The Morphodynamic characteristics of a mega tidal delta over decadal to centennial timescales: a model-based analysis

Abstract

About 500 million people live in the world’s river deltas, which are by definition morphologically active. Hence, an understanding of the geomorphological processes affecting deltas is essential to improve our understanding of the risks that delta populations face. Unfortunately, there is little reliable data on river deltas, meaning that the task of demonstrating the links between morphodynamic and environmental change is challenging. There have been significant human impacts on the morphodynamics of deltas (e.g. reduced sediment inflows, anthropogenic subsidence, etc.) in the past century, and these impacts may intensify in the future. This research aims to answer the questions of how tidal delta morphology evolves over multi-decadal timescales under multiple drivers. A 2D idealised morphodynamic model was developed (in Delft3D) that represents the essential morphological attributes of an idealised tide-dominated delta (inspired by the contemporary GBM delta, a classic tidally dominated delta with highly vulnerable people and environment). A series of model simulations over 100 years, were then used to explore the influence of three key drivers, both individually and together, on deltaic mophodynamics: (i) varying combinations of water and sediment discharges from the upstream catchment, (ii) changes in the rate of relative sea-level rise (RSLR), and (iii) selected human interventions within the delta, such as polders, cross-dams and changing land cover.

Model simulations revealed that all of the drivers considered here influence tidal asymmetry (defined as differences in intensity and duration between ebb and flood tidal flows) over the delta. However, the relative magnitude of flood:ebb flow ratio varies between 0.10 (high fluvial discharge) and 1.0 (Cross-dams). This is important because the tidal asymmetry and rate of sediment supply together effect residual flows, patterns of erosion and accretion, aggradation and progradation of the delta and hence the overall sub-aerial delta morphodynamics. As expected, the area of the simulated sub-aerial delta increases with increasing sediment discharge, but decreases with increasing water discharge. However, delta progradation rates are more sensitive to variations in water discharge than variations in fluvial sediment supply. Human modifications are important. For example, the sub-aerial delta shrinks with increasing RSLR, but it does not when the sub-aerial delta is polderised. Indeed, the use of polders is found to lead to an increase in delta area over time, provided the polders are restricted from erosion. However, the polders are vulnerable to flooding as they lose relative elevation. Cross-dams built to steer zones of land accretion within the delta accomplish their local goal, but may not result in net land gain at the scale of the delta. When these factors were combined, the simulations revealed that the effect of human interventions in the form of cross-dams and polders dominate the pattern of erosion and accretion over the delta with the effect of environmental change such as fluvial discharge and RSLR embedded on top of that.

Lessons learnt for the contemporary GBM delta implies that the cross-dams and polders have been the main control of the morphodynamic evolution of the delta over the last 60 years. However, with likely increased fluvial water discharge and RSLR, sediment starved polders will only increase the vulnerability of the poldered area from regular flooding. But, the future trend of morphological evolution of the GBM delta vastly depends on the local human intervention to the continuing trend of RSLR and varying fluvial discharges. This methodology could be used to analyse the prognosis for other delta types. In summary, this research (i) provides guidelines to understand the drivers stimulating morphodynamic changes on large deltas, (ii) helps to evaluate the possible effects of future scenarios, and (iii) forms a basis to plan future action.

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Identifiers

ORCID for Stephen Darby: orcid.org/0000-0001-8778-4394

ORCID for Robert Nicholls: orcid.org/0000-0002-9715-1109

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