Blocking nature to nurture a lake ecosystem?

Te Waihora (Lake Ellesmere) is New Zealand 5th largest lake with a wind fetch of up to 30 km. Wave exposure means the shallow lake bed, at not much more than 2 m depth, and shorelines can be pounded by the elements. The same storm that caused the Wahine maritime disaster in 1968 was the last straw for beds of submerged plants that were once widespread in the lake. These beds were ripped out by the massive waves and never re-established. Mana whenua and locals consider the loss of lake plants has reduced water quality and fisheries values for Te Waihora. Re-establishing these plant beds is a priority for the Whakaora Te Waihora Programme, who have funded the work. The Programme is a partnership of Te Rūnanga o Ngāi Tahu, Environment Canterbury, and the Ministry for the Environment, with the vision to restore the mauri and ecosystem of Te Waihora and its catchment.

Plants (Ruppia) in cultivation for later transplantation. [photo credit: NIWA]

The first challenge is how to protect plantings from wave action at restoration sites. NIWA is trialling a floating wave barrier that was installed in June of this year. The 100 m long barrier is constructed from a raft of buoyant logs anchored into the lake bed.

Construction of the wave barrier. [Photo: NIWA]

The design was based on commercial breakwaters used in harbours overseas. Papatipu Rūnanga of Ngāi Tahu were involved in the design and approving the location for the test barrier.

To confirm the wave barrier was working, NIWA placed instruments to measure wave action at locations inside and outside of the raft.

Deployment of wave measuring instruments [Photo: NIWA]

Location of the instruments along the 100 m barrier. [Photo: N-Viro Mooring Systems]

Monitoring continued for six weeks and captured several high wind events. Results showed measures of wave height was consistently reduced by about 20-40% inside of the barrier. Wave reduction was not related to wind speed alone but is influenced by the direction of the wind to the orientation of the barrier.

Figure 1. The top graph shows the percentage reduction in measures of wave height caused by the barrier, measured near the water surface (black dots) and near the bottom (red dots). The bottom graph shows the wind speed during the monitoring period.

 

 

From here the next steps are to trial plantings of submerged plants behind the test barrier and additional wave barriers.

Contacts

For more information about the wave barrier project please contact:

Iain MacDonald

Mary de Winton.