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

Tide-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 estuaries is not well documented. Water level and current velocity observations were used to analyze tide-surge interaction at multiple tidal and overtide frequencies inside of a shallow estuary. Near the head of the estuary, higher frequency harmonics dominate tide-surge 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 resonance. Extreme high frequency tide-surge interactions in estuaries present a significant threat to human life, as the onset of flooding (in < 1.5 hrs.) is more rapid than coastal storm surge flooding. Sea level rise is expected to increase the frequency of extreme tide-surge interaction events by increasing water levels in vulnerable systems, allowing for resonant conditions to be more easily reached. Commonly used storm surge forecasting models neglect high frequency tide-surge interaction and thus can markedly underestimate the magnitude and timing of inland storm surge flooding.