The Ōtorohanga District Council will continue to maintain the Aotea seawall under existing consent conditions.
THE regional council consent for the Aotea seawall expired in March this year, and the Ōtorohanga District Council is working through the consent renewal process.
The Aotea seawall was constructed to protect the Aotea foreshore and community from coastal erosion. The ongoing cost of maintenance is funded by the Aotea community.
The consent renewal has been lodged with Waikato Regional Council and the council will continue to maintain the seawall under existing consent conditions while ŌDC staff work through the renewal process to assure retention of the existing Aotea seawall while investigating the possible nature and scope of upgrade requirements.
The seawall was constructed in 2003 and was expected to limit shoreline retreat – provided the wall was adequately maintained, a Tonkin and Taylor report to then Environment Waikato stated in October 2007.
That report said the seawall was having “significant adverse effects” on the beach along the shoreline, including loss of high tide beach along significant lengths.
Over time, wall removal might be considered, the report said. But added options could be limited by landward development and advised caution was required in managing development in the hazard risk areas behind the wall.
The seawall was built because the shoreline in front of Aotea township was “extremely dynamic”.
Shoreline surveys dating from 1889 revealed major shoreline fluctuations and changes.
The most recent significant shoreline changes occurred in the 1960s and 1970s when the shoreline retreated by up to 125 metres, over a distance of several hundred metres.
A survey of residents conducted in 1979 indicated coastal erosion started to affect the initial subdivision about 3-5 years after its development in 1963. Residents were divided about whether the erosion began with a storm in May 1966 or during the “Wahine” storm of April 1968.
Severe erosion occurred in 1977, when a particularly large storm caused erosion of more than 60m of shoreline over a distance of several hundred metres.
Other reports suggested about 180m of shoreline retreat occurred in the period from the late 1960s to 1979.
But survey work commissioned by Environment Waikato in 1995 indicated the maximum shoreline retreat between 1963 and 1995 was nearer 125m, which incorporated additional relatively minor erosion between 1979 and 1995.
Between 1963 and 1995, 20 to 22 private sections were completely lost, and another dozen suffered partial or significant loss. Significant areas of reserve were also lost.
The severe erosion appeared to have been associated with complex changes both on the adjacent flood tide delta and at the harbour entrance – in particular changes to the entrance spit, which appeared to have allowed increased penetration of swell waves into the harbour.
Swell waves entering the harbour appear to have played a major role in most erosion noted to date, with increased tidal flows through the area immediately offshore from the settlement.
Deeper channels and increased tidal flows in these areas are thought to have increased the susceptibility of the shoreline to erosion during periods of heavier swells.
Studies of shoreline changes in other near-entrance locations indicated that changes in tidal flow patterns and channel depth can result in very significant shoreline changes.
The causes of the depth and shoreline changes are not well understood Tonkin and Taylor conceded, but guessed they could be associated with largely northward bypassing of beach sands across the harbour entrance and/or complex subtidal changes associated with the large volumes of sediment re-circulating between the flood and ebb tide deltas and the adjacent shorelines.
Shoreline change data compiled by Environment Waikato suggested that the 1990s erosion might have been part of complex shoreline fluctuations that occurred over long periods.
For instance, a short length of the shoreline surveyed in 1912 was located 50 to 100m landward of the position shown in a more extensive survey conducted in 1955 – suggesting a period of shoreline advance between those surveys similar in scale to the erosion of the 1960s and 1970s.
Similarly, a shoreline survey from 1889 lay 200 to 300m seaward of the 1955 and 1963 surveys in places towards the western end of the Aotea township, but 50 to 100m landward towards the eastern end.
The maximum extent of shoreline changes that can occur over long periods of time is unknown, but Tonkin and Taylor advised the regional council that a precautionary approach is warranted for all areas on sand given the significant shoreline changes that have occurred historically.
Waikato Regional Council and Ōtorohanga District Council have been approached for comment.
