On the Geometry of Tidal Channels

Aitor Ichaso and Robert W. Dalrymple
Queen's University, Kingston, ON

Virtually all tidal deposits, except for those on continental shelves, are channelized to some degree. The geometry of these channels, and especially their width, depth, sinuosity and curvature, will have a strong influence on the nature of the deposits that they create. Qualitative observations, theoretical considerations and quantitative measurements obtained from high-resolution satellite images indicate that the planform geometry of channels is strongly dependant on the relative importance of river and tidal currents. Channels with moderate to strong tidal influence show the expected exponential seaward increase in width because of the seaward increase in the tidal flux, whereas channels with less tidal flux (e.g., fluvially dominated distributary channels) do not show the same degree of seaward flaring. The sinuosity and radius of curvature, which influence the lateral extent of point-bar units and the longitudinal variability of facies within the point-bar deposit, show corresponding changes: the sinuosity decreases and the radius of curvature increases in a seaward direction through the tidally influenced/dominated part of the system. The “straight-meandering-straight” channel pattern reported in earlier studies appears to be restricted to systems with relatively little fluvial influence and even occurs in the tidal channels that characterize the abandoned or inactive portions of delta plains and the tidal marshes of microtidal and mesotidal back-barriers areas. These observations suggest that there is a geometric continuum between tidal point bars and the elongate tidal bars that characterize the seaward part of tide-dominated systems.