Changes in Sedimentation Rate of Tidal Flat and Environmental Significance at Qidong Foreland of the Yangtze Estuary

Yunfeng ZHANG1*, Zhenke ZHANG2, Hang REN2, Yingying CHEN

1. Research Institute of Jiangsu Coastal Development, Yancheng Teachers University, Yancheng 224007, China; 2. School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210023, China; 3. School of Geography, Geomatics and Planning, Jiangsu Normal University, Xuzhou 221116, China

AbstractThe sedimentation rate is an important environmental parameter to understand the evolution of coastal geomorphology. The tidal flat around Qidong foreland is located in the junction between Yangtze Estuary and Jiangsu coast, where the land-ocean interactions are strong and highly sensitive to environmental changes. The QDZ-1 core sediments collected at Qidong foreland was analyzed for 137Cs dating and grain size. The results showed that silt is the main sedimentary type with a trend of gradually fining from the bottom to the top, conforming to the sedimentation characteristic of the silt muddy tidal flat. The sedimentation rate could be divided into three distinct stages: 1963 to 1986 was 2.61 cm/yr, 1963 to 2011 was 1.82 cm/yr, and 1986 to 2011 was 1.10 cm/yr. Based on these, further explanation was made for the significance of environmental changes. According to the estimation of sedimentation rate, the impact of extreme environmental change was reflected by the sudden increase in sand proportion at the depth of 172.5 cm. Since the introduction of Spartina alterniflora in the 1970s, it has played an important role in tidal flat development. The tidal flat has a high sedimentary rate during this time. With continuous accretion of the tidal flat, low tidal flat turns to high tidal flat, the sedimentation rate gradually declines, and tidal flat sedimentation appears.

Key wordsThe Yangtze Estuary, Tidal flat at Qidong Foreland, Sedimentation rate, 137Cs dating, Environmental significance

1 Introduction

The sedimentation rate is an important indicator for study of the sedimentary environment. Long-scale sedimentation rate can reflect the evolution of geological history, while short-scale sedimentation rate can reflect dynamic conditions and material transport. The sedimentation rate of silt muddy tidal flat represents the sedimentation thickness of sediments per unit time. It is an important environmental parameter for the changes in tidal flat landforms, can comprehensively reflect the "silting-erosion" process and is a quantitative environmental indicator for determining tidal flat sedimentation and erosion[1]. There are many methods for studying the sedimentation rate of coastal environment, such as the comparison of chart at different periods, GIS spatial information method, river sediment discharge method, and radioisotope dating method[2]. The principle of the radionuclide dating method is to determine the age of the sediment based on the decay law of the nuclide and the time period corresponding to the half-life of the nuclide. Since Krishnaswamyetal. proposed the radionuclide137Cs time-scale dating method[3], the137Cs dating has been widely applied in studying the sedimentation of lakes, rivers and oceans. At present, the radionuclide137Cs time-scale dating method has become an important method for the reconstruction of modern sedimentation processes in the estuary and offshore waters[4]. According to the distribution characteristics of137Cs in tidal flat sediments in northern Jiangsu, Liu Zhiyongetal. analyzed the changes in sedimentation rate and environmental significance of the Xinyanggang estuary[5]. Using high-resolution137Cs sedimentation records, Wu Xiaoetal. analyzed the change of the sea-entering channel near Yellow River estuary and achieved excellent results in the cross check with the210Pb method[6]. Based on the137Cs dating method, Bing Zhiwu carried out a quantitative analysis of the changes in the sedimentation rate of the main estuary area of the coastal zone of Liaoning Province in the past 100 years[7].

Qidong foreland tidal flat of the Yangtze River estuary is under the administrative management of Yinyang Town, Qidong City, Jiangsu Province, and geographically belongs to the alluvial plain of the north estuary of the Yangtze River estuary. Due to the special geographical conditions, the interaction between the ocean and the river is strong, the intertidal zone is wide, and the sediment source is rich, Qidong foreland tidal flat is a typical silt muddy tidal flat landform, and it is the core area of coastal wetland provincial nature reserve of the north branch of the Yangtze River. Since the construction of coast defended road in the 1950s, human reclamation activities in this area have continued to increase, and significant changes have taken place in the tidal flat sedimentary environment[8-9]. Qidong foreland tidal flat of the Yangtze River estuary is gentle in flat surface, and mainly consisted ofS.alterniflorasalt marshland and silt muddy flat.S.alterniflorasalt marshland extends from Evergrande Group reclamation area to the southwest, and to the Yangtze River Dike Monument.S.alterniflorahas strong environmental adaptability and reproductive proliferation. After being introduced and promoted by humans, it has quickly become a dominant species of high tidal flats[10], forming a vast distribution ofS.alterniflorasalt marshland, and effectively playing an important role in protection of the coast and dike, and promotion of siltation. The outer side of theS.alterniflorasalt marshland is the intertidal sedimentary material of the silt muddy flat, and the erosion escarp widely exists between theS.alterniflorasalt marshland and the silt muddy flat. Therefore, using the radionuclide137Cs dating method to analyze the modern sedimentation rate of the tidal flat near the Qidong foreland of the Yangtze River estuary is helpful to understand the change laws of the tidal flat environment, and has important environmental indication significance; and it can provide decision-making reference for the development and utilization of tidal flat resources, and is favorable for protecting the tidal flat environment.

2 Sample collection and study method

2.1 Sample collectionThe tidal flat at the Qidong foreland of the Yangtze River estuary is one of the most active areas of modern human activities in the form of tidal flat reclamation in the north branch of the Yangtze River. In recent years, the tidal flat reclamation activities have been constantly strengthened. In the 1950s, the local government built coast defended road to improve regional traffic and resist storm surges. After entering the new century, further reclamation development was undertaken. In 2006, Guangzhou Evergrande Real Estate Group Co., Ltd. built a high-grade reclamation dike in the tidal flat near Qidong foreland. On the outside of coast defended road and inside of the Evergrande reclamation dike, there are three dikes built in 1970, 1989 and 1992 respectively. In August 2011, we carried out a three-day field survey at Qidong foreland tidal flat. In order to ensure that the sediment samples can truly reflect the changes of the tidal flat sedimentary environment, we collected sediment samples fromS.alterniflorasalt marshland in the Yuantuojiao Wetland Protection Area, which is not directly disturbed by human activities. Using a gravity sampler (Fig.1), we collected the core sediment QDZ-1 with the length of 215 cm. The sampler is a semi-circular portable hand drill made by Eijkelkamp (Netherlands). It can well avoid compression and deformation of core sediments. After the samples were collected, we gently removed the surface debris with a scraper, and undertook the characteristic description, photographing, and sample division for the sediment core. In order to prevent the samples from being damaged and contaminated, we divided samples into 43 samples at the interval of 5 cm. Immediately after the division, we placed them into a polyethylene sealed bag, and finally placed in a freezer, sealed and took back to the laboratory for experiment.

Fig.1 Study area and sample site

2.2 Experimental analysis

2.2.1Sediment grain size analysis. The grain size of the sediment samples was measured by laser grain size analyzer, taking 0.25 Φ as the grain size grading distance. The tester was a Mastersizer 2 000 laser grain size analyzer manufactured by Malvern (UK). The test range was 0.02-2 000 μm, and the repeatability error is less than 3%. The collected 215 cm long core sediment QDZ-1 was divided on site, and 43 samples were obtained at intervals of 5 cm. The sample test was undertaken at Key Laboratory of Coast and Island Development in Nanjing University (Ministry of Education). The basic procedure for grain size analysis and testing of sediment samples is as follows[11]. (i) Sampling: the uniformly mixed samples were sampled and placed in a 100-mL beaker for treatment. Then, took 0.1-0.2 g of sediments dominated by clayey silt or silty clay, 0.3-0.4 g of sediments dominated by fine sand or silt, and 0.5-0.6 g of sediments dominated by medium coarse sand. (ii) Removal of organic matter: added hydrogen peroxide (H2O2) solution with 10% concentration to a small beaker and placed for 24 h till more bubbles. If there were still bubbles, added an appropriate volume of hydrogen peroxide until there was no reaction at all, removed the supernatant with a pipette. (iii) Removal of calcium cement and living organism shell: added 10% hydrochloric acid to a small beaker and placed for 24 h till no more bubbles. It was better when there was no debris shell, and removed the supernatant with a pipette. (iv) Added 0.5 mol/L sodium hexametaphosphate (NaPO3)6to a small beaker to thoroughly disperse the sample, and placed for 24 h. (v) Test: to remove the residue in the instrument, washed it with distilled water for 3-5 times; after pouring 800 mL of dispersant (distilled water) into a large beaker, turned on the instrument to test the background value; added the pre-treated samples to the large beaker, oscillated 30 s with ultrasonic wave to form uniform suspension; when the shading was kept within the test range (10%-20%), the samples were tested. After the test was completed, the instrument would automatically output and save the data of different grain size of samples.

2.2.2Sediment dating analysis. The dating analysis of the sediment samples adopted the radionuclide137Cs time-scale dating method. The tester adopted GMX30P-A high-purity Ge coaxial detector manufactured by ORTEC (USA), with a detection energy range of 3-10 000 keV. The137Cs specific activity of standard source sample was 0.737 Bq/g, supplied by Bedford Institute of Oceanography (Canada), and the reference time was September 1, 2009. The sample test was undertaken at Key Laboratory of Coast and Island Development in Nanjing University (Ministry of Education). The basic procedure for sediment sample treatment and testing was as follows[12-13]. (i) Sampling: First, the vacuum freeze-dried sediment samples were ground to powder form and stirred uniformly with an agate mortar; then, about 40 g of the samples were weighed by an electronic balance with precision of 0.000 1 g, placed in a special cup and shook uniformly. (ii) Testing: The samples were placed in the lead chamber, and the intensity of the sample137Cs was directly measured by the detector, and the measurement time was generally about 72 000 s. The whole test process was controlled by Gamma Vision spectrum analysis software specially designed for the instrument. After the completion of the measurement, the137Cs specific activity value of of the sample was calculated according to the peak area of energy at 661.62 keV of γ-ray, expressed in Bq/kg.

2.2.3Calculation of the sedimentation rate. According to the depth position of the accumulation peak of the radionuclide137Cs, the dating time scale could be determined, and combined with the sedimentation thickness, the average sedimentation rate could be estimated. According to the area data of137Cs energy peak at 661.62 keV, the137Cs specific activity could be calculated by the relative method[14]using the following formula:

(1)

whereQ0denotes the specific activity of the radionuclide137Cs of the standard source sample, expressed in Bq/kg;A0denotes the count area of the standard source sample, expressed in NA;m0denotes the mass of the standard source sample, expressed in g;t0denotes the count time of the standard source sample, expressed in s.Qxdenotes the specific activity of the radionuclide137Cs of the sediment sample to be tested, expressed in Bq/kg;Axdenotes the count area of the sediment sample to be tested, expressed in NA;mxdenotes the mass of the sediment sample to be tested, expressed in g;txdenotes the count time of the sediment sample to be tested, expressed in s. Then, according to the relative thickness of the accumulation peak position of the radioactive specific activity on the137Cs sedimentary profile, combined with the determined possible time scale and sampling date, the average sedimentation rate of one stage could be calculated[13]using the following formula:

V1=d1/(t0-1963)

(2)

V2=d2/(t0-1986)

(3)

whereV1andV2denote the average sedimentation rate of sediments in different time periods, expressed in cm/yr;d1andd2denote the depth corresponding to the accumulation peak of radionuclide137Cs, expressed in cm, andt0denotes the sampling time of sediments.

3 Result analysis

3.1 Sedimentation characteristics and grain size changesThe core sediment QDZ-1 had a length of 215 cm, and the rhythmic bedding was significant on the whole. The upper plant roots were dense, and the middle and lower parts contained shell debris enriched layer. From the top to the bottom, it can be divided into the following layers: 0-30 cm was a dark brownS.alternifloraroot layer; 30-50 cm was a gray root-containing layer; 50-70 cm was a brown root-containing layer, and the plant root system was relatively small; 70-100 cm was a gray sandy silt containing shell debris; 100-120 cm was a blue-gray silt; 120-215 cm was a dark brown mud layer with a blue-gray layer in the middle, containing shell debris. The appearance of multiple shell debris sedimentation layers may be strongly influenced by extreme weather conditions. The analysis results of sediment grain size were shown in Fig.2. According to Fig.2, the core sediment QDZ-1 was mainly silt, and from the bottom to the surface layer, it can be divided into three sections: in the upper section (0-100 cm), the sediment was silt mingled with different thickness of clayey silt; in the middle section (100-125 cm), the sediment was silt and sandy silt; in the lower section (125-215 cm), the sediment was silt mingled with different thickness of sandy silt. The sediment composition had the highest content of silt, the average value was 73.3%, the distribution range was 59.6%-81.0%, followed by the sand content with the average value of 13.8% and the distribution range of 2.8%-35.7%, the lowest was the clay content with the average value of 12.9% and the distribution range of 4.7%-23.2%. The variation curves of silt and clay content were consistent, both increasing from the bottom layer to the surface layer, increasing from 68.9% and 6.6% to 75.4% and 20.3% respectively. However, the variation of sand content was opposite, showing a decreasing trend, from 24.5% at the bottom layer to 4.3% at the surface layer.

Fig.2 Curves for composition and grain size of core QDZ-1 sediments

The average grain size of the sediment was in the range of 4.600-6.833 Φ with the average value of 5.733 Φ; the median grain size was in the range of 4.296-6.800 Φ with the average value of 5.386 Φ. The average grain size and the median grain size curves were extremely similar, both showed a trend of increasing from small at the bottom to large at the top. In other words, sediment grain size gradually became smaller from the bottom to the top. The sorting coefficient of the sediments was in the range of 1.413-1.998 with an average of 1.726, so the sorting was poor. The skewness was in the range of -0.316-2.176 with the average of 0.862, which was positive skewness; the kurtosis was in the range of 2.639-8.428 with the average of 3.847, and it was wide-very wide.

It should be noted that at a depth of 172.5 cm, the sand content suddenly increased to 35.7%, and correspondingly, the average grain size and the median grain size also showed consistent changes, reaching a maximum value of 4.600 Φ and 4.196 Φ, indicating that hydrodynamic condition suddenly increased, leading to coarser sediment grain. The sorting coefficient reflects the uniformity of the grain size of the sediment. At this time, the sorting coefficient reached the extreme value of 1.413. From the classification and qualitative description of the grain size parameters[15-16], it can be seen that the sorting was better, indicating that the grain size with coarse grain as the main parts was more prominent.

3.2 Sediment137Cs time scale and sedimentation rateThe changes of the specific activity of radionuclide137Cs with the depth were shown in Fig.3. From Fig.3, it can be seen that there were two obvious accumulation peaks. The accumulation peak of radionuclide137Cs was the largest at the depth of 87.5 cm, and the specific activity was 3.78 Bq/kg, followed by the accumulation peak at a depth of 27.5 cm with the specific activity of 4.06 Bq/kg. According to the decay period of radionuclide137Cs (30.2 years) and the sedimentation law in the northern hemisphere, it can be known that the accumulation peak at a depth of 87.5 cm represents the 1963 time scale, and the accumulation peak at a depth of 27.5 cm represents the 1986 time scale.

Fig.3 The profile curve of137Cs specific activities of QDZ-1 core sediments

On a global scale, the atmospheric sedimentation of radionuclide137Cs began in the early 1950s. Theoretically, the earliest time scale that can be detected in sediment samples from the northern hemisphere is 1954. However, it is actually difficult to identify due to long-term decay. The atmospheric deposition of radionuclide137Cs was the largest in 1963, so the maximum accumulation peak should represent the 1963 time scale. This is an internationally recognized dating mark[17-18]. In 1986, a serious nuclear accident occurred at the Chernobyl nuclear power plant in the former Soviet Union, which had an important impact on sediment samples in the northern hemisphere, and thus 1986 also had a time-scale significance[19]. However, the137Cs leaked in the Chernobyl nuclear accident exerted a greater impact on the settlement in Europe but had little effect on the settlement in East Asia[20]. The study area Qidong foreland tidal flat is located in the Yangtze River estuary, belongs to a typical monsoon climate zone. The terrain is mainly plain, the climate is humid, and the rainfall is abundant. Therefore, there appeared an obvious accumulation peak in 1963. Apart from the atmospheric sedimentation, the reason for the accumulation of peak in 1986 may be connected with many factors such as river input, offshore sand transport, and no high terrain blocking in the north.

According to the137Cs time scale indicated by the core sediment QDZ-1 profile and the sample collection time (field survey and sediment samples collected in August 2011), using the above formula, we calculated the annual average sedimentation rate of the Qidong foreland tidal flat in the Yangtze River estuary. The annual sedimentation rate was 2.61 cm from 1963 to 1986, and 1.82 cm from 1963 to the present, and 1.10 cm from 1986 to the present. Since the 1960s, the sedimentation of the Qidong foreland tidal flat has experienced a process from fast to slow. With the gradual increase of the height of the tidal flat, from the low tidal flat to the high tidal flat, since the 1980s, the sedimentation rate began to decline, reflecting the general evolution law of tidal flat. According to the radionuclide137Cs time-scale dating method, although the specific sedimentation rate of each layer of the core sediment profile can not be obtained, only the annual average sedimentation rate in a certain period of time can be obtained, but according to the calculation results, we can see the general change trend and process of the sedimentation rate of the Qidong foreland tidal flat in the past 50 years.

4 Discussions

4.1 Introduction ofSpartinaplants in wave attenuation and siltation promotionIn the development process of tidal flats, when siltation reaches to a certain height, salt marsh plants are usually grown[21-22]. Through the wave-attenuating and flow-slowing effects of waves and tidal currents, as well as changes in the turbulent structure of water bodies, the salt marsh plants will ultimately produce a series of effects on the sedimentary geomorphology of the tidal flat surface[23]. In 1963, China successfully introducedSpartinaanglicaHubb. from the United Kingdom. In December 1979, China introducedS.alterniflorafrom the United States[24]. Under suitable habitat conditions, the introducedS.alternifloraoccupied the ecological niche of the original salt marsh, and rapidly expanded in the lower part of the high tidal zone of the muddy tidal flat and the upper part of the middle tidal zone, leading to significant changes in the hydrodynamic conditions and sedimentation process of the salt marsh tidal flat[25-27]. Now, it appears that the introduction ofS.alternifloramay influence the spatial distribution and diversity of salt marsh biological community[28-29]. However, at that time, the introduction ofS.alternifloraplayed a significant role in the protection of banks[30].

Due to the significant role of salt marsh plants in bank protection and siltation promotion, since the introduction ofS.anglicaandS.alterniflorain Qidong foreland tidal flat in the Yangtze River estuary, the sediments in tidal flat rapidly grow, and the sedimentation rate obviously accelerates. From Haozhigang to Lianxinggang, about 45 km long bank from the north to the south showed significant protrusion to the sea side, and the intertidal zone is 3.5-5.5 km wide with the slope of 1.1%-1.2%. Before 1960, the tidal flat was dominated by erosion and scour, and the local shoreline receded obviously. Because there was no large sandbank development outside the tidal flat, the intertidal zone was vulnerable to erosion under the action of strong waves[31]. Since the large-scale artificial introduction ofS.anglicain the 1970s, the sedimentation outside the dike has increased significantly, and the average flat surface above the high tide line was silted up quickly, with an average annual siltation rate up to 2.3 cm[31], which is very close to the sedimentation rate (2.61 cm/yr) estimated by the radionuclide137Cs time scale, and further demonstrates the reliability of the sediment dating and sedimentation rate calculation results. According to field survey data, the average high tide line was pushed to the sea by about 50 m from October 1980 to April 1983. By the early 1990s, the level elevation outside the dike was nearly 50 cm higher than that inside the dike[32]. Large scale introduction of Spartina plants significantly protects the tidal flat and promoted siltation. Qidong foreland tidal flat in the Yangtze River estuary has changed from scouring to siltation from Haozhigang to Lianxinggang. However, with the gradual increase of the height of the tidal flat, the elevation gradually increases, it gradually changes from the low tidal flat to the high tidal flat, and the sedimentation rate starts to decline.

Through comparing the sedimentation rate of the core sediment QDZ-1 in the study area with the adjacent area, it is found that the sedimentation rate is almost the same (Table 1), indicating that the effect of introduction ofS.alternifloraon protection of dikes and promotion of siltation is very significant. After the introduction ofS.anglicaandS.alterniflorain the Jiangsu Wanggang tidal flat on the north side of the study area, sediments showed rapid siltation in the intertidal zone. Using the210Pb and137Cs dating methods, we estimated the sedimentation rate of 3.3 and 3.1 cm/yr, respectively[33]. Under the action of protection of dikes and promotion of siltation by Spartina plants, Rudong tidal flat also presents the characteristics of constant siltation to the sea. According to the analysis results of210Pb, the sedimentation rate in the center of the middle tidal flat (middle of the Spartina tidal flat) is 4.40 cm/yr[34]. In the Nanhui tidal flat on the south side of the study area, especially in the middle and high tidal flats, the shoreline is rapidly pushed to the sea, mainly based on the siltation length. Using the210Pb dating sediment rate estimation method and the digital elevation model method established with the aid of historical chart digitization, we inferred that the average sedimentation rate of the middle and high tidal flats in Nanhui is 4-5 cm/yr in the past 50 years[35]. Thus, the sedimentation rate of Qidong foreland tidal flat in the Yangtze River estuary is closely associated with the reproductive expansion of the salt marsh plants, especiallyS.anglicaandS.alterniflora. When the rising tide reaches the beach of salt marsh (such asS.anglicaandS.alterniflora), the hydrodynamic force will be obviously weakened due to the frictional hindrance of the salt marsh plants, and the suspended matter will begin to settle and accumulate. At the time of ebb tide, because the initial hydrodynamics are small, the suspended matter settled on the beach surface can not be resuspended, leading to a higher sedimentation rate of the salt marsh beach. As the tidal flat elevation increases and exceeds the water level, the sedimentation rate will gradually decline.

Table 1 Comparison of sedimentation rate with the adjacent tidal flats

Adjacent areaMeasurementmethodSedimentationrate∥cm/yrDatasourceQidong foreland tidal flat137Cs2.61This studyWanggang tidal flat137Pb3.30Literature[33]137Cs3.10Middle of Rudong tidal flat210Pb4.40Literature[34]Middle and high tidal flat of Nanhui210Pb4-5Literature[35]

4.2 Vertical changes of tidal flat growthIn the natural state, in different areas of the tidal flat landform, under different hydrodynamic conditions, with the changes in the movement intensity of sediments along the way, the sediments have obvious zoning on the tidal flat surface, forming their respective sedimentary characteristics and showing obvious zoning. From sea to land, it is low, middle, and high tidal flats, and supratidal zone, and the sediments also become gradual coarser from high tidal flat to low tidal flat[36]. During the constant accumulation of the tidal flat, the four sedimentary zonings are also continuously pushed towards the sea, causing the sedimentary zoning on the land side to cover the sedimentary zoning on the sea side in sequence. Finally, the tidal flat becomes matures and forms a complete sedimentary facies sequence. From the bottom to the top, the sediment grain size becomes thinner, the lower part being coarser silt and fine sand sediment, and the upper part being finer muddy sediment[37]. The core sediment QDZ-1 is located in the Qidong forelandS.alterniflorasalt marsh wetland protection area. Although a dike was newly built outside the protection area in 2011, theS.alterniflorasalt marsh wetland still maintains the original natural state and is not directly disturbed by human activities. The average grain size (expressed in Φ value) shows a trend from small to large from bottom to top, indicating that the sediment grains gradually become thinner from bottom to top, which is consistent with the sedimentary characteristics of silt muddy tidal flat. This is consistent with the sedimentary characteristics of the Yangtze River estuary in the south of the study area[30]and the Haizhou Bay tidal flat[38]in the north side. The tidal flat sedimentary dynamics are affected by such factors as the tidal prism and water level changes, resulting in different velocities at different elevations. From the low tidal flat to the high tidal flat, the velocity is continuously reduced, causing fine grain matters to settle and accumulate in the upper part of the tidal flat and the coarse grain matters to settle and accumulate in the lower part of the tidal flat, forming a unique vertical distribution pattern of tidal flat sediments[39].

The core sediment QDZ-1 continuously showed several sudden increases of the sand content in the depth of 100-180 cm, which was 23.3% at 102.5 cm, 23.5% at 122.5 cm, and up to 35.7% at 172.5 cm, also accompanied by biological shell sediments, possibly due to the influence of extreme environmental changes. The tidal flat near the Qidong foreland of the Yangtze River estuary is close to rivers and sea. Due to influence of weather systems such as the subtropical zone, meteorological disasters occur from time to time, and the storm surge is a major disaster[40]. Through analysis of typhoon yearbook data and historical observation data in the past 50 years[41], it is known that the north and east routes of typhoon landing will bring obvious wind and rain to this area. The tidal flat of the Qidong foreland tidal flat in the Yangtze River estuary is wide and has a width of 12 km. The coastal environment is vulnerable to the high-energy dynamic environment of the ocean such as storm surges and strong waves. Under the action of the storm, the tidal flat sediments were strongly scoured, and such coarse grains as shells and gravels that were picked up settled on the scouring surface and near the high tide line; with gradual weakening of the storm energy, the accumulated sediments accordingly showed the decreasing grain size[42]. Generally, in normal tidal flat sediments, silt takes up the dominant position, and sand does not exceed 10%; however, in storm surge sediments, sand content can reach 30%[43].

Studies have shown that every 8-10 years, a flood will occur in the Yangtze River Basin, and it often occurs when heavy rains are concentrated in the flood season[44-45]. The summer rainfall in the Yangtze River Basin is closely related to the monsoon intensity. The monsoon intensity determines the rainfall intensity and the monsoon location determines the location of the main rain belt[46-47]. According to the sedimentation rate estimated by the radionuclide137Cs dating method, and considering the large sedimentation rate at the early stage of the tidal flat, we estimated the time scale at the depth of 172.5 cm using the maximum sedimentation rate, and the result was the year 1930. In the 20th century, the three major floods in the Yangtze River Basin occurred in 1931, 1954 and 1998, respectively. Considering the error of the radionuclide137Cs dating, the peak value of the sand content at the depth of 172.5 cm may be caused by the 1931 flood in the Yangtze River Basin. The reason for failure to identify similar floods in 1954 and 1998 is possibly due to the resolution. After the core sediments were collected, they were sampled at 5 cm intervals on site, and the sampling interval was large, so the resolution was too low. Consequently, similar large-scale floods were not effectively identified. The sediment grain size directly indicates the dynamic size and characteristics of the medium carrying it[48]. Strong rainfall in the Yangtze River Basin leads to strong hydrodynamics. The flood season has a large flow and strong sand carrying capacity. It carries almost all coarse grain matters, and the sorting property is also good, so there is no phenomenon of mixture of different grain sizes.

5 Conclusions

Through the analysis of the grain size parameter of the core sediment QDZ-1 collected in the tidal flat of the Qidong foreland of the Yangtze River estuary, and the cadmium137Cs time scale dating analysis, we reached the following conclusions. (i) The silt is the main core sediments, it content is in the range of 59.6%-81.0%, the average is up to 73.3%, taking up a dominant position. The sand content takes up the second position, and the lowest content is the clay. The core sediment QDZ-1 is located inS.alterniflorasalt marsh wetland which keeps excellent natural state. From the low tidal flat to the high tidal flat, with the gradual siltation of the tidal flat, the grain size becomes thinner from the bottom to the top, conforming to the sedimentary characteristics of silty muddy tidal flat. (ii) The sedimentation rate experienced a process from fast to slow. From 1963 to 1986, it was 2.61 cm/yr; from 1963 to the present, it was 1.82 cm/yr; from 1986 to the present, it was 1.10 cm/yr. After introduction of Spartina plants, the effect of bank protection and siltation promotion is significant. With gradual increase of the tidal flat elevation, it gradually changes from the low tidal flat to the high tidal flat, and the sedimentation rate also starts to decline. (iii) At the depth of 172.5 cm of the core profile, the sand content suddenly increased up to 35.7%, possibly due to the impact of extreme environmental changes. Considering the large sedimentation rate at the early stage of the tidal flat, we estimated the time scale at the depth of 172.5 cm using the maximum sedimentation rate, and the result was the year 1930. According to the historical flood records in the Yangtze River Basin, the peak change at the depth of 172.5 cm may be caused by the 1931 flood in the Yangtze River Basin.