Tidal channels on tidal flats and marshes

Abstract

In shallow coastal settings channels provide a pathway for the tide to propagate and are, thus, a primary control on the sedimentation and ecology of these environments. Being shaped by bidirectional flows, tidal channels exhibit morphologies, which, despite apparent similarities, bear significant and fundamental differences to fluvial channels, specifically their scaling with size. This chapter considers the classification of tidal channels and the networks they form. We examine the hydrodynamics of shallow tidal channels, including asymmetry in period or velocity between the ebb and flood tides, and the hysteresis seen in stage-velocity curves in regions with large intertidal areas. Channel initiation may occur either through incision or by variations in rates of deposition. Tidal channels evolve over time and a number of relationships are presented that have been derived to describe the geometry of tidal channels. Mutually-evasive pathways of flood and ebb flows may produce cuspate meanders; a morphology unique to tidal channels. Of particular importance, in terms of preservation potential, is the development of meanders in channels and the resulting pointbars. Pointbars in tidal environments are often fully or partially detached from the bank by a channel formed by the subordinate tidal current, however their exact morphology varies being dependent on channel sinuosity and tidal asymmetry. Channels are preserved through infilling (as tidal prism is reduced) and through lateral accretion, particularly at meanders. Pointbars in tidal regions are generally heavily bioturbated in the upper tidal range, and mid-tidal zones will exhibit inclined stratigraphy, often with intercalated beds of muddier and sandier deposits.