St Clair Sea Wall Erosion

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Effects of Sea WallsThe problematic St Clair Esplanade has dominated the news in Dunedin recently. Since the undermining and collapse of the paving areas Dunedin City Council staff and contractors have been feverishly working to stabilise and protect the eastern end of the wall. The wall has been problematic for the City Council since it was rebuilt in 2004, with on-going issues over the construction of access stairs to the beach. This latest slumping will test that most recent construction and just how the City Council will deal with the problem will be an interesting and costly engineering project for the City.

Particularly interesting will be whether the City Council take this opportunity to deal with the continued erosion of nearly 900 metres of dunes immediately east of the end of the wall. This portion of dunes has been significantly effected by the presence of the wall accompanied with the lowering of the beach immediately adjacent to the wall. The “end wall” erosion has been a constant battle for the Council since the first wall was established in 1872 and destroyed in the early 1880’s. Rebuilt in 1888 the wall was totally destroyed by high seas in 1890 and rebuilt in 1913. Episodes of dune and beach erosion continued regularly throughout the 20th century, culminating in a periods of dramatic dune loss in the mid-late 1990’s and early 21st century. A history of the St Clair sea wall can be found here.

The erosion during the 1990’s revealed the various attempts during the early to mid 20th century to utilise building and demolition waste along the dune toe along with the use of clay capping on the crest of the dunes east of the wall and towards Moana Rua Road. This material was considerably exposed and while some was removed much of it remains creating a highly erosional dune face. Attempts to deal with continual erosion immediately adjacent to end of the wall were made by the construction of a reno mattress in the 1990’s and filter cloth bags during the reconstruction of the wall in 2004. However despite these attempts the central issue of the effects of the wall on beach width and wave velocity remained and the measures were largely destroyed.

In the wake of the recent wall slumping the City Council must look more holistically at the wall and its effects on the dunes to ensure a more sustainable and pragmatic outcome for the conservation of the beach.

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