Doubly threatened – by rains and storm surgesAnother example of a sinking coastal metropolis that is also plagued by flooding is Manila, the capital of the Philippines. It is likewise threatened by heavy rainfall events and storm-induced high seawater levels. Between 1900 and 2013, parts of Manila sank by 1.5 metres, and further subsidence of around 40 centimetres is expected by the year 2025. This is worrying because Manila is in a region that is often affected by typhoons, huge cyclonic storms. They bring large quantities of rain and churn up the sea so violently that huge breaking waves arise. Typhoon Ketsana in 2009 was a cyclone of catastrophic proportions. The rain and the storm surge inundated some districts of Manila with 2 metres of floodwater.
The fact that flooding is increasingly affecting coastal cities that are not notably subsiding is demonstrated by the example of Mumbai. A large-scale flood hit the Indian megacity on 26 July 2005, when almost 950 millimetres of rain fell within a 24-hour period. That is roughly equivalent to the total volume that normally falls in Mumbai during the entire month of July. In the hour from 15.30 to 16.30 alone, there was 190 millimetres of rain. Since this coincided with a relatively high sea tide, the rainwater could barely seep away and instead backed up in the drains and particularly in the River Mithi which flows through the middle of the city.
Whereas most previous flooding had only affected the unauthorized (informal) settlements on the city periphery, this time the city centre was also hit, and the water rose by more than 1 metre in next to no time. Streets, shops and office buildings were inundated and traffic came to a complete standstill for many hours. In the end 22 per cent of Mumbai’s area was flooded. The flood that day took a tragic toll: over 400 dead and around 100,000 badly damaged homes and businesses. 30,000 vehicles were reduced to scrap metal.
The analysis of the events made it clear that such large-scale flooding in Mumbai could only have happened because major swathes of the natural inundation areas along the rivers had gradually been sealed over, partly by the construction of informal settlements and partly in the course of official projects like the extension of the airport. Moreover, many watercourses were blocked with waste and building debris, impeding the rainwater’s run-off. Elsewhere, rainwater sewers were poorly maintained, river banks had slipped and concrete walls collapsed.
USA also has self-inflicted problemsFrom a worldwide comparison of coastal metropolises, it is clear that the cities worst affected by flooding are mainly megacities in emerging economies. Experts attribute this to the fact that the required standards are not always adhered to in the planning of structures – particularly with regard to forward-looking land-use planning, disaster preparedness, and functional infrastructure, which includes such features as flood channels, a functioning sewerage system, flood banks and flood control barriers. On the other hand, cities in industrialized countries certainly also have to contend with subsidence. That is evident from the New Orleans conurbation, for example, in which around 1 million people live. New Orleans is in the US state of Louisiana, directly in the Mississippi Delta. Between 1900 and 2013 the city sank by a full 1 metre and is subsiding every year by a further 6 to 26 millimetres.
This is happening for several reasons. As in other affected coastal cities, the abstraction of groundwater plays a part in the subsidence – but how much of a part is very difficult to assess, because the particular characteristics of the soil in New Orleans give rise to another phenomenon: if the groundwater table falls, oxygen from the air penetrates deeper ground layers and activates the soil bacteria that live there. These decompose soil organic matter. Since the soil contains large quantities of organic material that has been carried into the delta by the Mississippi over centuries, the bacteria find large amounts to decompose. This loss of organic substance contributes substantially to the subsidence.
Additionally, in the region around New Orleans, the drilling of petroleum and natural gas and the emptying of those deposits are also causing the ground to sink. The loss of sediment that used to be transported into the delta by the Mississippi should not be underestimated, either. Today, because it is slowed down by numerous weirs, the Mississippi carries distinctly less sediment into the sea. The old sediment packages deposited in the delta are so heavy that the delta is slowly subsiding naturally. Whereas this subsidence used to be evened out by fresh sediment from the Mississippi, today the markedly reduced sediment load is far from sufficient to compensate.
- 2.34 > Hurricane Katrina struck the southeast of the USA in late August 2005. Months later, traces of the destruction were still plain to see.
- These problems had been known about for decades, just like the fact that New Orleans was vulnerable to flooding in the event of even moderately severe storms. Experts had last published a warning in a scientific journal in 2003 about the consequences of severe hurricanes, which often threaten the coast on the Gulf of Mexico in the summer months. The existing flood control works were too low, and others were badly designed, maintained or constructed, their report read. The authorities had in fact planned a hurricane protection system, but due to a shortage of funding it was not built. In the view of experts, even this protection system would have failed because it would have been constructed according to outdated and unduly low design criteria.
Thus, New Orleans was relatively poorly protected when the extremely forceful Hurricane Katrina hit the city at the end of August 2005. It caused a 7-metre rise in the coastal water level. Consequently the flood banks (levees) were breached in around 50 places and the city, which lies in a hollow, filled up with water. Only after that did the authorities react and decide to construct a modern and effective flood protection system, the Hurricane and Storm Damage Risk Reduction System (HSDRRS). This was completed in 2011. It includes higher and more resilient levees and flood control barriers as well as floodgates and emergency pumps at the outlets of the pipes that drain runoff from New Orleans. These measures have considerably reduced the risk of major flooding.
Furthermore in 2012 the US authorities passed a new master plan for the protection of the delta, which will help to protect not only the city of New Orleans but the entire delta region from hurricanes and especially floods in future. A complete package of measures, including dredging works or the pumping of sediment, will allow the delta to grow again over the next 50 years. A good 700 square kilometres of new delta will be created in this way. Added to that will be 500 square kilometres of salt marshes to temper the force of the waves in the event of storms and hurricanes.
Halting the subsidenceThe examples of Tokyo and Shanghai demonstrate that the subsidence of a city can be stopped if appropriate action is taken. After parts of the Japanese capital had subsided by around 4 metres since 1900, the decision was taken in the late 1960s to drastically restrict groundwater abstraction. Thereupon the soil strata carrying the groundwater slowly filled up again, so that towards the mid-1970s the subsidence had already been halted.
The Chinese metropolis of Shanghai was faced with similar problems. Its response was not only to heavily regulate groundwater abstraction but also to deploy pump- ing technology, which permitted more rapid replenishment of the groundwater reservoirs with water.
The largest agglomeration on EarthThe development and settlement of coastal areas is probably the most conspicuous change to these habitats. In many cases species-rich wetland biotopes like mudflats and salt meadows, marshes and peatlands have been dried out and irreparably destroyed by construction measures. An extreme example of this urbanization in coastal wetlands is the Pearl River Delta in the middle of the coastal province of Guangdong in the south of China. This is the location of a huge agglomeration consisting of eleven cities, including Hong Kong and Macao. The entire region, covering an area of almost 40,000 square kilometres, is almost the size of the Netherlands.
The region comprises several Special Economic Zones and has undergone a rapid economic upturn since the 1970s. Back then the delta was still characterized by small villages and expansive wetlands, but by the year 2000 the increasingly urban area that formed within it through the merging of the cities took up 3500 square kilometres, which is roughly four times the area of Berlin. Today the drained area covers as much as 4500 square kilometres and the population density is immense. Currently around 60 million people live in the Pearl River Delta – around 3.5 times more than in the Netherlands, which is a densely settled country by European standards. Thus, within a few decades it has developed into the most highly populated agglomeration on Earth. Experts expect growth in this region to persist until the Pearl River Delta is home to around 100 million people by 2030.
With the drainage and redevelopment of the wetland areas, the habitats of many amphibians and birds have disappeared. Furthermore, as a consequence of water pollution, today around 90 fish species in this area are threatened. Aside from that, many river branches in the region carry less water, particularly during the dry months, since numerous dams and power plants were built for drinking water abstraction and electricity generation. Overall this means that considerably less freshwater flows into the delta, and at times when sea levels are high, such as spring tides or storm surges, seawater can penetrate deeper into the delta. Plants and animals that are not adapted to brackish water or higher salt content retreat from the affected zones. The habitat is changing enormously.
- 2.36 > The Chinese sturgeon Acipenser sinensis is considered to be at acute risk of extinction.
- Another consequence of dam construction is to interrupt the upstream migration routes of some fish species between the sea and their spawning grounds. In the opinion of experts, this substantially contributed to the collapse of stocks of the threatened Chinese sturgeon Acipenser sinensis, for example.
The removal of sand and stones for building projects represents another extreme case of interference with nature. The building material is taken from the river beds with dredgers and special ships. This alters the rivers’ flow regimes, which in turn leads to changes in the composition of species assemblages. Many water organisms lose their breeding and spawning sites as a result of dredging works.
Big business built on sandIt is not just in China that sand and minerals are extracted but in many of the world’s regions. According to estimates by the United Nations Environment Programme (UNEP), every year between 47 and 59 billion tonnes of minerals are mined worldwide, of which sand, gravel and crushed rock make up between 68 and 85 per cent. Since there is no standardized recording of the statistics, the quantities can only be estimated approximately. Between 25 and 30 billion tonnes of sand are needed for the cement industry alone. But in numerous places this colossal demand entails major encroachments on the landscape. Such resource extraction is therefore viewed very critically in many regions. In South Africa, for instance, dune sand is extracted for the construction industry. Critics fear that the coasts will be less well protected as a result because dunes are a natural bulwark against the breaking waves. In India fishers are protesting against sand extraction from beaches. They are critical that the dredging up of suspended sediments is driving fish away. For the local small fishers, this means the loss of their livelihood.
Characteristic plant communities which typically grow in sandy sediment in coastal waters and on tidal flats. They have long, herb-like fronds and thus resemble – but are unrelated to – the grasses that grow onshore. They are important habitats, providing young fish with food and protection from predators. Various species of fish lay their eggs directly on seagrass, so these meadows are often key nurseries for fish. They are also a vital foraging ground for birds, such as Brent geese during their autumn migration across Western Europe’s Wadden Sea.
- Similarly in Indonesia and Cambodia, the removal of large quantities of sand – mainly for export to Singa-
pore – has led to major destruction of the coasts, which prompted the government of Indonesia to prohibit the export of sand completely in 2008 while the government of Cambodia markedly restricted official exports in 2009. Nevertheless, sand extraction in Cambodia is continuing on a grand scale. Cambodian nature conservation organizations draw attention to the fact that sand is being traded in some cases by Mafioso groups. They complain that Cambodian authorities are not carrying out any controls. To what extent the government has secretly awarded permits for this extraction, or how far the corruption of officials is involved, remain unanswered questions. Sand is being extracted using suction dredgers in a coastal protection area in the Koh Kong region west of the capital Phnom Penh, among other sites. This is destroying mangroves and seagrass meadows – important habitats for the dugong, a species of sea cow.
Turning now to Singapore, the South East Asian city-state is an extreme example of sand imports. Being an island, Singapore constantly requires sand for the enlargement of its urban area. Between 1995 and 2014, around 500 million tonnes of sand were imported – for the most part from Indonesia and Cambodia. Since those countries adopted their export restrictions, sand has been imported to Singapore illegally, say the Cambodian nature conservation organizations.
Overexploited fishing groundsThe overexploitation of coastal areas is particularly evident when it comes to fishing. Not only is too much fish taken from the oceans, but fishing can also destroy marine habitats such as coral reefs. Humans take more fish out of the sea than the sea can continue to produce. As a result, fish stocks decline over time. It is not possible to state exactly the degree to which coastal regions are overfished, since the Food and Agriculture Organization of the United Nations (FAO) does not differentiate in detail between coastal fisheries and deep sea fishing. However, given that coastal waters worldwide constitute the most productive marine regions they no doubt provide the bulk of fish, and fisheries experts believe that they are the most intensively fished waters. According to current FAO data roughly 30 per cent of all commercially fished stocks worldwide are overfished. Given that there are more than 38 million fishers in the world who could lose their livelihoods or their food source, this situation is alarming. At particular risk are the small-scale fisheries on which approximately 20 million fishers depend in the developing countries alone, where fishing makes a significant contribution to the food supply and livelihoods of coastal people. In the tropics, small-scale fishing is primarily carried out on coral reefs. The number of coral reef fishers is estimated to be in the order of 6 million, 1.7 million of whom live in Indonesia, roughly 950,000 in India and 910,000 in the Philippines. Roughly half of the 6 million reef fishers primarily dive for sea cucumbers. These sausage-shaped echinoderms are related to starfish and are exported to Hong Kong in particular where the dried animals are valued and traded as medicine.
In many areas, coral reef fisheries are not currently managed in a sustainable manner. Not only are their stocks being overfished but the reefs are also damaged in a variety of ways. This is tragic as in the long-term people are destroying their own livelihoods.
- 2.38 > South East Asia, and especially Indonesia, has particularly high numbers of reef fishers. However, as a ratio of reef fishers to rural coastal population, Western Pacific island nations have the highest percentage, as these islands offer scarcely any other livelihood options.
Stronger engines, greater destructionJust how strongly intensive fishing impacts on inshore marine areas can be illustrated by the example of the Belgian, German and Dutch North Sea coast where fishing has very strongly altered seafloor habitats over the last hundred years. In these habitats, flatfish species such as dab, plaice and sole that camouflage themselves by partially burying themselves in the soft seabed are caught with bag-like bottom trawls that are dragged across the sea floor on heavy metal frames. These beam trawls and the chains attached to them are designed to penetrate and basically plough up the top few centimetres of the soft seabed as they are towed along.
In the nineteenth and early twentieth century, flatfish were in many areas still caught using small rowing or sailing boats and correspondingly small beam trawls. However, with the introduction of larger, motorized vessels ever larger beam trawls came to be used in the North Sea. Due to their high weight and the attached chains they plough up the seafloor and crush many of the larger benthic organisms. Studies conducted in Belgium, Germany and the Netherlands have shown that this is the reason for the significant decline in larger, long-lived or sensitive benthic species observed since the middle of the last century. The sensitive organisms include the tiny moss animals, organisms that form delicate colonies on the seafloor, some of which resemble corals in shape.
- 2.42 > The English port town of North Shields in 1904. At the beginning of the twentieth century North Sea fishing was still mostly carried out by means of sailing boats.
- Ocean quahog populations are also affected. This species of clam lives buried only a few centimetres deep in the ocean floor and their shells therefore are easily crushed by beam trawls. Normally, ocean quahog can grow to a size of more than 10 centimetres and live for up to 120 years. But as a result of intensive beam trawling they have been decimated in the inshore areas of the North Sea; with multiple beam trawl passes per year and area the populations are hardly given a chance to recover. Today the heavily fished areas are dominated by fast-growing bristle worms and small bivalves such as the tellinids which measure only 1 to 2 centimetres in size. Starfish and hermit crabs have also increased in abundance, both of which feed on the remains of the organisms killed by beam trawls. Hermit crabs are protected from beam trawls as they reside in thick-walled empty seashells. Starfish survive as they are relatively robust; if any of their arms are severed by a beam trawl they can regenerate these arms and survive. >