Resilience of River Deltas in the Anthropocene

Abstract

At a global scale, delta morphologies are subject to rapid change as a result of direct and indirect effects of human activity. This jeopardizes the ecosystem services of deltas, including protection against flood hazards, facilitation of navigation, and biodiversity. Direct manifestations of delta morphological instability include river bank failure, which may lead to avulsion, persistent channel incision or aggregation, and a change of the sedimentary regime to hyperturbid conditions. Notwithstanding the in‐depth knowledge developed over the past decades about those topics, existing understanding is fragmented, and the predictive capacity of morphodynamic models is limited. The advancement of potential resilience analysis tools may proceed from improved models, continuous observations, and the application of novel analysis techniques. Progress will benefit from synergy between approaches. Empirical and numerical models are built using field observations, and, in turn, model simulations can inform observationists about where to measure. Information theory offers a systematic approach to test the realism of alternative model concepts. Once the key mechanism responsible for a morphodynamic instability phenomenon is understood, concepts from dynamic system theory can be employed to develop early warning indicators. In the development of reliable tools to design resilient deltas, one of the first challenges is to close the sediment balance at multiple scales, such that morphodynamic model predictions match with fully independent measurements. Such a high ambition level is rarely adopted and is urgently needed to address the ongoing global changes causing sea level rise and reduced sediment input by reservoir building.

Effect of Small Tidal Fluctuations on the Stability and Equilibrium Configurations of Bifurcations

“…As reported by Lentsch et al (2018), the reduced mobility of distributaries emerges even for weak tides, and notably, when new accommodation space is created by rising sea level. The erosive character of the current which promotes channel deepening (Rossi et al, 2016) tends to hinder the development of a topographic gradient advantage due to backfilling (e.g., Hoyal & Sheets, 2009), mainly responsible for an avulsion event (Hoitink et al, 2020).…”

“…Despite including the effect of the angle may be a difficult task, other important factors can be eventually included in the present formulation like the presence of a meander bend in the upstream channel (Kleinhans et al, 2008), a different phase of the tides at the mouth of the distributaries, or bifurcation width adjustment (removing the hypothesis of fixed banks) (Miori et al, 2006). In particular, the latter factor may be relevant in presence of relatively fast bank retreat phenomena as reported in different natural deltas (Hoitink et al, 2020).…”

“…In this context, from a reductionist point of view, single bifurcations of the delta represent the building blocks, controlling the distribution of water, sediment, and nutrient fluxes among the branches. Understanding their dynamics is therefore important in an era of climate changes and anthropogenic pressure on coastal areas that makes deltas vulnerable elements, threatened by degradation due to extraction of natural resources (oil and gases, and sand mining), land reclamation, sediment retention by reservoirs, and harbors development (Hackney et al, 2020;Hoitink et al, 2017Hoitink et al, , 2020.…”

“…In a more holistic approach, results from physics-based morphodynamic simulations and laboratory experiments (e.g., Lentsch et al, 2018;Rossi et al, 2016) have shown how the mobility of the channels decreases with increasing of the tidal energy, due to an enhanced ability of tidal currents to flushing away bed material sediment. Tidal action then prevents aggradation through backfilling, a process considered to be one of the main triggering mechanisms for channel avulsion (Hoitink et al, 2020;Slingerland & Smith, 2004).…”

Chaos in a simple model of a delta network

“…prediction | bifurcation | attractor | geomorphic models D eltas are landforms arising from the deposition of sediment by a river entering a standing body of water. Deltas worldwide are highly populated and widely relied upon for agriculture and navigation but are under threat of collapse due to relative sea-level rise and sediment-supply reduction (1)(2)(3). Additionally, delta deposits are important reservoirs for groundwater and hydrocarbons, and better understanding their stratigraphic architecture would improve prediction of subsurface fluid flow (4).…”