Infragravity currents in a small ría: Estuary-amplified coastal edge waves?

• Estuarine measurements show infragravity waves with periods at 4.3–4.8 min.
• A model shows that 3.5–4.5 min wave periodicities correspond to 3λ/4 resonances.
• The model shows that an effect of lowered runoff is to greatly enhance resonance.
• Water level amplitudes of 0.02 m at the mouth can drive currents of 0.4 m s−1.
• Time-scales show that the infragravity waves are probably coastal edge waves.

Observations are presented of estuarine infragravity oscillations in a very small estuary, the Ría de Santiuste, northern Spain, that discharges into a bay. Time-scale calculations and measurements indicate that the most likely source of amplified infragravity waves within the estuary are coastal edge waves, which appear to be resonating between the headlands of the bay, and onshore-offshore seiching of waters that may be resonating between the beach and the bay's seaward limit. Infragravity waves in the estuary have a narrow, power-dominant periodicity range of 4.3–4.8 min. Generally, only very weak waves, and at other periodicities, occur within the tidal river when it is unaffected by the tide. Although infragravity water-level amplitudes are small in the tidal river, typically ∼0.01 m during high water, the corresponding velocity amplitudes are significant, typically ∼0.1 m s−1, despite the occurrence of high runoff conditions during these measurements. Within the estuary and close to the mouth, near-bed salinity shows considerable variability that is a consequence of salt wedge oscillations with periodicities that are within the range of 4.3–4.8 min. Model results for simulated neap to spring high-water water levels show that wave periodicities in the range 3.5–4.5 min correspond approximately to 3λ/4 resonances. The model also indicates that an effect of low runoff is to greatly enhance resonant behaviour within the estuary, such that water level amplitudes of 0.02 m at the mouth can produce wave currents as fast as 0.4 m s−1 within the estuary, compared with ∼0.1 m s−1 during high runoff.