Tidal asymmetry and velocity skew over tidal flats and shallow channels within a macrotidal river delta

Abstract

It is often assumed that, in short, shallow estuaries, at leading order velocity and elevation are exactly out of phase, so that duration asymmetries in the rise and fall of the tide should be manifest as skewed velocities. We observed alternating ebb‐ and flood‐dominant velocity skew in response to the spring‐neap modulation of incident asymmetry generated by the mixed, mainly semidiurnal astronomical tides within the macrotidal Skagit River delta in Puget Sound, Washington. We describe three factors that may contribute to local asymmetries: (1) ebb dominance caused by phase lags between the surface gradient and local depth, (2) ebb dominance due to fluvial discharge, and (3) near‐bed flood dominance due to baroclinicity. Large spring tides led to greater frictionally generated phase lags and resulted in ebb‐dominant velocity skew. This ebb dominance caused by tidal drainage was reinforced by fluvial discharge across the tidal flat at lower‐low water. The baroclinic component of this discharge, however, produced flood‐dominant near‐bed velocity skew that countered the ebb dominance of the frictional effects. The balance of these processes depends strongly on the spring‐neap cycle, magnitude of river discharge, and position within the tidal flat and channel system. Our observations are notable in the context of previous studies describing these processes because our analyses indicate that these mechanisms are relevant over very short spatial scales of just a few kilometers and in very shallow systems.