The shifting sands of St Clair Technical Report
- Author
- Radich, J.
- Year
- 2019
- Pages
- 65
- Summary
- The St Clair poles are famous. People love them but this iconic structure also had function. Not a wharf, it was a groyne. It was built as a sand trap to raise the beach level in order to protect the Esplanade as well as the sand dunes. A series of wooden groynes performed this task for ~100 years in cycles of repair and neglect corresponding with sand accumulation or erosion. The poles that remain on the beach were installed in 1955 as a repair on a groyne constructed in 1921 but since about the year 2000, the poles have steadily deteriorated with ever-increasing dune damage. Textbooks state that groynes accrete sand on one side with erosion occurring on the other. This pervasive belief has cost Dunedin dearly because it is simply not the case at St Clair. Not a single photo exists showing leeside erosion on a St Clair groyne yet some commentators insist it will occur. Distinguished experts who have studied this beach agree that longshore drift occurs in both directions here, and that loosely planked wooden groynes accumulate sand evenly, just as the photographic record proves. Therefore, it follows that the first step in dealing with the current erosion problem should be to rebuild the one remaining groyne. It is easy to restore these poles as no resource consent is required to repair an existing structure and it is inexpensive with estimates ranging from $100 - $150, 000 depending on pole type. A vast quantity of sand lies just offshore showing no decrease from the Clutha dams. This repeatedly successful mechanism will trap some of that sand and grow the beach to become wider and deeper giving much needed protection to the seawall, sandhills and South Dunedin. With climate change bringing larger storm surges and sea level rise threatening the hinterland, accumulating more sand on the beach is an effective defence that uses the power of natural forces to work for us instead of against us. Reinstatement can be undertaken as a trial because not only is it quick and economical to install, it is easily removable if and when better overall solutions are determined. At the very least, knowledge gained will usefully inform future discussions. Even if you are sceptical, it is well worth the investment because Dunedin Ratepayers have spent hundreds of thousands on consultants and millions repairing coastal damage over the last 20 years with the situation only getting worse.