High‐Frequency Tide‐Surge‐River Interaction in Estuaries: Causes and Implications for Coastal Flooding

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AbstractTide‐surge interaction creates perturbations to storm surge at tidal frequencies and can affect the timing and magnitude of surge in tidally energetic regions. To date, limited research has identified high‐frequency tide‐surge interaction (>4 cycles per day) in coastal areas, and its significance in fluvial estuaries (where we consider it tide‐surge‐river interaction) is not well documented. Water level and current velocity observations were used to analyze tide‐surge‐river interaction at multiple tidal and overtide frequencies inside of a shallow estuary. Near the head of the estuary, higher frequency harmonics dominate tide‐surge‐river interaction and produce amplitudes more than double that of wind and pressure‐driven surge. Bottom friction enhanced by storm‐induced currents is the primary mechanism behind the interaction, which is further amplified by within‐estuary resonance. High‐frequency tide‐surge‐river interactions in estuaries present a significant threat to human life, as the onset of flooding (in <1.5 hr) is more rapid than coastal storm surge flooding. Commonly used storm surge forecasting models neglect high‐frequency tide‐surge‐river interaction and thus can markedly underestimate the magnitude and timing of inland storm surge flooding.