392
13 Transport and Mixing in Coastal Ecosystems
sections we will illustrate the relative importance of both these mechanisms by
examples from various coastlines and estuaries.
13.2 Transport and Mixing in Estuaries
13.2.1 A Brief Orientation
As was discussed in Chap. 8, estuaries are the regions where fresh and salt
waters mix. Because nutrient fluxes are spatially concentrated within the estuarine mixing zone, estuaries usually have a higher biological productivity than
adjoining coastal and continental shelf water. Estuaries also provide convenient, sheltered locations for ever expanding human development. This trend
yields increasing volumes of nutrients, human-derived materials and pollutants.
As the river flow rates, tides and winds are variable, the distribution of salinity, nutrients and plankton population within estuaries are rarely stable. In
particular, during flood event, the discharge of freshwater may be so high that
sea water cannot intrude into the estuary, and the mixing will take place on
the boundaries of the flood plume (see Sect. 13.3). Many biological, chemical
and geological effects in estuaries are dependent, in part, on the dynamics of
circulation and mixing processes. Despite many efforts, a multidisciplinary perspective of how coastal estuaries function is only now emerging, with scientists
realizing the crucial importance of the interaction between physical, chemical,
geological and biological processes to sustain estuarine development. A complete summary of these interactions is beyond scope of this book. Rather, in this
section we will describe selected examples which illustrate interesting phenomena in estuaries. Some of these are sufficiently well understood that analytical
or numerical models have been developed. In other cases only a qualitative
description is given. For better clarity of explanation, we will separately consider the transport and fate of sediments, and influence of hydrodynamics on
nutrient transport and primary production.
13.2.2 Sediment Transport in Estuaries
An understanding of the distribution and movement of suspended particulate matter in estuaries is relevant to many other related studies, such as
dredging/spoil dumping operations and transport and fate of particle-bound
contaminants. In estuaries the transport may be inland or seawards due to
tidal and freshwater flow variations. Sediment may be stored for long periods
as bed material in intertidal zones. However, oscillatory motion produced by
surface waves generates additional vorticity and turbulence in the bed boundary layer, and enhances the diffusivity and hence transport entrainment and
suspension capacity of the flow. For example, Kana and Ward (1980) showed
that in a shallow coastal zone, sediment transport may increase up to 60 times
during storm wave conditions as opposed to calm conditions. For the estuarine environment this increase can be up to 40 times as was detected by Owen
13 Transport and Mixing in Coastal Ecosystems
sections we will illustrate the relative importance of both these mechanisms by
examples from various coastlines and estuaries.
13.2 Transport and Mixing in Estuaries
13.2.1 A Brief Orientation
As was discussed in Chap. 8, estuaries are the regions where fresh and salt
waters mix. Because nutrient fluxes are spatially concentrated within the estuarine mixing zone, estuaries usually have a higher biological productivity than
adjoining coastal and continental shelf water. Estuaries also provide convenient, sheltered locations for ever expanding human development. This trend
yields increasing volumes of nutrients, human-derived materials and pollutants.
As the river flow rates, tides and winds are variable, the distribution of salinity, nutrients and plankton population within estuaries are rarely stable. In
particular, during flood event, the discharge of freshwater may be so high that
sea water cannot intrude into the estuary, and the mixing will take place on
the boundaries of the flood plume (see Sect. 13.3). Many biological, chemical
and geological effects in estuaries are dependent, in part, on the dynamics of
circulation and mixing processes. Despite many efforts, a multidisciplinary perspective of how coastal estuaries function is only now emerging, with scientists
realizing the crucial importance of the interaction between physical, chemical,
geological and biological processes to sustain estuarine development. A complete summary of these interactions is beyond scope of this book. Rather, in this
section we will describe selected examples which illustrate interesting phenomena in estuaries. Some of these are sufficiently well understood that analytical
or numerical models have been developed. In other cases only a qualitative
description is given. For better clarity of explanation, we will separately consider the transport and fate of sediments, and influence of hydrodynamics on
nutrient transport and primary production.
13.2.2 Sediment Transport in Estuaries
An understanding of the distribution and movement of suspended particulate matter in estuaries is relevant to many other related studies, such as
dredging/spoil dumping operations and transport and fate of particle-bound
contaminants. In estuaries the transport may be inland or seawards due to
tidal and freshwater flow variations. Sediment may be stored for long periods
as bed material in intertidal zones. However, oscillatory motion produced by
surface waves generates additional vorticity and turbulence in the bed boundary layer, and enhances the diffusivity and hence transport entrainment and
suspension capacity of the flow. For example, Kana and Ward (1980) showed
that in a shallow coastal zone, sediment transport may increase up to 60 times
during storm wave conditions as opposed to calm conditions. For the estuarine environment this increase can be up to 40 times as was detected by Owen