17 Jan Coastal Properties – Be Aware of the Risks
In this contribution first published in the NZ Property Investor Magazine, Director Mason Reed explains that developers of coastal properties are advised to engage suitably qualified expertise from the outset.
New Zealand is blessed with stunning natural beauty. In particular, being an island, a large proportion of our country is coastal. Some of the prime sites in the major centres, and other holiday areas, are sites which are close to the sea. This coastal land, because of its location, is typically very sought after (and therefore expensive).
However, there are geotechnical hazards which are normally associated with coastal land, which purchasers need to be aware of.
The land is continually being battered by erosional forces, such as wind, rain and ice. These forces have been eroding the land for millions of years, and have resulted in the formation of many of the soils in New Zealand. The erosion process, for steep sites, is often accelerated by land slippage, typically caused by extreme weather events (and large earthquake events).
Steep coastal cliff sites, which are popular sites for expensive houses, are also subjected to an additional force, that being coastal erosion by wave action.
The cliff toes adjacent to coastal sites are typically exposed to wave action from the sea. The wave forces have the effect of eroding the toe of the clifflines, which results in oversteepening of the cliffline, which can result in subsequent cliffline instability and regression of the cliffline.
Coastal cliff lines will be slowly and relentlessly retreating under the influences of continuing geological processes. The likely rate of retreat of seaward exposed cliff lines ranges between approximately 3 m and 10 m per century.
The main factors affecting stability along the cliff line are the water content, slope angle, and in situ undrained shear strength of the veneer soils, and the erosion of the cliff line toe and subsequent regression of the cliff face.
Factors influencing the stability of the cliff line slopes at the site include variations in lithologies, their hardness, extent, thickness and orientation, vegetative cover, groundwater seepage, stormwater or surface water infiltration, and the presence of subsurface discontinuities, such as subsurface erosion tunnels, and the influence of coastal processes.
Properties located on coastal cliff lines are subject to a greater risk of slope instability than properties located elsewhere, due to the risk of cliff line regression occurring. The increased risk to any building can however be mitigated by appropriate foundation design, taking into account the various factors affecting cliff line regression.
Given that the erosion of the toe of the cliffline is a critical factor to cliffline regression, an obvious remedial measure would be to protect the toe of the cliffline from wave force action. However, this is often easier said than done. The cliffline toe is often located outside the property boundary in the “Queen’s chain”. Works to protect the toe of the cliffline often involve civil engineering building works and the use of machine excavators on the beach. It is very difficult to obtain the necessary consents to undertake building and earthworks on beaches (which are sensitive receiving environments).
That being said, there are ways to prevent the regression of land adjacent to clifflines. A common method is to construct what is called a “buried palisade wall” at the crest of the cliffline. This structure is essentially a large “buried” retaining wall, which has the structural capacity to provide permanent retention to a theoretical height of soil, located upslope of the wall, which is determined by assuming the ground to the seaward side of the wall, has been removed i.e. has slipped away.
These walls are expensive to design and construct, but, if appropriately designed, can prevent the loss of land into the sea, as a result of coastal cliffline regression. Given the value of coastal land, these types of measures are sometimes justified to protect the land and the structures on it.
My advice to anyone considering purchasing coastal land, is to engage a suitably qualified and experienced geotechnical engineer to undertake a pre-purchase assessment, in order to determine the risk of the land being adversely affected by cliffline regression. Buyers should be aware that coastal properties are subject to a greater risk of slope instability than properties located elsewhere.