What is a dyke?
Well Jimmy, to put it bluntly, rather than a pejorative term for a lesbian, a dyke is in fact an artificially-engineered construction that works to prevent flooding in areas of low-lying land. Usually earthen, these dykes (sometimes referred to as levees, from the French lever, “to raise”) run parallel to the course of a river or coastline. Dykes can also be used to confine the flow of a river, resulting in higher and faster water flow, as well as for the purpose of ‘empoldering’, or as a boundary for an area prone to flooding.
Dykes are usually built by mounding earth on a levelled surface. Broad at the base, they taper towards the top, where temporary embankments or sandbags, as well as the planting of Bermuda grass, can be used as additional protection. Often, due to the increase in land acquisition over time, a number of dykes are built in series. Because a dyke is only as strong as its weakest point, the height and standards of construction have to be consistent along its length, which makes their planning and maintenance vital in ensuring the success of the system.
Some of the earliest ‘dykage’ occurred in Pakistan and India from around 2600 BC. Levees were found to have been constructed in Ancient Egypt over 3000 years ago, where a system of dykes was built along the left bank of the Nile stretching nearly 1000 kilometres. The Mesopotamians and ancient Chinese also built large dyke-like systems.
Dykes au naturel
While dykes/levees are commonly thought of as artificial constructs, they can also exist naturally. When a river spills out over its banks, the water spreads out and slows down, depositing its load of sediment. Over time, the river’s banks are built up above the level of the rest of the floodplain, resulting in natural ridges. When the river is not in flood it may deposit material within its channel, raising its level. This can raise not just the surface, but even the bottom of the river above the surrounding country. Sand dunes can also be considered natural dykes for the protection they offer to the adjacent land.
Hurricane dykes
The devastation caused by Hurricane Katrina in 2005 brought the words levee and dyke to the forefront of the public conscious. During the hurricane, the levees in place to prevent such a tragedy were breached at over 50 different points, submerging 80 per cent of the city. While most levees failed due to water breaching their tops, some failed when water passed underneath the levee foundations, causing the levee wall to shift. This sudden breaching released such high volumes of water that many houses were moved off their foundations and cars were tossed into trees like a scene from an apocalyptic blockbuster. The failure of the system put in place to prevent such devastation caused 1836 deaths and much of the blame has been placed with engineers, local contractors, and local levee boards for their lack of foresight.
Bikes on dykes
The modern word dyke (closely related to the English verb dig) is most probably derived from the Dutch word dijk, and it would be a gaffe of unforgivable proportions not to make reference to their clog-donning champions. The Dutch are synonymous with all manner of dyke-ery and they have used their extensive knowledge and skill to reclaim nearly 8000 square kilometers of land since the twelfth century. Currently, up to 60 per cent of the Netherlands is below the current mean sea level, and the population relies on this immense system of flood defence to stay dry.
Our very own namesake, Zeeland (or ‘sea land’) is a coastal province of Holland bordering Belgium, with a large part of its arable land a direct result of coastal reclamation. Much of the province lies below sea level, posing a considerable risk to its inhabitants. Historical breaches of the dyke system have taken their toll on the Dutch population.
“De Ramp” (the disaster) refers to a fateful breach which occurred in 1953. During the flood, many dykes in the provinces of Zeeland were unable to withstand the combination of spring tides and a northwesterly storm, eventually giving way to a torrent of water which took the lives of 1835 people, inundated 400,000 acres of land and decimated around 50,000 homes. The event boosted both research around, and the practice of, coastal engineering. It also spurred a rather drastic reaction in the form of the controversial Deltaplan, the implementation of which saw the almost complete closure of the tidal outlets of the rivers Maas and Rijn, and had significant ecological impact.
Local dykes
Closer to home there are a number of low-lying areas at particular risk of flooding. The Taieri plain (Otago) sits two metres below mean sea level. In May this year the plains were inundated after a considerable deluge. Other low-lying areas around the country are susceptible to occasional flooding, however most susceptible areas have a number of floodbanks in place that (usually) contain the overflow in ponding and drainage areas. Attitudes to flood plain areas have been criticised in the past, as a number of developers have undermined potential flooding risks by developing high-risk areas for residential subdivisions, such as that of Mill Creek in Otago. Fools.
It doesn’t take a wizard to know that rising sea levels are having a significant impact the world over. It is estimated that approximately 20 per cent of the global human population lives in coastal cities and of these, 100 million people live one metre or less above mean sea level. Here in New Zealand, this percentage is higher still, and with sea levels currently on the rise, we should be looking towards our namesake as a means to protect our precious landscape.
Unnatural dykes
Coastlines are often composed of a variety of rock and sediment types. From sheltered harbours, inlets and estuaries, long sandy dunes and exposed gravel beaches, to more pronounced rocky and cliffed coasts, they present us with a wide range of dynamic landforms. When left naturally, these environments have the ability to soak up vast amounts of wave and tidal energy. So why would we try to go against Mother Nature in the search for greener pastures?
When we reclaim land for development and roading we are interfering with the natural coastal system and the flow of sediments that nourish a coastline. Although reclamation has been managed sustainably in the past, there is a point at which the construction and maintenance of dykes becomes questionable in ecological terms. A growing movement called ‘soft engineering’ (again, pioneered by the Dutch) involves more ecologically responsible methods of protection from inundation, in stark contrast to the idea of erecting large protection barriers. By restoring marine marshes, growing trees and strengthening barrier islands, engineers are able to use strategies of protection that limit the need for bigger and badder dykes, following their adage ‘niet met fortsigheit maar met soetigheit’: not with force, but with sweetness.
Dykes against global warming
In 2008, two climate change reports released by the Ministry for the Environment that requested local governments consider hazards such as storm surges and high waves, and provided specific advice on sea level rise, particularly in regards to the Rongotai isthmus (Kilbirnie), which sits particularly low relative to sea level. Wellington City lies in a relatively well-restrained harbour, at about 2 to 2.5 metres above mean sea level, with a change in tide of, on average, just a little over one metre. In the event of dramatic sea level rise, we’re unlikely to ever try to ‘dyke-out’ by blocking off the entrance to the harbour with an engineering solution of Dutch proportions, however, in the coming years, protection may be required for the more vulnerable areas.
A five metre rise would inundate a substantial portion of Wellington’s central business district and all the low-lying parts of Eastbourne, Petone and Lower Hutt as far north as Avalon—although it’s worth pointing out that many experts have trouble with predictions of a one metre rise over the next century. A rise of one metre would probably flood some areas of Petone and Eastbourne, though it would have a minimal effect on the rest of the Hutt Valley, and the Rongotai isthmus would remain largely intact. In any case, it would probably be foolish to rush down to your local gondola dealer as the scenario of an antipodean Venice would be most unlikely.
Dykes are so ghey
So the jury remains ‘out’ as to whether attempts to contain and reclaim coastal areas through the proliferation of dykes is truly an effective means of managing the threat of inundation.Whether these rather drastic measures of ecological engineering are an efficient way to combat significant changes to the climate, or whether their impact will prove ultimately more damaging, is contentious and should be brought to the forefront of ecological discussion. At the end of the day, it no doubt all comes down to politics, and New Zealanders should be greatful our coastline can enjoy its current status as a dyke-free paradise.