Erosion at beach not so new for East Coast

A109 Light Utility Helicopter flight with mayor Gisborne City from the air in November 2023.

The beach erosion that occurred north of Gisborne last week was not unusual for this coast.

Gisborne District Council records show the dramatic give and take of the sea over the decades when catchment staff used to measure the Wainui beach profile and sand budget.

That is, they would regularly measure the slope of the beach and do the calculations to reveal how much sand had been deposited and eroded.

In some years winter storms would scour the beach down to its papa base — the mudstone and other soft stones that underlie the sand itself.

Over the years the sand came and went,  in the hundreds of thousands of tonnes, sometime building up the foredune and at other times taking it away from under any nearby stairways down to the beach.

There is also a constant movement of the inshore coastal current which carries sand with it as it flows from south to north.

Last week the perigean or king tides (the highest tides of the month) coincided with the weather to produce extra wave activity directed at the East Coast.

A  low pressure system off the South Island effectively placed a large mountain of water off the coast, and the south-westerly weather rolled the waves downslope towards Gisborne beaches.

The severest erosion occurs when there is a very deep cyclonic low to the east, creating a steep downslope towards the coast.

If this combines with the lunar high tide cycle, as well as powerful southerly or south-easterly winds, the waves get extra acceleration and can cause considerable damage onshore.