Tropical Cyclones Significantly Alleviate Mega‐Deltaic Erosion Induced by High Riverine Flow

Abstract

The drastic decline in sediment discharge experienced by large rivers in recent years might trigger erosion thus increasing the vulnerability of their extensive deltas. However, scarce information is available on the erosion patterns in mega-deltas and associated physical drivers. Here a series of bathymetries in the South Passage, Changjiang Delta, were analyzed to identify morphodynamic variations during high riverine flow and tropical cyclones (TCs). Results indicate that high river flow during flood season triggers large-scale net erosion along the inner estuary, generating elongated erosion-deposition patches. Erosion magnitude gradually weakens moving seaward with few localized bottom variations in the offshore area. TCs transport sediment landward and are accompanied by an overall weak erosion, with a less organized spatial pattern of erosion-deposition. TCs can therefore significantly alleviate erosion, reducing the sediment loss induced by riverine flows by over 50%. These results highlight the role of TCs on the sediment dynamics of mega-deltas. Plain Language Summary In recent years, climate change and reduced riverine sediment inputs have significantly affected the morphology of large deltas. However, it is unknown whether typhoons cause erosion or deposition in the submerged part of a delta, due to challenge in capturing infrequent events. Here we study the impact of high river flow and typhoons on the South Passage, Changjiang Delta. Results show that high riverine flows can cause large-scale erosion. Typhoons reduce fluvial erosion by transporting offshore sediments in the delta. The relative accretion caused by a typhoon is more than half the net erosion induced by riverine floods. This research sheds light on the sediment budget of large deltas and inform policy measures that can mitigate the effect of increasing anthropogenic pressure, extreme events, and sea level rise on delta morphology.

Sources, Mechanisms, and Timescales of Sediment Delivery to a New England Salt Marsh

“…Elsewhere in the world, much of the research around marine storm impacts to marshes has focused on major river systems experiencing extreme human modifications and tropical cyclone‐induced flood events. For example, in the Mississippi River and Changjiang Deltas, studies have found that the modern decline in riverine sediment inputs caused by dams and levees has been partially counteracted by marine sediment mobilized and delivered by waves and surge during tropical cyclones (Sanks et al., 2020; Smith et al., 2015; Wang et al., 2020). Our findings are therefore complementary to those along these major river systems in that we provide insight into another endmember: small estuaries impacted by more frequent and less intense extratropical storms.…”

A novel approach to discriminate sedimentary characteristics of deltaic tidal flats with terrestrial laser scanner: Results from a case study

“…The Nanhui tidal flat is located in the Nanhui Shoal which lies in the southern margin of the Yangtze Delta, China. The Nanhui Shoal is adjacent to the South Passage and is the fastest growing area in the delta, benefitting from previous abundant sediments transported into the subaqueous delta (Dai et al, 2015;Fan et al, 2017;Wang et al, 2020;Fig. 1B).…”

Luminescence characteristics of muddy sediments in the turbidity maximum zone of the Yangtze River mouth and implications for the depositional mechanisms

“…The offshore inputs into the Yangtze River mouth have been reported in many previous studies (e.g., Du et al, 2010;Ge et al, 2020;Wang et al, 2021) and surficial sediments in the subaqueous delta have yielded OSL ages exceeding 1,000 a (e.g., core A5-4 in Cheng et al, 2020; see Figure 1B for the core location). When older sediments are imported into the river mouth by processes such as tidal pumping, estuarine circulation induced by saltwater intrusion, and storm events (Warner et al, 2008;Cho et al, 2012;Li et al, 2016;Burchard et al, 2018;Wang et al, 2020), they are less likely to be bleached because the stratified water column suppresses turbulence and the sediments have no chance to be dispersed to the water surface. Moreover, thick benthic layer of fluid mud in the channel of Yangtze River mouth have been observed in the typhoon condition; numerical experiments indicate that the typhoon wind strongly enhances saltwater intrusion and therefore intensifies the degree of water stratification, leading to the formation and near-bed movement of the fluid mud layer from offshore region into the channel (Ge et al, 2018;.…”