Engineered Coasts

The coastal zone has long been one of the most intensely utilized segments of the landscape. Its juxtaposition between land and sea and its varied and abundant resources have been so attractive to humans that today within the industrialized countries of the world some 50% of the population lives near a coast^[1]. With such a concentration of people within such a finite area, coastal problems including resource destruction, flooding, pollution, and shoreline erosion are becoming increasingly important. Although these problems are very serious along many coasts today, there is concern that they will be amplified in the near future because of global warming induced acceleration in the rate of sea-level rise^[2].

Just when humans first renourished a beach is unknown. However, the first example may well have occurred more than 2000 years ago when Cleopatra had sand from Egypt shipped to “...Turkey so that she would not have to step on foreign soil”^[1]. Nevertheless, the artificial nourishment of beaches is relatively new as a method of coping with coastal erosion.

Among the major rivers in the temperate and tropical areas of the world the Mississippi River was one of the last to be densely populated and intensively utilized. Other rivers, such as the Nile, Tigris-Euphrates, Ganges-Brahmaputra and Yangtze, have been centers of dense population for millennia. The Mississippi River and delta only began to be utilized intensively after becoming a center for colonization in the 18th and 19th centuries and as a major transportation route only after the passes at the river’s mouth were opened to commercial traffic in the mid-19th century.

Tokyo Bay is a long, oval-shaped body of water surrounded by Tokyo, Chiba Prefecture, and Kanagawa Prefecture (Fig. 1). It is connected to the Pacific Ocean by Uraga Channel (90 m deep, 7 km wide). Tokyo Bay, from its mouth to its most distant point, has a length of approximately 90 km; it has a width of about 20 km. The bay encompasses a water area of about 1,200 km². It is relatively shallow, 18 m on the average. The total length of the coastline inside the bay is approximately 180 km. Tokyo Bay has 31 ports, of which Yokohama, Yokosuka, Kawasaki, Tokyo, Chiba, and Kisarazu are the largest. As a result, many ships navigate the bay and the traffic is always heavy.

Japan is a small country with a dense population. It consists of four major islands and more than 4000 smaller islands. Combined, the length of the shoreline is more than 34,000 km or about equal to that of the contiguous United States which has an area more than 20 times larger. Because Japan rests on the western part of the Pacific plate it is mountainous and subject to frequent seismic activity. The bulk of the population and most of the industry and commercial activities are concentrated on the coastal plain which is small in area. Because of the pressure this concentration places on the landscape, there is little natural room for expansion. As a result, the Japanese have been very aggressive in protecting the shoreline from erosion, in ocean space utilization and in reclamation^[1].

The Qiantang Estuary, with an upstream boundary at the tidal limit near the Fuchunjiang hydro-power station and a downstream boundary at the mouth of Hangzhou Bay, is a macro-tidal estuary that is world-renowned for the magnificent tidal bore in the estuarine stretch between Wenyan and Ganpu. It lies south of the Changjiang River Estuary and north of the hilly region of eastern Zhejiang. The rich sediment supply from the Changjiang River and the coastal regions of the East China Sea has formed a vast area of coastal plains on both banks. The plain at the north bank, known as the Taihu Lake Plain, is connected with the Changjiang delta; the plain at the south bank is the Xiaoshan-Shaoxing-Ningbo plain (Fig. 1). The total area of the plains stretching from both banks is about 50,000 km². The land is fertile, has well developed transportation systems, developed production, a prosperous economy, and a galaxy of talent in Chinese history. The evolution of the estuary has combined natural and cultural processes resulting in the construction of an artificial coastline. This artificial coastline reflects the development of the area’s social economy and the progress of science and technology. In order to understand the historical development of the area it is helpful to review the interaction of man and nature.

The seawall (sea dike) situated in the estuarine reach of the Qiantang Estuary is subjected to the strong, destructive action of the world-renowned Qiantang bore; in Hangzhou Bay, on the other hand, the dike is subjected to strong wave action. The seawall protects the vast lowlands of the Taihu Lake plain on the north bank and the Xiaoshan-Shaoxing-Ningbo plain on the south bank; these plains are prosperous and densely populated. This seawall attracted much attention from the supreme sovereigns during China’s history. Emperor Qianlong of the Qing Dynasty inspected the seawall in Haining County four times during his six patrols to the southern area of the Changjiang River. After many attempts at construction, repairing and reconstruction, large amounts of money and labor were expended and, finally, a magnificent seawall, nearly 300 km long, was completed. Along with The Great Wall and Grand Canal, the Qiantang Seawall is one of the three most famous civil engineering works of ancient China.

Humans have modified coastlines in a variety of ways and for a variety of reasons throughout much of history. Some of the earliest modifications were nothing more than re-arranging boulders along a rocky coast to trap fish during tidal variations, building mounds in coastal swamps and marshes and creating middens along the shore (Fig. 1). For example, shell middens dating back to the last interglacial are present along the coast of South Africau^[1] and underwater cameras have recently (2000) photographed remains of a dwelling site at 91 m deep on the bottom of the Black Sea^[2].

The Changjiang is the largest river in China. Its mouth is advancing seaward gradually. For example, in the 7^th century B.C. the mouth was in the reach from Zhenjiang to Yangzhou. Then in the 17^th century A.D., it moved eastward to the reach of Jiangying, and in the middle of the 20^th century to the area of Xuliujing (Fig. 1). Now downstream from Xuliujing, the Changjiang mouth is firstly divided into the North and the South Branches by Chongming Island. The South Branch is likewise sub-divided into the North and the South Channels by Changxing Island. Furthermore, the South Channel is again divided into the North and the South Passages by Jiuduansha Island. Thus, today, there exists a three-order bifurcated estuary with four outlets to the sea (Fig. 2).

Quaternary sea level changes have played a key role in the development and evolution of the Pearl River delta. About 10,000 years ago, sea level was 100 m lower than today. There were deeply incised river valleys in the modern Pearl River delta area at that time. They formed the delta base topography and large amounts of coarse riverine sediment were deposited on top of that base. Approximately 6000 years ago the Holocene transgression reached its maximum and sea level approached today’s level. Upstream deposition in the backwater reaches of the river valleys ended at that time; these deposits are widely distributed beneath modern network channels. The topography was more or less ria in form, consisted of shallow bays with a weak hydrodynamic sedimentary environment. Many rocky islands were scattered in the ancient drowned estuary^[1,2]. During that period, the most northward deltaic coastline was located along a line linking Daojiao, Huangpu, Shiqiao, Chencun, Shunde, Jiangmen and Shafu^[3].

Coastal morphology changes continuously because of (changing) natural conditions. But also human interferences in the coastal system in past and at present (e.g. building manmade coastal structures like breakwaters to shelter port entrances) have affected the coastal morphology to a large extent. The coastal system is, however, a very vulnerable system, so one has to be aware of thoughtless interventions in the system.

The history of protection against the sea in the Netherlands is a long one and a good example of leap frogging between man made solutions and man made problems. It started 2000 years ago with small scale interventions in the water management of the low lying soils of the Rhine—Meuse—Scheldt Delta, see Figure 1. Culverts for drainage, made of a hollowed trunk with a hinged cover, were placed in a dike, in order to prevent flood tides entering agricultural land. see Figure 2.

Rotterdam, located on the most northern branch of the Rhine—Meuse deltaic system, existed already as a fishing village in the 11 th century A.D. and as early as 1100 A.D. was protected by (low) dykes. Flooding was a regular threat: in 1164 the entire village was destroyed during a flood and again in 1421. The nucleus of the present town existed from the 15th century on and gradually developed into a sea port. The first, small, harbour had been made in 1350 (van Dam, 1990) and the connection with the North Sea was along the main river branch that passed Rotterdam towards the west and opened in a funnel-shaped mouth towards the North Sea. The shifting sand banks and narrow channels were not a large problem as long as the ships were small. They were more vulnerable to stormy weather: the harbour, some 35 km inland, provided a better shelter than would have been provided by a harbour on the coast. At those times such harbours did not exist on the North Sea coast: fishing ships were drawn high on the beach.

The closing off and the partial reclamation of the Zuyder Zee has resulted in the gain of 166,000 hectares of new land (Figure 1). This new land appears to lend itself for agriculture, urban development, recreation and nature conservation.

The history of the British East coast is different from the continental coast along the North Sea, like in Holland, where over 2000 years the peopie lived in alluvial lowlands, that were frequently flooded and had to protect themselves anyhow against the Sea. The British topography provided sufficient high territory for people to live and to protect themselves against the sea until the era of the industrial revolution, when a fast increasing population started to live and work on land that was seriously under threat of being flooded by the Southern North Sea. Apart from this demographically defined need for an artificial improved coast, the sea itself has caused reauirements for coastal improvements as well.