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Wave field around submerged breakwaters

Sharif Ahmadian, A; (2013) Wave field around submerged breakwaters. Doctoral thesis , UCL (University College London).

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Shoreline response to submerged breakwaters is particularly influenced by the wave field behind the structure driven by coastal processes. The 2D aspects of wave transmission behind submerged breakwaters have been extensively studied by researchers. However, available 2D engineering design tools are inefficient in breakwater design due to not being able to provide any information on the spatial distribution of the nearshore wave field around the breakwater. There are very few studies considering 3D effects in the literature and consequently no reliable guidance for engineers. This encouraged the author to investigate this subject experimentally and numeri¬cally, with the aim of contributing to this important research topic. A comprehensive set of 2D and 3D experiments has been conducted in three wave tanks with different scales. A method has been prepared for predicting the waves transmitted behind the breakwaters based on the data-driven algorithms called Artificial Neural Networks (ANN) and using some of the experimental data collected. Multi-layer perceptron (MLP) and Radial basis function (RBF) models were designed and trained by the Levenberg-Marquardt learning algorithm (LM) and a derivative- based algorithm of gradient descent (GD) respectively. To verify the numerical model, wave simulation was also carried out using DHI MIKE 21 BW (2DH Boussinesq Wave module) based on the numerical solution of the time domain formulations of Boussinesq type equations. The spatial variations of wave energy and wave pattern around the breakwater were generally found to depend on incident wave climate and whether or not wave breaking occurred over the breakwater as well as degree of breaking, with different wave patterns observed for different wave conditions. In cases with waves breaking over the breakwater the lower wave heights were observed behind the breakwater crown on the shorewardside; for nonbreaking wave conditions passing over the submerged breakwater lower wave heights were observed in the gap between the end of the breakwater and the flume wall. Investigations illustrated that the dimensionless Cartesian coordinates x/L₀ and y/L₀ were the most significant parameter in the 3D wave field around the breakwater, with wave height and energy varying spatially around the structure. This confirms the importance of 3D effects on wave height prediction and highlights the inadequacy of 2D models that are unable to deal with spatial variation of wave height behind the breakwater. The RBF model trained by non-dimensional parameters was determined as the most appropriate tool and was proven to be more capable of handling wave transmission prediction comparing with other ANN models. Predictions from the proposed ANN model were found to be in very good agreement with new laboratory data never seen by the model before. The ANN model predictions have also been compared with results from the MIKE 21 BW model. The proposed ANN model was validated in three distinct cases of interpolation, extrapolation and larger scale tests. The model gave the most reliable and convincing predictions within a specific range of input parameters (interpolation) while outside this range (extrapolation) to some extent, reasonable results were still achieved. The proposed model was assessed under larger scale conditions with data collected in another wave tank with different laboratory facilities. Outputs under these conditions also showed good agreement. This shows that the performance of the model is not affected significantly by scale changes and the model has the potential to be used in real applications. The Boussinesq wave model was found to overestimate wave-induced breaking dissipation over the crest of the submerged breakwater leading to underprediction of wave transmission. The evaluations showed more consistency between the measured experimental data and predictions from the ANN model in comparison to those from the Boussinesq wave model. These demonstrate the accuracy and reliability of the model and its capability in predicting the wave field around submerged breakwaters. A simplified version of the numerical model and wave prediction scheme is provided in this thesis for practical applications. The proposed ANN model is a significant advance in that it can be used to predict 3D wave pattern around submerged breakwaters in the range of dimensionless Cartesian coordinate -0.26<x/L₀<O.54 and -O.29<y/L₀<1.7 whereas existing models in the literature are restricted to a 2D field only. The proposed model considers combined transmission and diffraction in a wide range of water depths and wave climates whereas existing models in the literature are restricted to calculating overtopping and diffraction coefficients separately and in a limited range of input parameters which make them less accurate and reliable with lower usefulness and applicability. The agreement with experimental data is encouraging and the evaluations show that the proposed model could be applied in practical applications by engineers and scientists as a more precise and dependable design tool than existing models.

Type: Thesis (Doctoral)
Title: Wave field around submerged breakwaters
Language: English
Additional information: Permission for digitisation not received.
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Civil, Environ and Geomatic Eng
URI: https://discovery.ucl.ac.uk/id/eprint/1414995
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