that after a breach the inlet mouth is narrowing and the cross-sectional area reduces until it reaches a certain stable equilibrium by the end of the northeast monsoon period. During the southwest monsoon there is little wave action and the inlet is tide-dominated, it slightly widens but a status quo is maintained. By reproducing the present day behaviour of the inlet the model has been qualitatively validated and is considered suitable for an investigation into the effects of climate change. Four climate change scenarios consisting of combinations of sea level rise, a change in fluvial run-off and a change in wave climate have been applied to the present day model. The model shows that future changes in river flows and waves are of minor importance compared to the effects of sea level rise with regard to inlet stability. The narrowing behaviour which was apparent in the present day is not observed in the climate change scenarios involving sea level rise. An increase in basin area caused by a higher mean sea level enlarges the tidal prism which results in more flushing capacity of the flows through the inlet. The increased flushing capacity ensures that a larger inlet cross-sectional area is maintained and also enhances the locational stability of the inlet. The predicted increase of the morphological stability can be considered a positive development. An increased cross-sectional area and the lack of a narrowing behaviour means that the future morphological behaviour will not have negative consequences for inlet navigability, water quality, ecology and other socio-economic factors. Even though the predictions show that climate change is likely to have a positive impact on inlet stability for Tu Hien, it is emphasised that the overall impact of climate change on low-lying coastal zones may be significant in a negative sense and deserves continuous attention.Coastal EngineeringHydraulic EngineeringCivil Engineering and Geoscience
Probabilistic modelling of extreme beach erosion using XBeach
No full textThe 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.Hydraulic EngineeringCivil Engineering and Geoscience
The morphological response of large tidal inlet/basin systems to relative sea level rise
No full textHydraulic EngineeringCivil Engineering and Geoscience
Low frequency wave resonance on fringing reefs
No full textReef 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.CoMEM - Coastal and Marine Engineering and ManagementHydraulic EngineeringCivil Engineering and Geoscience
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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