Human utilization of mega-deltas: the importance of tidally modulated ground surface elevation

Human development of mega-deltas is influenced by the elevation of delta ground surface. The elevation, modulated by the largest tidal range, influences the inundation pattern during storm surges, or the degree of risk in the presence of a sea dyke. However, the tidal modulation may be interrupted by nature or human induced subsidence or sediment starvation. Thus, the dynamics of the elevation should be studied in order to optimize the techniques to maintain the tidally modulated elevation. Furthermore, appropriate engineering schemes may be adopted to improve the deltaic geomorphological condition.

Among the numerous river deltas all over the world, 20 or so are giant deltas with an area of more than 10,000 km 2 . They have received much attention because of their rich natural resources and large populations. Ironically, the urban areas in these giant deltas rarely become a centre of regional economy and social development. Some of them lack large cities; the others have large cities, but tend to be located either in an adjacent catchment basin, or in the inner part of the delta, at considerable distance from the shoreline ( Table 1). The Mekong Delta, for example, has Ho Chi Minh City, but this city is situated in another basin nearby and farther from the coast. Likewise, in the case of Dhaka and Kolkata in the Ganges-Brahmaputra delta, the distances between these cities and the delta front are around 200 km and 100 km, respectively. Perhaps the only exception is Shanghai, which is located right at the mouth of the Yangtze River; furthermore, at the apex of the Yangtze River delta, there is another large city, Nanjing.
A number of factors should be considered to explain the above mentioned phenomenon, e.g., the climatic zone to which the delta belongs, the conditions of ports and waterways and land resources, and the timing and stage of economic-social development. However, one factor that should not be overlooked is the elevation of the delta ground surface, which is modulated by tidal conditions. The tidal range of world's deltas differs substantially between < 0.5 m and > 10 m (see Fig. 8, Yang et al. 2020). Thus, the link between land elevation and tides implies diverse environmental conditions for humans living in deltas.
In tidally dominated environments, sediment is transported to landward and accumulates over the intertidal zone, with sandy material being deposited in the lower part and muddy sediment over the upper part. The highest position of accumulation is determined by tidal range or characteristic tidal water level (Gao 2019). The tidal range is the vertical distance between high and low water levels during a tidal cycle (Carter 1988 to describe the characteristic water levels. Thus, the tidal ranges for spring and neap phases can be approximately defined as the differences between MHWS and MLWS, and MHWN and MLWN, respectively. The implication is that if the mean tidal range during the spring phase is 5 m, then the fine-grained sediment can reach an elevation of 2.5 m above sea level. In reality, the upper limit of sediment accumulation or the potential maximum surface elevation can be higher, because it is determined by Highest High Water on Springs (HHWS), rather than by MHWS. In Shanghai, the mean tidal range during the spring phase is close to 5 m, whilst the ground elevation reaches as high as 4.5 m (Xu 1997). Here, caution should be taken for the datum of elevation measurements, which was not explicitly shown in Xu (1997). If the elevation is expressed against mean sea level, as is the case in most topographic maps in China, then it is straightforward to observe that 4.5 m is much higher than 2.5 m. However, for the Shanghai region the local datum is sometimes used, which lies some 1.66 m below the mean sea level. In terms of the local datum situation, the ground is now 2.8 m above the mean sea level, still being higher than 2.5 m.
Apart from tides, can river flooding or storm events also raise the ground? Indeed, flooding in the middle and upper reaches of the river generates water levels that are much higher than normal, bringing with them muddy sediments that accumulate on top of the floodplain. As a result, the sediment layer of a river terrace also has the dual structure of coarse-grained material at the bottom and fine-grained material at the top. The terrace elevation increases with each flooding event. For deltas, however, the river flooding levels are not so high, because the river becomes wide as it flows into the estuarine and coastal waters. The situation of storm events is somewhat different. During inundation, the storm surge brings with it suspended sediment towards the upper parts of tidal flats, causing accretion in salt marshes or mangroves. Thus, sediment accumulation during storms is beneficial to the enhancement of the ground elevation. However, this effect should not be overestimated. In tidal flat environments, the deposits tend to be tidally dominated with a small contribution by storm layers (Reineck and Singh 1980). The reason is that the sedimentary material transported by storm flows is derived from resuspension over the tidal flat bed, which causes partial reworking of the normal sedimentary sequences. Hence, the net contribution by storms is in the form of redistribution of sediment.
A typical example is the Mississippi River in the United States: this large delta is characterised by enormous water and sediment discharges from the river, and it is frequently hit by hurricanes (Coleman et al. 1998). Here, the small tidal range is responsible for the low-lying landscape.
The ground surface elevation has a great environmental impact. Saline water intrusion, water table position, and inundation and water logging patterns are all influenced by this factor. Storm surge is one of the major natural hazards for deltas. The consequences of storm events are correlated with the elevation.
Inundation patterns may be evaluated by simplified, conceptual calculations. If non-linear interaction between storm surge and tides is neglected, then the timing of inundation/exposure of the land surface can defined by the following equations: where y is water level, x is tidal phase, R is the tidal range, H is storm surge magnitude, and H 0 is the ground surface elevation. Once the timing is determined, the Inundation time in terms of percentage during the storm event, can be derived. Although the system formulated by Eqs. (1) and (2) include the necessary mechanism of inundation, the result of the calculations should be considered as qualitative because the peak tidal water level and the tidal water level curve are not adequately accurate to reveal the inundation details.
In the calculations on the basis of Eqs. (1) and (2), it is assumed that the deltas involved are not protected by a sea dyke; for the three hypothetical deltas, their average spring tide ranges are set to be 5, 3 and 1 m, with their ground surface elevations being 4, 2.4 and 0.8 m, respectively. It is further assumed that the storm surge has a duration of several days and occurs simultaneously with a maximum astronomical tide, which approximately represents the most devastating storm surge hazard.
The result (Table 2) shows that significant differences exist in terms of inundation patterns. For the delta with the elevation of 4 m, the inundation period in response to a surge of 2 m accounts for only 21% of the period of storm event, which increases to 63% for a surge of 5 m. In contrast, for the delta with an elevation of 2.4 m, these two values become 41.6% and 100% (i.e. complete inundation during the entire event), respectively. The situation of the delta with an elevation of 0.8 m is the worst, with 100% inundation when the the magnitude of storm surge exceeds 1.5 m. Thus, the inundation period will be relatively short for the deltas with a high elevation, but the situation of a lower elevated delta will be devastating. With several days of inundation, one can imagine the difficulties faced by the people living there. (1) In addition, there is an extreme situation. Located at the inner part of the Bay of Bengal, the Ganges-Brahmaputra River delta is subjected to a water level increase induced by storm surges of up to 7 m (Mohit et al. 2018). This explains why this delta still has a serious inundation risk despite the large tidal range. A high ground surface elevation can resist the storm surge well, but when the storm event reaches a certain threshold, this resistance will be ineffective.
Unlike the Ganges-Brahmaputra delta, the Yangtze delta, where Shanghai is located, has certain advantages in coping with storm surges. According to historical records and numerical calculations, the largest water level increase due to storm surge in this region reaches 3.67 m. Taking into account the nonlinear interaction between storm surge and astronomical tide, the highest water level would be 6.77 m above local datum when the maximum storm surge and astronomical tide occur at the same time (Ying and Yang 1986). If the nonlinear interaction is neglected, then the highest water level would be 7.87 m (Duan et al. 2004). In either case, a 4 m elevation (in terms of the local datum) implies that the ground in this area cannot be flooded for the entire duration of the storm surge. With additional protection provided by sea dykes, the risk of being inundated by storm surges is greatly reduced. Such a geomorphologic stability is highly beneficial to the economic and social development of Shanghai.
It is worth noting that the peak high tidal level sets the upper limit of upward accretion, but this is not a sufficient condition for the surface to be elevated to such a limit. Many other factors can affect the actual elevation, such as the artificial removal of oil, gas and water from the delta's underlying sediments, the trapping of sediment in reservoirs upstream, the human activities like reclamation, and sea level rise (Syvitski et al. 2009); even without the influence of human activities, land subsidence can be caused by sediment compaction and dehydration.
Furthermore, poorly designed reclamation projects prevents further input of sediment and modifies the evolution of the delta geomorphology, which also reduces the elevation (Auerbach et al. 2015).
The major parts of the Yangtze delta were formed before the period of large-scaled reclamation, so most of the areas became above 4 m in elevation. Presently, the upper part of the intertidal zone in the delta front is generally below 3 m in elevation (local datum) (Mao et al. 2014), whilst the recent reclamation is aimed at this part of the land. Inevitably, the newly reclaimed land is much lower in elevation than historically developed land that was associated natural accretion, making it more vulnerable to storm surge hazards.  The above analysis does not necessarily lead to any pessimistic views. On the positive side, there exist many supporting technologies for human utilization of megadeltas. Here, in addition to the monitoring of sea level rise and land subsidence, the temporal changes in delta surface elevation is also important. However, detailed information on elevation appears to be scarce in literature (e.g., Bomer et al. 2020). Since land subsidence and sea level rise rates alone do not provide a complete picture of future delta risks without addressing the initial surface elevation and its dynamics, more data sets in the form of, e.g., hypso-metric curves, should be made available with high spatio-temporal resolutions.
One important measure is to raise the elevation of the ground. Because the elevation of the ground is restricted mainly by the tidal range, it would be rare to observe a naturally high area in a large delta with a small tidal range. In this case, how should the development of the delta proceed? On the one hand, the management of land subsidence should be strengthened to reduce the rate of subsidence (actually, the delta with a large tidal range also has this task). On the other hand, efforts should be made to improve the ground elevation, and at the same time special attention should be paid to the protection of ecosystems. By gradual modification with a careful design, some areas of the delta will be raised to a suitable height; the value of the land will be enhanced by this process.
In this respect, the historical development of the Yangtze River delta deserves attention (Bao and Gao 2021). The delta is characterized by a network of waterways whose density is so high that it covers almost one-tenth of the land surface. Some of these were natural tidal waterways of the past, and instead of filling them up to expand the land area they were retained as drainage channels to desalinate the soil and, on the other hand, as shipping channels. Moreover, in the network construction, a large number of the waterways were artificially excavated, with their functions being the same as those of the preserved tidal channels. During the excavation of these waterways, a large amount of sediment was produced, which was deposited onto the surrounding land. If the height of the canal bottom is set at -5 m, then the sediment generated would be at least 5 m thick, which could raise the elevation of the surrounding land by 0.5 m. Similar practices still continue today, as can be seen in the recent reclamation at the delta front.
The practices mentioned above should comply with economic feasibility, but the new land transformed in this way should have a relatively high market value and be ecologically and socially acceptable. The point made here is that the elevation condition for delta development, in relation to tidal dynamics, storm surge and sediment accumulation, must be adequately considered.