Sensitivity of Subsurface Permeability in Coastal Deltas to Their Morphodynamic and Geomorphic Characteristics

The amount of fresh water moving through coastal deltas worldwide is controlled by the complex subsurface structures within a delta. Morphodynamic influences produced by the feeding river, waves, and tides, in addition to sea level transgressions and regressions, have resulted in deltaic aquifers that are highly heterogeneous. We use 171 unique two-dimensional morphodynamic models to explore the range of subsurface permeability, hydraulic gradient, and groundwater flux within three end-member delta types (fluvial, wave, and tidal). We quantify the connectiveness of the subsurface permeability and estimate the horizontal heterogeneity and anisotropy of the permeability. A distance-based generalized sensitivity analysis is used to investigate the impact morphodynamic influences (fluvial, wave, and tidal), basin conditions (sediment concentration and bathymetric gradient), and geomorphic characteristics (number of channels, shape of the delta plain, and shoreline rugosity) have on the subsurface permeability, hydraulic gradient, and connectivity. We find that the median permeability in deltaic landforms is 4.0 x 10-12 m2 (relating to a hydraulic conductivity of 2.1 x 10-5 m/s), the average hydraulic gradient is 3.9 x 10-4, and the mean specific discharge is 1.3 x 10-8 m/s. High permeability bodies are highly connected and are associated with channelization. Subsurface permeability, hydraulic gradient, and the connectiveness of high permeability areas are sensitive to morphodynamic influences (fluvial, wave, and tidal) and the geomorphic characteristics (number of channels and shoreline rugosity) within a delta. Since morphodynamic influences and geomorphic characteristics are easily identified by looking at the surface of the delta, we suggest that the deltaic subsurface can be characterized by identifying features on the delta surface. Delta morphodynamics controls the permeability, hydraulic gradient, and connectivity of high permeability bodies in coastal deltasHigh permeability bodies are associated with current and previous channelization and are highly connectedWave deltas may be especially susceptible to inundation and groundwater salinization due to high permeability and low hydraulic gradient

Automated summary

The amount of fresh water in coastal deltas is controlled by subsurface structures. This study explores the range of subsurface permeability and hydraulic gradient in three types of deltas (fluvial, wave, and tidal) using morphodynamic models. The results show that deltaic landforms have a median permeability of 4.0 x 10-12 m2, an average hydraulic gradient of 3.9 x 10-4, and a mean specific discharge of 1.3 x 10-8 m/s. The study also finds that high permeability bodies are associated with channelization and that morphodynamic influences and geomorphic characteristics have a significant impact on the subsurface permeability and connectivity.