3 - Estuarine sediment dynamics

Abstract

The estuarine hydrodynamic regimes define the type of bed sediments and these, in turn, the fauna and flora both on the bed and in the water column. Sediments are mobile, and their dynamics occur at time scales of millennia (the geomorphological time scales), of the tides, of storms, of river floods, of the seasons, and of turbulence (seconds to minutes). Erosion-deposition events occur in estuaries on daily (ebb-flood), weekly (spring-neap), lunar (tidal), equinoctial, and seasonal (wet-dry seasons) cycles, as well as at the time scales of millennia (the geomorphological time scales), of storms, of river floods, and of turbulence (seconds to minutes). This creates the removal, settlement, and storage of sediments. In addition, sediment delivery occurs both from the catchment and the coastal zone. Coupled with the underlying geomorphology, often with coastal soft coasts, this creates estuaries that are both sediment rich and also highly vulnerable. Estuaries are also sites of tidal pulsing of sediments moving upstream under relatively low river flow and high tidal flow conditions, and the freshwater pulsing downstream of sediments under high river flow and weaker tidal conditions. Storms in coastal waters may result in coastal sediment being transported and deposited in estuaries. All of these factors combine to create predominantly sedimentary environments and what are regarded as ephemeral environments, which may be either infilling or migrating seawards. The dynamics of the sediments, both on the bed and in the water column, determine their biological structure and functioning, and has repercussions for contaminant fate and effect, for the use and export of nutrients, for productivity, and for all users and uses of the estuary. This chapter explains those dynamics.