The call of the ocean has always been strong with me. I was born in it, grew up around it and feel lost when I cant see it. I respect the might and power of the ocean and am thankful for its offerings.
I felt at ease when I moved to Dunedin a few years ago, I left the farm, left the glorious high flyer lifestyle of travelling the world, I had no idea what I was doing with my life but upon driving up to St Clair beach I instantly felt calm. The Ocean to me is what a soft blankie does to a child. The ocean is my security blanket.
I could think nothing better than buying a house (only when I marry a rich dying businessman or win lotto, so never!) right by the ocean, but I know that the closer I get to the coast, the risk of erosion and sea level change increases. YET people still buy multi-million dollar houses right on the waters edge, then continue to complain that the ocean is encroaching on their perfectly manicured lawn and everyone should take notice! Shit me its frustrating!
But for those who have a too much money ( and not enough cents (sense) ), the powers to be within local body governments are trying their best to alleviate the ‘wrath of mother nature’. Personally, I like to think that mother nature is just too busy fixing our f***ups to give a crap about us, but others seem to take it personally.
SO WHY DO WE THINK WE CAN CONTROL HER?
It hasn’t worked in many places. People study the hell outta which is the best way to save our beaches, where in a few places I’ve experienced the most ridiculous solution: Lets take sand from this beach, and dump it on the beach where the rich people live! Hi Auckland! How ya going up there??
Anyway this isn’t about my disappointment in societies quest to control the ocean, while we single handily kill it, this post is going to be about the science behind the erosion occurring at St Clair beach.
Coastal erosion has been a hot topic in Dunedin over recent year with the ongoing damage to the St Clair sea wall and the erosion of the dune areas at St Kilda at Ocean Beach Domain. Yet these issues are not new, this has been happening for over a century now. Recently, more people have attributed the decline of Ocean Beach to the effects of climate change, but isn’t everything blamed on climate change? That seems to be the ‘hip’ thing to do.
People need to understand that the weakening of Otago’s dunes and dunes nationally are a product of historic land use, poor development choices, widespread removal of indigenous vegetation and attempts to tame untameable natural processes.
Coastal erosion involves the breaking down and removal of material along a coastline by the movement of wind & water. It leads to the formation of many landforms and, combined with deposition, plays an important role in shaping the coastline.
The biggest factor affecting coastal erosion is the strength of the waves breaking along the coastline. A wave’s strength is controlled by its fetch and the wind speed. Longer fetches & stronger winds create bigger, more powerful waves that have more erosive power. As waves approach a coastline they lose energy though because friction with the seabed increases. This means that the bathymetry (the underwater elevation) of the ocean or sea bed also impacts the strength of waves.
Certain landforms reduce wave’s erosive power. Beaches increase the distance a wave travels before it reaches the coastline’s cliffs and so reduces its energy. Headlands refract waves around them, reducing their erosive power at one location while increasing it at another.
This video helps explain this ( I apologise, I just cant seem to find any short and interesting videos that aren’t voiced by annoying people!) .
Impacts of Seawalls on Beaches
Building a seawall on a beach has several inevitable impacts and additional potential impacts. Sound familiar St Clair residents?
Wherever a hard structure is built along a shoreline undergoing long-term erosion, the shoreline will eventually migrate landward to (and potentially beyond) the structure. The effect of this migration will be the gradual loss of beach in front of the seawall or revetment as the water deepens and the shoreface moves landward. While private structures may be temporarily saved, the public beach is lost. Passive erosion will eventually destroy the recreational beach area unless this area is continually replenished. Excessive passive erosion may impact the beach profile such that shallow areas required to create breaking waves for surfing are lost.
Seawalls are placed on the beach. In many cases, construction of seawalls is on public property (beach), which is then lost.
Refers to the interrelationship between wall and beach whereby due to wave reflection, wave scouring, “end effects” and other coastal processes the wall may actually increase the rate of loss of beach.
Public access impacts
These can be a result of passive erosion, placement loss or active erosion. Seawalls built on eroding beaches will lead to the loss of access.
Seawalls are generally not attractive and can detract from a natural beach experience.
Scientific studies have documented a loss of ecosystem services, loss of habitat and reduction in biodiversity when seawall-impacted beaches were compared to natural beaches.
The most important thing to remember is that a seawall is never built to protect the beach. Rather, it is built to protect property, structures or a cliff from erosion.
According to Dyer (1994) in his thesis ‘Geography is knowing where the surf is pumping’ he concluded that:
- Erosion at St. Clair occurs during extended periods of strong southwest winds which are associated with increased wave heights, decreased periods and enhanced longshore currents. Under these conditions sand is transported both offshore and alongshore to the east, away from the western St. Clair corner, resulting in a lowering of the beach profiles.
- Erosion is accentuated at St. Clair by the presence of a sea wall, resulting in exceptionally low profiles which may allow fill to be eroded fom behind the concrete seawall face. While erosion is shown to be associated with predominantly steep waves, accretion at St. Clair was shown to often be unrelated to wave steepness.
- Within the significant accretion periods the longshore current direction was considered to be of greater importance. Thus currents moving to the west under the influence of easterly quarter swells transport sand in to the western St. Clair corner.
- The presence of the western headland blocking these currents results in rapid deposition, and accretion of the St. Clair profiles
This video below was made by a local by the name of Paul Pope who is an advocate for coastal conservation. If you are interested in this topic further, his Facebook page “The Beginners Guide to Coastal Conservation“ is worth a look. It gives a good interpretation of the processes behind what is happening at St Clair.
Going back in time, have a gander at these next two videos. Only posted in the last year, they show what has been done to ‘save the beach’, but considering what has happened in the past, I worry the DCC’s 10 year plan isn’t long enough, with only more and more money going to be spent in the future.
So the Solution? Move!
Says the girl with no fixed abode and 2 cats.
But realistically, in my opinion, there isn’t one! Well that’s if moving isn’t an option. We stuffed it up when it comes to St Clair, with bad land management and overuse. We just have to put our big kid pants on and adapt. We cant control the force of the ocean. Fact.
Thanks for reading,
Till next time!
Dyer, M. J. (1994). Geography is knowing where the surf is pumping. Christchurch: University of Canterbury.
Images and Gif’s have been sourced under the creative commons license.
‘The Physics of Erosion’, Paul Pope, Published on Mar 24, 2016