Storm-driven morphodynamics of a dredged inlet channel revealed by multi-timescale bathymetric analysis at the Mondego River mouth, Portugal

This study analyzes the storm-driven sediment dynamics of the periodically dredged, wave-dominated tidal inlet of Figueira da Foz (Portugal), using high-resolution bathymetric surveys and dredging records spanning more than 14 years. The Digital Elevation Models derived from repeated surveys reveal the development of a persistent submerged sandbar that forms an ebb-delta system, in which wave-driven processes largely exceed tidal forcing, promoting predominantly alongshore sediment bypassing. Sediment budgets computed at multiple temporal scales show a marked seasonal contrast. During summer periods, morphological changes are limited and sediment variability is largely controlled by dredging operations. In contrast, winter conditions are dominated by storm-driven sediment fluxes, with individual storm events mobilizing several hundred thousand cubic meters of sediment, frequently exceeding annual dredging volumes. For example, the winter of 2010/2011 recorded a net accretion of +287,329 m³ despite seven storm events, while the February 2017 storm event (PSI ≈ 7,222 m²·h) mobilized over 100,000 m³ within a two-week period. These events drive rapid reorganization of the ebb shoal, channel infilling, and updrift beach progradation, periodically compromising navigation depth within the inlet channel. Despite repeated dredging interventions (annual volumes consistently exceeding 100,000 m³ and reaching up to 400,000 m³ in recent years), the submerged sandbar consistently re-establishes at depths of approximately −5 to −6 m, indicating a strong tendency toward a storm-controlled dynamic equilibrium. The long-term natural infilling rate of approximately 1,000 m³ day⁻¹, derived from 14 years of sediment budget analysis, underscores the system's high resilience to anthropogenic interventions. The results demonstrate that storm-driven sediment dynamics impose fundamental constraints on dredging efficiency and channel maintenance strategies, highlighting the need to explicitly account for extreme wave conditions in the engineering management of wave-dominated inlet channels.