Fluvial sediment supply to a mega-delta reduced by shifting tropical-cyclone activity

Abstract

The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , 17 with a significant fraction being sequestered in large deltas, home to over 500 million 18 people. Most (>70%) large deltas are under threat from a combination of rising sea 19 levels, ground surface subsidence and anthropogenic sediment trapping 2,3 , and a 20 sustainable supply of fluvial sediment is therefore critical in preventing deltas being 21 'drowned' by rising relative sea levels 2,3,4 . Here, we combine suspended sediment 22 load data from the Mekong River with hydrological model simulations to isolate the 23 role of tropical cyclones (TCs) in transmitting suspended sediment to one of the 24 world's great deltas. We demonstrate that spatial variations in the Mekong's 25 suspended sediment load are correlated (r = 0.765, p < 0.1) with observed variations 26 in TC climatology, and that a significant portion (32%) of the suspended sediment 27 load reaching the delta is delivered by runoff generated by TC-associated rainfall. 28Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 29 explaining past 5,6,7 , and anticipating future 8,9 , declines in suspended sediment loads 34 reaching the world's major deltas. However, our study shows that changes in TC 35 climatology affect trends in fluvial suspended sediment loads and thus are also key to 36 fully assessing the risk posed to vulnerable coastal systems. 37 Mt over recent years (1981-2005The world's largest rivers contribute a disproportionately large fraction (Extended 38Data Table 1) of the terrestrial sediment flux, which has both created, and is critical in 39 sustaining, their great deltas. Moreover, river borne sediments are a key vector for carbon 40 and nutrients, thereby playing a vital role in global biogeochemical cycles 10,11 . However, a 41 significant majority (>70%) of large deltas are now recognized as being under severe 42 threat from rising relative sea levels 2,3 , in part due to reported anthropogenically-driven 43 reductions in sediment loads 5,6,7 . Many large rivers are located in tropical regions 44 (Extended Data Figure 1) that exhibit highly seasonal flow regimes affected by tropical 45 cyclones (TCs). The potential destructive or constructive impacts of tropical cyclones that 46 directly strike deltas are well established 12,13 . However, when they strike further upstream 47TCs deliver much higher than normal levels of rainfall, effectively triggering landslides 48 and mobilizing sediments into the river network, thereby generating very high 49 instantaneous sediment loads 14,15,16 . Such high sediment loads could compensate for the 50 potential destructive effects of TCs striking deltas proper but, notwithstanding some prior 51 studies in smaller drainage basins 17,18 , the role of TCs in driving sediment delivery to the 52 lowlands and coast remains unclear. As noted, this is particularly the case for large rivers 53 that carry much of the terrestrial sediment flux because these rivers are, in t...

A Review of the Integrated Effects of Changing Climate, Land Use, and Dams on Mekong River Hydrology

“…While Xayaburi dam has been built to fulfill the growing regional energy needs (its installed capacity will be 1,285 MW), experts believe that the energy supply will come with unprecedented and devastating costs to the environment and livelihoods of tens of millions of people in the region [9,17,39]. The dam will likely cause irreversible and permanent ecological change to the Mekong by altering the natural flow regime and adversely affecting fisheries and other aquatic resources; it will also affect flood-recession ecosystems in the lowlands and impede sediment delivery to the Mekong delta region [39][40][41][42][43]. Growing transportation networks, especially the mountain roads in headwater catchments, some of which are linked with the construction of new dams are causing significant changes in sediment production and deposition in river channels [44].…”

“…In the Mekong, most of the sediment upstream of Cambodia is stored inside the channel and, unlike in other rivers such as the Amazon, there is little sediment exchange between channels and floodplains except in the Cambodian lowlands and Mekong delta [43,113]. There are a number of sediment monitoring stations along the mainstem Mekong [42], but there is a lack of basin-wide sediment monitoring network, especially for small tributaries and headwater catchments where a multitude of factors affect sediment generation, deposition, and transport. Studies conducted over different regions in rural Southeast Asia suggest that road networks, especially the mountain trails, which are often associated with agricultural expansion and dam construction, are one of the major drivers of changing sediment dynamics in the region [44,[53][54][55]114].…”

Carbon dynamics and CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; outgassing in the Mekong delta

“…Several studies predict that a large fraction (70-95 %) of the sediment load could be trapped by hydropower reservoirs if all of the planned infrastructures are effectively built (Kummu et al, 2010;Kondolf et al, 2014). In addition, sediment river delivery could also vary in response to changes in climate (Västilä et al, 2010;Lauri et al, 2012;Darby et al, 2016). This would have important consequences on the sediment deposition in the delta that seems to have already shifted from a net depositional (accretion) regime into a net erosion regime (Anthony et al, 2015;Liu et al, 2017).…”

On the Holocene evolution of the Ayeyawady megadelta

“…The reason is the highly energetic tidal, wind and wave regime of the northern Andaman Sea that exports most sediments offshore despite the large load of the river, as described by Ramswamy et al (2004) and Hedley et al (2010). In addition to their effects upstream (Brakenridge et al, 2017), the expected sediment deficit after dams are constructed on the river and tributaries may significantly impact the fragile delta sedimentary equilibrium (Giosan et al, 2014) rendering it more vulnerable to the accelerating sea level rise (Syvitski et al, 2009) or changes in frequency and intensity of cyclones hitting the coast (Darby et al, 2016) compounded with increased subsidence linked to the rapid development of the region (e.g., Van der Horst, 2017).…”