Natural and nature-based solutions for coastal protection, restoration, and resilience improvement
Our research focuses on the potential of natural and nature-based infrastructures for coastal protection, restoration, and resilience improvement. We use analytical theories, numerical simulations, and experimental techniques to investigate the physics in flow-structure(vegetation) interaction at microscale (individual scale), wave dissipation by vegetation meadows and aquaculture structures at mesoscale (canopy scale), and coastal wave-current-sediment processes at macroscale (regional scale).
Wave-current-sediment processes in Lake Michigan
FVCOM-SWAN-CSTMS model results
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The world’s largest freshwater system, the Great Lakes and St. Lawrence River, is facing an estimated minimum of $1.94 billion in coastal damages from climate change by 2025 (recent survey). Associated with climate variability, the water level of Lake Michigan increased rapidly in the recent decade from the recorded low water level in 2013 to record highs in 2020. In the meantime, the significant wave height over the lake also showed an increasing trend with a strong correlation with water level change with a time lag (Huang et al., 2021, Front. Mar. Sci.). As a result, the basin-wide total nearshore sediment erosion also accelerated dramatically in the past decade (Zhu et al., in review). Our recent research with a regionally high-resolution version of the model indicates significant influences of coastal structures on sediment transport and coastal erosion. These results are critical for coastal management planning for projected regional climate impacts.
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Wave attenuation by vegetation and aquaculture structures
Modeling the interaction between waves and suspended canopies based on OpenFOAM
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In addition to producing seafood, aquaculture structures may also provide coastal protection benefits either alone or with other nature-based structures.
We derived a generalized three-layer theoretical model for both regular and irregular (random) wave attenuation due to the presence of biomass within the water column. The biomass can be characterized as submerged, emerged, suspended and floating canopies that can consist of natural aquatic vegetation and potential aquaculture systems of kelp or mussels. The present theoretical model incorporates the motion of these canopies using a cantilever-beam model for slender components and a buoy-on-rope model for elements with concentrated mass and buoyancy. The model is able to predict the effective blade length or bulk drag coefficient of flexible blades for wave attenuation. We found that suspended aquaculture farms more effectively attenuate waves with a smaller period or the higher frequency components of wave spectrum. The performance of suspended aquaculture farms is less affected by water level changes due to tides, surge and sea level rise, while the wave attenuation performance of SAV decreases with increasing water level due to decreased wave motion near the sea bed. Incorporating suspended aquaculture farms offshore significantly enhance the coastal protection effectiveness of SAV-based living shorelines and extend the wave attenuation capacity over a wider wave period and water level range. The combination of suspended aquaculture farms and traditional living shorelines provides a more effective nature-based coastal defense strategy than the traditional living shorelines alone. More details are referred to Zhu et al. (2020, 2021, Coast. Eng.; 2022, Adv. Water Resour.). |
Fluid-structure interaction
Fluid-structure interaction modeled with immersed boundary method on OpenFOAM
Modeled blade motion compared with the observed motion of the blade tip
Asymmetric motion of submerged vegetation in linear waves
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To understand the interaction between waves and flexible blades, we developed a fluid-structure interaction (FSI) solver based on OpenFOAM® with immersed boundary method (Zhu et al., ICCE2018). The FSI model is also useful for other coastal and offshore flexible structures.
To simulate the blade motion, we introduced a cable model, which can capture the asymmetric "whip-like" motion of submerged aquatic vegetation (SAV). Wave asymmetry would cause blade motion to be asymmetric. However, asymmetric blade motion may also occur in symmetric waves. We found that the asymmetric blade motion in symmetric waves is induced by two major mechanisms: (i) the spatial asymmetry of the encountered wave orbital velocities (wave motion relative to blade) due to blade displacements and (ii) the asymmetric action on the blade by vertical wave orbital velocities. The peak asymmetric blade motion increases with wave height and blade length but decreases with increasing blade flexural rigidity. The asymmetric blade motion of SAV could hinder sediment suspension by providing a "shelter". Blade motion characteristics play an important role in wave‐vegetation interaction, wave‐driven currents, wave‐attenuation capacity, breakage of vegetation and ecosystem services (Zhu et al., 2020, JGR Oceans). |
Dynamics of offshore engineering structures
3D numerical model for the dynamics of longline kelp farms
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A 3D consistent mass-based model, cable model, is proposed to simulate the large deflection of the culture line structure. The 3D model couples the components of the entire aquaculture system. The simulations are performed with results compared with published data for suspended blades in both steady and oscillatory flow. Dynamics of a kelp longline system are studied for tidal currents scenarios. Results show that the longline tension follows water level to the peak at high tide and sensitive to water level in high tide. The non-parallel currents could enhance the tension as well as the deflection of the longline system. (Zhu et al., ISOPE2019)
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Jacket platform
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Aging jacket platforms. We developed a monitoring method for aging jacket platforms based on the pushover analysis using SACS. The measurements include the platform displacement, the bearing loads of the pile end, and the platform subsidence. Based on the structural characteristics and design requirements, early warning conditions were proposed. The monitoring method and warning system have been applied to a jacket platform in South China Sea. (Tang et al., 2015, Ocean Eng.)
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