Number 9. 

 'Tetrapods are designed in such a way that they dissipate the force of incoming waves by making the water flow around rather than against them... A number of tetrapods arranged together form an interlocking, porous barrier that dissipates the power of waves and currents...The main feature of the tetrapod lies in its design which is not found in nature. Tetrapods are a symbol of artificiality rather than aesthetics'