Browsing by Author "Corbella, Stefano."

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Coastline impacts of tropical cyclones and climate change on Mauritius.
(2014) Douce, Yanick.; Stretch, Derek Dewey.; Corbella, Stefano.
The tourism industry has grown to become the third pillar of the Mauritian economy, due mainly to its prestigious beaches. However, Mauritius is prone to severe tropical cyclones which can have detrimental impacts on its coastal environment and ultimately its economy. It is predicted that the frequency and intensity of these tropical cyclone will increase under the influence of climate change. This may accelerate the rate of coastal erosion as a result of rising sea level, tidal waves and storm surges. Prior to 2003, no preventive coastal management framework was available in Mauritius to address the impact of tropical cyclones. Since, various studies have recommended that wave prediction models be adopted to assess and quantify the amount of sediment transport resulting from tropical cyclones. In this research, the relative coastal erosion due to the predicted impact of climate change on tropical storm waves at Belle Mare beach in Mauritius was studied. This was achieved by, firstly, the generation of cyclone wind fields as the primary driving mechanism for the wave model using the MIKE 21 Cyclone Wind Generation model. This required cyclone parameters such as the cyclone tracks, maximum wind speeds and maximum pressures which were extracted from the “Australian Severe Weather” centre. Mike 21 SW model, a 2D-spectral numerical modelling software, was then used to generate cyclone-induced waves and surges. This required the creation of a regular grid defining the shoreline and bathymetry which was obtained from “NOAA data centre”. Based on the published expected effect of climate change on cyclones, the intensity of cyclone Davina (1999) was increased by 5%, 10% and 15% respectively. The resulting wave outputs for each scenario were applied in the XBeach model to assess and quantify the coastal erosion at Belle-Mare. The outputs from XBEACH revealed that the reef surrounding Mauritius reduces the cyclone wave height before it reaches the beach by some 85%. Despite the beneficial influence of the coral reefs on storm surge, the relative volume of sediment erosion resulting from the various intensified cyclone scenarios for Davina is significantly high; 15.36m3/m for a 5% cyclone intensification, 18.16m3/m for 10% and 21.45m3/m for 15%. XBEACH was found to produce reliable results in modelling processes in the coastal zone with accuracy. The implementation of vegetated dunes was proposed as a suitable and natural coastal defense measure at Belle Mare beach for environmental, aesthetical, socio-economic and financial reasons.
Eﬀect of coral reefs on wave attenuation and erosion: Mnemba Island, Zanzibar.
(2017) Swanepoel, Curtis Russell.; Stretch, Derek Dewey.; Corbella, Stefano.
Global warming and sea level rise are some of the greatest threats humanity faces today with low-lying coral reef islands most vulnerable to these threats and their repercussions. This study focuses on the mechanisms that have resulted in the severe erosion of low-lying coral reef islands. Mnemba Island, a small island oﬀ the north-east coast of Zanzibar is used as a case study. It is a premium high value resort that is currently experiencing severe erosion. The erosion has resulted in the collapse of various structures and utilities littering the eastern beach with debris and fallen Casuarina trees which dominate the Island’s eastern border. Mnemba Island is a typical low-lying island surrounded by a coral reef platform. This study identiﬁed the key role these reefs play in wave attenuation and protecting the island from the large swell that surrounds the reef. The reef crest was found to attenuate wave heights by up to 70%. Overﬁshing and changes in the climate have resulted in the destruction of the coral reef communities that protect the island because of their increase hydraulic roughness and shallowness. Modelling shows that the impact of reduced bed roughness from coral degradation and just 10 cm sea level rise can result in up to 40% increase in wave height at the Island’s eastern beach. Coupled with the large tidal range and increased wave heights propagating over the reef platform, wave driven forces and currents are enhanced thus disturbing the quasi-equilibrium wave climate resulting in major erosion. The ﬁne sediments are lifted into the water column and transported into deeper sections of the reef platform. The low wave energy in the following seasons is insuﬃcient to overcome the critical shear stresses that lift the transported sediments back into the water column and hence, a net loss of sediment oﬀ the reef platform is experienced.
Modelling sand bypass schemes on the KwaZulu-Natal coastline.
(2015) Wells, Calvin Paul.; Stretch, Derek Dewey.; Corbella, Stefano.
Coastal structures such as breakwaters cause a disruption of longshore sediment transport along coastlines. The result of this disruption creates sand accumulation up-drift and beach erosion down-drift of these structures. Therefore, sediment bypass schemes are implemented by dredging the sand out of the sand trap up-drift of the structures and nourishing the beach down-drift of them. The beach north of the Richards Bay harbour entrance in KZN, South Africa was used as a case study to model and compare alternative nourishment schemes to alleviate chronic beach erosion due to disruption of the longshore sediment supply. This study used the Delft3D 2DH sediment transport models to investigate the nourishment schemes and a calibration study was done to test the capability of the models to maintain a theoretical equilibrium profile over a long term simulation. Subsequently the model was used to investigate and compare three nourishment schemes at a case study site over a period of a year to determine the beach response to the nourishment. The sediment budget for the nourishment schemes was limited to 1 000 000 m3 per year. The first scheme comprised of a continuous steady nourishment throughout the year and the second scheme was a bulk nourishment where the sediment is dumped onto the beach at the maximum dredging capacity, in this case 10 000 m3/day. The last was a bimonthly sediment nourishment scheme. The model calibration results revealed that a single wave related transport factor governs the cross-shore movement direction. A single set of parameters does not produce offshore sediment movement during large wave events and onshore movement during smaller wave events as observed in reality. Therefore, the model was unable to reproduce a quasi-equilibrium behaviour unless the cross-shore transport factors are allowed to vary as a function of wave height. It was possible to define a cross-shore factor within the Van Rijn transport model that limited the cross-shore movement over a long term morphological simulation resulting in only the longshore transport affecting the morphology within the model. This model setup was used for the case study since a lack of sediment supply was the main focus of this study. The continuous steady nourishment results showed a natural longshore shore movement of sediment down-drift of the harbour entrance and a uniform beach width increase along the entire beach. The bimonthly nourishment closely emulated the continuous nourishment resulting in a net increase of beach width along the modelled coastline. The bulk nourishment revealed significant differences to the previous cases. The sheltering effect of the northern breakwater kept the main recreational beach in a nourished state while the northern beach outside of the breakwater’s shadow-zone returned to its initial sand starved state.
Predicting shoreline response to wave and sea level trends.
(2012) Corbella, Stefano.; Stretch, Derek Dewey.
In March 2007 the KwaZulu-Natal coastline was devastated by an extreme storm event. There is international concern that such events are associated with climate change. There is evidence of global changes in climate but there is still uncertainty as to whether they are anthropogenic or part of natural decadal (or longer) cycles. The increase in frequency and intensity of extreme storm events will impact on the sediment dynamics of coastlines and the associated risks need to be modelled and quantifed so that they can be included in coastal planning and management. Durban is a coastal city on the east coast of South Africa and has been used as a case study to identify trends in wave parameters and beach profile volumes. The correlation between profile erosion, waves and tides was explored using singular spectral analysis. The dependence between wave parameters was modelled using copulas. The decadal trends were introduced into these models using a nonstationary generalised extreme value distribution. Numerical models (SWAN, SBEACH, XBEACH) were used to transform the statistical model to near shore waves and estimate the associated erosion. The copula model was used to investigate the relationship between multivariate return periods and erosion return periods. Coastal defence options were reviewed and those appropriate for Durban were identifed. This study provides a review of Durban and Richards Bay's 18 years of Waverider data. It presents wave parameter exceedance statistics and wave height return periods for Durban. Durban's wave data showed increasing trends in maximum significant wave heights, peak wave period, storm event frequencies and a trend towards a more southerly mean wave direction. However, only the increase in peak period and wave direction was statistically significant. The trend in wave direction is considered a potential coastal hazard as it has the potential to increase the littoral drift by 1 % per annum. Durban's beach profiles have shown a long term erosion trend which is due to a combination of wave and sea level trends, and a reduction in sediment supply. The reduction in sediment supply from rivers was found to be both anthropogenic and natural. Storm, wave parameter and sea level trends were estimated to contribute more than 75 % to the total long term erosion. It was found that it takes an average of 2 years for a beach to recover to its pre-storm volume. Different types of coastlines recover at different rates and these recovery rates should be considered in risk assessments. A method for estimating future impacts due to storm and sea level trends has been proposed in the form of a non-stationary copula based statistical model. In general a bivariate return period of wave height and duration was found to approximate erosion return periods, while a method for estimating an analogous multivariate storm and erosion return period was developed. Geotextile sand filled containers were found to be a suitable coastal defence as they satisfy social, environmental and political pressure.