Journal of Coastal Research 32-44 Royal Palm Beach, Florida Winter 1999

A Study of Coastal Morphodynamics on the Muddy Islands in the Changjiang River Estuary

Yang Shilun

State Key Lab of Estuarine & Coastal Research, East China Normal University, Shanghai, 200062, P.R. China

. ~ .. SHILUN, Y., 1999. A Study of Coastal Morphodynamics on the Muddy Islands in the Changjiang River Estuary .tlllllll,:. Journal of Coastal Research, 15(1), 32-44. Royal Palm Beach (Florida), ISSN 0749-0208. ~ ~. Based on field work for morphology and collected data on hydrodynamics, winds as well as artificial structures, this article deals with characteristics of coastal profiles, shoreline migration, erosion-accretion cycles and influence of ~~ a-+; 1&-- human activities on coastal development. Due to silt-dominated sedimentary environments and current-dominated hydrodynamic conditions, coastal morphology in the studied area is different from that on sandy beaches. On the other hand, the ringlike shoreline of the islands and the strong influence of river discharge make the coastal processes more comprehensive than normal muddy coasts. Coastal profiles in the islands were classified as three types: channel bank, tidal beach and shoal head. In the light of these terms, three kinds of erosion-accretion cycles were explored; (a) several years cycle, controlled by the changes in the offshore river channels; (b) annual cycle, influenced by the seasonal variations in winds, sea level, tidal range and nearshore suspended sediment concentration and (c) storm cycle caused by typhoons or cold waves. Due to the protection of vegetation, morphological process in the marsh was apparently different from that in the adjacent bare flat. Protective structures have stoped the retreat of the erosive coasts and slowed the migration of the islands, while reclaimation on accretional coasts greatly narrowed intertidal profiles and speeded the advancement of shorelines. The potential impacts of sea-level rise and reduction in sediment supply in the next century were also related.

ADDITIONAL INDEX WORDS: Coastal profile, erosion-accretion cycle, muddy tidal flat, sediment island, the Chang­ Jiang River estuary, China.

INTRODUCTION (BRUUN, 1954; STRAHLER, 1966; DEAN, 1977; WRIGHT, 1979; LARSON, 1991; DEAN et al., 1993); (d) response of beach pro­ The study of coastal morphology has direct application in file to sea-level rise (BRUUN, 1962; HANDS, 1976; HANDS, coastal engineering (LARSON et al., 1994) and should be in­ 1980; LEATHERMAN, 1987); (e) experimental study especially valuable to future planning and management of shorelines in wave flume (KEULEGAN, 1945; KING et al., 1949; RECTOR, (CARTER, 1988). The shape and slope of coastal profiles are 1954; SCOTT, 1954; IWAGAKI et al., 1963; KAJIMA et al., 1982) related not only to sedimentary grain-size and water energy and (D mathematical modeling (HARRISON, 1969; HARRISON (BASCOM, 1951; SHEPARD, 1963; WIEGEL, 1964; KING, 1972), et al., 1971; SONU et al., 1973; WINANT et al., 1975; FELDER but also to erosive and accretional processes and offshore sub­ marine morphology. Response to variations in energy or sed­ et al., 1980; BOWAN, 1981; KRAUS et al., 1983; LARSON et al., iment supply may be traced through shoreline migration of 1989; LARSON et al., 1991). changes in sediment elevation. The study of coastal morpho­ In contrast to the work done on beaches, morphological dynamics is to show the mechanism responsible for the for­ study of muddy coasts has come relatively late. Some re­ mation and development of coastal topography. searchers te.g. STRAATEN et al., 1958; POSTMA, 1967; COL­ Significant progress has been made in the study of sandy LINS 1981; YUN, 1983; ZHANG, 1986; CHEN, 1991 and WANG beaches. The studies mainly included the following aspects: et al., 1993) described the transportation and sedimentation (a) temporal profile changes such as seasonal cycle (SHEP­ of fine sediments on tidal flats. Others (WELLS et al., 1981; ARD, 1950; WINANT et al., 1975; AUBREY, 1979; FIElD et al., ANDERSON, 1983; YUN, 1983; YANG, 1991; JIANG, 1993 and 1980; AUBREY et al., 1985; LARSON et al., 1994), storm-fair­ ZHANG, 1993) showed the erosive and accretional changes in weather cycle (SONU, 1970; Fox et al., 1973; HOWD et al., muddy flat profiles as well as their controlling factors. Gen­ 1987; LARSON et al., 1994; LEE, 1995; SEXTON, 1995), tidal erally speaking, a muddy flat is different from a sandy beach cycle (OTVOS, 1965; STRAHLER, 1966; SCHWARTZ, 1967) and in three aspects: (a) the former is mainly made up of silts and long-term cycle (JIMENEZ et al., 1993; LARSON et al., 1994); clayes (viscous sediments) while the latter principally sands (b) relationship between profile and dynamics (LAFoND, (non viscous sediments). (b) The slope of the former is grad­ 1939; SHEPARD et al., 1940; GRANT, 1948; KING, 1953; DUN­ ual, usually less than 1.0%; but the latter is sharp, normally CON, 1964; SUNAMURA et al., 1987); (c) equilibrium profile greater than 1.0%. (c) The former is dominated by tidal dy­ namics while the latter by waves. 96005 received 30 June 1996; accepted in revision 17 October 1997. Up to now, no systematic research has been made on mor- Coas ta l Morphodyn amics in the Changjiang Estua ry 33

5.------.--.., .~..!~ . 0° 4 &...... ;.."1... , . 0° .''''"f'. 3 _, ... -..... ~ .--·r . 2 / -.~.~~ //C I '. ' I ye E 0 " If? '" /i'l/ /-'''xl.. . I-I I A.I _ ./\ ~ -2 •B Ii / . Q) " -3 F /1/ .. \ I ' e D\ -4 31'20' - 5 I/f \E

A"'P: Pr ofile -6 /~ 1. Changxin -7 L...... l!...-J__'---l~:__'~--'---:::--'~:-'/ 2. Hengsa 3 4 5 6 7 8 3. Jiuduansa Mz(et» o 30km Figure 2. Changes in sedime nt mea n grain-size ( <1» along the st udied 1 I profiles (et> = - log2d[mm]) the dotted line A,B . . .F re pres en ts the pro­ files accor d with those shown in Fig. 1; the real line is the mea n of th e 121'30' dotted ones .

Figure 1. Map of the Cha ngjiang River estuary showi ng th e setting uf th e islands and th e sites of the cross-shore profiles studied (int ertida l shoa ls were not dr awn out except th e biggest one, th e -Iiuduansa). th e sea are dominant, with th eir frequ ency 1.6 tim es as th e average; mean wind velocity averages 3.9 mis, 4 to 5 storms affect th e area each year, half being tropical cyclones and half being strong cold wind s (cold waves), with a maximum veloc­ phodynami cs along th e island coasts in the Changjiang River ity of 26 m/s recorded on April 28, 1983. The mean tidal ran ge estua ry although some meaningful results have been ac­ is 2.59m-3.08m in the North Branch, 1.96m-2.47m in the quired in the adjacent mainland coasts (YUN, 1983; CHEN, South Branch, 2.45 m at Cha ngxin and 2.62 m at Hengsh a; 1991; YANG, 1991 and ZHANG, 1993). These island coasts, the maximum tid al range is 5.95 m in th e North Branch, 4.67 developed in a large bran ching river mouth, are different ei­ m in the South Branch, 4.49 m at Cha ngxin and 4.64 m at th er from sandy beaches or from normal muddy coasts in Hengsha (GSCll, 1996). In normal conditions, mean and morphodynamics. A study of th em is needed. maximum current velocity in th e river courses is less than 1.0 m/s and 2.0 mis, respectively. On tid al flats and marshes, PHYSICAL SETIING th e current weak ens landward, with th e maximum less th an Changjiang, th e third largest river in th e world, carries 924 0.5 m/s in marshes and 1.0 m/s in lower flats (YANG, 1994). X 109 m3/a water and 468 X 106 t/a sediment (GSCll, 1996) Dominant waves are wind-driven in the estuary. Positive cor­ into th e East China Sea at 310 Nand 1210 E. The river mouth relati on exists between wind and wave direction and between (Figure 1) encompasses an area of 3830 km". The width of wind velocity and wave height (YANG, 1991). Average and th e river mouth (about 50 km on th e average) mad e it pos­ maximum wave heights in th e history was respectively 1.0 m sible for shoals to form in ar eas where currents were rela­ and 6.2 m at th e Yingshuichu an Station (a few kilomet ers tively slow due to th e effect of Coriolis force (CHEN et al., outside th e front of the river mouth) and 0.2 m and 3.2 m in 1979). Three islands, named Chongming, Cha ngxin and the South Channel (GSCll, 1996). Hengsa, have been developed from shoals which were first Sediments in the intertidal zone are dominat ed by silt and formed respectively in 7th, 17th and 19th centuries. Each clay. Based on 128 represe nta tive samples from tidal flats island ha s been reclaim ed and inhabited by man; th e history and marshes on th e islands and th e J iuduan sha shoal, the can be traced to 1300 BP in Chongming (GSCll, 1996). Now mean of th e middle grain-size (dso) was 0.027 mm which ex­ th eir ringlike shoreline are 210 km, 59 km and 30 km, re­ actly accords with th e feature of the sus pended sediments spectively (Figure 1). Besides th ese islands, a large intertidal supplied by the Cha ngjiang River (YANG, 1994). On th e bed shoal, named J iuduan sha, exists at th e front of th e river of th e river courses around th e islands and th e shoal, th e mouth, with its top elevatio n a little above th e mean high mean of dso (from 64 samples) was 0.066mm, wit h most of tidal level. The area of intertidal zone (outside th e seawall on the South Bran ch, th e North and South Channel as well as the three islands) is 293 km 2 in Chongming, 44 km 2 in Chang­ th e upp er reac hes of the North Bran ch bein g covered by fine xin, 16 km 2 in Hengsha and 114 km 2 in J iuduan sha (no sea­ sands and th e North and South Passage as well as the lower wall on it ). reaches of the North Branch being deposited by muds (YANG, The area is characterized by a Monsoon climate. According 1994). From the course bed to supertidal marsh , sediments to the multi-year records in th e island s: SE-SSE winds from become finer (Figure 2). Along the shoreline, intertidal sedi-

Jo urnal of Coastal Research , Vol. 15, No.1, 1999 34 Shilun ments are relatively coarse on the eastern sides (facing to the profile. In this type, the slope is gradual with no deep concave open sea) (Figure 2) with a coarsest sample (d5o = 0.15mm) in the submarine part because there is no deep channel there. found on the tidal beach at profile P (YANG et al., 1994). The average gradient between the 5 m isobath and the top Along accretional and stable coasts, about 0.5 m above the shoreline at the northwest head of the Jiuduansha was mean sea level is the border between the bare flat and the 0.051 % in 1996, about 1/30 to 1/50 as in type (a). However, marsh (vegetated area). In erosive coasts, the elevation of the sediments on flats (usually silty sands) are coarser than those border is dependent on the site of the eroded scarp. The lower in type (a) perhaps due to stronger water energy (in marshes part of the marsh is habitated by the pioneer plants of Scir­ of different types, deposits are similarly very fine because of pus and the higher part by reeds. the energy-reducing effect of vegetation, so it is meaningless to make a comparison among the types). METHODS AND MATERIALS Type (c) represents the open coasts situated in the eastern side of the islands and shoals. Slopes are very gradual here. Intertidal profiles were based on level surveys made by the For example, the average gradient between the 5 m isobath author and his colleaques. In these surveys, the distance be­ and the top shoreline in the eastern coasts of the Chongming, tween two rods was 200 m at profile F and 100 m in the other Hengsa and Jiuduansa was seperately 0.024%, 0.0390/0 and sections. The submarine profiles were drawn on the basis of 0.037% in 1996, about 1/60 to 1/100 of the profiles in type (a). 1:10,000 topographic maps surveyed by the Shanghai Irri­ There is hardly any concavity in the profiles even to the depth gation Bureau. Other data on climate, hydrography and his­ of 30 m which is 60-70 km apart from the shore because no toric evolution originated from the Shanghai Coastal Zone deep channel is developed across the profiles due to the fact Comprehensive Survey from 1981 to 1986 and the Shanghai that the tidal current direction in these areas is nearly par­ Island Comprehensive Survey from 1990 to 1995 (in both of allel to the profiles. The formation of these gradual slopes which the author took part). In dealing with annual erosion­ can be attributed to the favorable tidal and wave conditions accretion cycles of intertidal profiles, a 12-month time series as well as the gentleness of the continental slope. In spite of of surveys was made. Most of the profile surveys extended the gentleness, deposits on flats (mainly sands) are coarser from the mean low tidal level landward to the seawall. So the than those in type (a) (Figure 2) and type (b) due to the open concept of 'average profile elevation' (mean of the elevation situation which makes them attacted by higher wave energy. numbers from the rods adopted) was relative and not abso­ In hydrodynamics, type (c) is co-controlled by tides and lute. But for each profile, the data were processed on the waves. principle of same rods in different months, which made the Profile E, Nand P are transitional forms between type (a) concept useful. Many enlightenments adopted in this paper and (c) because a shallow channel develops across them in originated from the author's in situ observations along coasts the submarine area. especially in the profiles showed in Figure 1. Based on 27 beaches, KING (1972) verified the relationship between gradient, grain-size and wave energy first found by RESULTS AND DISCUSSION BASCOM (1951) and WIEGEL (1964): gradient is positively cor­ Types of Coastal Profile related with grain-size and negatively correlated with wave energy. This theory only partly corresponds to the profiles in In accordance with shape and relationship to submarine the Changjiang Estuary. The sheltered condition is best in topography, coastal profiles in Figure 3 can be classified as type (a) and worst in type (c). This can interpret why the (a) channel bank, the main type (profile B,G,H,J,C,D,M, and slope in type (c) is the gentlest. But the slope in type (a) is 0), (b) shoal head (profile L) and (c) tidal beach (typically too sharp in consideration of muddy sediments on it, and the profile F). Profiles in type (a) are generally located on the slope in type (b) and (c) is too gentle in view of their sand main two sides, the northeast and the southwest of the is­ and silty-sand composition, if the theory from beaches were lands and the Jiuduansha shoal. They are perpendicular to applied. Perhaps the failure to apply the theory can be at­ the channel course and dominant-current direction. Normally tributed to the strong effects of tidal direction and velocity. the profile is narrow with a sharp bank and a deep concave According to the movement of shoreline, profiles can be bed only hundreds or a little more than one thousand meters classified as retreating (profile G, J, C, M and 0), advancing offshore. The fate of this type is dependent on the behaviour (profile F, L, E and N) and stable (profile H, D, Band P). The of the channel. So there is no harm in saying that the retreating shore is being eroded and an obvious scarp devel­ morphodynamics of this type is controlled by currents and ops between the naked flat and the marsh. As a result, the not by waves. Although the slope of the banks is from 0.8% intertidal zone is narrow. The scarp on profile G was sur­ to 8.5%, 2.4% on the average (the feature of sandy beaches veyed to have migrated landward for 56 m in a period of one in gradient), the sediments are silt-dominated with the mean year between October in 1990 and 1991. In the same time, grain-size (

Journal of Coastal Research, Vol. 15, No.1, 1999 Coastal Morphodynamics in th e Changjiang Estuary 35

T DL TDL

M

o

o 1.0 2.0 Offshore(km) N

p

2.0 3.0 4.0 Offshore(km)

L

~ scar p vertica l scale dis tance: l m

B.. ·P: Profile TDL: theo retic dat um level

o 1.0 2.0 3.0 4.0 5.0 6.0 Offshore(km) Figur e 3. Representative profiles of th e island coasts (profile locations are shown in Fig. 1).

tion . In this type, the profile is narrow without scarp and its shape looks like a standard reve rse's'. The 'stability' usua lly <:E4.f:==-= = = :o-_ benefits from the protection of coastal structures. .S 3 ~ 2 Shoreline Migration % 1 o 2 3 4 5 6 7 8 Each island in the Changjiang river mouth has developed Offshore(km) from shoalts), The primary form of Chongming was the Xisa Figure 4. Recent accumulation of the eastern Chongming shore. shoal. Since its birth, more than 50 shoals, some of them unnamed, have participated in th e development of the pres-

Journal of Coastal Research, Vol. 15, No. 1, 1999 36 Shilun

121'30' ~\ 121'53' t N ~ '-----,'.-...., . .-~ ( 1992 ' ..-.....~...... :...... - .--:.:.::._---,-~' - ' - ' - , ...1960'('S I I ., ' "', ~'" ( '\1940'S' \. " ',---- ..... i, . ( 1926 :'-, \ 1700'S'.: I/ '--A-_ '\ 31 '20' ...... ------..... ( = .... 'l ./ 1\ ~ J \ 1900 o 20km .' / V . / \ 31'30' " ...... I , v-, I '\':.:, ) 1860 Fig ure 5. The shoreline (seawall) history of the Chongming Island in .....---" ':;' th e past 300 years. o 6 km . ... ent island. Six shoals were united to form th e Changxin Is­ Figure 6, The shoreline history of th e Hengsa Island after its birth , land , the eldest of th em was th e Yawosa shoal. In the history of the river mouth, shoals used to experience changes of accumulation or erosion. In some cases, an old shoal died out somewhere while a new one was born in an­ to go mainl y through th e southern branch and broaden th e oth er place. The main (or net) migration trend of th e shoals river bed. The effects of th e two factors were combined with (or islands) has been northeastward. This trend resulted from each oth er to induce the island (or shoal) to migrate north­ accumulation on the northeastern side and erosion on th e ward. southwestern side. Xisa, the embryo of th e Chongming Is­ It can be deduced that after a catastrophic flood which land , was primarily located where th e present South Bran ch greatly enlarged th e river bed, th e river bed would become is situa ted. In th e past 2,000-3, 000 yea rs , about four groups broader th an it was necessary for normal discharges to go of shoal have been united into the northeastern bank of the through. Then a further bran ching would occur by forming a estuary (CHEN et al., 1979), of th em was the present Qidong new shoal. This is why it is a regular pattern th at th e sec­ spit (Figure 1). The cause for thi s migration has not been ondary branching occurs in th e former southern bran ch. For thoroughly explain ed up to now but th e Coriolis force may be example, th e South Branch was divided into th e North and th e key. It can be deduced th at, und er the effect of th e Cor­ South Channel by th e Cha ngxin and Hengsa island , and th e iolis force, th e domin ant ebb current (due to th e joining of South Channel was further separa ted into th e North and river water) would flow in a slight slanting direction to south, South Passage by the J iuduan sa shoal. In about 1820 AD, gradually making th e south ern br anch be the mai n course . th e South Branch took th e place of th e North Branch as the As a result, th e northern side of th e island or shoal would be main strea m of th e Cha ngjiang river mouth. Fr om th en to subjected to accretion and th e southern side to erosion, as the end of the 19th century, th e southern bank of th e shown in Figure 5. It has been an enigma that Hengsha mi­ Chongming Island migrat ed northward for 7 km with a rate grated northwestward for 10 km , 100m/a on th e average (Fig­ of nearly 90 m per year. The artificial protective structures ure 6). which started in 1894 have stoped th e erosion trend in th e In contrast with Hengsha, the shoal of Jiuduansha has mi­ southern bank of the Chongming Island and made it rela­ grated southeastward for 7 km since 1965 (the head retreated tively stable (Figure 5). The tran sfer of the main stream to at the rate of 230m/a and the tail part advanced at th e rate the South Bran ch promoted the formation of th e shoals which of 245m/a). J iuduan sh a is different from Hengsha in th at th e composed th e present Cha ngxin and th e Hengsa Island and former is situated in the branching point against th e ebb cur­ resulted in th e bran ching in th e downstream of the South rent and Hengsh a in th e shelter of the Chan gxin Island. The Branch . Later , with the Sout h Cha nnel becoming the main Jiuduansha's trend of migration accords with th e genera l ad­ stream, the Jiuduan sa was born and th e southern banks of vanc ement of th e delta as reflected by th e eastern Chongmin th e Changxin and th e Hengsa Island was subject to erosion. coast. While th e islands or shoa ls moved north ward, the front of The present coastal geomorphology of the three islands re­ the river mouth extended eas tward. Since th e 9th century th e veals th e law th at all of th e southern coasts would be subject eastern shoreline of th e Chongming Island has moved sea­ to erosion except for th e protection of th e artificial structures. ward for 15km (Figure 7) which is nearly equal to th e dis­ On th e other hand, accretion occurs in many parts of the tance the Qidong spit and th e Nanui spit (res pectively rep­ northern coasts with exception where a deep water course is resenting the north ern and southern bank of the river mouth) pressing on toward s th e bank. Under thi s effect, th e northern has moved sea ward in th e same period. branch would gradually be silted up and th e Changjiang river In conclusion, th e river sediment discharge has been the water would flow mainly through th e southern bran ch. More­ controlling factor of th e eas tward advancement of the delta, over, the effect of th e Coriolis force which mak es th e water while th e natural migrati on of the islands has been greatly pile up on the right side would lead the dominant ebb current influenced by the Coriolis force.

J ourn al of Coastal Research, Vol. 15, No.1, 1999 Coas ta l Morphodynami cs in th e Cha ngjiang Estuar y 37

t Profile B N

------~7l771r

Figure 7. The shoreline (seawall) hist ory of th e eas te rn Chongming Is­ land since th e 9th century.

Erosion-Accretion Cycles Several Years' Cycle ver tica l scale dista nce: 1m Thi s cycle mainly occures in th e channel bank along the northern and southern coasts of the islands. Figures 8-10 0.5 1.0 I.5 2.0 reveal the 2 to 4 years' cycle of subtidal profiles on the Offshore(km) northeastern side of the Chongming Island (sorrowfully few Figure 8. Severa l years' erosion-accret ion cycle of submarine area in comparative data were obtained for intertidal profiles). This profile B. kind of cycle is induced by th e changes of the offshore channel. Cha nnel's change has three bas ic types:(a) onshore­ offshore migration;(b) change in cross-section area and (c) pattern is different. In the eas te rn coast of Chongming, for change in cross-section shape. When the channel moves example, the highe st elevation occures in spring and th.e low­ shoreward, the bank is eroded. When th e channel becomes est in autumn (Figure 13), which is similar to that III the sma ller in cross-section area, accumulation usu ally happ en s. eastern Nanhui coast (YANG, 1997). In the northeastern coas t Type (a) might result from the longshore movement of th e of Chongming, the highest and lowest elevation respectively channel's meander s. Type (b) might result from the variation occures in winter and summer (Figure 14). In the southwest­ in water discharge which can be caused by changes in river ern coast of the island (profile H), lowest eleva tion occures in basin conditions. It might also result from th e migration of sprin g whil e the highe st one is difficult to discriminate be­ the subma rine sa nd spit. In Figu re 11, on the southwe stern tween in autumn and winter. According to the one-year-pe­ side of Chongming, the spit was 10-12 km in length and its riod mea suremen t in 1983 and 1984, the seaso nal chan ge in avera ge migration rate was 1.7 km/a after 1973, which could average profile elevation was 47 em in profile F, 22 em in result in a 6 to 7 years' cycle of profile change . Type (c) is profile E and 14 cm in profile H afte r net erosion or acc.retio.n more complex than typ e (a) and (b). Most of the changes in was taken off. The factors influencing the annual cycle III this Figures 8-10 and Figure 12 wer e mixed among the three area are not so simple as shown by SHEPARD (1950) and KING typ es. Some of them show a balance between erosion and (1953) who attributed beach cycles only to seasonal va riation accretion, but oth ers do not. It is concluded that the channel's in wind direction. In the Changjiang Estuary, seas onal vari­ change might be caused by mul tiple factors. ation occures not only in wind direction but also in sea level (a 49-56 ern amplitude of monthly-mean sea level between Annual Cycle the Augu st peak and th e January nad ir), tid al range (a 24­ Annual erosion-accretion cycle is widespread in the Chang­ 45 em differ ence between the Septemb er maximal and the jiang delta coasts. In different orientated coasts, th e cycle J anuary minimal ), river discharge of water and sediment

Journ al of Coasta l Research, Vol. 15, No. 1, 1999 38 Shilun

nom) TDL(Om) :!.;~\ Profi le D '. '.:~.\ TDL ::~ ..... 1984 /. : 1982··:~~':·~""""'· - ·-"·""':" ·7'.,., ·";O .-c:.:::::(;:t({::·:

/.

/.t :::".":. ~ :: ' . Profile C / TDL ~..

TDL

TDL ~ ~L wrt~l~ak ;I. , ", '::. ' 1986,/'(:::"1985 d~tanc~ 1m / .... =""""~ ~.::: : . , TDL 1.0 2.0 3.0 4.0 5.0 Offshore(km) Figure 10. Several years ' erosion-accretion cycle of submarine area in 6 // profile D.

is monthly mean value). The linear regressive coefficients are listed in Table 1. It can be concluded that only profile H accords with the theory that onshore winds induce erosion and offshore winds vertical sca le cause accretion, as shown by SHEPARD (1950) and KING distance: 1m (1953) in their studies of beaches. This may be attributed to o 0.5 1.0 1.5 2.0 2.5 the beach feature of profile H (its slope was 2.0% and its Offshore(km) sediments below the eroded scarp was sandy with middle grain-size larger than 0.063mm). In profile E, the onshore Figure 9. Several years' erosion-accretion cycle of submarine ar ea in winds in winter was accompanied by accretion and not by profile c. erosion as in beaches (SHEPARD, 1950; KING, 1953). The in­ tertidal accretion in winter might result from the erosion of the offshore shoal, which provided rich sediments and made the suspended sediment concentration much higher than in (monthly-mean discharge of water in July is 4.8 times as in summer (according to the survey at the offshore station, the January, monthly-mean discharge of sediment in July is 36.5 mean suspended sediment concentration in the winter three times as in February) and coastal suspended sediment con­ months is 4.1 times as that in the summer three months). In centration (the monthly mean value in February is 7.3 times profile E and F, R I was respectively - 0.77 and -0.53 and R2 as in July). Suppose RI , R2 , R3 , R4 , Rs' R6 , and R7 respectively seperately -0.56 and -0.74, which reveals the negative re­ represents the correlative coefficient between average inter­ lationship between intertidal elevation and sea level as well tidal elevation and the factors of sea level, tidal range, on­ as tidal range. It can be und erstood that (a) rise of sea level shore wind frequency, offshore wind frequency, river water increases the water depth in the intertidal zone and makes discharge (at the Datong Station, 640 km upstream from the the breaker zone migrate shoreward, which subjects the river mouth), river sediment discharge (also at the Datong shoreface to more wave energy and promotes erosion;(b) in­ Station) and coastal suspended sediment concentration (each crease in tidal range strengthens currents and enhances their

Journal of Coastal Research , Vol. 15, No.1, 1999 Coas ta l Morphodynam ics in the Cha ngjiang Estuary 39

Nanmen • -.. -. Baozheng --...... -...... :--_ -- ...... __ -----::.--°""'""'- 0 """'"'-~ ...... -::::..-:=...-.:..--_ .- .::-;.-:-:...... ~ ._ .-:--- .-: .- .- ...... - - ...... 1963 --._-)"".:,: - ::.::.:. : ~ . - . - . _ . _ ' ._'""C"" ---' 0 ••••• o'o°.:.c.. ' '-'" " " 19'"73 '\'; /976--.:.::::::::_._ '-,.- .- ._. _ ._ ....-----__.J •••••••• ••••••• •••••• •_ .....1978 '; ...... ':: .-.- ...... - -

o 5km ..../ migration trend ------

Figu re 11. Migrat ion of submarine sa nd spit (- 5 m contour) in the southern coast of th e Chongm ing Island.

capacity to carry sediments out of the intertidal zone. In Ta­ Why are they false in appeara nce? Sediment concentratio n ble 1, R6 (E) = - 0.67 and R6 (F) = - 0.57 are false appear­ in the river does not represent that in the estuary. In fact , ances which easi ly leads to a wrong conclusion that increase the correlative coefficient between suspended sediment con­ in river sedime nt discharge causes erosion in deltaic coasts. centration at the Datong Station (640 km upper the rive r mouth) and that at the Yingshuic huan Station (at the front of the river mouth) is - 0.86. What really reflects the rela­ tionship between condition of sediment concentration and

Profile E

/ I 4.0 ...... ------, / Profile F I ---===os--- I supertida l (ma rsh)

3.0 higher flat E C­ .s ~., 2.0 '0

1.0 storm

vertical scale __----....;s orm distanee: lm

2.0 3.0 4.0 5.0 6.0 7.0 N D J F M A M JJ AS 0 Offshor e(km) 1982 1983 Figure 12. Morph ological changes in profile E. Figu re 13. Seasonal variation in differ ent elevation in profile F.

J ournal of Coastal Resea rch, Vol. 15. No. 1, 1999 40 Shilun

Table 1. Linear regressive coefficients between average intertidal elevation Generally, storms induced not only an erosive trace but also and factors. an accretional one on the intertidal zone. Eroded mud pud­ dles, a few meters in diameter and 10-20 em in depth, oc­ Profile R n, R R R R6 R 1 3 4 5 7 cured in the middle and high flat, and a distinct scarp E -0.77 -0.56 0.83 -0.83 -0.94 -0.67 0.35 emerged between the flat and the marsh, while fine-sand -0.53 -0.74 0.02 0.03 -0.11 -0.57 0.72 F waves (several decimeters in height and several tens of me­ H 0.07 -0.28 -0.57 0.58 -0.25 0.13 ters in length) appeared in the low flat and coarse silt mound (several meters in width, several tens of meters in length and several decimeters in thickness) in the lowest part of marsh. shoreface change is R • RiE) = 0.35 and RiF) = 0.72 reveal 7 Two aspects of difference existed between storm cycles of the machanism that increase in sediment concentration caus­ the mud-dominated shores of the Changjiang delta and the es accretion. In the three profiles along the northern coast of beaches in the world: (a) storm-induced erosion on beaches the Hangzhou Bay, neighboring the Changjiang Estuary in usually occurs along the whole shoreface (including berm and the south, R was respectively 0.97, 0.69 and 0.57 (YANG, 7 foreshore), but in the study area it occurs only in the lower 1997). Why does a negative relationship exists between the part of the intertidal zone (the bare flat and sometimes to­ river sediment concentration and the estuarine one? In sum­ gether with the front fringe of the marsh); (b) the recovery of mer, it results from the joining of a great deal of extra sea the tidal flat after a storm offten needs only several days or water, with very low suspended sediment concentration, to a few weeks because during each tide the eroded surface will the estuary under the effects of sea-level rise and increase in be submerged and has chance to accrete, but that of the tidal range, which makes the estuarine sediment concentra­ beach, especially the berm, often needs a few months (and tion much lower than that in the river. In winter, wave en­ sometimes a few years; SEXTON, 1995) for normal waves can ergy in the estuary is high and deposited sediments are re­ not reach the high part of the beach unless during spring tide. suspended, making the sediment concentration much higher than in the river. Tidal Cycle In fact, what really controls sedimentation processes is bal­ ance between sediment concentration and water capacity to Although spring-neap tidal cycle of sediment elevation has carry sediment. When the concentration is lower than the been reported on beaches (KOMAR, 1976) and on the tidal flat capacity, erosion will take place; conversely, when sediment of the Hangzhou Bay (CHEN, 1987), no evidence has been concentration is higher than the capacity, accretion will re­ found to confirm the law in the Changjiang river mouth part­ sult. In brief, although seasonal erosion-accretion cycles are ly because of the lack of data. A 99-day period (from June 15 widespread in the studied area, they are different in pattern through September 21, 1987) of survey on the eastern Nan­ and their causes are complex. hui shore showed that, from spring to neap tide, 5 net ero­ sion, 8 net accretion and 1 balance occured while from neap Storm Cycle to spring tide, 6 net erosion, 6 net accretion and 2 balance occured (YANG, 1991). So it was concluded that although the The duration of storms in the Changjiang Estuary was usu­ current velocity in spring tide was much higher than that in ally 2 to 3 days. Half of them resulted in a 50cm-150cm in­ neap tide the sediment elevation was not subject to distinct crement in water level. The storm transfers a great deal of erosion-accretion cycle because of the dominant role of the energy to water and increases its capacity to carry sediments. wind-induced waves from the open sea (YANG, 1991). Combined with the increment in water level, storms tended to erode the tidal flat to a great extent. For example, during Difference Between Marsh and Bare Flat the storm from 25th to 28th, September 1983 with a maxi­ mum wind velocity of 19 mis, a 126 em of increment in water It is important to point out that erosion-accretion cycles in level and wave heights three times as the normal were ob­ the studied area happened mainly on bare flats instead of in served around the islands. As a result, the tidal flat in profile marshes. Benefitting from the protection and sediment-trap­ D, E, Hand F was respectively eroded by, on the average, 27 ping effect of the vegetation, sediment surface in the marsh em, 37 cm, 40 em and more than 50 em (during the same was seldom eroded, even during storms, unless the plants period, the average and maximum eroded depth was sur­ were harvested. This kind of difference can be seen in Figure veyed 30 em and 80 em, respectively, in the eastern Nanhui 13. In fact, sometimes during storms, quick accretion occured coast). Erosion on tidal flats were usually balanced by accre­ in a marsh while the bare flat was strongly eroded. For in­ tion on channel beds through the transportation of sedi­ stance, during the storm from 27 to 29 July 1987, the eastern ments. A week after the typhoon, continued from 17th to 19th Nanhui shore was surveyed and seen to have experienced a August 1997, with the maximum velocity in the storm center mean erosion of 12 em on the bare flat and a mean accretion being reported to have reached 11th grade, the main course of 0.8 cm in the marsh, with some particular tracts (each with bed of the North Passage was surveyed to accreted 40 em on decades of square meters in area) in the lower Scirpus marsh the average and 70 em as the maximum, while eroded scarps accreted by 5 to 8 em (as a result, the dense plants were about 50 ern on the average were observed along the island completely covered by deposits). In a fortnight period from 24 shorelines. In the same time, sand waves on the bed of the August to 7 September 1987, in which landward gales occu­ South Channel, with their height being found to be 50-100 red, the bare flat was eroded by 7.9 em while the marsh ac­ em before the storm, were covered by muds during the storm. creted for 1.4 em, both on the average (YANG, 1991).

Journal of Coastal Research, Vol. 15, No.1, 1999 Coastal Morphodynamics in the Changjiang Estuary 41

Impacts of Human Activities on Coastal Protective Structures Morphodynamics Some 310 groynes, 145 m in length on the average, are now Reclamation distributed along the coasts of the three islands. Most of them were constructed after 1950's. These groynes are mainly scat­ China has a long history of reclaimation from the sea. The tered in (a) the southwestern and northwestern side of seawall standed side by side with the Great Wall and the Chongming, (b) the southwestern side of Changxin and (c) Grand Canal as the three largest Chinese ancient the northern and southern side of Hengsa. In these sections engineering works. The first large-scale reclamation was in of coast, the mean distance between two adjacent groynes is the 7th Century. From the end of the 1940's to the early of 485 m. Besides these groynes, 146 km in total length of rocky the 1980's, the country reclaimed about 5000 km 2 lands from or concrete slope were constructed on the seaside of seawalls, the sea. About 620/0 of the Shanghai land area was obtained mainly along the above-mentioned sections. from reclamation (CHEN et aI., 1989). About half of the The first effect of protective structures was to change the Changming Island's area (1160 km 2 in 1992) was reclaimed morphological and sedimentary features of profiles. After the from the sea since the 1940's (Figure 5), although the island construction of groynes, eroded scarps used to be covered by was first habited and reclaimed by man in AD 696 (GSCII, sediments and the profile transfered to stable one with no 1996). The present Changxin Island was formed by linking apparant relief in it. The mean grain-size in the profile be­ the six former small islands, respectively named Yawosha came finer. (first reclaimed in 1844), Shitousha (first reclaimed in 1850), The second and the most important effect of the protective Ruifengsha (first reclaimed in 1925), Panjiasha (first structures was to stop the retreat of the shoreline as well as reclaimed in 1924), Jingdaisha (first reclaimed in 1924) and the whole movement of the islands. For example, before the Yuanyuansha (first reclaimed in 1907) through artifitial large-scale construction of protective structures, the site of structures for the purpose of large-scale reclaimation from Chongming county seat was northward migrated for three the 1950's to 1970's. The Hengsha Island was first reclaimed times (the former sites have sunk in the water of the present in 1880 (GSCII, 1996). Reclamation in the Chanjiang Delta South Branch), and eight towns at the small islands which has been quickened (Figure 7, Figure 5), partly because of latter formed the Changxin Island 'sank' in the South Chan­ nel. After the construction of the structures, the stableness the increase in population in the area, partly for the increase of the southwestern bank of the Chongming Island has im­ in river sediment flux caused by the exploitation of the river peded the broadening of the South Branch and then main­ drainage (CHENet aI., 1989), and partly due to the promotive tained the alloting ratio of the river-sourced water and sedi­ effect of reclamation itself on accretion. ment discharge between the South and North Branch. This Reclamation by constructing seawalls greatly changed the effect in turn slowed the silting-up of the North Branch and natural processes of the intertidal profile. On the islands, the the advancement of the northeastern coast of the Chongming base of seawalls were generally situated at 3.5 m to 3.6 m Island. The structures have also resulted in a stable North above the Theoretic Datum Level, a little higher than the and South Channel and hence the Changxin and Hengsa Is­ mean high tidal level (3.33 m, 3.28 m and 3.25 m above TDL land. From 1960's to 1992, the shoreline of Chongming respectively in Chongming, Changxin and Hengsha). So the changed much less than in the period from 1940's to 1960's, plane and broad supratidal zone (high marsh) between the especially along the southwestern and northwestern bank seawall and the extreme high tidal level (5.67 m, 5.65 m and (Figure 5) where protective structures were densely distrib­ 5.53 m above TDL respectively in Chongming, Changxin and uted after 1950's. Also benefitting from the protection of the Hengsha in the recoded history, about 2 m upper the base of artificial structures, the Hengsha Island moved for only a lit­ seawalls) was cut away from the sea. For example, when the tle from 1958 to 1990 (32 years) compared with the same eastern Chongming shore (profile F) was reclaimed in 1991, length of period from 1926 to 1958 (Figure 6). Similarly, the width between the new seawall (3.6 m above TDL at its base) Changxin Island changed less after 1970's when the whole and the old one (on its seaside, the top elevation of shoreface island was formed by artificial structures. was 3.8 m) reached 4 km, with the slope being 0.0050/0. As a Somewhere gronyes had the effect to promote accretion. In result, the intertidal profile was narrowed and the tidal 1972, a 3000m-in-Iength groyne, prolonged eastward at the current in spring tides weakened (in neap tides, flood water southern corner of the Hengsha Island, was built. After the can not reach the seawall), while more wave energy was construction of the groyne, the former eroded shore on the depleted in front of the seawall. In this way, the northern side of the groyne, with its Om-line only 30 m apart transportation of sediments was influenced and the from the seawall, was transfered to accretional type. In 1991, development of tidal creeks restrained. the Om-line was surveyed to have migrated seaward for 2950 Reclamation has a mechanism to speed up accretion m, 155 mJa on the average. As a result of the acretion of outside the seawall and therefore quicken the growth of the shoreface, a 450m-in width reed marsh was developed in the islands. After reclamation, the former supratidal zone (now top part of the profile. The accretional area was trigonal. The inside the seawall) is no longer silted up by sediments. This effect of the long groyne weakened northward: the longer the part of sediments has to deposit outside the seawall. In this distance from the groyne, the less the influence. In a word, way the advancement of the shoreline is quickened, providing the coastal morphodynamics in the Changjiang estuarine is­ chance for further reclaimation. lands has been greatly influenced by human activities.

Journal of Coastal Research, Vol. 15, No.1, 1999 42 Shilun

coasts in morphodynamics. Artificial structures for reclai­ c: mation and costal protection, furthermore, make the coastal .2 2.2 ~ Profile E evolution more complex than the primary natural one. As a v 2.1 result, the Changjiang estuarine island coasts have different o E'2.0 profile types and different regimes controlling the erosion­ § -- 1.9 accretion cycles, either compared with each other or com­ pared with other kinds of coast. Ev 1.8,...... , ! I JFMAMJJASOND 1983 ACKNOWLEDGMENTS

Figure 14. Seasonal erosion-accretion cycle ofthe mean elevation in pro­ The author wish to thank H.G. Xu, J.Q. Zhou, Z.C. Mao file E (the net accretion was taken off). and J.M. Qian for their cooperative work in surveying the profiles and collecting sediment samples. Appreciation is also extended to W.Li and L.X. Wang for analysing the sediment FORECAST ON FUTURE COASTAL samples and D.F. Shong for drawing the illustrations. This DEVELOPMENT research was supported by the Chinese Natural Sciences Foundation (No: 49611007). Two major factors will inevitably influence the develop­ ment of the Changjiang delta coasts in the next century: the LITERATURE CITED reduction of the river-sourced sediment flux and the sealevel rise. The Three Gorge Project, known all over the world, is ADERSON, F.E., 1983. The northern muddy intertidal: Seasonal now being constructed at the middle reaches of the Chang­ factors controlling erosion and deposition-A review. Canadian jiang River 1800 km from the river mouth. According to the Journal of Fishy Aquatic Science, 40 (Suppl. 1), 143-159. AUBREY, D.G., 1979. Seasonal pattens of onshore/offshore sedi­ computation of ZHU (1987), the river-sourced suspended sed­ ment movement. Journal of Geophysical Research, 84, 6347­ iment discharge into the sea will reduce by 11% due to the 6354. Three Gorge Project. In addition to the effect of this giantic AUBREY, D.G. and Ross, R.M., 1985. The quantitative descrip­ reservoir, the Northward Withdrawal Transfer of the Cha­ tion of beach cycles. Marine Geology, 69, 155-170. BASCOM, W.H., 1951. The relationship between sand size and ngjiang River Water Project will reduce the sediment dis­ beach face slope. Transcription ofAmerican Geophysics Union, charge by 1-2% (CHEN, 1995). By the end of the 21st Cen­ 32, 866-874. tury, sealevel in the Changjiang estuary will rise about 1.0m, BOWAN, D., 1981. Efficiency of eigenfunctions for descriminant 2h of which is from the optimum forecast on the globe sea­ analysis of subaerial non tidal beach profiles. Marine Geology, level rise made by the IPCC and the remainder is based on 39, 243-258. BRUUN, P., 1954. Coast erosion and development of beach pro­ land subsidence by rough estimates. A famous example of files. U.S. Army Beach Erosion Board Tecnical Memorandum coastal erosion due to reduction in river sediment discharge No. 44, US Army Engineering Waterways Experiment Station, is seen in the Nile Delta. After the construction of the Aswan Vicksburg, Mississippi. Dam, the deltaic coast retreated at a mean rate of 30m/a BRUUN, P., 1962. Sea-level rise as a cause of shore erosion. Pro­ ceedings ofAmerican Society ofCivil Engioneers, Journal ofWa­ during a period of 50 years and once at a rate of 143-160m/a terways and Harbors Division, 88, 117-130. (CARTER, 1988). Another example was the Skokomish River CARTER, R.W.G., 1988. Coastal Environments. San Diego, Aca­ delta in the America, where an erosive rate of 1.1-3.3 em/a demic, 559p. on the outer delta resulted from the withdrawal of 40% of the CHEN, J.Y., C.X., YUN and Y.F., DONG., 1979. The development annual runofffrom the upper reaches (JAY, 1996). Up to now, model of the Changjiang Estuary During the Last 2000 Years. Acta Oceanologica Sinica, 1, 1-8. (in Chinese). numerious reports have related the effect of sea-level rise on CHEN, J.Y., WANG, B.C. and Yu, Z.Y., 1989. Developments and coastal erosion. It is difficult to foretell if the whole Chang­ evolution of China's coast. Shanghai Scientific and Technical jiang delta will retreat in the next century. But it is safe to Publishers, Shanghai (in Chinese): 519p. say that some, if not all, of the precent advancing coasts will CHEN, H.D., 1987. Sedimentation of the northern coast of the Hangzhou Bay. Master's Thesis, East China Normal Universi­ be transfered to erosive types and the others will slow their ty. Shanghai, China. (in Chinese). accretional rate under the combination of sediment reduction CHEN, W.Y., 1991. Sediment transportation and sediment dy­ and sea-level rise. The present accretional profiles (e.g. profile namic environment of tidal fiat. Acta Oceanologica Sinica, 13, F and E) will become narrower and steeper. A dynamic loss 813-821. (in Chinese). of 50% of the tidal wetlands, about 200 km", will occur on the CHEN, X.Q., 1995. The impact of sealevel rise on the Shanghai coast. Institute of Estuarine & coastal Research, East China islands by the end of the 21st Century. Normal University. Shanghai, China. (in Chinese). COLLINS, M.B., C.L. AMOS and G. EVANS., 1981. Observation of CONCLUSIONS some sediment-transport processes over intertidal zone. Sedi­ mentology, 5, 81-89. Multiple factors control the coastal mophodynamics of the DEAN, R.G., 1977. Equilibium beach profiles: US Atlantic and Changjiang estuarine islands. The silt-dominated sediment Gulf coast Department of Civil Engineering Report, No. 12, Uni­ source and current-dominated hydrodinamics make it differ­ versity of Delaware, Newark, Delaware. DEAN, R.G., HEALY, T.R. and DOMMERHOLT, A.P., 1993. A ent from sand or gravel beaches. On the other hand, with the "Blind-folded" test of Equilibium beach profile concepts with great influence of the river discharge and the ring-like shore­ New Zealand data. Marine Geology, 109, 253-266. line trend, the island coasts are different from normal muddy DUNCON, J.R., 1964. The effects of water table and tidal cycle on

Journal of Coastal Research, Vol. 15, No.1, 1999 Coasta l Morphodynamics in th e Cha ngjiang Estu ary 43

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Journal of Coastal Research, Vol. 15, No.1, 1999