Hydrodynamics of Wave-Vegetation Interactions

The following sections are included:

• Preface
• Contents

The following sections are included:

• Relevance of Wave-Vegetation Interaction
• Types of Wave-Vegetation Interaction
• Scope and Outline of the Book
• References

Hydrodynamics–vegetation interactions follow the same physical principles, irrespective of vegetation type. However, magnitude of the relevant physical traits of vegetation vary strongly between species, in space and in time. This chapter starts by describing these traits and how they can be measured before it outlines their variations and what these depend on. It concludes with a discussion on how these variations can best be considered when assessing hydrodynamics-vegetation interactions in field, laboratory and numerical applications.

The past tsunami events, despite inducing extensive damage to the coastal community, cognized coastal vegetation as a buffer to the inundating tsunami. This, as well as the endangerment of coastal forest as ecosystems, necessitates a deeper understanding of coastal vegetation’s potential for tsunami damage mitigation. The literature highlighted the energy reduction of up to 60% due to the presence of vegetation during tsunami inundation conditions. Thus, several studies in the past and recent times investigated the effect of vegetation on tsunami energy reduction. This chapter extensively reviews in detail the vegetation parameter and flow parameter influence on energy reduction with a review furthering on the hybrid defence system (combination of vegetation and hard structures). The chapter presents a clear idea for building a coastal forest, thereby helping identify the best combination of the parameters for availing the maximum advantage of the vegetation with the available land space.

Coastal areas are particularly vulnerable to climate change effects and are subjected to an increasing risk, mainly due to flooding and coastal erosion resulting from sea level rise and changes in the frequency and severity of extreme events. In addition, the rapid urbanization and subsidence of certain areas are increasing their exposure and vulnerability and, therefore, the need to manage these risks by adopting new adaptation measures. Among the most innovative strategies for coastal adaptation, are the nature-based solutions (NBS). NBS that include vegetated coastal ecosystems have demonstrated their ability to buffer incoming flow energy and retain sediment. However, the consideration of these NBS in coastal protection policies and adaptation plans is very limited. This is mainly due to the lack of guidelines and recommendations for their design, implementation and monitoring. These procedures should be based on the correct quantification of the coastal protection services provided by these solutions. This chapter presents a review of the different approaches followed at different scales to study the flow-vegetation interaction and to quantify the resulting ecosystem services in terms of coastal protection. The different ecological and hydrodynamic parameters involved at each scale and the obtained results are presented. Finally, an integrated approach incorporating the different scales is outlined and the main future needs to advance in the implementation of these NBS are identified.

The advantage (reduction of fluid forces) and disadvantage (driftwood production) of a coastal forest as a tsunami buffer zone are summarized. Considering the functional limitations and disadvantages, a hybrid defense system (HDS) that strengthens existing coastal forests as a mitigation measure for a future large tsunami by combining a forest with an embankment and/or a moat has been proposed. Utilizing knowledge obtained from flume experiments and numerical simulations, two hybrid defense systems (HDSs), VME (vegetation (V), moat (M), and embankment (E) from seaward) and EMV, the reverse order of VME, have been installed in Japan based on the existing forest conditions and land availability. For the coastal forest construction, stand structures of vegetation (density, diameter, tree crown height, etc.) and their critical breaking values are important for increasing the mitigation effect. Forest management (in forestry, thinning) should be discussed based on the parameters in each species.

Flexible coastal vegetation such as salt marsh grasses and seagrass blades can dampen incoming waves, which benefits coastal protection. Flexible vegetation bends and sways under wave forcing. These dynamics lower the capacity of flexible vegetation to dampen waves in comparison to rigid vegetation. Solving the reduction of wave damping capacity remains an engineering challenge as it depends on the applicable wave-vegetation interaction regime. We identify six wave-vegetation regimes that depend on wave and vegetation characteristics. The non-buoyant vegetation regimes can be classified based on the Cauchy number and the excursion ratio. Under the common quasi-flexible vegetation regime, wave damping can be solved using a cantilever beam model. The reduction of wave damping by flexible vegetation compared to rigid vegetation can be quantified using a so-called work factor, which depends on the wave and vegetation properties. The solution extends towards the small tilt and rigid regimes, but not to the fully flexible regime. The analysis and model presented here provide a tool that can be applied to solve wave attenuation over flexible coastal vegetation.

This chapter presents the mathematical and numerical approaches for wave interaction with porous structure. The porous structure can either represent a breakwater or a vegetation. Two modelling approaches such as microscopic and macroscopic are discussed. In macroscopic approach, the mathematical model is based on unified governing equation which is used for pure fluid and porous region. Additional drag force terms representing either the porous or vegetation is incorporated into the governing equation. Further, treating the interface between the pure fluid and porous region has been described. The implementation in phase resolving and phase averging models are reviewed and discussed.

In this chapter methods for simulating wave-vegetation (WVI) interaction are reviewed, with an emphasis on approaches that are resolving numerically the large and small-scale dynamics in the near-field of aquatic plants or surrogates thereof. The fluid flow around such (fixed or moving) is complex, turbulent and highly three-dimensional and local flow instabilities result in flow separation, turbulence production in the form of coherent flow structures and energy dissipation. The accurate representation of these processes in numerical models that are aiming at gaining fundamental insights into the mechanisms of wave-vegetation interaction and/or at deriving physics-based subgrid scale models for large-scale applications of WVI. First, the governing equations suitable for near-field wave-vegetation large-eddy simulations are introduced together with selected numerical approaches to handle the complex physics. Secondly, examples of eddy-and plant-resolving simulations of flow through vegetation surrogates are presented.

The coastal vegetation is now closely regarded as one of the nature-based solution for protecting the coast against hazards such as storm surges and tsunamis. This chapter presents a comprehensive review on the interaction of vegetation with extreme flow due to the hazard as mentioned above through physical, numerical and field studies. Signature studies from the post Great Indian Ocean tsunami revealed the effectiveness of vegetation as a measure of reducing the inundation distance and height. This opened a new dimension forcing researchers to examine the wave-vegetation interaction problem. Conventional hard measures such as seawalls, bulkheads and revetments have been used successfully as coastal protection, although negative effects due to such structures have also been reported in the literature. The vegetation is considered as an eco-friendly measure that plays both the role of coastal protection and in ensuring the safety of the habitat if planned and managed effectively.

The following section is included:

• Index