| Coastal deltas house more than 335 million people worldwide in some of the largest population centers in the world, including growing megacities such as Shanghai, Dhaka, and Bangkok. These populations often rely heavily upon groundwater resources to meet domestic, agricultural, and industrial water demands—making the sustainability of fresh groundwater resources critical to ensuring the longevity of coastal communities. This research uses delta morphology as a tool for understanding the distribution of fresh and saline groundwater within coastal deltas.
The overarching goal of this research is to characterize the amount and spatial distribution of fresh groundwater resources in coastal deltas and determine the vulnerability of deltaic groundwater to salinization. The research hypothesis is that the amount of fresh groundwater within a delta is dependent on the permeability distribution within the subsurface of the delta, which is ultimately controlled by the morphodynamic influences within the deltaic basin. The hypothesis was examined by developing over 200 unique coastal delta models using 2D coastal hydrodynamic and sediment transport modeling (Delft3D-FLOW). The sediment models were linked to a density-dependent groundwater flow model (SEAWAT, run through iMOD) to understand how delta morphology impacts the fresh and saline groundwater volume in coastal deltas. (2018-2023). A short description of methodology and results for three studies encompassed in this research is provided.
Additional information can be found in: Anderson, A.M. (2023). Fresh Groundwater Availability in Costal Deltas. (Doctoral Dissertation). ¶¡ÏãÔ°AV, Burnaby, British Columbia, Canada. |
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Study 1: Controls on coastal delta formation under varying morphodynamic and basin conditions. |
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| Data from 51 of the world’s largest and most populated deltas are collected from previous scientific publications to determine the range of fluvial discharge, wave height, and tidal amplitude that exist within deltas around the world. Additionally, bathymetric gradient, incoming sediment concentration, and median grain size are also analyzed. The numerical morphodynamic model Delft3D-Flow is used to simulate the formation of coastal deltas under varying morphodynamic and basin conditions. The models mimic a sediment bearing river debouching into an ocean basin. A total of 207 models are used to span the full range and combination of fluvial and marine influence. A distance-based generalized sensitivity analysis is used to determine the sensitivity of delta formation to changes in fluvial discharge, bathymetric gradient, wave height, and sediment concentration. Large deltas can form across a wide range of wave height and tidal amplitude, indicating that the presence of marine influences does not inhibit sediment deposition. The numerical models created in this study support these observations; models that have high wave and tidal influence can form deltas in environments where there is adequate fluvial discharge supplying sediment. For a delta to form in the models, the following criteria must be met: 1) average annual discharge is greater than > 2,000 m3/s; 2) sediment concentration is 0.05 kg/m3, and 3) fluvial discharge comprises over 50% of the morphodynamic influence. When one or more of these conditions are not met, there is a greater likelihood that a delta will not form. Differences in sediment concentration and bathymetric gradient were found to impact whether models with the same balance of morphodynamic influences either form a delta or do not. Although the classic Galloway ternary diagram is a useful tool to compare delta morphology, it does not explain why some combinations of fluvial, wave, and tidal influences result in delta formation while others do not. |
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Study 2, Sensitivity of subsurface permeability and groundwater flux in coastal deltas to their morphodynamic and geomorphic characteristics. |
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| Two-dimensional morphodynamic models are used to explore the range of subsurface permeability, hydraulic gradient, and groundwater flux within three end-member delta types (fluvial, wave, and tidal). The connectiveness of the subsurface permeability is quantified in addition to estimating the horizontal heterogeneity and anisotropy that exists within deltaic permeability. A distance-based generalized sensitivity analysis is used to investigate the impact morphodynamic influences (fluvial, wave, and tidal), basin conditions (sediment concentration, bathymetric gradient), and geomorphic characteristics (number of channels, shape of the delta plain, delta from rugosity) have on the subsurface permeability, hydraulic gradient, and connectivity. Key findings include: 1) The overall permeability in deltaic landforms has a median value of 4.0x10-12 m2, relating to a hydraulic conductivity value of 2x10-5 m/s. The average hydraulic gradient is 4x10-4. Wave deltas are the most permeable and have the lowest hydraulic gradient while tidal are the least permeable and have the highest hydraulic gradient. 2) The high permeability bodies are associated with current and previous channelization in a delta and are highly connected horizontally. This high connectivity potentially allows for salinization of the subsurface through permeable pathways as sea level rises. The channel network is most evenly distributed in tidal deltas, resulting in high permeability bodies located throughout the entire delta plain. High permeability bodies in wave deltas are only located in the channel levees and are not pervasive throughout the entire delta. 3) Wave deltas may be most susceptible to inundation and groundwater salinization through marine encroachment due to the lower hydraulic gradient and increased permeability. Anderson, A.M., D.M. Allen, and J.G. Venditti. Sensitivity of groundwater flux and subsurface permeability to morphodynamic and geomorphic characteristics of coastal deltas. Water Resources Research. November 27, 2023. DOI: https://doi.org/10.1029/2022WR034136. |
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Study 3, Impact of delta morphology on vulnerability to groundwater salinization in large coastal deltas. |
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| The numerical modeling code iMOD-WQ is used to determine the susceptibility of shallow deltaic aquifers to groundwater salinity in fluvial, wave, tidal deltas.
The role of the river network and recharge in freshening of the shallow subsurface is investigated.
Numerical modeling, along with previously published information on morphodynamics, climate, and population, is used to determine the vulnerability of large coastal deltas around the world to future groundwater salinization. Key findings include: 1) Wave deltas are more susceptible to groundwater salinity in the shallow subsurface than fluvial and tidal deltas. Wave deltas also have the highest salinization rates when groundwater is abstracted. These deltas are particularly sensitive because of the increased hydraulic conductivity within the delta, low hydraulic gradient, and fewer river channels. 2) The volume of saline water in the shallow subsurface within deltas around the world varies between 36% and 89% of the total groundwater volume, depending on the delta type and the amount of recharge the delta receives. Deltas receiving less recharge are more susceptible to groundwater salinity compared to deltas in wet climates that receive substantial recharge (more than 100 mm/yr). 3) Although deltas located in a wet climates (receiving more than than 100 mm/yr of recharge) have greater freshwater volume in the shallow subsurface, these deltas are most impacted by groundwater abstraction. Freshwater volumes of groundwater in wet climates can decrease by up to 24% in a wave delta and 5% in a fluvial delta within 500 years. Deltas located in dry climates are more resistant to decreases in groundwater recharge. 4) The number and placement of rivers in a delta play an important role in determining the fresh groundwater volume. In dry climates, rivers are responsible for most of the freshening that has occurred in the past 8 ky of modern delta formation. In wet climates, the role of the river network to supply or remove water from the shallow subsurface is highly sensitive to changes in groundwater removal and supply when channels are not abundant throughout the delta. 5) The freshwater lens that develops in the shallow subsurface is often deepest surrounding the river networks in fluvial and tidal deltas. In wave deltas, the freshwater lens is deepest adjacent to the river network in the subaerial levees. Salinity is most likely found in areas with low hydraulic conductivity that are not near a river channel. |
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