Several studies have highlighted the negative impact of underwater impact pile driving on marine life, underscoring the need for effective noise mitigation systems (NMS). While bubble curtains are commonly used, recent research also focuses on near-pile NMSs. For instance, with resonating devices such as Helmholtz resonators, which are known for their tunability and effectiveness in specific frequency ranges. However, the potential for broadband noise mitigation using arrays of Helmholtz resonators in offshore impact pile driving remains uncertain. This thesis aims to address this research gap.

Locally resonant acoustic metamaterials, characterised by unusual properties such as negative or zero-valued effective mass, have shown potential for broadband noise mitigation. This thesis defines an acoustic metamaterial composed of Helmholtz-type resonators and investigates its combined effects. The study employs the finite element method (FEM) and the lumpedcomponent method to define the resonator characteristics of a reference Helmholtz-type resonator. Subsequently, the boundary element method (BEM) is used to examine the acoustic behaviour of horizontal arrays of Helmholtz-type resonators, appropriately named Helmholtz-type acoustic metamaterials, in the frequency domain. Multiple configurations of these systems are analysed. Finally, a case study approximating the pressure field radiated by a vibrating monopile excited by an impact hammer is conducted in the time domain using the FEM.

The results indicate that Helmholtz-type acoustic metamaterials amplify sound pressure at frequencies below the natural frequency of the individual resonators and reduce it at frequencies above. Functional grading, achieved by incrementally decreasing the natural frequencies of the individual resonators in steps of up to 3 Hz along each horizontal array, can reduce the low-frequency amplification while maintaining the high-frequency attenuation. In a vertical system of Helmholtz-type acoustic metamaterials similar behaviour is observed. A vertical system comprising of horizontal arrays of 20 resonators with a horizontal spacing of 0.1 meter and a vertical spacing of 1 meter between each array shows a promising balance between attenuation and unwanted amplification. However, the system’s performance is sensitive to maintaining optimal volume of encapsulated air within the resonators, as the target frequency can significantly increase if a large percentage of the encapsulated air is lost. The transient response behaviour aligns with frequency domain observations, showing low-frequency amplification and high-frequency mitigation. Additionally, it is found that the orientation of the resonators does not significantly affect the transient response. Finally, it is important to note that mechanical coupling effects, which are not included in this study, may introduce additional low-frequency interactions.

This study demonstrates the potential of using Helmholtz-type acoustic metamaterials in near-pile NMSs for offshore impact pile driving, emphasising the system’s sensitivity to the frequency of the applied force and the importance of air volume maintenance. The findings suggest that while Helmholtz-type resonators can effectively mitigate noise in specific frequency ranges, careful configuration is crucial for achieving broadband noise mitigation and for minimising the risk of unwanted amplification of the pressure field. ... Read more

The Vietnamese Mekong Delta, a vital region in the country’s economy, faces the dual challenges of coastal erosion and mangrove degradation, which threaten its long-term sustainability and flood protection capabilities. This research focuses on the coastal area of the Bac Lieu province, characterized by severe erosion and degrading mangrove forests. The study investigates the applicability and potential impacts of hydraulic measures to decrease the net rate of coastal erosion, utilizing numerical modeling with Delft3D and a comprehensive socio-economic analysis. The research hypothesizes that the coastal erosion is partly driven by the placement of a sea-dike to protect aquaculture farms, initiating a positive feedback loop. This loop explains the relation between coastal erosion and mangrove degradation. The proposed hydraulic measures to interfere with this feedback loop are a porous detached breakwater, a shoreface nourishment and the removal of the existing sea-dike. The socio-economic analysis involves questionnaires for local residents, field investigations, and insights from experts in Ho Chi Minh City. While the questionnaires provide inconclusive results, the overall socio-economic impact of the nourishment and breakwater is deemed positive and worth further exploration, particularly in light of the critical role of mangroves in future flood protection. On the other hand it is concluded that the measure of removing the sea-dike will have a negative impact on the coastal area of Bac Lieu due to the intensive land-use and the lack of individual protection of the farms and villages. Therefore, this measure is not modelled. Numerical modeling with Delft3D assesses the hydraulic impact of the breakwater and nourishment on the heavily eroded and partially eroded coasts of Bac Lieu. Results indicate that the nourishment method exhibits a positive effect in reducing net erosion, especially in low energy conditions. Conversely, the porous breakwater shows minimal impact on cumulative erosion and sedimentation. Since this is against all expectations, the validity of the schematization of the porous breakwater is questioned. It is observed that the schematization does not grasp the complex behaviour of the breakwater and therefore it is concluded that Deft3D is not a suitable modelling tool for modelling a porous breakwater. The findings suggest that the nourishment method is a promising approach for reducing erosion in Bac Lieu, benefiting both the heavily and partially eroded coasts. To determine the best course of action for Bac Lieu, further research into the long-term effects and configurations of nourishment is recommended. Additionally, informing local inhabitants on the threats of relative sea-level rise and flood protection, and fostering consensus between the government and engineering agencies on the importance of protecting the Mekong Delta and its mangrove ecosystems are essential steps toward a more resilient future. ... Read more