Summer Series Week 3: Beaches buffer our shores from Mother Nature’s might
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 Nature's 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. Sand that dries out at low tide is picked up by the wind and blown into the dunes where is it is stabilised by dune-binding plants such as spinifex and pingao. The build-up of this buffer of sand in the beach and dunes completes the erosion/accretion cycle. However, the intensity of the next storm will determine whether the slow fight back was enough to buffer against the next bout of erosion.
"Understanding how beaches buffer themselves against storms has taught us how to fight like Nature," says Dr Hume.
"Coastal management authorities are now using the natural buffering processes of beaches in their own approach to beach erosion control and preservation. For example, beach nourishment (the process where sand lost by erosion is replaced by sand from another source by mechanical means) is often now used instead of hard structures such as sea walls, which cause waves to scour the beach in front. Dune conservation, by fencing and planting, is also encouraged as a means to build up the buffer of sand."
Dr Terry Hume is a marine geologist and coastal oceanographer at NIWA's Hamilton office. His research interests focus on large-scale coastal processes, including the natural transport and storage of sand, coastal hazards, and beach erosion; and developing and applying beach and estuary classification for research and management purposes. He holds an Honorary Associate Professorship at the University of Auckland and an Honorary Lectureship at the University of Waikato. He's also a keen surfer.
Fast facts about sand:
- Origins: Rock or shell
- Time to form: Decades (soft shell, such as cockles and horse mussels) to millennia (hard rock, such as feldspar and quartz).
- Distance travelled: Hundreds of kilometres over millennia. Longest journey in New Zealand is about 400km (Mt Taranaki to North Cape).
Uses in New Zealand:
- sand paper (e.g., from garnet sands like those at Hunts Beach and Bruce Bay, Westland)steel industry (e.g., Taharoa ironsand, ilmenite)glass (e.g., Parengarenga quartz sand, Northland)concrete (Pakiri Beach, Kaipara Harbour)filler under roads, pavements, drains and bowling greens.
- Sand trivia: To qualify as sand, grains must be between 0.06mm and 2mm in diameter.1200oC is the temperature needed to convert quartz sand to glass.Quicksand is sand that has been so saturated with water that the friction between sand particles is reduced and they can no longer support weight. Sand can produce high- or low-frequency sounds under certain conditions – nobody is really sure how. This strange phenomenon occurs at some New Zealand beaches – such as Waihi Beach (Bay of Plenty) when conditions are right.