Relative sea‐level rise in the coming century will increase the risk of flooding and shoreline retreat on most major river deltas. River deltas can counteract flooding and shoreline retreat by depositing sediment on their surface. Yet, it is unclear what processes influence sedimentation and its variability on deltaic surfaces. Towards this end, we conducted a numerical modeling study in Delft3D to understand how floods, tides, and vegetation affect sedimentation rates and their spatial variability on islands in a deltaic system. Our experiments use a fully calibrated and validated hydrodynamic model of Wax Lake Delta, LA, USA. We analyzed eight numerical experiments that include a control simulation with no floods, tides, or vegetation, and seven simulations where we add in floods, tides, and vegetation. Our results clearly show that floods and tides have opposing effects. Compared to the control, floods introduce more sediment and increase the mean sedimentation rate, whereas, tides spread sediment over a larger area and decrease the mean sedimentation rate. Vegetation has a negligible effect on mean sedimentation rates but does shift sedimentation closer to the shoreline and to higher elevations. Overall, the amount of sedimentation on an island depends on its hydrological connectivity with the surrounding distributary channels. These results show that hydrologically well‐connected deltaic islands subject to tidal and riverine flooding aggrade their surfaces more evenly, which may be ideal for preventing inundation from relative sea‐level rise.
A major thread of theoretical research on the response of shorelines to changing boundary conditions has adapted the moving‐boundary approach from heat transfer and solidification/melting. On sufficiently short time scales, shorelines respond to changes in relative sea level in a simple, geometrically predictable way. On longer time scales, their behaviour becomes far more complex and interesting, because the surface over which the shoreline moves is itself continually modified by morphodynamics that depend sensitively on shoreline location. This makes the shoreline the archetype of moving‐boundary problems in morphodynamics, and subject to potentially counterintuitive behaviours over time scales on which the sediment surface modifies itself as relative sea level changes. We review existing moving‐boundary theories and propose two significant extensions to allow inclusion of first‐order effects of waves and tides. First, we show how wave effects can be included via planform diffusion linked to river‐mouth location, which results in shoreline smoothing during delta‐lobe growth and localized transgression after channel abandonment. Tides produce a low‐gradient region in which the sea and land overlap; we show how this can be treated in a moving‐boundary framework by replacing the shoreline with a ‘mushy region' so that the handoff from land to water occurs over a zone rather than a line. We also propose that the moving‐boundary approach can be readily generalized to other dynamic moving boundaries, such as those separating different regimes of river transport. The shoreline thus serves as a prototype for modelling dynamic facies boundaries along the whole source–sink system. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
more » « less- NSF-PAR ID:
- 10456253
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Earth Surface Processes and Landforms
- Volume:
- 45
- Issue:
- 1
- ISSN:
- 0197-9337
- Page Range / eLocation ID:
- p. 96-108
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Tide‐influenced deltas are among the largest depositional features on Earth and are ecologically and economically important as they support large populations. However, the continued rise in relative sea level threatens the sustainability of these landscapes and calls for new insights on their morphological response. While field studies of ancient deposits allow for insight into delta evolution during times of eustatic adjustment, tide‐influenced deltas are notoriously hard to identify in the rock record. We present a suite of physical experiments aimed at investigating the morphological response of tide‐influenced deltas subject to relative sea‐level rise. We show that increasing relative tidal energy changes the response of the delta because tides effectively act to remove fluvially deposited sediment from the delta topset. This leads to enhanced transgression, which we quantify via a new methodology for comparing shoreline transgression rates based on the concept of a ‘transgression anomaly’ relative to a simple reference case. We also show that stronger tidal forcing can create composite deltas where distinct land‐forming processes dominate different areas of the delta plain, shaping characteristic morphological features. The net effect of tidal action is to enhance seaward transfer of bedload sediment, resulting in greater shoreline transgression compared to identical, yet purely fluvial, deltaic systems that exhibit static or even regressive shorelines. © 2019 John Wiley & Sons, Ltd.
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Tide and salinity data collected at minute intervals over multiple semidiurnal tides were used to investigate the source of water (e.g., seawater, river, groundwater and rain) and their relative timing in mixing at the mouth of a river, a tidal creek at mid-estuary and a tidal creek at the shoreline at the head of a tropical mangrove estuary. Our objectives were to document the temporal changes in tide induced water level changes and salinity at each location and to use the relationship between salinity and water level to elucidate the sources of water and the timing of different sources of water in the hydrologic mixing processes. The data trends in tide vs. salinity (T-S) plots for the river mouth revealed mixing with seawater during rising tides and freshwater diluted seawater (brackish) drainage from the mangrove forest during ebb tides. In the mangrove creek at mid-estuary, the data trends in the T-S plots for rising tides initially showed constant salinity, followed by sharp rises in salinity to peak tide caused by seawater intrusion. The salinity decreased precipitously at the start of tidal ebbing due to influx of freshwater (rain) diluted brackish water from the mangrove forest. The data trends in the T-S plots for the tidal creek at the shoreline located at the estuary head showed constant salinity which decreased only near peak rising tide because of river dilution. During tidal ebbing, the salinity further decreased from groundwater influx before increasing to background salinity, which stayed constant to low tide. Establishing T-S patterns for multiple locations in mangrove estuaries over sub-tidal to tidal scales define the expected salinity variations in seawater-freshwater mixing which can be used to (1) establish baseline hydrologic and salinity (hydrochemical) conditions for temporal and spatial assessments and (2) serve to guide short to long-term sampling regimes for scientific studies and estuarine ecosystem management.more » « less
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Abstract Coastal rivers that build deltas undergo repeated avulsion events—that is, abrupt changes in river course—which we need to understand to predict land building and flood hazards in coastal landscapes. Climate change can impact water discharge, flood frequency, sediment supply, and sea level, all of which could impact avulsion location and frequency. Here we present results from quasi‐2D morphodynamic simulations of repeated delta‐lobe construction and avulsion to explore how avulsion location and frequency are affected by changes in relative sea level, sediment supply, and flood regime. Model results indicate that relative sea‐level rise drives more frequent avulsions that occur at a distance from the shoreline set by backwater hydrodynamics. Reducing the sediment supply relative to transport capacity has little impact on deltaic avulsions, because, despite incision in the upstream trunk channel, deltas can still aggrade as a result of progradation. However, increasing the sediment supply relative to transport capacity can shift avulsions upstream of the backwater zone because aggradation in the trunk channel outpaces progradation‐induced delta aggradation. Increasing frequency of overbank floods causes less frequent avulsions because floods scour the riverbed within the backwater zone, slowing net aggradation rates. Results provide a framework to assess upstream and downstream controls on avulsion patterns over glacial‐interglacial cycles, and the impact of land use and anthropogenic climate change on deltas.
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Abstract It is widely recognized that waves inhibit river mouth progradation and reduce the avulsion timescale of deltaic channels. Nevertheless, those effects may not apply to downdrift‐deflected channels. In this study, we developed a coupled model to explore the effects of wave climate asymmetry and alongshore sediment bypassing on shoreline‐channel morphodynamics. The shoreline position and channel trajectory are simulated using a “shoreline” module which drives the evolution of the river profile in a “channel” module by updating the position of river mouth boundary, whereas the channel module provides the sediment load to river mouth for the “shoreline” module. The numerical results show that regional alongshore sediment transport driven by an asymmetric wave climate can enhance the progradation of deltaic channels if sediment bypassing of the river mouth is limited, which is different from the common assumption that waves inhibit delta progradation. As such, waves can have a trade‐off effect on river mouth progradation that can further influence riverbed aggradation and channel avulsion. This trade‐off effect of waves is dictated by the net alongshore sediment transport, sediment bypassing at the river mouth, and wave diffusivity. Based on the numerical results, we further propose a dimensionless parameter that includes fluvial and alongshore sediment supply relative to wave diffusivity to predict the progradation and aggradation rates and avulsion timescale of deltaic channels. The improved understanding of progradation, aggradation, and avulsion timescale of deltaic channels has important implications for engineering and predicting deltaic wetland creation, particularly under changing water and sediment input to deltaic systems.
