what is a groyne?
A groyne is a structure built out from the shore into the water. At the Wairoa River mouth, we are investigating a permanent training groyne – so called because it is designed to help guide, or "train," the awa into forming and holding a stable, open path to the sea, east of Pilot Hill.
The mouth of the Wairoa River is always on the move. Sand and gravel shift with the tides, currents and river flows, and the river bar can build up or even close over. When the mouth is constricted, the river cannot drain as freely to the sea, which can hold water levels up in the lower river. A more open, stable mouth helps the river discharge more freely, particularly during high flows, which supports flood resilience over the long term.
The mouth of the Wairoa River is always on the move. Sand and gravel shift with the tides, currents and river flows, and the river bar can build up or even close over. When the mouth is constricted, the river cannot drain as freely to the sea, which can hold water levels up in the lower river. A more open, stable mouth helps the river discharge more freely, particularly during high flows, which supports flood resilience over the long term.
How Does it Work?
Rather than fighting the river, a training groyne works with it. By guiding the flow, the structure helps the river's own energy keep a channel scoured open through the bar, so the mouth is less likely to shift positions or close over entirely. The earliest concept developed for Wairoa proposed a combination of three elements – an Ecocell hexagonal concrete crib wall (a type of heavy-duty interlocking retaining wall made from honeycomb-shaped concrete blocks), a bed of crushed limestone underneath it to provide a stable foundation, and a layer of rock along the outside to protect against erosion.
These details are at still very much at concept stage only and will be refined as the investigations progress.
Turning a concept into a buildable design takes careful work. The next steps identified for the groyne include an engineering site survey, further design refinements, and more detailed construction cost estimates. Some of this investigative work is now underway (see below).
These details are at still very much at concept stage only and will be refined as the investigations progress.
Turning a concept into a buildable design takes careful work. The next steps identified for the groyne include an engineering site survey, further design refinements, and more detailed construction cost estimates. Some of this investigative work is now underway (see below).
Where is the Project at?
The groyne is at concept stage only – this is where we build a clearer picture of the project and explore the key questions of: can we build it, will it achieve what we are aiming for, is it affordable and are there funding options?
This information is needed before any decisions about detailed design or construction can be made. To support this, several pieces of investigative work are now underway:
This is investigative work only. Seeing survey crews or divers near Pilot Hill does not mean construction is beginning, it simply means we are gathering the information needed to understand what might be possible.
This information is needed before any decisions about detailed design or construction can be made. To support this, several pieces of investigative work are now underway:
- Engineering and geotechnical investigations, including an underwater seabed survey near Pilot Hill (subject to weather and diver availability) to help determine seabed depth and foundation conditions
- Ecological studies to better understand the natural environment of the river mouth, and
- Initial cultural assessment activity for the river mouth and surrounding area.
This is investigative work only. Seeing survey crews or divers near Pilot Hill does not mean construction is beginning, it simply means we are gathering the information needed to understand what might be possible.
