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| Breaching of coastal barriers under extreme storm surges and implications for groundwater contamination Elsayed, S.M. (2017). Breaching of coastal barriers under extreme storm surges and implications for groundwater contamination. Technische Universität Braunschweig: Braunschweig. 208 pp.
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| Documenttype: Doctoraat/Thesis/Eindwerk | |
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| Abstract |
For these specific purposes, this study starts with a comprehensive literature review of the causes and forms of coastal erosion and barrier breaching. The latter results in a summary of the reasons and components of extreme sea levels during extreme storm surges and how these extreme sea levels interact with coastal barriers until causing their damage and inducing full breaching. Therefore, hydrodynamic processes that might initiate a breach from the seaside and landside are highlighted. In addition, the geomorphological processes that might deepen and widen the initiated breach are addressed. Thereby, the state of the art breaching models are examined in order to select the most appropriate model for further analysis. As a result, the XBeach model is selected. The identified limitations of XBeach are then discussed, showing that XBeach overestimates coastal erosion and thus breaching dimensions for high overtopping rates and high flow velocities. Thus, two model limitations related to the sediment stirring in XBeach are selected as the model limitations that need to be urgently addressed in order to predict adequate breaching dimensions and reliable inland discharges through breach-induced inlet(s). Such a comprehensive literature review includes also a summary of the possible consequences of breaching-induced floods, especially for coastal aquifer contamination by salt water. Moreover, the review of the state of the art modelling tools for predicting coastal flood extent, water depths and associated kinematics showed that the current approach for breaching-induced flood is still based on modelling these two processes separately. Thus, combined modelling of these two processes using XBeach is suggested. The review, in addition, showed that the most recent studies on storm-driven salt water intrusion lack proper modules to simulate the processes leading to coastal floods (e.g. overtopping, breaching). Thus, XBeach is introduced, after the suggested improvements and extensions, as the most suitable model to perform the coupling among the involved processes. In order to examine the performance of XBeach before any extension or development, it is preliminarily applied to reproduce 17 large-scale laboratory tests that were performed in 2013 in the large-scale flume (GWK) of the Coastal Research Centre (FZK) in Hannover to simulate the coastal dune erosion at the western coast of Wangerooge Island (northern Germany). The numerical results showed that XBeach overestimates dune erosion. Moreover, such overestimation increases dramatically with significant overtopping rates on coastal barriers. As a result, this step confirmed the urgent necessity of the pre-selected two model improvements related to the sediment stirring in XBeach. These improvements are related to (i) the wave nonlinearity effect on sediment transport, which is described in XBeach by a calibration factor for the time-averaged flow depending on the wave skewness and asymmetry and (ii) the considerable excess of the actual shear stress required to initiate the sediment particle motion as compared to that predicted by the common Shields criterion. As a first step toward improving the prediction capability of XBeach, a novel formula is developed that predicts the calibration factor for the time-averaged flow depending on the wave nonlinearity. On the other side, this study introduces a novel approach to account for the grain-stabilization effect in reducing sediment transport and coastal erosion in compacted to highly compacted soils. These two improvements are implemented in XBeach and the improved model is then very successfully tested for dune erosion, for barrier breaching as well as for a barrier island overwash under an extreme storm surge event. Particularly, the second model improvement opens the way toward further improvements to account for spatially varying soil resistance, which is crucial for a reliable prediction of breach locations along the barrier. In a further step, this study has shown that the scope of XBeach, initially developed for near shore hydrodynamic processes and associated morphodynamics, can be extended for coastal inundation, so that XBeach can be used to simulate both barrier breaching and subsequent hinterland inundation in a single model system. In addition, the study has examined the feasibility of using the groundwater module of XBeach to simulate the vertical salt water intrusion induced by coastal inundation. The latter step aimed at examining the feasibility of simulating the breaching, induced inundation and subsequent saltwater intrusion in a single model system that considers the mutual interaction among the involved processes so that the outcomes of one process is “automatically” transferred to the next model. Regarding the feasibility of using XBeach as a salt water intrusion model, it was shown that XBeach still needs to account for the advection-dispersion of density dependent transport. Thus, at this stage, a separate modelling of the flood-induced salt water intrusion using Visual Modflow/SEAWAT was found as the most feasible alternative. As coastal flooding is one of the major threats to groundwater quality in coastal aquifers, the study has also addressed this issue, its implications for sustainable development in coastal zones and the current modelling approaches. Moreover, the common structural approaches to mitigate saltwater intrusion are also summarised. None of these measures is suitable for mitigating vertical salt water intrusion. Therefore, the study suggested using subsurface drainage network so that percolating salt water might be drained before contaminating the aquifer. To highlight the value of the study outcomes, the modelling system applying the improved XBeach to simulate both inland discharges and induced hinterland inundation in addition to Visual Modflow/SEAWAT to simulate the subsequent saltwater intrusion is used to draw the implications of possible coastal flood near Bremerhaven, northern Germany. The outcome of this case study showed that a flood event for 2.8 hours might contaminate the aquifers near Bremerhaven so that they might remain contaminated for around 45 years, i.e. until they get remediated naturally. The application of the subsurface drainage system shortens the latter interval to three years and prevents the contamination of the deeper aquifers. Finally, the new contributions of the study and the lessons learnt from the case study are summarised and further improvements are suggested. |
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