Modelling the influence of varying sediment sources on coastlines.

Masters Degree, University of KwaZulu-Natal, Durban.Coastal erosion is of concern to developed shorelines worldwide and has largely intensified due to anthropogenic influences. Sea-level rise, reductions in sediment supply and changes to wave behaviour due to changes in climate were identified as potential causes of chronic erosion. With climate change expected to increase the frequency and intensity of storms, coastline management and planning will require greater attention. A major obstacle of coastal planning is the lack of available models for predicting long-term changes. Furthermore, reliable long-term wave data are often unavailable or unreliable. Predicting long-term changes is essential for effective management of coastal defence schemes. One-line models present a reduced-physics and reduced dimension approach and provide an efficient and viable alternative to 2D and 3D models while being less computationally intensive. The long-term impacts of varying sediment inputs on the stretch of coastline between uMhlanga and the uMngeni River mouth in Durban are explored using a one-line model. Site selection was based on ongoing erosion and known operations of sand-mining, damming and a sand-bypass scheme. Existing models are used as a framework to develop a coastline model that uses statistically modelled wave climates as the input source of wave data. Results indicated that a minimum longshore sediment supply (460,961 m3/year) required to maintain beach volume in the study region exceeds the estimate by Corbella & Stretch (2012) of 418,333 m3/year. Observed beach erosion by eThekwini Municipality indicated a current longshore sediment supply of 410,276 m3/year. Furthermore, volume conservation did not ensure beach width conservation along the entire coastline, with a minimum sediment influx of 596,183 m3/year required for beach width and beach plan area conservation. Shore nourishment behaviour were analysed in the form of alongshore sand waves with results showing that multiple, smaller nourishments results in more realistic sand wave amplitudes that are required for diffusion dominant waves. Smaller nourishments allow for more diffusive effects while maintaining a diffusive state whereas larger nourishments tend to become advection dominant following rapid diffusion. vii An investigation of the advection-diffusion relationship of river sediment discharges inferred that sand waves along the Durban coastline are advection dominated. A critical aspect ratio of between 0.037 and 0.041 represented the equilibrium point between advection and diffusion. River sediment discharges of this aspect ratio are potentially significant in preventing erosion given the relatively high diffusive rate and slow advection speed associated with the value. Furthermore, extreme river discharges exceeding 200,000 m3 remained in coastal systems for between 3 and 4 years and are potentially important mechanisms behind coastline recovery after storms.Only available in English

Integrated Methodology for Physical and Economic Assessment of Coastal Interventions Impacts

Due to economic, environmental, and social interest of coastal areas, together with their erosion problems, different coastal management strategies can be considered, with different physical (shoreline evolution) and economic (net present value, ratio benefit-cost, break-even point) consequences and impacts. Therefore, this work presents an integrated methodology that aims to compare and discuss the most promising coastal intervention scenarios to mitigate erosion problems and climate change effects, considering costs and benefits related to each intervention. The proposed methodology takes a step forward in assessing the coastal erosion mitigation strategies, incorporating three well-defined and sequential stages: shoreline evolution in a medium-term perspective; structures pre-design; and a cost-benefit assessment. To show the relevance of the methodology, a hypothetic case study and several intervention scenarios were assessed. In order to mitigate costal erosion two different situations were analyzed: the reference scenario and the intervention scenarios. 34 intervention scenarios were proposed and evaluated to mitigate the erosion verified. Depending on the parameter considered (reduce erosion areas, protect the full extension of urban waterfronts, improve the economic performance of the intervention by increasing the net present value, the benefit-cost ratio or decreasing the break-even time), best results are obtained for different scenarios. The definition of the best option for coastal erosion mitigation is complex and depends on the main goal defined for the intervention. In conclusion, costs and benefits analysis are demanded and it is considered that the proposed methodology allows choosing better physical and economic options for future coastal interventions, helping decision-making processes related to coastal management

Tidal hydrodynamic response to sea level rise and coastal geomorphology in the Northern Gulf of Mexico

Sea level rise (SLR) has the potential to affect coastal environments in a multitude of ways, including submergence, increased flooding, and increased shoreline erosion. Low-lying coastal environments such as the Northern Gulf of Mexico (NGOM) are particularly vulnerable to the effects of SLR, which may have serious consequences for coastal communities as well as ecologically and economically significant estuaries. Evaluating potential changes in tidal hydrodynamics under SLR is essential for understanding impacts to navigation, ecological habitats, infrastructure and the morphologic evolution of the coastline. The intent of this research is to evaluate the dynamic effects of SLR and coastal geomorphology on tidal hydrodynamics along the NGOM and within three National Estuarine Research Reserves (NERRs), namely Grand Bay, MS, Weeks Bay, AL, and Apalachicola, FL. An extensive literature review examined the integrated dynamic effects of SLR on low gradient coastal landscapes, primarily in the context of hydrodynamics, coastal morphology, and marsh ecology. Despite knowledge of the dynamic nature of coastal systems, many studies have neglected to consider the nonlinear effects of SLR and employed a simplistic bathtub approach in SLR assessments. More recent efforts have begun to consider the dynamic effects of SLR (e.g., the nonlinear response of hydrodynamics under SLR); however, little research has considered the integrated feedback mechanisms and co-evolution of multiple interdependent systems (e.g., the nonlinear responses and interactions of hydrodynamics and coastal morphology under SLR). Synergetic approaches that integrate the dynamic interactions between physical and ecological environments will allow for more comprehensive evaluations of the impacts of SLR on coastal systems. Projecting future morphology is a challenging task; various conceptual models and statistical methods have been employed to project future shoreline positions. Projected shoreline change rates from a conceptual model were compared with historic shoreline change rates from two databases along sandy shorelines of the. South Atlantic Bight and NGOM coasts. The intent was not to regard one method as superior to another, but rather to explore similarities and differences between the methods and offer suggestions for projecting shoreline changes in SLR assessments. The influence of incorporating future shoreline changes into hydrodynamic modeling assessments of SLR was evaluated for the NGOM coast. Astronomic tides and hurricane storm surge were simulated under present conditions, the projected 2050 sea level with present-day shorelines, and the projected 2050 sea level with projected 2050 shorelines. Results demonstrated that incorporating shoreline changes had variable impacts on the hydrodynamics; storm surge was more sensitive to the shoreline changes than astronomic tides. It was concluded that estimates of shoreline change should be included in hydrodynamic assessments of SLR along the NGOM. Evaluating how hydrodynamics have been altered historically under a changing landscape in conjunction with SLR can provide insight to future changes. The Grand Bay estuary has undergone significant landscape changes historically. Tidal hydrodynamics were simulated for present and historic conditions (dating back to 1848) using a hydrodynamic model modified with unique sea levels, bathymetry, topography, and shorelines representative of each time period. Changes in tidal amplitudes varied across the domain. Harmonic constituent phases sped up from historic conditions. Tidal velocities in the estuary were stronger historically, and reversed from being flood dominant in 1848 to ebb dominant in 2005. To project how tidal hydrodynamics may be altered under future scenarios along the NGOM and within the three NERRs, a hydrodynamic model was used to simulate present (circa 2005) and future (circa 2050 and 2100) astronomic tides. The model was modified with projections of future sea levels as well as shoreline positions and dune elevations obtained from a Bayesian network (BN) model. Tidal amplitudes within some of the embayments increased under the higher SLR scenarios; there was a high correlation between the change in the inlet cross-sectional area under SLR and the change in the tidal amplitude within each bay. Changes in harmonic constituent phases indicated faster tidal propagation in the future scenarios within most of the bays. Tidal velocities increased in all of the NERRs which altered flood and ebb current strengths. The work presented herein improves the understanding of the response of tidal hydrodynamics to morphology and SLR. This is beneficial not only to the scientific community, but also to the management and policy community. These findings will have synergistic effects with a variety of coastal studies including storm surge and biological assessments of SLR. In addition, findings can benefit monitoring and restoration activities in the NERRs. Ultimately, outcomes will allow coastal managers and policy makers to make more informed decisions that address specific needs and vulnerabilities of each particular estuary, the NGOM coastal system, and estuaries elsewhere with similar conditions