Yangtze floods recorded on Mt. Mufu and Swallow Cliff in Nanjing, China

ABSTRACT The Yangtze drainage basin is the most densely populated and prosperous area of China; however, it is frequently threatened by floods. The Holocene flood lines of the Yangtze River have been preserved on Mt. Mufu and Swallow Cliff in north-eastern Nanjing, and they are characterised by the presence of particular colour tones and the horizontal arrangement of erosional pits and holes. Four major paleo-flood lines at altitudes of 8.51, 9.43, 10.47, and 12.84 m were identified via a field survey along the river bank. Historical literature and instrumental flood records extend the paleo-flood stage to the contemporary era and indicate that the highest flood line of 12.84 m represents a maximum limit for future floods. Besides, the flood line at 10.47 m can be regarded as a foreseeable extreme flood risk level with a recurrence interval of 100–200 years, especially considering the current circumstances of rising sea levels and decreasing flood storage capacities caused by human activities.


Introduction
The Yangtze River is one of the largest rivers in the world in terms of river length, water discharge and drainage area. This river originates from the Tibetan Plateau, crosses the country from the west to the east, and finally debouches into the East China Sea at Shanghai. Nearly 440 million people live in the Yangtze drainage basin, which has an area of 1.94 £ 10 6 km 2 (Chen et al. 2001). The annual precipitation in this basin is approximately 1070 mm, and it is dominated by the eastern Asian subtropical monsoon climate. Because of the highly uneven spatio-temporal distribution of rainfall, with values ranging from 500 mm in the west to 2500 mm in the east and more than 70% of the rainfall occurring in summer, annual floods in this basin are inevitable. These floods usually occur from June to July  and only vary in their magnitude, frequency, and impact on modern human societies (Ding et al. 2008;Yu et al. 2009;Ge et al. 2011;Deng et al. 2016).
Long flood series are essential for understanding the local climate, projecting future changes, and managing and assessing the safety of infrastructure along rivers, such as artificial levees and series of locks and dams, especially in the most densely populated and prosperous areas. However, instrumental flood records of the Yangtze River only extend 150 years into the past, which was when the earliest gauging station was established at Hankou in 1865 (Qian & Zhu 2001;Jiang et al. 2007;Zhang et al. 2008;Guan et al. 2015). However, because of the prevailing ancient custom of writing local chronicles, abundant historical records of flood events are valuable resources for reconstructing longer flood series (Jiang et al. 2006). Other paleo-flood records can be derived from slackwater sediments in bedrock gorges, from boulder berms in upland areas (Macklin & Rumsby 2007), and from layers of sand or gravel strata in floodplain sediments through the identification of textural reversals in the sedimentary sequences (Goman & Leiph 2004;Li et al. 2013). These methods have been deployed successfully in temperate catchments and semiarid or arid environments (Sheffer et al. 2008).
In the 1990s, the Yangtze River experienced extraordinary flooding that caused billions of dollars' worth of damage. Many of these extreme floods were larger than expected or predicted. During the summer of 1998, a heavy flood caused more than one thousand deaths and rendered millions homeless. The economic damage of this flood event was estimated at 20 billion dollars (Zong & Chen 2000). Knowledge of prior floods of similar or larger magnitude in this area might have improved the flood hazard estimations and prevention and preparation strategies for such events. In this research, we explored a series of caves and eroded watermarks recorded naturally on Mt. Mufu and Swallow Cliff as proxy records of the major Holocene flood stages of the lower Yangtze reach in the Nanjing section. Common sense dictates that what has really happened may happen again; therefore, investigating this paleo-flood state has many practical ramifications for studies of the lower Yangtze reach as well as for local economic development.

Geological setting
The Yangtze River can be divided into the upper, middle, and lower reaches based on the geology, geomorphology, and regional climate (Figure 1(a)). The upper section is more than 4300 km long from the source to Yichang. The middle section is 950 km in length from Yichang at the end of the Three Gorges reach to Hukou, which is an outlet of the Yangtze to the Poyang Lake. The final 930 km below Hukou constitutes the lower section (Chen et al. 2001;Xu et al. 2008).
The study site is located in the northeast section of the city of Nanjing, which is in the lower reach of the Yangtze River (Figure 1(b)) and well known for its cultural and historical significance as an ancient capital of six dynasties with its recorded history spanning over 2000 years. The Yangtze River flows through Nanjing from the southwest to northeast. The north bank is 98 km in length, and the south bank is 88 km (Figure 1(c)). The width of the water surface is 1.1 to 2.5 km; the average water depth is ¡20 to ¡30 m; and the deepest trough is ¡72 m. Several sand islets and shoals are scattered throughout this section because of the tidal backwater caused by the river-tide interaction, and the average annual discharge is 2.88 £ 10 4 m 3 /s (Pan 1990;Chen et al. 2012).
Since the Middle Pleistocene, the river course in this area has been quite stable along the valley between two groups of mountains and hills. Two major mountains occur on the northern bank: Mt. Laoshan, which is mantled by forest and underlain by Sinian neritic sedimentary rock dated ca. 600 Ma BP; and Mt. Lingyan, which is composed of a set of Pleistocene sandy gravels overlain by basalt rock. One mountain occurs on the southern bank: Mt. Mufu (6 km long, 1 km wide, and 205 m high), which consists of ancient Lunshan limestone of Ordovician age (Xia 1998).
A series of limestone caves similar to small rock shelters is distributed along Mt. Mufu. Three bigger caves have been well preserved as historical and scenic spots: Toutaidong (1st cave), Ertaidong (2nd cave), and Santaidong (3rd cave), from east to west. The 3rd cave has the most scenic view and consists of three smaller caves layered vertically at altitudes of 7.56 -25.1 m. The average annual lowest water stage of the Yangtze River here is 2.98 m. Larger floods have inundated these caves for a long time and are believed to have formed all the caves.
Swallow Cliff (alt. 36 m) is located on the river channel to the northeast of Mt. Mufu. A series of clear flood marks is shown on this vertical cliff that appear similar to strokes on a wall, and this series is an ideal source of information for flood studies.

Material and methods
The instrumental flood data were collected from local meteorological stations and hydrological gauging stations; and historical data were collected from various records, including local chronicles. Documentary entries on hydrographic events used in this paper were critically reviewed to ensure the reliability of the reconstructed flood records.
Field surveys were conducted to identify paleo-flood stages along the riverside. Total station surveying and D-GPS technologies (Leica TCR702/SR510, Leica Geosystems Ltd., Swiss) were deployed to collect feature data at the field sites, and they provided detailed relative elevation data for constructing continuous paleo-flood lines. Sediment samples were collected from caves, erosional pits, and river flats.
A Mastersizer 2000 particle size analyser (Malvern Instruments Ltd., UK) was used to measure the sediment samples, and their consistency was verified by frequency curve characteristics and used to authenticate the flood lines. The designed size range was 0.02-2000 mm, and the reproducibility was better than 2%. Each of the surficial samples was thoroughly mixed, and about 1 g was selected, placed into a 25 ml glass beaker, and then stirred with distilled water for 2 min; subsequently, 10 ml of 5% hydrogen peroxide (H 2 O 2 ) was added and stood for 24 h to eliminate organic matters. The sample was then treated by adding 10 ml of sodium hexametaphosphate ((NaPO 3 ) 6 , 0.5 mol¢ L ¡1 ) and standing for 24 h to aid dispersion. The computer program GRADISTAT (Blott & Pye 2001) was run to obtain the statistics for the grain size data exported from the laser granulometer. The statistical parameters, i.e. mean, standard deviation (sorting), skewness, and kurtosis were calculated using the logarithmic method of moments.
Pieces of flowstone collected in two caves were transported to the Stable Isotope Lab of the Department of Geology and Geophysics, University of Minnesota, USA for U-series dating, which confirmed that the old cave-generation period ended at ca. 400-500 ka BP (Tan et al. 2010).

Characteristics of the flood lines
The flood lines, which are represented on the limestone rock and wall along the south river-side of Mt. Mufu and Swallow Cliff, have a particular dark tone with several small eroded pits distributed horizontally, and these features demonstrate that Mt. Mufu has experienced heavy floods over a long period.
We found four clear paleo-flood lines preserved in the 3rd cave ( Figure 2) and unclear flood marks in other caves. In the 2nd cave, which is 200 m east of the 3rd cave, paleo-flood lines are difficult to identify because this cave was once used as a residential shelter. In the 1st cave, which is 270 m east of the 2nd cave, the light-grey paleo-flood lines are faint and blurred, and they are horizontally discontinuous because of the anthropogenic interference. The reason for the relatively good preservation of flood lines in the 3rd cave is mainly because of its superior scenic view, which made it an ideal location for monks to practice. Therefore, we studied the flood marks first in the 3rd cave and then traced them into other locations.
(1) The first flood line (FL1) is deep-grey coloured at an altitude of 8.5 m, which is 5 m higher than the present river surface and the bottom line in this cave. Small erosional pits and holes are developed along this flood line. Although hydrological records indicate that modern floods frequently reach this level, the erosional indentation in the surface of limestone rock shows that the flood line was initiated a long time ago.
(2) The second flood line (FL2) is dark grey at an altitude of 9.4 m and characterised by horizontal tiny erosional holes, and it is not as clear as FL1 because of rock collapse. (3) The third flood line (FL3) is the clearest line, and it is coloured deep grey at an altitude of 10.5 m. Small erosional pits are distributed horizontally, and small embryonic caves in the form of vaults and uprights along the line show that the formation period was longer and earlier than those of FL1 and FL2. (4) The fourth flood line (FL4) is grey at an altitude of 12.8 m. Small vault caves developed at this level, thus reflecting changes in the flood stages. An unclear flood line with smaller erosional pits (FL4-3) between FL3 and FL4 is observed at an altitude of 11.7 m. The altitude of FL4-3 is at the bottom of a small vault cave, which was subsequently dug through by an upright cave, thus forming a water tunnel into FL3. This relationship indicates that FL3 formed later than FL4 and was more stable than FL4-3 and FL4.
Based on these remnant paleo-flood lines in the 3rd cave, further investigations were conducted in the upstream and downstream regions to consolidate the results. Certain findings are described below.
(1) In the 1st cave (450 m downstream), several flood lines are identified on the rock cliff, and two are clear and coloured dark grey at altitudes of 12.68 m and 12.34 m, which are approximately equal to FL4 in the 3rd cave. show that their substance composition is the same as that from the Yangtze River. The major component is silt (80%) with an average diameter of 5.97-6.46 ' and a unimodal symmetrical frequency curve (Table 1, Figure 4). This result also reveals that FL1 is the remnant water mark of Yangtze River floods.
The characteristics of the water marks preserved in the limestone rock and cliff provided a preliminary confirmation that they represent the fading relics of paleo-floods, and four distinct paleoflood lines were distinguished by their horizontal continuity ( Figure 5, Table 2).

Documentary entries on hydrographic events
After the last glacial maximum, the water level of the Yellow Sea (into which the Yangtze River debouches) presented the lowest level at ca. 15 ka BP and the highest level in the Holocene Megathermal at ca. 6.5 ka BP, and since then, the water level has been decreasing gradually with several fluctuations. At the epoch of the high sea level, the riverbed was rising, the runoff was increasing, and the river valley was widening via enhanced lateral erosion. Rising waters cascaded through a hierarchically arranged system in an efficient flood distribution network that was stable for all but the heavy floods. The overarching Holocene history of the lower Yangtze River represented a metamorphosis from a Late Pleistocene braided floodplain into an anastomosed river system. Along the riverside from Yichang, Anqing, to Nanjing, scores of buried subtropical broadleaf paleo-trees were excavated and 14 C dated to 6-5 ka BP (Yang & Xu 1980), which represented the first peak of the frequent large flood events. In the second terrace on the north bank in Nanjing, buried paleo-trees and alluvial sandy gravel dating to 4.2-3.6 ka BP (Zhu et al. 1997) revealed the second flood peak. From 190 BC to 1999, 240 flood disasters in the lower Yangtze reach were recorded in various historical literature; thus, heavy floods occurred on average every nine years. The dates of these floods were accurate in the Chinese lunar calendar along with the traditional reign titles, which we converted to the current international Gregorian calendar by the Academia Sinica Department of Information Technology Services. The flood depths indicated in the literature were usually imprecise because of the lack of datum reference, ambiguity or exaggeration in the flood descriptions, and variations of the length unit (Chinese ruler) throughout the long history of the region. Table 3 shows several of the historical flood entries related to the Nanjing area.

Instrumental flood record
Continuous instrumental hydrographic monitoring began at Xiaguan gauging station in Nanjing in 1912, and it recorded 12 heavy floods above 9.0 m (the warning level is 8.5 m). The altitudes of these floods corresponded to the flood line between FL2 and FL3. The largest recorded flood occurred in 1954 and had a peak of 10.22 m and maximum runoff of 9.26 £ 10 4 m 3 /s, and this flood persisted  for 62 days above the high stage of 9.5 m ( Table 4). The statistics of the maximum flood stages grouped by decades show an upward trend after the 1960s that continued into the 1990s (Figure 6).

Causal factors of the high flood stages
(1) The lower reaches of the Yangtze River include an enormous alluvial plain and many tributaries, and the elevation is only 2-7 m above mean sea level in 95% of this region, thus flash floods tend to come after short periods of heavy rain and most often affect small streams and flood prone areas. In the Plum Rain Season, usually from June to July when a slowly drifting cold front meets a moist and stable subtropical air mass to form the Jianghuai Quasi-stationary Front that fluctuates over the lower Yangtze reach, general flooding tends to affect major rivers and the whole region. Additionally, Jiangsu Province is one of the targets of typhoon storms in summer, which bring extraordinary rainfall within a short period. Furthermore, the periodic spring tide produces stagnant flood discharge and raises the high water stage along the tideway. Moreover, the storm surge and astronomical spring tide may Heavy flooding in Nanjing, water depth of 10 feet or more on the south bank; private houses drifted; dykes burst. Aug 1298 Yangtze River overflowed; Nanjing experienced a serious disaster; water height up to 40-50 feet; houses and huts were submerged or drifted. Aug 1502 Yangtze flood inundated Nanjing with depth of up to 5 feet; more than a thousand private and military houses collapsed; part of the city wall and some bridges were destroyed. Aug 1560 Yangtze flood reached the Sanshan gate; several feet of water in the Qinhuai residential area. SU 1589 Yangtze River overflowed; in Nanjing, the water reached heights of tens feet on the ground; houses in farmlands were submerged. Jul 1608 Heavy rain lasted nearly 20 days in Nanjing, 'the flood was extremely abnormal, people drowned, ..., this is a disaster that hasn't happened for 200 years'. SU 1755 Heavy rain in Jiangsu; high water stage lasted more than 40 days. SU 1848 Rainy in many provinces along the middle and lower reaches of Yangtze River; 'Rivers and lakes rose together, bank breached at many places in Jiangsu, flood overflowed on both sides of the Yangtze River at depths of several feet'. act together in the rainy season to form an extraordinarily high water stage that persists over a long time, and such events constitute disasters for human beings. In the Nanjing section, the flood discharge from the upper reaches is the main reason for the high water stage, according to the flood records. The highest water stage at the Xiaguan station in Nanjing exhibits an excellent correlation with the Datong station upstream in Anhui Province (r D 0.97) but a poor correlation with the tidal stage of the Wusong station downstream at the river mouth (r D 0.2) (Rui 1994).
(2) The flood storage capacity has decreased at this river section, which is a primary reason for the persistently high water stage. From Mt. Mufu to Zhenjiang, the river bed has suffered the most severe siltation in the middle-lower reach of the Yangtze River, which is accompanied by a gentle slope of 0.5 ¡ 1.0 £ 10 ¡5 (Xu 1999). Shallows along the river channels function as water barriers and dam the downstream flow, thereby extending the time of flood discharge, raising the high water stage, and eventually enhancing floods events (Hu & Luo 1992). (3) The tributary Huaihe River plays a backwater role when its flood meets the Yangtze flood between Datong and Nanjing, especially when it coincides with heavy rainfall in this area. A hydrological model shows that the water stage at the Xiaguan station of Nanjing could rise 0.15 m once the flood discharge of the Huaihe River reaches 1.2 £ 10 4 m 3 /s (Rui 1996). (4) Current anthropogenic changes in land use and land cover are expected to enhance the risk of forming the high water stage, especially when the lake reclamation in this region has significantly decreased the flood storage capacity (Yin & Li 2001;Nakayama & Watanabe 2008;Yang et al. 2016).  (2) Major flood fluctuation can be identified by comparing the altitude change of these flood lines. The differential values are 0.92 m between FL1 and FL2, 0.95 m between FL2 and FL3, and 2.37 m (in the 3rd cave) and 2.0 m (on Swallow Cliff) between FL3 and FL4. The minimal change is 0.33 m between FL4 and FL4-3 in the 1st cave, and the maximal change is 2.37 m as noted above. Therefore, the flood stage change is mainly between 1 and 2 m for a given period over the long run.

Implications of the flood lines
(3) The rock caves and cliff consist of the Lower Ordovician Lunshan limestone, which is a stable bedrock bank and an ideal medium to preserve vestiges of ancient floods. The local flood control standard for dyke projects is 12.8 m (10.8 m of designed water level plus 2 m for additional safety) based on a recurrence interval of 100 years. This specification is approximately equal to the paleo-flood line FL4, which we estimated could not recur. Therefore, the investigation of paleo-flood lines provides scientific evidence for local protection works against Yangtze floods. (1) The highest flood line (FL4) is the earliest flood relic, and it corresponds to the Middle Holocene at 6-5 ka BP when the sea level was at its high stage. On the second terrace at an altitude of 10 m on the north bank, the stratigraphic record shows rapid alluvial sedimentary facies, peat bogs, and buried paleo-trees with coarse sand and gravel, which indicates a warm moist environment as well as the prevalence of floods in the lower Yangtze reach. The particle size analysis also reveals a higher proportion of medium and fine sand and a lower proportion of clay, which indicates flood activities during this period. In the Middle Holocene, the Yangtze River mouth was at Zhenjiang, which is 65 km downstream from Nanjing, and the water stage was ¡2 to ¡3 m (Li and Min 1981). The current average high water stage is 7.15 m between June and August, and the mean slope is 0.5-1.0£ 10 ¡5 (Chen et al. 2001) from Nanjing to the river mouth at a distance of 480 km. Therefore, we can deduce that the water stage of Nanjing was approximately 6-8 m in the Middle Holocene, and paleo-floods frequently reached the altitude of FL4 when an extraordinary flood, storm surge and spring tide coincided, particularly considering the long geological span. However, it is nearly impossible for floods to reach this altitude at present because of the migration of the river mouth and the lower sea level.
(2) The second highest flood line (FL3) is consistent with several historical flood events in the literature. We can deduce the altitude of these flood stages to be approximately 12 m by their imprecise description in ancient Chinese, such as 'depths of more than ten feet', 'five feet in the city', 'houses submerged in farmlands', etc. The corresponding time series of 1170, 1298, 1502, and 1589 may indicate that the flood recurrence interval at this stage is approximately 100 to 200 years. The extraordinary flood recorded in 1298 has a counterpart event in the Old Walled City of Shibam, Yemen, which is inscribed on the World Heritage List by UNESCO. At this site, torrential rains and floods are rare except for two disastrous floods in 1298 and 1532.
(3) The lower flood lines (FL2 and FL1) are within the range of instrumental flood records. Although they are currently disengaged from the river flow, heavy floods could still reach these altitudes. For example, the flood stages in 1954 and 1998 surpassed these lines. Horizontal flood lines with dark grey tones were found in the rock bank eroded as linear dented marks. The current average highest water stage is approximately 9.3 m, and the flood recurrence interval at this stage is 10-50 years.

Flood lines under the scenario of global warming
Since the industrial revolution, the global average surface temperature has increased with a 100-year warming trend  of 0.74 C, and the global average sea level has risen at an average rate of 1.8 mm/a since 1961 (IPCC 2007). Such global climate change enhances ocean dynamics and causes incremental increases of climate extremes, such as storm surges, torrential rainfall, and floods. Although many dams, dykes, and reservoirs have been constructed throughout the Yangtze catchment in recent decades to enhance flood control, including the project of Three Gorges Dam, their effectiveness for withstanding extreme events, such as a 100-year flood, has been largely weakened under the current circumstances (Zong & Chen 2000). Recently, sudden changes in drought/ flood have frequently occurred in the middle and lower reaches of the Yangtze River (Tian et al. 2016), which may be related to environmental changes and could exacerbate the flood events. The lack of relevant quantitative research as a basis for decision-making is a challenge for flood monitoring and risk mitigation. Holocene deposits that still retain primary sedimentary structures are rare in this region, thus demonstrating the importance of identifying more ground features that could reveal complex hydrodynamic processes, such as caves and cliffs, and utilising multiple sources to reconstruct long flood series. The results of the presented analysis highlight the potential for expanding available databases for the study area. The proposed methodology may be utilised in other regions with similar geological settings affected by flood risk and applied for environmental analyses, hydraulic engineering, and disaster mitigation.

Conclusions
The watermark archive preserved in the caves of Mt. Mufu and Swallow Cliff has recorded significant regional Yangtze River floods since the Middle Holocene. The overall stability of the Yangtze River in the Nanjing section and the characteristics of the Ordovician Lunshan limestone fostered the development of flood lines. Historical literature and instrumental flood records extend paleofloods to the contemporary era and indicate that the highest flood line of 12.84 m (FL4) can be considered a maximum limit for floods in the foreseeable future. However, the level of 10.47 m (FL3) could still be reached, which indicates the threat of extraordinary floods, especially under circumstances that heavy rainfall, storm surges, and astronomical spring tide act together. The development of more detailed flood recurrence intervals depends on the precise dating of paleo-flood lines, which will be conducted in further studies.