Forgotten half of coastal erosion

OPINION PIECE

A “sea” of positives. With Wainui Beach — and erosion — in our news feed, it creates a perfect opening to discuss an essential natural coastal process on sandy coastlines: sand movement.

Let’s begin with remembering that sand is meant to move. And when the dunes and beach are made of sand, they too will move. An improved understanding of sand movement will allow us to move beyond the most obvious physical manifestation — erosion — and appreciate its greater purpose.

Sand eroding from beaches and dunes performs a function, and that function also explains why Wainui Beach has world-class surf breaks.

First off, we need some context. For headland-bound sand beaches, like Wainui, we have a defined spatial domain for sand movement: across the beach, and along the beach.

For the former, sand moves from the dunes and beaches seaward into the ocean (and vice versa); for the latter, sand swings back and forth (northward or southward) between Tuaheni and Makorori Points. The direction of movement is determined by wave direction.

Given Wainui’s exposure, it receives an abundance of ocean swells from the north, east and south — year-round. Not to skite, but when we recall that our predominant wind (NW) is “offshore”, nature has provided all the natural elements of a “surfer’s paradise” for Wainui Beach. But I digress — back to sand movement.

We often see the physical signs of erosion, but ignore its function. I call it the “forgotten half” of coastal erosion.

The process of erosion involves a temporary movement, or shifting, of sand from on the dry walkable part of the beach to the wet, underwater area beneath the waves. Imagine a speed bump on the beach; it moves seaward towards the horizon and creates a similar speed bump (“sandbank”) on the seafloor.

On roads, we build permanent speed bumps to slow cars. Along sandy coastlines, nature builds temporary, as-required, sandy speed bumps using beach and dune sands (the nearest sources of sand). We call that process “erosion”.

Why create an underwater speed bump, or sandbank? The sandbank forces incoming waves to slow down and break on top of them out at sea — away from the shoreline, far away from where we have our houses and infrastructure. It means the maximum wave energy in breaking waves is dumped in the surf zone, over that underwater sandbank.

Therefore, the formation of sandbars — via the process of erosion — is a natural protection mechanism for beaches, dunes and manmade infrastructure along the coast.

To complete the cycle of sand movement, a period of erosion is followed by accretion — a time when the underwater sandbanks dismantle and the mound of sand returns to the beach and dunes.

Complicating factors include storm clusters (back-to-back storms) and rips that disrupt this cycle, and generate severe dune erosion.

There is another valuable perspective to this process of erosion, too. Those speed bumps, or underwater sandbanks, constitute our “surf breaks”. The persistent sand movements and formation of sandbanks makes Wainui a world-class surfing beach, and explains why it has three surf breaks of national significance. Therein lies the importance of sand movement.

Sand moves, through erosion, to create sandbanks so that energy-laden waves break out at sea. And this function benefits us all in two primary ways: a natural protection mechanism for our infrastructure along the shoreline, and creation of hollow, A-frame-shaped surfing waves.

This knowledge can inform our approach to surf-break protection and private property protection. I also hope it will remind others that beaches do not need protecting; what we build on them, and next to them, does.

A “sea” of positives. With Wainui Beach — and erosion — in our news feed, it creates a perfect opening to discuss an essential natural coastal process on sandy coastlines: sand movement.

Let’s begin with remembering that sand is meant to move. And when the dunes and beach are made of sand, they too will move. An improved understanding of sand movement will allow us to move beyond the most obvious physical manifestation — erosion — and appreciate its greater purpose.

Sand eroding from beaches and dunes performs a function, and that function also explains why Wainui Beach has world-class surf breaks.

First off, we need some context. For headland-bound sand beaches, like Wainui, we have a defined spatial domain for sand movement: across the beach, and along the beach.

For the former, sand moves from the dunes and beaches seaward into the ocean (and vice versa); for the latter, sand swings back and forth (northward or southward) between Tuaheni and Makorori Points. The direction of movement is determined by wave direction.

Given Wainui’s exposure, it receives an abundance of ocean swells from the north, east and south — year-round. Not to skite, but when we recall that our predominant wind (NW) is “offshore”, nature has provided all the natural elements of a “surfer’s paradise” for Wainui Beach. But I digress — back to sand movement.

We often see the physical signs of erosion, but ignore its function. I call it the “forgotten half” of coastal erosion.

The process of erosion involves a temporary movement, or shifting, of sand from on the dry walkable part of the beach to the wet, underwater area beneath the waves. Imagine a speed bump on the beach; it moves seaward towards the horizon and creates a similar speed bump (“sandbank”) on the seafloor.

On roads, we build permanent speed bumps to slow cars. Along sandy coastlines, nature builds temporary, as-required, sandy speed bumps using beach and dune sands (the nearest sources of sand). We call that process “erosion”.

Why create an underwater speed bump, or sandbank? The sandbank forces incoming waves to slow down and break on top of them out at sea — away from the shoreline, far away from where we have our houses and infrastructure. It means the maximum wave energy in breaking waves is dumped in the surf zone, over that underwater sandbank.

Therefore, the formation of sandbars — via the process of erosion — is a natural protection mechanism for beaches, dunes and manmade infrastructure along the coast.

To complete the cycle of sand movement, a period of erosion is followed by accretion — a time when the underwater sandbanks dismantle and the mound of sand returns to the beach and dunes.

Complicating factors include storm clusters (back-to-back storms) and rips that disrupt this cycle, and generate severe dune erosion.

There is another valuable perspective to this process of erosion, too. Those speed bumps, or underwater sandbanks, constitute our “surf breaks”. The persistent sand movements and formation of sandbanks makes Wainui a world-class surfing beach, and explains why it has three surf breaks of national significance. Therein lies the importance of sand movement.

Sand moves, through erosion, to create sandbanks so that energy-laden waves break out at sea. And this function benefits us all in two primary ways: a natural protection mechanism for our infrastructure along the shoreline, and creation of hollow, A-frame-shaped surfing waves.

This knowledge can inform our approach to surf-break protection and private property protection. I also hope it will remind others that beaches do not need protecting; what we build on them, and next to them, does.

Your email address will not be published. Comments will display after being approved by a staff member. Comments may be edited for clarity.

Paul Hansen - 2 years ago
Precisely Ms Dunn, the beach is not too close to some of the infrastructure, humans decided to build it too close to the beach. The beach and its near fore dune areas of any coast are always going to be dynamic zones - the more ocean swell exposure, the more dynamic.

Poll

  • Voting please wait...
    Your vote has been cast. Reloading page...
    Do you like the new committee structure brought in at Gisborne District Council?

    See also: Committee shake-up