Coastal residents of Japan used to withhold the strong belief of the capability of seawalls in protecting them from the harsh tidal and tsunami waves but the beliefs were shattered with the occurrence of the March 11 earthquake and tsunami in Japan. About 40% of the 35,000 kilometre coastline is lined with concrete seawalls, breakwaters or other structures that seek to protect Japan against high waves generated from typhoons and tsunamis. Besides the development of quake-resistant buildings, coastal defences are also the main initiatives against earthquakes and tsunamis. Devastation of the coastal regions and death tolls that have exceeded the 10,000 mark led to queries related to the design of the seawalls and the necessity of rebuilding and redesigning these structures. The risk of reliance on seawalls was evident at the Daiichi and Daini nuclear power plants, which were both seated along the coast in close proximity to the earthquake zone. This attributed to the failure of the diesel generators that is vital in maintaining the cooling systems of the reactors during meltdown. The seawalls at the power plants were believed to have succumbed to the tsunami waves. Furthermore, the power plants were located at a lower elevation, based on the impression that the seawalls would be able to protect them from the impacts of tsunami waves. In Kamaishi, 4 metres of tsunami waves surmounted the breakwaters which was constructed several years ago in the harbour of the city at 63 metres deep, spanning 2 kilometres. The city centre was eventually subject to inundation. Preliminary investigations conducted by independent civil engineers revealed that sections of the Kamaishi’s breakwater is broken in the harbour, blunting the force of the tsunami, waves. In Kamaishi, 648 deaths had been confirmed, while 630 people were classified as missing. In the coastal village of Taro, the tsunami waves swept away people who have climbed atop the seawall, wiping off most of the Taro town. This exhibited that the seawalls have created a false sense of security among the people. Construction of the seawall began in 1933 after the tsunami event in 1933. There was reinforcement and expanded in the 1960s. Tsunamis are closely related to the history of Japan’s Sanriku territory. It houses one of the world’s most elaborate concrete seawalls that have transformed the coastal view into garrison-like towns. However, the recent tsunami have destroyed many coastal defences which are designed and erected to protect coastal communities against tsunami waves. Researchers are beginning to assess the effectiveness of seawalls and breakwaters in dissipating the energy of tsunami waves as people begin campaigning for the halting of such construction projects. Furthermore, the construction of these structures incur high cost. Instead, funds that are allotted for the erection of such coastal defences could be diverted into education and evacuation drills which could better prepare the people in Japan for earthquake and tsunami events.
The Yellow River stretches over a distance of 4,845km, draining over most of Northern China. The upper reach flows eastwards from the Tibetan highlands and the middle section reaches south from Baotou and the deserts and plains of Inner Mongolia.
There is substantial supply of sediments to the Yellow River as the vast areas of arid and semi-arid hilly areas carry eroded silt and feeds the river with its sediments. The supply of sediments is so intense that the river carries concentrated loads of yellow coloured sediment, giving the river its name. The lower stretch of the river, downstream from Zhengzhou, plagues the area with frequent floods as it flows across a densely populated and cultivated alluvial plain. Areas in proximity to Zhengzhou had been affected by siltation which raises the river bed by an average of 6 to 10 cm annually, increasing the severity of floods and the subsequent rapid filling of the reservoirs behind the dams.
Each episode of flood led to the raising of the river channel via siltation, dykes were breached and existing dykes were raised. In 1887, the river topped 20m high levees and drowned people and their crops and a concomitant famine that took the lives of more than 1 million people. In 1960, the San-men Gorge was completed, towering at a height of 122m and more than 900m wide. Its reservoir had been designed to hold back water. There was also a major initiative to plant trees and irrigate vast stretches of the silt plateau in the aim of reducing the amount of silt reaching the Yellow River.
Repeated construction of levees has not prevented the occurrence of catastrophic floods and river course changes. The lower 800km stretch of the Yellow River has repeatedly shifted its course by hundreds of kilometres. With each flood, the people built the levees to greater heights but this is also met with frequent failures of these levees. Source: The Straits Times, 26th September 2010
Of recent, there had been a series of dams and rivers being constructed to store water meant for flushing the silt away and this is carried out on an annual basis over a 15 to 20 day period. The flushing period takes place every July and reservoir gates would be opened to prevent the drying up of the water flow. Water would be blasted through the gates at speeds ranging between 2,600 and 4,000 cubic metres per second, depending on the amount of silt in each section. The force of the water would then scour the river channel and carry silt for deposition in the sea instead of the river channel. This has enabled the Yellow River to gain depth by an average of 1.5m since the first flushing was undertaken in 2002.
With reference to:
Hyndman, D. (2006) Natural Hazards and Disasters
Abott, P.L. (2007) Natural Disaster
The Straits Times (September 25, 2010). ‘Taming the Yellow River’.
Singapore adopts a combination of hard and soft engineering measures to protect its coastal areas. Hard engineering measures adopted in Singapore would include the construction of seawalls and breakwaters. Soft engineering measures would include the planting of mangroves and protecting coral reefs.
Seawalls are constructed out of materials such as reinforced concrete and granite. They are constructed in front of the coastal area and serve the function of slowing down direct wave erosion of a beach cliff. A wave striking a seawall is forced abruptly upward, such that the swash comes down more steeply and with greater force, eroding the sand in front of the seawall. Instead of protecting the beach, bigger waves would now approach closer to shore, accelerating the rates of erosion and removal of the beach. When the water in front of the beach becomes sufficiently deep, the beach would then be completely removed and the waves may undermine the barrier. Seawalls can be found in areas such as the coastal areas of Seletar facing the Straits of Johor. Seawalls over here are built low due to the calm and sheltered waters.
Revetments are a form of seawall consisting of large angular boulders that are used to protect the seaward end of the East Coast Reclamation Scheme in various phases. They have also been constructed at the fringe of the reclaimed islands at the southern zones of Singapore. Gaps have been placed in the revetments to facilitate the creation of beaches, especially in the sheltered areas.
Breakwaters are built offshore and parallel to the shore to promote deposition in the protected area behind the barrier and erosion on the downdrift sides. They trap sand moving along the beach and cause erosion further down the beach. Without continuous supply, beaches that are starved of sand would then be eroded away. Marine erosion has been removing much sand from the 1.3km long Siloso Beach and the worst affected is the middle section of the beach. Four islets, acting like breakwaters have been erected about 100m from the shore to ameliorate the force of the waves before they hit the beach. Breakwaters are also erected in the Pasir Panjang Wharf Complex to protect the coast against squalls and waves reaching 1.2m high during the southwest monsoon season between June and September.
Mangroves serve as a natural buffer zone between the sea waves and coastline. The muddy, waterlogged soils are capable of absorbing much of the energy between from strong winds, waves and storms. The authorities such as National Parks Board (NParks) have been stepping up efforts to manage and conserve mangrove areas. NParks designed a set-up of mud-filled bags, rocks, mangrove seedlings and poles to replace lost mangrove seedlings and prevent repeated episodes of erosion at the 1.65 stretch of coastline at Pulau Tekong.
Coral reefs are also effective in protecting coastal areas but are increasingly threatened by activities such as sedimentation brought about by reclamation works and dredging of shipping channels, which are smothering the corals to death. Corals in the deeper waters are the greatest victims as the silt block out much of the sunlight and food.
Coral reefs in Singapore
There are about 54 square km of coral reefs in Singapore, with most of them concentrated in and around Sentosa. Coral reefs are able to buffer the shorelines from waves, currents and storms, absorbing about 90% of the energy produced by wind generated waves.
The Three Gorges Dam built along the Yangtze River has been put up to the greatest challenge since its completion in 2009. Floodwaters in the dam rose by 4m in height overnight and as of July 23, 2010, the Three Gorges Dam registered a record high level of 158.8m.
For years, the Three Gorges Dam had been labelled as the best way to end the centuries of floods that haunt the Yangtze River basin. It had put off complaints and feedbacks with regards to the environmental damage of the US$23 billion reservoir, which displaced more than 1.4 million people in order to facilitate its construction. Source: http://www.internationalrivers.org/files/images/3g_map_graphic.preview.jpg
In view of the Three Gorges Dam close to exceeding its capacity, the Chinese officials have started to acknowledge the limits of the huge hydroelectric facility. Series of landslides were also triggered as a result of the torrential downpours that battered China. China is now confronted with the worst floods since 1998, which saw the death of at least 4,150 people and the evacuation of 18 million people and the destruction of more than millions of houses due to the 1998 floods in the Yangtze River.
The ongoing floods are set to worsen as China moves into the typhoon season. More than 230 rivers in China have risen above the warning points and two dozen of them exceeding historic highs. The flooding mainly affects the southern areas of China and has brought about economic losses of at least US$22 billion and affected about 120 million people.
