by Dr J Floor Anthoni (2000)
|Dams across rivers stop sediment from reaching the sea. They reduce the sand budget of beach/dune systems.|
|Due to global climate change (warming), sea levels are rising. Beaches could be drowned or pushed further inland. But strangely, some beaches appear unaffected.|
An argument often touted to explain why beaches are disappearing, is that of the large and increasing number of dams in rivers for energy generation, irrigation and flood control. They trap up-river sediment and indeed reduce the amount of sand that could otherwise have reached the sea. In some major rivers such as the Nile, this has affected the sediment budget considerably but overall, their influence is rather small. Every year more and more forests are burnt to make room for agriculture, a practice that increases soil erosion and hence the quantity of new sand, enormously. Here in New Zealand, the loss of sand from hydro dams is negligible.
Note that most of the sand along our coasts is very old, having been laid down thousands to millions of years ago. The hardness of the sand's quartz grains (SiO2) makes it wear very slowly. Although these grains get thrashed by waves, each is enveloped by a film of lubricating water, while each grain also has a round, smooth shape.
|Rising sea levels
As a consequence of global climate warming, it has been predicted that sea levels would rise, on the one hand by the sea water column expanding and on the other by the melting water from glaciers and polar ice caps.
Measuring the ocean level, however, is very difficult because continents drift and can be pushed up or down. Where heavy ice caps once pushed a continent down, it is still found bobbing up many thousands of years later. But scientists agree that overall, the sea level is rising. The picture above gives an idea by how much in two places half a world apart, Auckland and New York.
scientific work has been done to correlate the world wide beach erosion
with rising sea levels. The argument is shown in this diagram of the continental
Each continent is surrounded by a continental shelf ranging to 100-200m deep. The slope of the continental shelf varies enormously, between 1:100 and 1:500. If the sea level rises by 1-2mm/yr, the shelf expands and beaches are expected to be pushed inland by 0.1-1m each year. In a lifetime this could amount to a noticeable shift, which is roughly what has been measured on many beaches.
But this argument is not sound for four reasons.
|Before the last ice age, about 65000 years ago, the sea stood almost exactly where it is found today, evident from the location of old dune sands (picture A). During the last glaciation (ice age), the ocean's level sank by about 120m because of the amount of water locked up in ice caps and glaciers. As the seashore moved outward toward the edge of the continental shelf (but not quite), the old sand was left behind, but new dunes and beaches formed (picture B). By about 6000 years ago, the oceans started to rise again, sweeping the new sand dunes before it, all the way from the shelf edge to where they are found today, overlaying the old dune sand (Picture C). The rapid rise in sea level ended some 4000 years ago.|
the seas are slowly rising further, these new dunes will just shift slowly
further inland, but remember that they have been where they are today for
some 4000 years! However, what we are experiencing today is that the dune
sand is moving seaward as the sea is moving landward, something quite contrary
to what has been happening in the past 6000 years! Obviously, we have to
find better answers to why many beaches are eroding so rapidly today.
The graph shows how the average world temperature and sea levels have been changing over the past 300,000 years, which makes the whole story a bit more complicated. About 100,000 years ago, the sea level stood approximately where it stands today.
|Interestingly, picture D shows what happened to our hard sea coasts, where mountain ridges border the sea. Remember those mysterious rock platforms extending out in sea, just above low tide level? They show us where the coast was 65,000 years ago (and 100,000 years ago), before the last ice age, and 4000 years ago, when the oceans slowed their rapid rise. The distance between the edge of these platforms and the current cliff faces represents some 4000 years of coastal erosion. The reason that the platforms are so resilient against erosion by the sea, is probably because they are covered with a protecting film of life. This film of life consists mainly of algae, and in particular the stone-leaf alga Lithothamnion, need to be covered in water, and need sunlight as well. This also explains why their level is just above low tide, even though erosive forces there are strongest.|
|Knowing the thermal expansion co-efficient of water,
we can make a preliminary estimate of how much the sea level could rise
due to it becoming warmer. The average depth of the world's oceans is about
3000m. All the oceans of the world can be simplified as a bar 3000m long.
The water expands inside a basin that stays the same (assumption). So a
larger volume has to fill the same basin. The thermal volume expansion
coefficient of water is 0.2E-3 but decreases for water under ten degrees
(we'll ignore this). So the sea level may rise by at most 60cm for each
degree of warming.
The diagram shows specific gravity (density) versus temperature.