Most Kiwis appreciate the recreational opportunities our beaches offer - whether for beachcombing, surfing, walking the dog, bird-watching, swimming or simply snoozing in the sun. But did you know that beaches are also Natures way of buffering and protecting seafront real estate and infrastructure, such as parks and roads, against high winds and waves during powerful storms or rough seas?

If you live at the beach or visit it regularly, you will have seen how sand on the beach goes through cycles of erosion and accretion (build up).

The erosion process can be dramatic - taking place when big steep seas, combined with high tides, cause waves to pound the shoreline. Big surges of water rush up the beach with considerable force, scarping the dune face.

However, the beach fights back by allowing a proportion of the uprush to percolate into the sediments. As a result, the backrush has less power than the uprush, protecting the beach from erosion.

Gravel beaches are particularly good at protecting themselves from erosion, says NIWA marine geologist and coastal oceanographer Dr Terry Hume, "because they are very permeable".

"With a lot of the uprush water lost into the gravel, the backrush has little power to erode the beach face. Furthermore, the powerful uprush carries with it gravel that gets tossed up by the waves to build up a gravel ridge above high-tide level. This ridge prevents waves tipping over the ridge, creating further erosion."

Sand stripped from the beach and dunes during storms is carried out to sea by the backwash and undertow to the shallow nearshore, where it is deposited in banks. Surfers know this well as waves peeling over these banks in shallow water provide excellent surfing.

Dr Hume, a keen surfer himself, says the formation of these banks is the beach fighting back.

"When waves break on the nearshore banks they absorb wave energy so that the waves have less power when they arrive at the beach, buffering it from further erosion."

Accretion of sand takes place during calmer periods of long, low sea swell and is very gradual. The banks slowly migrate to the shore where they weld onto the beach.

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Beaches buffer our shores from Mother Nature's might