|Hard infrastructure or hard engineering structure||Inflexible/rigid structures constructed both cross and longshore, for coastal protection (e.g. groins, seawalls and breakwaters) [33, 34]||In nature, not all systems work as flexible systems. There are inflexible “hard” systems (e.g., cliffs, reefs) and others that are very flexible or “soft” (e.g., sandy beaches, wetlands). Depending on the benefits of constructing the infrastructure, it can be categorized as grey or green infrastructure.|
|Soft infrastructure or soft engineering structure||Flexible constructions for coastal protection that aims to work with nature and enhance the habitat: depending on their intensity, frequency and persistence, drivers (e.g., wind, waves and runoff) modify the form and position of the structure at different scales of time and space (e.g. beach nourishments, artificial dune construction and saltmarsh creation) [35, 36].|
|Grey infrastructure||Traditional engineering infrastructure for coastal protection focused on controlling a physical factor that threatens human interests (i.e. seawalls and dikes). Also known as structural measures, ecosystem degradation is usually a consequence of this type of infrastructure [37, 38].||
Both grey and green infrastructure aim to solve a problem related to human interests.|
Grey and green infrastructure concepts are often taken to be hard/soft solutions. However, green infrastructure is not necessarily synonymous with soft solutions, nor are rigid solutions synonymous with grey actions.
|Green infrastructure||Multifunctional constructions for coastal protection, based on the conservation of the connectivity of the ecosystem, and the energy and mass fluxes .|
|Hybrid alternative||Combines natural and built infrastructure (see ). Similarly, green hybrid projects include infrastructure with different levels of naturalness.|
|Adaptive solution||The inclusion of green infrastructure (unique or hybrid alternative) following a monitoring program; adapting the solution, based on evidence of its performance, allows movement from one stage to the next, forward, or backward, if necessary . This type of solution can be implemented using a three-step process, as shown in Fig. 2. The range of possible actions must include the dismantling of the infrastructure and ensuring economic activities are at a tolerable level for ecosystem balances.||
When we speak of adaptive solutions, we assume that some degree of flexibility similar to that of a natural system is mimicked.|
This can be naturally self-adaptive in certain conditions, or be adapted by humans over time.