Journal of Coastal Research 297-308 West Palm Beach, Florida Spring 2001
Temporal Change in Bed Level of a River Mouth Channel, Yangtze River Mouth: with Emphasis on the Response to River Discharge and Storm Shi-Iun Yang, Ping-xing Ding, and Shen-liang Chen
State key Lab of Estuarine & Coastal Sediment Dynamics & Morphodynamics East China Normal University, Shanghai, 200062 P. R. China Email: [email protected]
ABSTRACT .
YANG, S.-L.; DING, P.-X., and CHEN, S.-L., 2001. Temporal Change in Bed Level ofa River Mouth Channel, Yangtze .tllllllll:. River Mouth: With Emphasis on the Response to River Discharge and Storm. Journal ofCoastal Research, 17(2), 297 ~ 308. West Palm Beach (Florida), ISSN 0749-0208. eusss~~ Based on 23 bathymetric surveys from January 1988 to December 1990 in the South Passage, Yangtze River mouth, -+4 along with corresponding data on river discharge and coastal storm, this paper examines the different time-scales of b--- bed level change, with emphasis on the response of bed level to river water discharge and coastal storm. Results are summarized as follows: (a) Under normal conditions, the range of change in bed level was on the order of a few centimeters during a period of half a month, 10 to 50 centimeters in the different seasons and 40 to 60 centimeters in the comparison between flood season and dry season, on average of the along-channel survey sites. A maximum change could be several times the average change. (b) The average along-channel depth was negatively correlated with the river water discharge. (c) The response of bed-level of the South Passage lagged changes in river discharge at the Datong Station by 1 to 2 months, which reflects the influence of the 600 km distance between the hydrological station and the river mouth. (d) In taking into account this lag effect, a correlative coefficient of 0.90 was found. Based on these data, the range of annual change in the channel bed level was forecasted to be the order of 50 em for normal discharges, 20 em for minimal difference in seasonal discharge and 100 em for maximum difference in seasonal discharge. (e) A ten year storm event was found to result in an average accretion of 60 ern, which superimposed upon the river-induced seasonal change. (D A good spatial unanimity of erosion and accretion events was found for the main part of the channel. Conclusions include the following: (a) The channel bed-level of the Yangtze River mouth is sensitive to both seasonal changes in river discharge and to storm events. Under conditions of normal river water discharge and severe storms, the combined annual difference in average bed level can reach the order of 1.0 m. It is probable that accretion in flood season could exceed 1.5 m in the case of co-occurrence of extreme high water discharge and extreme severe storms. (b) In view of a good positive correlation between river water discharge and riverine sediment discharge, the response to water discharge suggests a response of bed-level to the sediment discharge. (c) Due to its regulative function in annual distribution of the river discharge and its role in reducing sediment supply, the Three Gorge Dam will perhaps reduce the range of annual change in the bed level of the river mouth channels and increase the depth of the navigational channels.
ADDITIONAL INDEX WORDS: Channel bed-level, river discharge, storm, river mouth, the Yangtze River, China.
INTRODUCTION ZENKOVICH, 1974; Lv, 1980; MOBAREK and GOODMAN, 1980; NICHOLSON, 1983; CARTER, 1988) and China (XIA, 1993; Understanding of coastal morphological evolution has di QIAN, 1994; JI, 1996). In contrast to these studies, changes rect application in engineering and is invaluable to future planning and management (CARTER, 1988; LARSON and in the bed-level of river mouth channels as well as the influ KRAUS, 1994). In the last few decades, many articles con encing factors have been less studied (SHEN et al., 1983), cerning short-term changes in the bed level of beaches (Ko probably because of the difficulty in bathymetric surveying MAR, 1976; FELDER and FISHER, 1980; DUBOIS, 1988; Hou and, in some cases, for secrecy consideration in bathymetric THUYS et al., 1994; LARSON and KRAUS, 1994; LEE et al., data on navigational channels. 1995; SEXTON, 1995) and tidal flats (ANDERSON, 1983; The navigational condition of a river mouth is usually lim ZHANG, 1993; ALLEN and DUFFY, 1998; YANG et al., 1998a) ited by the water depth of the channel bar. The bed level of were written. Medium and long-term responses of deltaic a channel bar is influenced by river discharge as well as by coasts to changes in river discharge have also been paid great estuarine hydrodynamics. Storm and seasonal change in riv attention to in America (NORRIS, 1964; BOWEN and INMAN, erine sediment supply are probably two dominant factors in 1966; BROWNLIE and BROWN, 1978; JAY and SIMENSTAD, short-term erosion and accretion events on the channel bed. 1996), Africa (KHAFAGY and MANOHAR, 1979; ORLOVA and This is especially true in cases of broad intertidal-subtidal flats exposed to open sea waves and local winds, and in cases 99123 received 13 December 1999; accepted in revision 8 August 2000. of sharp annual changes in riverine sediment supply. A large 298 Yang, et al.
..c:: 14 o Ul water discharge "0 o E 12 ~ sediment discharge c '-../ • t idal range __ Q) Q) E bO ~ 10 ""0 Q) Ul ro X (],) ~ en bO '"'III ro ~ 'S c "0 25~m s:
1 Changxing o ....., 2 Hengsha en jan. Feb. Mar. Apr. May jun. jui. Aug. Sep. Oct. Nov. Dec . 3 Jiuduansha ro month -- coastline w 0N Figure 2. Seasonal variation in monthly riverine water and sediment 31 0 m discharge at the Datong Hydrological Station, 600 km upstream of the -5m South Passage; also illustrates tidal range in the central South Passage ~ Study area (based on multiple-year data).
Figure 1. The Changjiang (Yangtze) River mouth within which the stud ied area is located. sediment (YANG, 1999a) into the East China Sea at 30.9° ---31.7°N and 121.1° ---121.9°E. These discharges stand the part of sediment eroded from the neighboring tidal flats can fifth largest and the fourth largest in the world's rivers, re be transported and deposited onto the channel bed of the spectively (EISMA, 1998). The grain-size of 67.3% of the riv Yangtze River mouth during storms (SHEN et al., 1983; YUN, erine sediment is <0.05mm, with d50 = 0.027mm (YANG, 1983). Sharp annual change in sediment influx of the Yang 1994). About 1/4 of the river discharge pours into the open tze River results in a cycle of erosion and accretion on chan sea through the way of the South Passage (YANG et al., nel beds of the river mouth (SHEN et al., 1983). 1998b). According to multi-year records of a hydrological sta The South Passage is one of the major navigational chan tion at the middle of the South Passage, the average and nels in the Yangtze River mouth. The passage was not maximum tidal ranges are 2.67m and 4.62m (GSICI, 1996). dredged between 1988 and 1990. During this period the The river discharges vary annually (Figure 2). On the aver bathymetric surveys for this study were conducted. A study age, the monthly river water flux in July is 4.4 times as large of changes in the bed level of this passage (especially in the as in January and the monthly river sediment discharge in section of bar which is tens of kilometers in length and 3-4 July is 36.5 times as large as in February. In relation to this, m in height) is meaningful both for the understanding of river the monthly riverine suspended sediment concentration mouth channel evolution and for the economic purpose of (SSC) changes from 0.1 gil in February to 0.8 gil in July. maintaining the navigational course. Although the maximum monthly tidal range (in September) This paper examines the magnitude and time-scale of ero is only 1.17 times as high as the minimum monthly tidal sion-accretion changes in bed level of the South Passage in range (in January), this variation is meaningful in view of relation to variation in river discharge and the occurrence of the fact that the flood tidal water discharge is 8.8 times as storms. The results may be useful in modeling the bed level large as the riverine fresh water discharge under the condi cycles of the river mouth channels. Moreover, the work may tions of the average tidal range and average riverine water be a foundation for forecasting the influence of the world discharge (SHEN et al., 1983). Current speed in the South wide-known Three Gorge Dam project; because, the project Passage sharply changes in flood-ebb cycle, spring-neap cycle, will greatly change the annual distribution of river water dis storm cycle and seasonal cycle, with the average velocity of charge and result in a decrease in sediment supply. both the vertical profile and tidal cycle (under normal tidal conditions) being around 0.6 --- 0.8 mls and the maximum STUDY AREA velocity being more than 2.0 mls (personal observations). To a great extent, sedimentation in the South Passage is influ The South Passage is one of the major passageways in the enced by local winds. Although the wind velocity averages forward of the Yangtze River mouth. Its length and width only 4 mis, a maximum velocity of 30 mls was once found between the two 5m isobaths are around 40 km and 2 to 5 (YANG, 1999b). Annually several storms (with a wind velocity km, respectively (Figure 1). The bed of the South Passage larger than 10.8 m/s) affect the river mouth. During a storm, composes part of the river mouth bar complex (SHEN et al., hydrodynamics and related transport of sediment are greatly 1983; CHEN et al., 1988b). Erosion and accretion of this pas changed, especially when taking into account the sediment sage is influenced by the river water and sediment discharg exchange between the channel and the tidal flats. South Pas es, as well as by the estuarine hydrodynamic conditions sage is located within the turbidity maximum zone of the (SHEN et al., 1983). On an average, the Yangtze River trans river mouth (SHEN et al., 1992), with SSC being 0.1 --- 0.7 gl ports 924 X 109 ms/yr water and 468 X 106 tlyr suspended I on the surface and 1.0 --- 8.0 gil near the bottom (SHEN et
Journal of Coastal Research, Vol. 17, No.2, 2001 Yangtze River Mouth China 299
Table 1. Scale of wind velocity (NTSBC, 1992
Scale o 2 3 4 5 6 7 8 9 10 11 12 Velocity (m/s) 0-0.2 0.3-1.5 1.6-3.3 3.4-5.4 5.5-7.9 8.0-10.7 10.8-13.8 13.9-17.1 17.2-20.7 20.8-24.4 24.5-28.4 28.5-32.6 >32.6
al., 1988). Sedimentation of the suspended particles is gov relating the bed level to river discharge at the Datong Sta erned by the match and mixing of river water and the saline tion, different correlative analyses were compared and data water (SHEN et al., 1983; HE and SUN, 1996; PAN and SUN, were presented in a table. Correlative coefficient between bed 1996). In contrast to the sandy beds in the upstream channels level and river water discharge (D) was computed. Do rep of this river mouth (the South Channel and The South resents the average river water discharge in the same month Branch), beds of the South Passage are typically muddy, with as the bed level. Do.s represents the distributive river water dso being 0.008 '"'-' 0.032mm (DONG and DING, 1988; YANG, discharge (from the distributive curve of the annual dis 1994), which is related to flocculation of the fine-grained sus charge) 10 to 20 days earlier than the survey of bed level (the pended sediments (QIU et al., 1988). number of days was dependent on the primary amount of discharge, i.e. the larger the discharge, the fewer the days).
MATERIALS AND METHODS D1 represents the average river water discharge a month ear lier than the bed level survey. D , represents the distributive Bathymetric maps of 1/50,000 scale were surveyed by 1 river water discharge one month earlier than the bed level Shanghai Marine Survey Bureau between January 1988 and survey. D1.s represents the distributive river water discharge December 1990 in the South Passage. The interval of the sur 1.5 months earlier than the bed level survey, and D repre veys was from half a month to four months. The duration of 2 sents the distributive river water discharge two months ear a survey was normally a few days. The spatial interval of lier than the bed level survey. In these ways, the time lag measurement grid system was 1 em (equal to 500 m in the between the river water discharge and the response of the field). On each of 23 maps, 74 cross-channel sections, num channel bed level could be taken into account in view of the bered 1 to 74, were drawn in downstream order. Due to the 600km distance from the Datong Station to the South Pas absence of reference marks and changes in topography, some sage. The scale of wind velocity was classified according to of the transects failed to be made strictly perpendicular to The Specification for Oceanography Survey in China as is the isobaths. For comparative study, the 23 transects in each seen in Table 1. of the 74 sets were overlapped by locating Transect 1 through 31°10'N-121°52.2'E and 31°12'N-121°53.4'E and the other RESULTS AND DISCUSSION 73 transects parallel to Transect 1 with intervals of 1 em on the map (or 500 m in the field). The maximum depth in each Morphology of the Channel Bed of the 1702 transects was found out and listed. In this way, the 74 numbers from each map compose an along-channel Along-Channel Profile profile and the comparison of these profiles gives rise to tem Based on the 23 surveys, the average along-channel (along poral and spatial changes in the maximum depth course of the maximum depth) profile of the South Passage is given in the passage. Although this method failed to ensure the strict Figure 3. The middle and lower reaches (the abscissa is from overlap in space of the 23 along-channel profiles, it is more 10 to 35 km in Figure 3) of the profile indicate a river mouth reasonable than the traditional method that fixes the location bar. The height of this bar is around 2.5 m. In contrast to of the profiles because the maximum depth is migrating in this average profile, the instantaneous profiles given by the transect. Along-channel bed-level changes during different single surveys are generally wavy, and will be described in periods were studied through comparison of these profiles. In the following paragraphs. There are two mysteries for the profile: (a) Why does the average profile have two peaks on the bar? (b) Why is the instantaneous profile wavy? The first is likely to be related to the two balanced zones between river ,...... j cd C) 5 discharge and sea water, corresponding to the dry season and 'r-1 +-i (1) the flood season respectively. As shown by SHEN et al. (1983), H 6 0 (1) CHENet al. (1988b) and PAN and SUN (1996), the river mouth ...c:~ 7 +-i ~ bar on the whole reflects the hydrodynamic balance and (1) ...c: (1) 8 chemical mixing of the river water and the sea water. It may +-i C cd ~ also be related with passageways which connect the North ,...... j 0 0. 9 ,...... j (1) Passage to the South Passage by extending across the Jiu ...0 10 duansha intertidal island in the central part of the bar. As ...c: +-i 0. 0 5 10 15 20 25 30 35 for the second mystery, it is unbelievable to attribute the (1) '"0 downstrem distance (km) wavy configuration to sandwaves because sandwaves found in this river mouth are composed of sands (YANG et al., 1999) Figure 3. Average profile along the maximum depth (based on 23 bathy rather than mud which composes the South Passage bed. Lo metric surveys from January 1988 to December 1990). cated in the maximum turbidity zone of the mouth, the South
Journal of Coastal Research, Vol. 17, No.2, 2001 300 Yang, et al.
...-t o 6 (ij 0 ''''';
Figure 4. Characteristic cross-channel profiles in the upstream reaches, Figure 5. Changes in the maximum depth during a half month period middle reaches and downstream reaches. in a winter-spring season (average erosion of the whole profile was 5 ern; average erosion of the top bar was 1 em; maximum erosion was 45 cm; maximum accretion was 30 ern).
Passage bed is dominated by silt and clay, which contrasts the sandy bottom of its upstream channels (YANG et al., Changes in the Bed Level in Different Time-Scales 1999). Furthermore, in contrast to the irregular and long wavy relief in the South Passage, sandwaves found in the Terms Shorter than a Month upstream South Channel are regular and the length of them Figure 5 and Figure 6 show changes in the maximum depth is only from several meters to several tens of meters (YANG during periods of half a month which filter the influence of et al., 1999). In addition, the along-channel scale of the bar spring and neap tides. No intense wind occurred during and in the South Passage is greater than those in the river just before the two periods. So the data reflects the evolution mouths of the Mississippi, the Niger, the Nile and the Sen under normal weather conditions. The magnitude of average egal, as pointed out by CHEN et ale (1988b). change was only a few centimeters, although the maximum change amounted to several decimeters. Figure 7, seems to Cross-Channel Profile illustrate the impact of severe weather (although tidal change is a factor). The effect of a spring-neap cycle on the bed level The cross-channel profile of the South Passage includes two of the channel could be at most 1 cm in view of the facts that basic types respectively occurring in the deep part (the upper follow. (a) The local suspended sediment concentration is reaches) and the shallow part (the middle and lower reaches) about 1.0 gil more in spring tide than in neap tide. (b) The of the channel. Type (a) (Section 9 in Figure 4) can be divided average water depth in this channel is around 10 m. (c) The into aU-like deep course and a gentle slope, between them unit weight of the sediment is approximately 1.5 g/cm". A being a sharp slope-break. Type (b) (Section 26 and Section simple estimate gives rise to a bed level change of only 0.67 45 in Figure 4), on the other hand, is much shallower with a cm for the sediment exchange between the channel bed and gradual change in the gradient. Figure 4 illustrates the fol the water column in the spring-neap cycle. Elevation mea- lowing 3 points: (a) The gradient of the side slope is from 0.4%0 to 5%0. The former is approximately the same as the seaward slope of the subaqueous delta (YANG, 1999a). (b) The slope on the side of Jiuduansha intertidal island (i.e. the o '''''; 5 +-> northeastern side) is sharper than that on the other side (the (1) ~ mainland side), which implicates the presence of a flood tidal 0 (1) 6 ..c channel under the effect of the Coriolis force. Based on hy +-> ---. E (1) <;»: drodynamic measurements in 1988, PAN and SUN (1996) in ..c 7 +-> (1) ~ dicated the dominance of the flood tidal current in the South ~ (ij o ...-t 8 Passage. It is significant that the bathymetric surveys pro ...-t 0. (1) viding data for this study were conducted from 1988 through .D -----Sept. 22-23, 1989 ..c +-> -Oct. 5-6, 1989 1990, suggesting that hydrodynamics in the South Passage 0. 9W (1) was dominated by the flood tidal current in spite of a seaward '""Cl 10 residual transport of water and sediment due to the river 0 5 10 15 20 25 30 35 discharge. Under the influence of Coriolis force, flood tidal downstream distance (km) currents in the South Passage veer obliquely towards the Jiu duansha side, which results in the formation of a sharp slope Figure 6. Changes in the maximum depth during a period of half a on the southwest side of the Jiuduansha. (c) In close prox month in an autumn (average erosion of the whole profile was 1 em; average erosion of the top bar was 1 em; maximum erosion was 30 em; imity to the sea, the channel becomes wider and its side maximum accretion was 30 ern). slopes more gentle.
Journal of Coastal Research, Vol. 17, No.2, 2001 Yangtze River Mouth China 301
Q) 5 ~ (\l ~ 0.. 6 ~ (\l 0 .~ +-l 7 Q) 8~ Q) ...c:: 8 +-l -----May 9-13, -----May 10-15, 1988 4-4 1989 0 9 .....Aug.8, 1989 .....Jun.2-5, 1988 ...c:: +-l 0.. Q) 10 --0 0 5 10 15 20 25 30 35 o 5 10 15 20 25 30 35 down-stream distance (km) downstream distance (km) Figure 9. Changes in the maximum depth during the summer season of Figure 7. Changes in the maximum depth during a period of 18 to 26 1989 (average accretion of the whole profile was 29 em; maximum accre days in a spring-summer season (average erosion of the whole profile is tion was 85 ern). 23 em; average erosion of the top bar is 30 em; maximum erosion is 70 ern). Different Seasons Figures 8 to 11 provide examples of bed level changes in surements on the intertidal flat in the southwestern bank of different seasons. Within a season, the average and maxi the South Passage indicated similar spring-neap changes in mum changes were about 10 to 50 em and 50 to 90 em, re sediment surface (personal observation). The average erosion spectively. The figures indicate that the channel bed tends to of more than 20 em in Figure 7 might be attributed to a sud accrete in spring and summer and erode in autumn and win den accretion generated by a storm that occurred immediate ter. It seems that summer is the major season of accretion ly before the former survey (from May 3 to May 6, the wind while winter is the major season of erosion. Nevertheless, velocity was in the magnitude of Scale 7 to Scale 9). After the this is only a general tendency or net result of the relevant sudden accretion, the decreased channel volume would be in seasons. It does not mean progressive accretion or erosion sufficient for the water discharge. As a result, the channel within the season. Shorter erosion or accretion events oppo bed would be eroded. This erosion is inevitable in view of the site to the general tendency might occur during the season. mutual adaptation between morphology and hydrodynamics. For example, an erosion of 23 em was found in the period A similar mechanism has been found in beach environments. from the middle of May to early June within the summer of After the storm, a swell profile (which represents normal 1988 when the whole season had a net accretion of 12 cm. weather conditions) would be developed from the storm-in The wind condition was normal (wind velocity was less than duced profile (KOMAR, 1976). Lacking surveyed data on the Scale 6, with gusts of Scale 6 occasionally) through the month channel bed before the storm (between February and May, before each of the surveys shown in Figures 8 to 11, except no survey was conducted) resulted in direct research of its for when a gale occurred on August 4 in 1989 (the wind ve influence not being able to be undertaken. locity of which was from Scale 7 to Scale 9). The bed level changes illustrated in Figures 8 to 11 seem mainly a function of river discharge.
5 ~ 5 0 ~ .~ 0 +-' 6 .~ Q) +-' ~ Q) 6 0 ~ Q) 0 ...c:: 7 Q) +-,~ ...c:: .+oJ 7 Q) Q) ...c:: c Q) +-' (\l 8 ...c:: ~ ~ .+oJ 8 o 0.. ~ ~ 0 -----Aug. 8,1989 Q) -----Feb. 15-16, 1989 ~ ..0 9 Q) 9 .....Nov. 7-23, 1989 ...c:: .....May 9-13, 1989 ..0 +-' ...c:: 0.. .+oJ Q) 0.. --0 10 Q) 10 0 5 10 15 20 25 30 35 --0 0 5 10 15 20 25 30 35 downstream distance (km) downstream distance(km)
Figure 8. Changes in the maximum depth during the spring season of Figure 10. Changes in the maximum depth during the autumn season 1989 (average accretion of the whole profile was 13 em; maximum accre of 1989 (average erosion of the whole profile was 12 em; maximum erosion tion was 50 ern). was 70 em).
Journal of Coastal Research, Vol. 17, No.2, 2001 302 Yang, et al.
o 5 .~ 5 +J :§ (l) ~ (l) 0 ~ (l) 6 (l) 6 +J C ...c ro +-'~ ~ ~ E o 0. (l) "--/ 7 ~ 7 (l)~ ...c +J (l) ,.D ro C o ~ ~ .~ ro 8 o ~ 8 +J +J ~ 0. 0. (l) -----Nov. 7-23, 1989 (l) ~ (l) ,.D -----Feb. 15-16, 1989 ~ 0 (l) 9~ .....Feb.5-10, 1990 ...c 9 ~'W ~ +J .....Aug.8, 1989 +J 0. (l) 10 ~ 10 0 5 10 15 20 25 30 35 0 5 10 15 20 2,5 30 35 down-stream distance (km) downstream distance (km)
Figure 11. Changes in the maximum depth during the winter season Figure 12. Dry season-flood season changes in the maximum depth from from 1989 to 1990 (average erosion of the whole profile was 49 em; max February to August 1989 (average accretion of the whole profile was 43 imum erosion was 80 em). em; maximum accretion was 90 em).
Flood Season vs Dry Season Between-Years Change Of the changes in the bed level of the South Passage, those The between-years difference (between a certain month between dry season and flood season are most cyclic and no and the corresponding month of the next year) in bed level is ticeable. This is undoubtedly related to the river discharge. changeful, with the beginning and the end in different sea As seen in Figure 2, the minimum and maximum river water sons. Generally, it is larger between autumns and summers discharges occur respectively in January and July, and the than between winters and springs (Table 3). The randomness minimum and maximum sediment discharges occur in Jan of storm and river flood peaks in the seasons of summer and uary-February and July. Differences in the bed level between autumn seems to be the main causes for this. For example, flood season and dry season are listed in Table 2. No severe the maximum difference of 60.2 em in Table 3 is related to a storm had happened during at least a month before each of severe storm blown through the river mouth before the Sep the surveys. The difference is typically in the order of 20 em tember survey of 1990, about which discussion in detail will between January-February and July (No.1, 2, 3, 4, 5), 40 to be given below. On the other hand, the 35.9 em accretion, 60 em between February-March and August (No.6, 7, 8, 9, from comparison of the June survey in 1989 with the June 10), and 30 em between March and September and between survey in 1988, seems related to the significantly higher river April and October (No. 11, 12, 13, 14). It is interesting to note discharge during the period of spring and early summer in that the largest difference occurs between February-March 1989. The river water discharge in this period was 16% more and August rather than between January-February and July than that in the same period in 1988 and 11% more than the (when the lowest and highest river discharge at the Datong multiple-year average water discharge of the corresponding Station occurred). This lag in response reflects the influence period. In contrast with this, the 18.6 em erosion from com of the 600-km distance from the hydrological station to the parison of the February survey in 1990 with the February river mouth. Based on the flow velocity, YANG (1999a) gave survey in 1989 might be associated with a much higher water a rough estimate of about a month for the water (as well as discharge in the winter of 1989. The water discharge in this the suspended sediment) at the Datong Station to reach the winter was 28% greater than the previous winter and 26% river mouth. The time lag implicated in Table 2 approxi greater than the multiple-year average of the corresponding mately confirms this deduction. Figures 12 and 13 are two period. Based on multiple-year records at the Datong Station, representative examples of vertical shifts in the along-chan the riverine suspended sediment concentration in winter was nel bed profile between the dry and flood seasons. only 0.19 gil on the average which is only 34% of the yearly
Table 2. Comparison of average changes (em) in the bed level between surveys in dry season and flood season without influence of severe storms ("+" represents accretion and <:» represents erosion)
No. Period Change Season No. Period Change Season 1 Jul. 21-22, 1988 vs Jan. 6-18, 1988 +21 W 8 Aug. 8, 1989 vs Feb. 15-16, 1989 +43 W 2 Jan. 6-10, 1989 vs Jul. 21-22, 1988 -21 D 9 Feb. 5-10, 1990 vs Aug. 8, 1989 -61 D 3 Jul. 15-24, 1989 vs Jan. 6-10, 1989 +9 W 10 Aug. 8, 1989 vs Mar. 2-3, 1989 +47 W 4 Feb. 15-16, 1989 vs Jul. 21-22, 1988 -24 D 11 Sep. 22-23, 1989 vs Mar. 2-3, 1989 +29 W 5 Jul. 15-24, 1989 vs Feb. 15-16, 1989 +25 W 12 Apr. 14-25, 1989 vs Oct. 10-11, 1988 -31 D 6 Feb. 5-10, 1990 vs Jul. 15-24, 1989 -44 D 13 Oct. 5-6, 1989 vs Apr. 14-25, 1989 +32 W 7 Feb. 15-16, 1989 vs Aug. 5-Sep. 8, 1988 -23 D 14 Mar. 2-3, 1989 vs Aug. 5-Sep. 9, 1988 -28 D W means from dry season to wet season and D means from wet season to dry season. Wet season is from May through October and dry season from November through April.
Journal of Coastal Research, Vol. 17, No.2, 2001 Yangtze River Mouth China 303
o 5 .~ +oJ Q) So-i o 6 Q) ..c:: +oJ ,---.... S Q) '--" 7 ..c:: +oJ Q) ~ ~ co o ~ 8 ~ 0.- Q) --Aug. ..0 8, 1989 ..c:: 9 .....Feb.5-10, 1990 +oJ 0. Q) '""d 10 o 5 10 15 20 25 30 35 down-stream distance (km)
Figure 13. Flood season-dry season changes in the maximum depth from August 1989 to February 1990 (average erosion of the whole profile was 61 em; maximum erosion was 130 em).
averaged value and 23% of the flood season value (from May through October). It seems that increase of water discharge in winter will result in erosion on the channel bed because of the low suspended sediment concentration, although an op posite tendency exists in the rest of the year due to higher suspended sediment concentration.
Response of Channel Bed Level to River Discharge and Storms Response to River Discharge A comparison of maximum channel depth and river water discharge reveals a linear correlation. The correlative coeffi cients between them are listed in Table 4. A few points of information from this table will be discussed: (a) The average of the maximum depths is negatively cor related to the water discharge. Without taking into account the relationship between water discharge and sediment dis charge (and the relationship between water discharge and suspended sediment concentration), one might reach a wrong conclusion that an increase in water discharge will result in erosion on the channel bed. In fact, based on the multiple year data at the Datong Station, a very good positive corre lation exists between the sediment discharge and the water discharge as well as between the sse and the water dis charge (Figure 14 and Figure 15), where R2 values are 0.9908 and 0.9708, respectively. So, as for the influence of river dis charge, the profound cause for seasonal changes in the bed level of the channel is likely to be the variation in sediment discharge and/or in riverine suspended sediment concentra tion, rather than variations in the water discharge. In other words, increase in sediment discharge and/or suspended sed iment concentration results in accretion on the channel bed, and vice versa. Because data on riverine sediment discharge is unavailable for the period of bathymetric surveys, water discharge is utilized as substitute in this study. (b) The lag in response of the South Passage bed level when compared to the change in the Datong Station river discharge is about 1 to 2 months, because the absolute values of the correlative coefficients are typically larger in the cases of Dl'
Journal of Coastal Research, Vol. 17, No.2, 2001 304 Yang, et al.
Table 4. Correlative coefficient of the linear regressive function between the average of the maximum depth along the South Passage and water discharges at the Datong Station, showing the lag response of bed level to river discharge because of the 600-km distance between the station and the river mouth.
Water Discharge Correlative Coefficient (r) in Different Section** Statistic at the Datong Number of No. Station* Whole Profile Deep Section Shallow Section Sample (n) Period of Surveys 1 Do -0.2973 -0.3667 -0.2294 23 Jan. 1988 to Dec. 1990 2 Do. 5 -0.523 -0.4929 -0.505 23 Jan. 1988 to Dec. 1990
3 D1 -0.6426 -0.5703 -0.6408 23 Jan. 1988 to Dec. 1990
4 D1 , -0.6644 -0.6036 -0.6545 23 Jan. 1988 to Dec. 1990
5 D1. 5 -0.7411 -0.6418 -0.7428 23 Jan. 1988 to Dec. 1990 6 Dz -0.6727 -0.4975 -0.7285 23 Jan. 1988 to Dec. 1990 7 Do -0.527 -0.6807 -0.3651 13 Dec. 1988 to Jan. 1990 8 Do. 5 -0.7562 -0.7111 -0.6371 13 Dec. 1988 to Jan. 1990
9 D1 -0.8963 -0.7163 -0.8093 13 Dec. 1988 to Jan. 1990
10 D1 , -0.8656 -0.714 -0.7719 13 Dec. 1988 to Jan. 1990
11 D1. 5 -0.8291 -0.5569 -0.7937 13 Dec. 1988 to Jan. 1990 12 Dz -0.7568 -0.3693 -0.7838 13 Dec. 1988 to Jan. 1990 * As shown in the paragraph of "Methods and materials": Do is the average riverine discharge of the exact month in which the bed level was surveyed,
Do. 5 is the distributive riverine discharge (from the distributive curve of the corresponding annual discharge) 10 to 20 days earlier than the survey of bed level (the number of days depended on the primary amount of discharge, i.e. the larger the discharge, the less the days), D 1 is the average riverine discharge of the month just earlier than the month in which bed level was surveyed, D1, is the distributive riverine discharge one month earlier than the bed level survey; D1. 5 is the distributive riverine discharge 1.5 months earlier than the bed level survey, and D, is the distributive riverine discharge two months earlier than the bed level survey. ** The "whole section" means the whole along-channel profile (0 to 36.5 km), "deep section" means from 0 to 9.5 km and "shallow section" means from 10 to 36.5 km.
D1" D1.5' and D2 • This coincides with the conclusion that the sponse of the shallow section of this channel bed. The abso river mouth accretion peak lags the river flood peak at the lute value of the correlative coefficient is larger in the former
Datong Station because of the 600 km distance. section than in the latter section in the cases of Do and DO.5. (c) The absolute coefficient for the period from December Conversely it is less in the former section than in the latter
1988 to January 1990 (n = 13) is generally larger than that section in the cases of D1 , D1" D1.5 and D2 ; on the whole, the from January 1988 to December 1990 (n=23). This may be a absolute value in the former section is less than in the latter function of two effects: the inequality in the number of sta section. It seems that sediment dynamics is different in these tistic sample and the interference of a severe storm which two sections, which may be related to spatial changes in geo occurred just before the survey in September 1990. According morphology, hydrodynamics and the chemical environment. to statistic knowledge: with equal correlative coefficients, the The correlation corresponding to the best coefficient in Ta greater the number of samples, the better the correlativity. ble 4 is given in Figure 16. Based on this formula, mean and The severe storm that occurred before the survey in Septem extreme values of annual change in average maximum depth ber 1990 resulted in deposition on the channel bed of the of the South Passage bed are estimated and listed in Table South Passage, and will be discussed below. 5. These values forecast the impact of the river water dis (d) It may be of importance to note that some differences charge on the along-channel average of bed level, without exist between the response of the deep section and the re-
c 0 0.9 o 5 ',.-i o +-l o t\l 0.8 o ~ y = 0. 0991xl. 2552 ~ 2 +-l 4 y = O. 0991x . 2552 c O. 7 2 Q) R = 0.9708 Q) 2 o eo R = 0.9908 ~ O. 6 0 ~ ~ 3 o ..c U) C,) <, +-l--- 0.5 U) tl.O ~~ :0 ~ 2 ~~ O. 4 ',.-i +-> '"0 c Q) O. 3 Q) U) E ',.-i '"0 O. 2 '"'0 Q) Q) '"0 U) ~ O. 1 o Q) 0- U) o 234 5 6 ;:1 U) ° 2 6 water discharge (10000 m3/s) 4 ° water discharge(* 10,000m3/s) Figure 14. Statistic relationship between the multiple-year monthly wa ter and sediment discharges of the Yangtze River at the Datong Hydro Figure 15. Plot of suspended sediment concentration versus water dis logical Station. charge at the Datong Station (based on multiple-year data).
Journal of Coastal Research, Vol. 17, No.2, 2001 Yangtze River Mouth China 305
Table 5. Estimates ofmean and extreme values ofseasonal change in the average maximum depth (m) of the South Passage bed based on the cor 60000. 00 00 relative formula in Figure 15. =0 ~ 50000. 00 0 Mean Flood Month Maximum Flood Minimum Flood (I) ,...--...... c: U) ...... " 40000. 00 Discharge Month Discharge Month Discharge ~~ • vs vs vs (1)"--" 30000. 00 Mean Dry Month Minimum Dry Maximum Dry 00~ =0 ~ ..... Discharge Month Discharge Month Discharge -5 ~ 20000. 00 .~ C/) "0 Y -61539x + 443818 Flood month 6.859 6.432 7.047 ~ (I) 10000.00 2 Dry month 7.340 7.385 7.249 ~ R = 0.7323 ~ Difference 0.481 0.953 0.202 o. 00 The "flood month" ("dry month") means the month with the highest water 6. 10 6. 30 6. 50 6. 70 6.90 7.10 discharge (lowest water discharge) in the monthly water discharge dis average ofthe maximum depth (m) tribution. The "maximum flood month" ("minimum flood month") means the month with highest (lowest) historical water discharge in the "flood Figure 16. Correlation between the average of the maximum depth and month". The "maximum dry month" ("minimum dry month") means the water discharge at the Datong Station (the bathymetric surveys were month with highest (lowest) historical water discharge in the "dry conducted between December 1988 and January 1990; the water dis month". The corresponding monthly water discharges used in the com charge used was a month earlier than the corresponding survey). putation is from GSICI (1996).
there would be a lower bed level (higher water depth) in the taking into account the influence of storms. In view of the September survey in 1990 than of the September survey of fact that the "maximum monthly discharge in flood season" 1989. This is due to the positive correlation between the an and the "minimum monthly discharge in dry season" did not nual channel bed level and water discharge (a negative cor occur in the same year in history, (nor the "maximum month relation between the channel maximum depth and water dis ly discharge in dry season" and the "minimum monthly dis charge). charge in flood season"), the difference of 0.953 m in the table would be larger than what really occurs (and the difference Combined Influence of River Discharge and Storm of 0.202 m would be less than any real annual change). Nev It is difficult to fully distinguish the influence of river dis ertheless, Table 5 suggests a useful range of 0.2 to 1.0 m and charge from that of storm because they are often superposed an average of 0.5 m for the annual change in average maxi one another. It is of greater importance to examine the mag mum depth of the South passage. The 40 to 70 em of annual nitude of bed level change under the combination of these two change observed in 1964 and 1968 on the bar of this channel factors. The comparison of the Sept. 1990 profile with the (SHEN et al., 1983) falls within this range. Feb. 1990 profile in Figure 17 gives an example of the com bined influence of a normal river discharge and a ten year Response of Channel Bed to Coastal Storm storm event. As shown above, a 10 to 12 scale storm hit the Comparison of the profile of Sept. 1990 with the profile of study area before the September survey in 1990. The average Sept. 1989 plotted in Figure 17 gives rise to an average dif difference between the two profiles reaches 103 em which is ference of 60 em and a maximum difference of 155 cm. These differences most likely reflect the impacts of a series of storms on deposition of the channel bed. Actually, Typhoon 5 No. 15, with wind velocity scale of 10 to 12, and Typhoon No. :§ 12, with wind velocity scale of 8 to 10, hit the Yangtze Es U 'M +-' 6 tuary around August 31 and August 20, 1990 respectively. In Q) ~ 0 contrast to these typhoon events, no storm with wind velocity Q) ...c higher than Scale 7 occurred from February through Septem +-' 7 Q) ber 1989. It is reasonable to attribute the main accretion in ...c +-' the period from February to September 1990 to Typhoon No. ~ 8 0 - - - - - Feb. 15-16, 1989 15, because this storm was much more intense and more ~ Q) III Sept. 22-23, 1989 closed (on time) to the September survey than Typhoon No. .-0 - ...c 9 ------Feb. 5-10,1990 +-' 12. It is possible that the real impact of these storms on ac 0.. Q) ...... Sept. 11-15, 1990 cretion might be larger than reflected in Figure 17 in view of '"0 10 the following causes: (a) The September survey was made 2 0 5 10 15 20 25 30 35 to 3 weeks after the typhoons. During the period between the down-stream distance(km) survey and the typhoons, the channel bed might have expe rienced erosion. (b) The total amount of water discharge from Figure 17. Comparison of bed level changes between two alike terms February through September was 50/0 less in 1990 than in from February to September, mainly showing the influence of Typhoon 1989, and that from July through September was 11% less No. 15 in 1990, average accretion and maximum accretion is 60 em and 155 em more in profile Sept. 1990 than in profile Sept. 1989). in 1990 than in 1989. If the typhoons did not hit the estuary,
Journal of Coastal Research, Vol. 17, No.2, 2001 306 Yang, et al.
(1) :§ 0 s:: 2 ~ 0 1 i--; ...... 90.2.5-10 0- Q) ...... Feb. 5-10, 1990~ N 2 - --- -90.9.11-15 ~ 3 '"0 o ---- -Sept. 11-15, 1990 I Q) 3 ------90.11.6-15 '~ E +..J 4 ------Nov. 6-15, 1990 ! ;:::j (1) 4 ~ OJ 0 OJ (1) cd 5 ...c /'""".. 5 s:: +..J~ cd 6 (1) ~ ...c 6 7 +..J r-S Q) ~ 8 0 .-0 r-! 7 (1) ~ 9 +-' ..0 ~ ...c 8 10 +..J '"0 0- 0 2 3 4 (1) 0 2 3 4 5 6 7 5 '"0 croos-channel distance from jiuduansha distance from the northeastern end (km) side (km)
Figure 18. Temporal shift of cross-channel profile, Transect No.9 (4 km Figure 19. Temporal shift of cross-channel profile, Transect No. 26 (12.5 from the upstream end). km from the upstream end).
approximately the sum of the 48 em estimate for the bed level bed level of the maximum depth line is estimated to reach change under normal flood season-dry season river discharge the order of 1.0 m. It is probable that accretion in flood season and 60 em (from the storm impact). It is probable that, under will exceed 1.5 m in the case of co-occurrence of extreme high extreme conditions, the combined influence of annual change river discharge and extreme severe storms. in river discharge and severe storm would amount to 150 em (b) The response of river mouth channel-bed level lags in difference of the average maximum depth. changes in river discharge at the Datong Station by 1 to 2 months, a function of the 600 km distance between the hy Spatial Difference in Response of the Bed Level drological station and the river mouth. (c) In view of the positive correlation between river water Spatially, most of the erosion and accretion events on the discharge and riverine sediment discharge, bed level re channel bed were similar, especially when the average sponse to river water discharge is likely due primarily to the change exceeded 10 em (Figures 7-13, 17, 18-20). Neverthe contribution of riverine sediment discharge. Without this pos less, some differences existed between the seaward end and itive correlation, the relationship between bed level and wa the main part of the along-channel maximum depth profile ter discharge would be reversed. (Figures 9, 10, 12, 13, 17). The seaward end of the along (d) Most of the erosion and accretion events show spatial channel profile seems less sensitive to the change in river unanimity in the studied passage. So the results obtained for discharge. Across the channel, there seems a critical depth the maximum depth profile is characteristic of the main on the slope of the Jiuduansha side. Erosion and accretion channel part in great extent. tends to be opposite above and below this critical depth. The critical depth is about 5 to 6 m below the theoretical plane (Figures 19-20). Theoretically, a critical depth would exist in view of the sediment exchange between the channel and the 0 ~ shallow beds (SHENet al., 1983; YUN 1983). Ifa critical depth o '~ exists on the southwestern slope of the channel, it would be +..J ~ -Peb. .5-10,1990 (1) ~ 2 - - - - -Sept. 11-15, 1990 shallower than 4 m isobath because the characteristics of ero 0 (1) sion and accretion are similar below this depth (Figures 18 ...c ------Nov. 6-15,1990 +..JE' 20: the right side of the figures represents the southwestern ~ ~ 4 bank slope of this channel). In addition, the deep part of the +..J co ~ r-! o 0- channel seems to collect a great volume of sediment during r-! (1) a storm-induced accretion event (Figures 17-20). ..0 6 ...c +..J 0- (1) SUMMARY AND CONCLUSIONS '"0 (a) The channel bed level of the South Passage is sensitive 8 to changes in hydrodynamics and sediment supply. The dis 0 1 2 3 4 .5 6 7 8 charge related seasonal change in bed level tends to be sev distance from the northeastern end eral decimeters. A ten year storm event was found to have (km) resulted in an average deposition of 60 em along the maxi mum depth profile. For conditions of normal river discharge Figure 20. Temporal shift of cross-channel profile, Transect No. 45 (22 km from the upstream end). and severe storm, the combined annual difference in average
Journal of Coastal Research, Vol. 17, No.2, 2001 Yangtze River Mouth China 307
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