Effects of different wave calculation methods on groundwater flow and salt transport in coastal aquifers under irregular waves

Abstract

This study evaluates the Nielsen set-up empirical equation's applicability for groundwater dynamics under irregular waves through numerical simulation, comparing phase-resolving and phase-averaged methods under three peak enhancement factor (γ) conditions. Key findings demonstrate that both numerical methods generate stronger horizontal pressure gradients than the empirical equation, yielding larger upper saline plume (USP) features (area, salt content) and greater saltwater penetration depths. The phase-averaged method produces more persistent pressure gradients, resulting in enhanced USP characteristics compared to the phase-resolving method, which promotes broader salt-freshwater mixing through wave-induced water level fluctuations. Reduced γ values correlate with more uniform wave energy distribution, amplifying swash extent and pore pressures, thereby intensifying USP quantities and method-dependent discrepancies. Interface flux analysis reveals phase-averaged conditions yield higher peak infiltration (30 % greater) and marginally stronger submarine groundwater discharge (SGD) than phase-resolving results, with γ reduction further accentuating these differences. The set-up empirical equation consistently underestimates flow and salt dynamics by 21∼53 % across main metrics (USP area: ∼53 %; mixing area: ∼49 %; SGD: ∼24 %; total influx: ∼21 %), demonstrating its unreliability for wave-groundwater coupling systems. These findings strongly support the need for physics-based numerical approaches in coastal aquifer management, particularly for regions experiencing intensified wave variability. The study establishes γ as a key modulator of USP dynamics and method selection criteria for subsurface transport modeling.