Auckland Council requested that AR & Associates (ARAL) identify management issues and investigate remedial options for the western portion of Mathesons Bay. Loose wire and collapsed gabion baskets presented a health and safety risk and a steep drop at the western end of the gabion wall restricts beach access and presents a further health and safety issue. There has also been issues raised around the stability of the western access road and the suitability of the existing seawall structure to support the bank.
A range of potential management options were considered and included the removal of the gabions, physically training the Kohuroa Stream, replacement with another erosion control structure, a training groyne, access structures and hybrid solutions within the longer term context. A basic comparison of the various options was provided, and concluded that a hybrid solution is the best approach in addressing the range of management issues along this part of the beach.
The conceptual design for a preferred long-term solution presented to AC involved removal of the gabion baskets and manually training the Kohuroa Stream in order to control erosion of the western stream bank. Discussions with the local community were held to gain a better understanding of how the beach is being used and what their desires are for management across this part of the beach. Further, a monitoring programme was proposed in order to better understand the interactions of coastal and stream processes.
However, it was recognised that there are management issues that require attention in the short term, and specifically the stability of the western access road. ARAL developed a plan to replace the existing rock rip-rap structure with an appropriately designed erosion protection structure preventing any further retreat of the existing bank. It was also proposed to realign the sealed road 2m landward in order to move the road edge away from the over steep section of bank. This will also allow for reshaping of the bank to a more stable slope.
Planting of the bank with appropriate species was also considered to assist in providing additional stability.
Overall both long and short term proposals were seen as innovative and adaptive responses that assisted in developing resilience at site from the expected increased pressures from climate change and sea level rise.