Tetrapods protecting the wave-spending beach at Brighton Marina. They were invented over 50 years ago in France as a substitute for large boulders which tend to get dislodged over time by the pounding of waves. They are designed to lock together in a random but open structure which dissipates the energy of the wave through turbulence. Numerous variations in shape have since been produced but all go under the generic name "tetrapod", borrowed from zoology.
Their use became very widespread at one time, but the pendulum has swung against "hard stabilisation' of coastlines as this interferes with natural currents, material drift and deposition. Tetrapods now are only used in special situations such as that shown above; and Japan has undertaken a large programme of removal of tetrapods in an effort to beautify its coast. However they are intriguing objects and collectively form a fascinating abstract sculpture. Moderation is the key.