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Coastal processes and tidal flat reclamation in Cixi City, China

Processus côtiers et restauration de la vasière intertidale à Cixi, Chine
Lihua Feng et Xiong Chen
p. 181-190

Résumés

La configuration géomorphologique du site de Cixi, constituée de terrains vallonnés au sud, de la mer au nord, et de la plaine littorale dans la zone centrale, a été créée vers 2500 ans BP. La zone intertidale de Cixi appartient à la classe ensablement d'une batture. Onze digues ont été construites depuis la dynastie des Song. Le taux moyen de progradation de la ligne de rivage avant la fondation de la République populaire était de 25 m/an, il a augmenté depuis de 50 à 100 m/an. Les facteurs intervenant dans la formation et l'évolution de la zone intertidale de Cixi comprennent principalement la contrainte du relief, les débits fluviaux et les activités humaines. L'influence des activités humaines est devenue encore plus grande dans les temps modernes. Le principe de régulation du chenal de marée dans le processus de restauration de la zone intertidale devrait guider le mode d'intervention dépendante des circonstances au même titre que la restauration de la zone intertidale s’accompagne de la régulation du chenal de marée. Le premier aspect de ce principe suppose que l’enlèvement du chenal fluviatile doit respecter la loi de l’écoulement et du transport des sédiments. Le dernier aspect signifie que la régulation de l’incision du chenal de marée doit être étroitement liée à la restauration longitudinale de la zone intertidale.

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Notes de la rédaction

Article soumis le 18 août 2011, accepté le 3 janvier 2012.

Texte intégral

Introduction

1Cixi City lies on the southern bank of Hangzhou Bay, China, between the east longitude of 121°02’-121°43’ and north latitude of 30°20’-30°34’. The City has a historical title of tidal flat in the Tang dynasty and successive forming of land in the Song dynasty. Labouring people of the city have been continuously engaged in the practice of enclosing the tidal flat and constructing ponds since the Song dynasty, and, therefore, the modern littoral plain has gradually emerged in the city. It is obvious that the social developmental history of Cixi City should, to a large extent, be the history of reclamation and development of the tidal flat on the southern bank of Hangzhou Bay (Ma, 1998; Wang, 2007; Dai, 2008). In fact, the Cixi tidal flat underwent a very long formation and evolution history including earth crust tectonic movement, glacial movement and human activities of pond construction and tidal flat reclamation in successive dynasties (Kirby, 2000; Zhang et al., 2002). Therefore, the Cixi tidal flat should be considered the result of the joint action of natural and artificial factors.

2Tidal flat reclamation of Hangzhou Bay has a long history, so there is extensive literature on the subject. Early in the 1980s, according to the situation of tidal flat reclamation in modern history, Y.H. Hu and D.Z. Shi (1984) considered that one of the effective measures for improving tideland resource utilisation is the use of bulkhead dams and mesh dams to promote siltation. Y.L. Lin and B.Q. Chen (1996) analysed the important role of tidal flat resources in Cixi City’s economic development and put forward a programme that set comprehensive tideland development as a goal and gradually formed the base of agriculture, fisheries and the salt industry. Using remote sensing technology, L.S. Song et al. (2007) studied the map of tideland resource changes in the Qiantang River estuary and Hangzhou Bay. Their investigation offered a scientific basis to explore the causes of tideland resource changes, the development trend and the environmental impact. X.L. Meng et al. (2010) took Cixi City as an example and analysed the change of land use during the rapid urbanisation in 1997 and 2007 by using Landsat TM/ETM and CBERS images; they then made a preliminary forecast for land use in 2010-2020 by the Markov model.

3From the above, some scholars have investigated the coastal evolution and land use of Cixi City and have drawn several useful conclusions. The author will discuss in depth the cause of coastal changes in Cixi City based on previous work. A large amount of the data in this paper derived from “Cixi Water Conservancy Records” (Xu, 1991), “Records on Qiantang River” (Ma, 1998) and “Comprehensive Finding Report of Coast Zone and Tidal Flats Resources in Zhejiang Province” (Dai, 2008). To obtain scientifically reasonable conclusions, the authors used the research methods of field investigation, data calculation and causal analysis to determine the common effects of natural factors and human factors in coastal processes and tidal flat reclamation in Cixi City.

4Along with the fast development of the social economy, a conflict of land use in the city has become more and more conspicuous. Moreover, the tidal flat is a potential land resource reserve. To mitigate the conflict between agricultural and industrial demands for land, the city will engage in larger scale reclamation and exploitation of the tidal flats. Therefore, the objective of this study is to provide a sound scientific basis for reclamation efforts and to offer substantial benefit to new engineering work in the future.

Study area

5The southern and northern sides of Cixi City are hilly areas and sea, respectively, and the middle part of the city is littoral plain. The total area of the city is 1,718 km2. Hilly area, littoral plain, tidal flat and sea account for 9.7%, 45.2%, 25.2% and 19.9% of the area, respectively. In the southern hilly area, the strata consist mainly of late Jurassic (J3) igneous rocks, which are divided into four formations: Dashuang Formation (J3d), Gaowu Formation (J3g), Xishantou Formation (J3x) and Jiuliping Formation (J3j). The lithology is complex, mainly including tuff, rhyolite, andesite, sandstone/conglomerate, etc., with a total thickness of 4,800-5,000 m (Ghosh et al., 2004). In the littoral plain (including the tidal flats) the strata are divided into two parts. The lower part comprises the early Cretaceous (K1) Fangyan Formation and the Eocene (E2)-Oligocene (E3) Changhe Formation. The former is sandy siltstone, with a thickness of 173 m, while the latter is argillaceous siltstone, with a thickness of 1420 m. They jointly form the solid basement of the littoral plain. The upper part consists of loose Quaternary strata (Hannah, 2004), including the middle Pleistocene (Qp2) Zhijiang Formation, the late Pleistocene (Qp3) Dongpu and Ningbo Formations and the Holocene (Qh) Binghai Formation, with lithology of siltstone, sandy loam, subclay, clay, etc. and a thickness of 94 m (Xu, 1991).

6Cixi City is located within the subtropical monsoon climate zone, with four distinct seasons, ample sunshine and an annual precipitation of 1,345 mm. Perennially, the wind direction varies along with the seasons, with mostly northerly wind between November and March, mostly southerly wind between April and July, a typhoon-affected season between May and October and a typhoon-stricken period between July and September. The average wind speed in the zone is 3.8 m/s, with a maximal wind speed of 28 m/s. The normal wind direction is E-SE, and the strong wind direction is N-NW. The average maximal wave height in the coastal area is 2.4 m, with a measured maximal value of 6.1 m. Typhoon and cold wave are the two major meteorological systems. Typhoons have especially strong damaging effects on engineering structures and contribute greatly to the historical record-making maximal wave heights (Dai, 2008).

7The coastal tide of Cixi is irregular semidiurnal, with flood tide duration of 6 h and ebb tide duration of 6 h and 25 min. Because of the effect of the horn-shaped (funnel-shaped) estuary, the tide wave from the outside sea deforms when entering Hangzhou Bay (called ‘funnel effect’). From the mouth to the inside of the estuary, the high-water level gradually increases, while the low-water level continuously drops. Thus, the tidal range on the eastern coast of Cixi is only 2-2.5 m, while that of the western coast can be as large as 5.5-6 m. Constrained by the topography of Hangzhou Bay, the flood and ebb (reciprocating current) flows in the coastal area of Cixi are nearly parallel to the shoreline (Ma, 1998).

Evolution of the Cixi tidal flat

Continental formation of Cixi City

8Towards the end of the early stage of the late Pleistocene, about 100000 BP, sea level rose globally, and the first marine transgression occurred in the region (Passchier et al., 2003). This caused the northern area, in the region between Andong and Xinpu, to finally become a shallow sea. After that, at the beginning of the middle stage of the late Pleistocene, sea level gradually fell, and the land area expanded. Afterwards, near the end of the middle stage of the late Pleistocene, the global climate warmed up and this again resulted in rising sea levels. The second marine transgression occurred, and the plain of the overall city area became part of the sea. During the late stage of the late Pleistocene, about 15000 BP, the Dali Glacial Maximum appeared, and sea level fell significantly, to 140-160 m lower than the present sea level. The overall area of the city was again exposed to form the land, and the coastline moved to the eastern area of the present Zhoushan archipelago. After that, during the early stage of the Holocene, about 12000 BP, the global climate again warmed up. Large-scale marine transgression appeared, and sea level rose to a height of 130 m. During the middle stage of the Holocene, about 7000 BP, sea level rose continuously to a height of -2 m. The maximum height of sea level in the Holocene was achieved about 6000 BP, when it was even 1-2 m higher than the present sea level (Isern and Anselmetti, 2002; Lemeille and Sorel, 2002). The overall area of the city again became part of the sea, and all the mountains in the southern area became isolated islands. Then, marine regression began to occur about 5000 BP. The mountains in the southern area were exposed to form swampland, and, at the same time, the northern area of the region was still part of the sea (Xu, 1991).

9After the late stage of the Holocene, about 2500 BP, Hangzhou Bay gradually took on a horn shape. Silt was continuously transported from inland by the action of water dynamics. The southern bank of the bay was gradually silted outward to form land, and, therefore, the overall area of the Cixi region was divided into two parts. The southern part belongs to the lacustrine deposit and marine deposit zone and has many lakes. A peat horizon and black organic soil remain in the stratum of the zone owing to lagoon swamping during continent formation. Within the northern part, the region north of Dagu dyke is the marine-deposit plain of Hangzhou Bay. Neither natural lake nor river exists in the region. The continent formation in the region is the result of deposition of silt from the upper stream of Hangzhou Bay, fine sand from the continental shelf of the East China Sea and sand from the collapsed land of Hangzhou Bay. The coastline had already been located in the area between Zhouxiang, Hushan, Guancheng and Longshan in the Song dynasty (Xu, 1991), as shown in fig. 1.

10Satellite photogrammetry conducted in 1986 indicates that there are two obvious gray lines in the region of Cixi City. They are equivalent to ancient shorelines. One line lies along a plane from Zhouxiang to Hushan to Longshan; the other along a plane from Xisan to Andong to Xinpu. The two lines are distinct, dividing the city into southern, middle, and northern parts. The southern area belongs to the plain of lacustrine and marine facies formed in the late stage of glaciation. The middle area is the sedimentary plain of marine facies formed in the middle stage. The northern area is the marine deposit plain of littoral facies formed in the most recent stage. This area is also called the ‘Andong salt zone’ (Ma, 1998). Outside the northern area of the dyke, a configuration of irregular semicircular tidal banks called the ‘Sanbei Riffle’ can also be observed (fig. 1).

Fig. 1 – Migration chart of coastline in Cixi City.
Fig. 1 – Carte de la cinématique du trait de côte dans la ville de Cixi.

Fig. 1 – Migration chart of coastline in Cixi City.Fig. 1 – Carte de la cinématique du trait de côte dans la ville de Cixi.

Modern evolution of the Cixi tidal flat

11The recent evolution of the Cixi tidal flat is closely correlated with the tide (Elgar and Raubenheimer, 2011). Observation shows that the Cixi tidal flat is always under continuous silting processes and belongs to the silting-up type tidal flat. In addition, the silting has obvious fluctuations and variation depending on the phases of the moon and the seasons (corresponding to tide). Silting during spring tide and scouring during neap tide are general phenomena. In addition, silting in summer and scouring in winter are also commonly observed phenomena. The beach slope is 0.25‰ in the silting stage, and the value increases to 0.27‰ in the scouring stage. The beach is easily scoured under strong dynamic conditions such as typhoons and sharp flood tides, and the same beach can easily shift to a silting state after the occurrence of a typhoon (Holgate and Woodworth, 2004).

12The changing processes along the coastline of Cixi City during various historical periods are unquestionably recorded in the dykes around the city. Construction of Xieling dyke was begun in the Seventh Year of Qingli of the Song dynasty (around AD 1047). Dagu dyke was constructed in the Yuan dynasty (about AD 1341; Xu, 1991). After that, ten dykes were in turn constructed along with the expansion of the bank beach as shown in fig. 1 and tab. 1. This resulted in the coastline of the city moving outwards to a distance of 18 km, with a reclaimed land area of 886 km2. The average outward migration rate of the coastline has been calculated to be 25 m/a within the most recent 600 years as indicated by the different coastlines in each historical period (Dai, 2008). The outward migration rate of the coastline increased after the founding of the People’s Republic of China owing to the implementation of large-scale silting and reclamation because of the urgent demand for land. The annual migration rate of the coastline is up to 50-100 m during this period.

Tab. 1 – Construction time of Cixi dike and outward migration rate of the coastline.
Tab. 1 – Construction de la digue Cixi dans le temps et taux de migration du trait de côte vers la mer.

No.

Dike name

Time of Construction (AD)

Distance from the former

(in m)

Migration rate of coastline (in m/a)

1

Xieling-dagu dyke

1047-1341

2

Jie dyke

1489

1700

6-12

3

Yuliu dyke

1724

4430

19

4

Liji dyke

1734

700

30

5

Yanhai dyke

1796

1000

20

6

Yongqing dyke

1815

2000

30

7

Chengqing dyke

1892

1000

21

8

Ba dyke

1952

3700

74

9

Jiu dyke

1968

1000

52

10

Shi dyke

1992

2600

104

11

Shiyi dyke

2002

1120

112

Genesis of the Cixi tidal flat

Landform restriction

13The Cixi tidal flat lies on the convex bank (southern bank) of Hangzhou Bay. Because of the inertial centrifugal force of the estuary’s reciprocating current, the water body converges towards the concave bank (near Zapu, northern bank; Yang, 2001). Because the flow path of the concave bank is relatively long and that of the convex bank is relatively short, the current velocity of the concave bank is greater than that of the convex bank. For example, during flood tide, the average current velocity at the concave bank is 1.95 m/s, while that of the convex bank is only 1.16 m/s; during ebb tide, the average current velocity of the concave bank is 2.18 m/s, while that of the convex bank is only 0.7 m/s (tab. 2). Because of this difference in current velocity between the banks, the sand contents of the currents are different too. For example, during flood tide, the average sand content at the concave bank is 3.1 kg/m3, while that of the convex bank is only 2.74 kg/m3; during ebb tide, the average sand content at the concave bank is 3.93 kg/m3, while that of the convex bank is only 1.55 kg/m3 (tab. 3; Dai, 2008). Because the sand content ratio between flood and ebb tide at the concave bank is 0.79 (R<1), and that of the convex bank reaches 1.77 (R>1), the general changing tendency of the two banks of Hangzhou Bay should be described as decreasing in the northern bank and rising in the southern bank.

Tab. 2 – Current velocity (V) in Ganpu section of Qiantang River.
Tab. 2 – La vitesse du courant (V) dans la section de Ganpu de la rivière Qiantang.

Bank

V of flood tide (in m/s)

V of ebb tide (in m/s)

Maximum

Average

Maximum

Average

Concave bank

(northern bank)

3.47

1.95

3.55

2.18

Convex bank

(southern bank)

2.38

1.16

1.06

0.70

Tab. 3 – Silt content (C) in Ganpu section of Qiantang River.
Tab. 3 – Teneur en limons (C) à la section de Ganpu de la rivière Qiantang.

Bank

C of flood tide (in kg/m3)

C of ebb tide (in kg/m3)

Maximum

Average

Maximum

Average

Concave bank

(northern bank)

3.98

3.10

6.91

3.93

Convex bank

(southern bank)

3.96

2.74

2.55

1.55

River runoff

14The existence of a tidal flat implies that that there is an interval that mainly presents as an accumulating landform, which is always formed by the movement of sediment in suspension under the dynamic action of coastal currents and waves (Dellwig and Hinrichs, 2000; Whitford, 2001). The Qiangtangjiang River at the estuary is wide and shallow, providing ample space for a large south-north oscillation of the main channel. In the water dynamics shadow zone, distant from the main channel, sand and silt sedimentation often occur, forming submerged sand bars in the estuary. For example, the North Sediment is such a shallow patch (fig. 2; Xu, 1991). During a continuous wet year of the Qiantang River, the stream is powerful enough to break through the North Sediment. Therefore, the main stream of the river moves through the North Channel (also called the ‘north tide’), and the sedimentation rate of the Cixi tidal flat then becomes higher. Conversely, at other times the main stream of the Qiantang River moves through the South Channel (also called the ‘south tide’), and thus the sedimentation rate of the Cixi tidal flat becomes lower, as shown in fig. 2.

Fig. 2 – Plane chart of Xisan tidal furrow of Hangzhou Bay.
Fig. 2 – Carte du chenal de marée du Xisan dans la baie de Hangzhou.

Fig. 2 – Plane chart of Xisan tidal furrow of Hangzhou Bay.Fig. 2 – Carte du chenal de marée du Xisan dans la baie de Hangzhou.

Human activities

15Human activities have gradually become an important influencing factor for the evolution of the Cixi tidal flat as the reclamation scales on the tidal flat have been enlarged. Furthermore, the degree of human influence has become more and more obvious in recent years. The important influence of human activities on the evolution of the Cixi tidal flat is demonstrated as follows, taking the Xisan tidal furrow as an example.

Formation of the Xisan tidal furrow

16The Xisan tidal furrow is located near Xisan sluice, which connects the Yuyao and Cixi dykes, as shown in fig.  2. The inlet of the furrow is in the Xisan sluice, and the tail of the furrow extends to the Fengshou sluice. The length of the zero-metre contour line of the furrow is about 11 km, and the average width of the furrow is about 1000 m (Xu, 1991). It can be seen from all previous survey charts of Hangzhou Bay since the 1950s that the Xisan tidal furrow was formed in 1976. It has existed for more than 30 years. Because of the large-scale reclamation on the tidal flat within the estuary of the Qiantang River since the 1960s, the river channel has been gradually silted, and the tide wave has also been gradually deformed. The rising rate of the tide level near the Xisan sluice has gradually increased. Therefore, the tide level of the flood tide has become relatively high, and two flood streams now appear near the Xisan sluice. One of the flood streams continuously moves towards the west (called the ‘westward stream’) and finally enters the South Channel. Another flood stream moves towards the east and finally enters the Andong beach face (called the ‘eastward stream’). This inshore eastward stream is the reason for the formation of the Xisan tidal furrow.

Origin of the Xisan tidal furrow

Increase of flood tidal level near the Xisan sluice

17Large-scale reclamation on the tidal flat within the estuary of the Qiantang River has been going on since 1968. By 1976, the enclosed land had already reached 33,000 hm2. The width of the river channel has been largely constricted owing to the said reclamation (Grams and Schmidt, 2002). The river section from Yanguan to Jianshan decreases in width by 47-64%, and the fluctuating magnitude of the main channel in the same section has also been reduced from 28 km to 14 km, as shown in tab. 4. Water-dynamic conditions of the river have been changed owing to the large-scale reclamation. This causes the appearance of a new upper beach outside the dyke because of silting. The area of the upper beach, with a height larger than 5 m in the Jianshan river bend in 1976, increased by 10,000 hm2 compared with that of the 1960s. Development of the upper beach finally decreased the capacity of the river and therefore resulted in silting in the riverbed. Aggradation of 1.3-2 m in the height of the riverbed was observed in 1977 in the river section from Yanguan to Jianshan compared with that of 1956 (Xu, 1991). Reduction of the river width, development of the upper beach and aggradations of the riverbed in the Jianshan river bend, as mentioned previously, contribute to the aggravated deformation of the tidal wave in the estuary. Deformation of the tidal wave can be manifested in several ways, such as variation of the alongshore tidal level, of the rising rate of the cross-sectional tidal level and of the duration of the flood and ebb tides (Nash et al., 2004; Sabatier et al., 2009). It can be seen from the long-term observations of the alongshore tidal level of Hangzhou Bay since the 1970s that the high tidal level has a tendency to increase year after year. The average high-tidal level in 1977 was 0.14 m higher than that of 1970 at the Ganpu hydrometric station, and the value also increased by 0.2 m at the Zhapu hydrometric station. The increase of the average high-tidal level during the flood period in September was even more obvious. The values were increased by 0.27 m and 0.3 m, respectively, at the Ganpu hydrometric station and the Zhapu hydrometric station during the same period (Ma, 1998). After comparing the sectional tidal level graph of the southern bank of the Xisan sluice with that of the northern bank, we can also see that the flood tidal level on the southern bank was lower than that of the northern bank before 1977. However, the tidal level 2 h after the flood tide on the southern bank exceeded the value at the same time on the northern bank after 1977, with a maximum differential of 0.5-0.7 m. Thus, it can be seen that the rising rate of the sectional tidal level on the southern bank of the Xisan sluice increased after 1977 and, therefore, resulted in an increase of the flood tidal level. This also increased the gradient between the westward and the eastward streams within the sea region near the Xisan sluice. The strengthened eastward stream is the main dynamic factor for the formation of the Xisan tidal furrow, as shown in fig. 2.

Expansion of aggradation of the Andong beach face.

18Area variations above the zero-metre line in the Andong beach face over a period of about 40 years are shown in tab. 5. It can be seen that the area of the Andong beach face nearly doubled during this period owing to the large-scale reclamation on the tidal flat. Furthermore, the magnitude of area increase before 1979 is larger than it was after 1979. The former value is 37%, while the latter value is 25%. The faster increase in magnitude before the 1980s can also be observed from the variation of the offshore distance of the zero-metre line of the Andong beach face. The zero-metre line of the section R87 (measured section; fig. 2) moved outward a distance of 3 km from 1974 to 1977 (Ma, 1998). Expansion of aggradation and decrease of the water depth within the Andong beach face leads to an increase of resistance force for transmission of the flood current along the beach face, and a decrease in the velocity of the flood current. Hence, the time used for transmission of the flood current into a certain position is retarded, and the rising rate of the tidal level decreases accordingly. Therefore, the differential of the water level of the flood tide between the higher western part and lower eastern part to the northern sea area of Andong is further increased. This finally strengthens the eastward stream and leads to further development of the Xisan tidal furrow.

Tab. 4 – Evolution of the typical sectional river width (in km) between 1956 and 1976.
Tab. 4 – Évolution de la largeur de la rivière dans sa section typique (en km) entre 1956 et 1976.

Sectional position

Yanguan

Daquekou

Jianshan

Ganpu

River width in 1956

12.5

28.0

26.5

21.7

River width in 1976

4.5

14.5

14.0

19.8

River width decrease

8.0

13.5

12.5

1.9

Tab. 5 – Area variations over the zero-metre line in Andong beach face from 1959 to 1998.
Tab. 5 – Variations de la zone au dessous du 0 marin de la plage face à Andong entre 1959 et 1998.

Year

1959

1979

1998

Area (in km2)

164.2

225.0

281.3

Increasing magnitude (in %)

37

25

Variation of scouring and silting of sediment in the Xisan tidal furrow

19The appearance of serial furrows in the outside lateral beach face (also called ‘coast spit’) of the Xisan tidal furrow is the main reason for the variation of scouring and silting in the tidal furrow. After the appearance of the serial furrows on the beach face, the flood current from the serial furrows and the flood current from the inlet of the tidal furrow will intersect. The current velocity will decrease, and the sediment-carrying capacity will decline after the intersection. Therefore, silting of the tidal furrow will appear. The phenomenon of the serial furrows has appeared twice since 1976. One occurred in 1979, and the other in 1999. The former serial furrows led to a silting of the tidal furrow up to -0.6 m, and the latter led to a silting of the tidal furrow up to 1-2 m (Dai, 2008).

20The serial furrows are unstable because the western lateral beach face (also called ‘concave bank’) is easily collapsed by opposite scouring of the flood and ebb currents, and the eastern lateral beach face (also called ‘convex bank’) appears to be easily silted. Therefore, the serial furrows gradually move towards the west and finally return to the inlet position of the tidal furrow. During this process, deep scouring of the tidal furrow again occurs, and the initial forms are restored. Moreover, variation of scouring and silting of the tidal furrow is closely related to the height of the outside lateral beach face. If the height of the beach face is large, then the velocity of the lateral flood current is small, and the tidal furrow is scoured deeper. Conversely, if the height of the beach face is small, then the velocity of the lateral flood current is large, and the tidal furrow will be highly silted.

Discussion of tidal flat reclamation

Damaging effects of the tide stream of the Qiantang River

21The estuary of the Qiantang River is a typical strong-tide estuary with a torrent stream and a broad tide range. The lowest tidal level of -2.5 m was measured at the Xisan sluice on August 22, 1951, and the highest tidal level of 8.2 m was surveyed on August 19, 1997. The above feature of the tide stream unquestionably has serious damaging effects on the inshore dyke. Three T-dykes and four tidal sluices were destroyed in Cixi City between July 1979 and December 1980. The length of the damaged dyke was 2,200 m, and the area of the damaged upper beach reached 800 hm2. The fastest slump speed was up to 20 m/a. For this reason, about 500,000 labourers have been put to work to protect the coast around the whole city. A stone riprap of 78,000 m3 has been finished, and a dyke over 5,000-m long has been repaired and strengthened (Xu, 1991). In spite of these measures, many incidents of damage to the dyke have occurred since 1983, and a number of emergency measures for dyke protection have been performed by the shore residents. At the present day, the entire region still remains unstable. The Xisan tidal furrow’s existence, especially, poses a tremendous threat to the structural toe of the dyke. The deepest position of -14 m of the tidal furrow has already reached 25 m away from the dyke, and this phenomenon is gradually becoming a huge latent threat to the Cixi dyke (Xu, 1991).

Regulation of the Xisan tidal furrow

22The tidal flats are an important land resource reserve for Cixi City and will play a pivotal role in the future social and economic development. As population and economic development increase, the land resources in Cixi City become more and more insufficient. The per capita area of the cultivated farmland of the city is only 0.044 hm2 at present. Therefore, the regulation of the Xisan tidal furrow must be closely linked with reclamation of land from the tidal flat. Hence, the principle for regulation of the Xisan tidal furrow should guide action according to circumstances, and reclamation of the tidal flat should be accompanied by regulation of the tidal furrow. The former part of this principle means that the strike of the river channel must accord with the law of stream movement and transport of sediment. The latter part means that regulation of the tidal furrow must be closely linked with alongshore reclamation of the tidal flat.

23At present, the tidal flat reclamation work on the western side of Sizaopu has already been completed. The project begins at a position 2 km to the west of the Fengshou sluice in the west and ends at the Sizaopu sluice in the east. The area of reclamation is about 4,500 hm2, and the length of the western dyke is up to 3.8 km. Dyke remedy and reclamation work in Yaobei have also been performed in Yuyao City. This project has a reclamation area of 1,300 hm2, and the length of the eastern dyke on the western side of the Xisan tidal furrow is up to 3.7 km. At present, a stone riprap has already been finished for the project.

24According to the principle of the said regulation, and in view of the accomplishment of the dyke project along the western and eastern sides of the Xisan tidal furrow, which has the effect of obstructing the stream in the tidal furrow, dam construction by stone riprap in the inlet position of the Xisan tidal furrow should be considered. After the connection of the dam with the outside lateral beach face (coast spit), the Xisan tidal furrow could be cut off thoroughly and would be further changed to an internal reservoir for cultivation purposes and a flat for cereal crops and cotton. This measure has effects not only on protection of the former riverside erodible dyke, but also on land area increase.

25The timing for dam construction by stone riprap must be carefully considered because of the close relationship between south-north divagation of the main channel of the Qiantang River and wet/dry year of the river basin (Ma, 1998). As previously mentioned, the main channel of the Qiantang River lies near the north during a continuously wet year, and the current velocity of the Xisan tidal furrow is therefore small at this time. Conversely, the main channel of the river lies near the south during a continuously low-water year, and the current velocity of the furrow is therefore large at this time. Thus, the correct timing for the dam construction by stone riprap in the Xisan tidal furrow is in the continuously wet year of the river.

Conclusions

26The geomorphic configuration of hilly land in the southern area, sea in the northern area and littoral plain in the middle area within the overall area of Cixi had already been formed in the later stage of the Holocene about 2500 BP, and there were two obvious gray lines in the configuration. The Cixi tidal flat belongs to the silting-up type tidal flat. Eleven dykes have been constructed since the Song dynasty. The average outward migration rate of the coastline before the founding of the People’s Republic was 25 m/a, and it has been increased to 50-100 m per year since the founding.

27The influencing factors for the formation and evolution of the Cixi tidal flat mainly include landform restriction, river runoff, human activities, etc. The influence of human activities has become even more important in modern times. The Xisan tidal furrow was formed in 1976. The main dynamic factors for the formation of the tidal furrow are an increase of the tide level of the flood tide near the Xisan sluice and an expansion of aggradation of the Andong beach face. Ultimately, the formation of the tidal furrow can be attributed to large-scale human activities.

28The principle for the regulation of the Xisan tidal furrow should guide action according to circumstances. Reclamation of the tidal flat should be accompanied by regulation of the tidal furrow. The former part of this principle means that the strike of the river channel must be in accordance with the law of stream movement and transport of sediment. The latter part means that the regulation of the tidal furrow must be closely linked to alongshore reclamation of the tidal flat. In view of the accomplishment of the dyke project along the western and eastern sides of the Xisan tidal furrow at present, dam construction by stone riprap could be considered to cut off thoroughly the tidal furrow. The correct timing for dam construction by stone riprap should be selected in the continuous wet year of the Qiantang River.

This work was supported by National Natural Science Foundation of China (No. 41171430, 40771044). We are very grateful to the two referees for their helpful comments and suggestions, and S. Suanez who edited the French texts.

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Annexe

Version française abrégée

La ville de Cixi se trouve sur la rive sud de la baie de Hangzhou, en Chine, entre 121°02' - 121°43’ E de longitude et 30°20' et 30°34’ N de latitude. Historiquement, la ville s’est installée sur cette zone estuarienne durant la dynastie des Tang, et le paysage a successivement été construit durant la dynastie des Song. Depuis cette période, les paysans ont constamment cloisonné cette zone intertidale tout en construisant de nombreux étangs : la plaine littorale moderne a donc progressivement émergé dans la ville. Les secteurs sud et nord de la ville de Cixi sont respectivement occupés par des collines et la mer ; la partie centrale de la ville est formée par la plaine littorale. La superficie totale de la ville atteint 1,718 km2. Le secteur de collines représente 9,7 % de la superficie totale de la zone d’étude, la plaine littorale 45,2 %, la zone intertidale 25,2 %, et le bras de mer 19,9 %.

L’analyse d’images satellitaires réalisées en 1986 indique qu'il y a deux lignes sombres bien marquées dans la région de la ville de Cixi. Elles ont été interprétées comme étant d’anciennes lignes de rivage. Un premier paléo-rivage se trouve le long d’une ligne joignant Zhouxiang à Hushan de Longshan, le second de Xisan à Andong à Xinpu. Ces deux paléo-rivages sont bien distincts et séparent la ville en trois zones méridionale, centrale et septentrionale. La zone méridionale appartient à la plaine littorale constituée de dépôts lacustres et marins accumulés durant la dernière phase glaciaire. La zone centrale constitue la plaine sédimentaire de faciès marins accumulés durant l’Holocène. La zone septentrionale se situe la partie la plus récente de la plaine littorale formée de dépôts marins. Cette dernière zone est aussi appelée la zone saline de Andong (Ma, 1998). Au-delà de la zone septentrionale, une organisation semi-circulaire irrégulière de bancs intertidaux, appelés Riffle Sanbei, peut être observée (fig. 1). Les processus de changement le long du littoral de la ville de Cixi au cours de diverses périodes historiques sont incontestablement enregistrés dans le paysage de digues qui ceinturent la ville. La construction de la digue Xieling a débuté dans la Septième Année de Qingli de la dynastie des Song (autour de 1047). La digue Dagu a été construite sous la dynastie des Yuan (environ AD 1341; Xu, 1991). Après cela, dix digues furent à leur tour construites avec l'expansion de la frange littorale (fig. 1). Cela a abouti à un déplacement du front de mer de la ville vers la mer sur une distance de 18 km, se soldant par un gain de surface terrestre d’environ 886 km2. Le taux moyen de progradation de la ligne de rivage a été estimé à 25 m/an au cours des 600 dernières années, comme indiqué par la position des différents traits de côte pour chaque période historique (Dai, 2008). Les vitesses de progradation de la ligne de rivage ont augmenté après la création de la République populaire de Chine, avec la mise en œuvre à grande échelle de poldérisation et de gain de zones humides en raison de la demande pressante de terres agricoles. Le taux de progradation de la ligne de rivage à atteint 50 à 100 m/an durant cette période.

Les facteurs intervenant dans la formation et l'évolution de la zone intertidale de Cixi correspondent principalement à la faiblesse du relief, à l'écoulement fluvial et aux activités humaines. En raison de la force d'inertie centrifuge du courant alternatif de l'estuaire, les flux hydriques convergent vers la rive concave (près de Zapu sur la rive nord, Yang, 2001). La tendance générale de l'évolution des deux rives de la baie de Hangzhou montre un abaissement de la rive nord et un exhaussement de la rive sud. Lors des années pluvieuses, le flux hydrique de la rivière Qiantang est assez puissant pour ouvrir des brèches dans la rive Nord. Par conséquent, le courant principal de la rivière se déplace à travers le chenal septentrional (également appelé marée nord), et la vitesse de sédimentation de la zone intertidale de Cixi augmente. Inversement, le courant principal de la rivière Qiantang se déplace à travers le chenal méridional (aussi appelé marée sud). Dans ces conditions, la vitesse de sédimentation de la zone intertidale de Cixi diminue (fig. 2). Les activités humaines sont progressivement devenues un facteur important de l'évolution de cette zone avec l’augmentation des surfaces de la zone soumise à l’influence de la marée. En outre, l’action de l'Homme est devenue de plus en plus évidente ces dernières années. L’incision du chenal de marée de Xisan a commencé en 1976. Les principaux facteurs dynamiques intervenant dans sa mise en place sont une augmentation des hauteurs de marée lors du flot, près de l'écluse Xisan, et une progradation par accrétion de la plage de Andong. L’incision du chenal de marée est également due aux activités humaines.

En vue de la réalisation d’un projet de construction de digues le long des rives orientale et occidentale du chenal de marée du Xisan, afin de réguler le flux, la construction d’enrochements à l’entrée (passe) du chenal de Xisan doit être envisagée. Après le raccordement des enrochements à la zone offshore de la plage, le chenal de marée de Xisan pourra être complètement court-circuité et transformé en un réservoir interne utile à la culture de céréales et du coton. Cette mesure aura des effets non seulement sur la protection de l’ancienne digue sensible à l'érosion mais aussi sur l'augmentation de la superficie terrrestre.

Le moment choisi pour la construction des enrochements doit être soigneusement défini en raison de la relation étroite entre les divagations sud et nord du chenal principal de la rivière Qiantang et l'année (humide / sèche) dans le bassin-versant (Ma, 1998). Le chenal principal de la rivière Qiantang se trouve vers le nord au cours d'une année humide, lorsque la vitesse du courant de marée du chenal de Xisan est faible. À l'inverse, le chenal principal de la rivière se trouve vers le sud au cours d'une année plutôt sèche, lorsque la vitesse du flux augmente. Ainsi, la bonne période pour la construction du barrage en enrochement dans le chenal de marée de Xisan correspond à une année humide.

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Table des illustrations

Titre Fig. 1 – Migration chart of coastline in Cixi City.Fig. 1 – Carte de la cinématique du trait de côte dans la ville de Cixi.
URL http://geomorphologie.revues.org/docannexe/image/10230/img-1.png
Fichier image/png, 60k
Titre Fig. 2 – Plane chart of Xisan tidal furrow of Hangzhou Bay.Fig. 2 – Carte du chenal de marée du Xisan dans la baie de Hangzhou.
URL http://geomorphologie.revues.org/docannexe/image/10230/img-2.png
Fichier image/png, 50k
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Pour citer cet article

Référence papier

Lihua Feng et Xiong Chen, « Coastal processes and tidal flat reclamation in Cixi City, China », Géomorphologie : relief, processus, environnement, vol. 19 - n° 2 | 2013, 181-190.

Référence électronique

Lihua Feng et Xiong Chen, « Coastal processes and tidal flat reclamation in Cixi City, China », Géomorphologie : relief, processus, environnement [En ligne], vol. 19 - n° 2 | 2013, mis en ligne le 26 août 2015, consulté le 30 avril 2017. URL : http://geomorphologie.revues.org/10230 ; DOI : 10.4000/geomorphologie.10230

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Auteurs

Lihua Feng

Department of Geography - Zhejiang Normal University - No. 688 Yingbin Road - Jinhua 321004 - China (fenglh@zjnu.cn).

Xiong Chen

Department of Geography - Zhejiang Normal University - No. 688 Yingbin Road - Jinhua 321004 - China.

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