Environmental Engineering Reference
In-Depth Information
ESTUARIES AND LAGOONS
Estuaries and lagoons partially enclose saline and freshwater bodies, dominated by fluvial
or tidal processes. Estuaries are the freshwater-fed, submerged lower reaches of
structural, river- or glacier-eroded valleys. They usually lie orthogonal to the coastline.
Lagoons impounded by barriers are coast-parallel, the direct product of coastal processes
and are also river- and/or rain-fed. Both types of embayment are classified by their water
chemistry and exchange processes. Stratification, or vertical separation, occurs in
estuaries lacking significant tidal or current mixing. Freshwater and denser salt water
override and undercut each other respectively in river-dominated estuaries. The inland
extent and slope of the
saltwater wedge
is determined by the opposing vigour of river
discharge. Its shifting boundary marks a concentrated zone of clay-silt particles where
flocculation
into larger aggregates encourages them to settle as mud. Tidal mixing
produces vertical homogeneity, with salinity increasing steadily seawards. Open lagoons
connected to the sea by tidal passes normally maintain balanced tidal exchanges, except
in arid-zone
sabkhas
, where strong evaporation precipitates salts and reduces return flow.
Closed lagoons are rain-fed and, whilst storm washover, lateral seepage through
permeable barriers and evaporation may restore some salinity, it varies between
freshwater-brackish-hypersaline extremes. Narrow
tidal passes
through barriers connect
lagoons, and estuaries with restricted mouths, with the sea. They create their own distinct
water and sediment fluxes to form microcosms of full-scale coastal land-systems (Figure
17.9).
Tidal flats
, protected from large waves and fringed by
salt marsh
and
mangrove
swamp
in temperate and tropical latitudes respectively, dominate the estuarine and
lagoonal intertidal landsystem. Beaches and dunes are rare. Clear patterns of particle size,
bed forms and related vegetation succession emerge, despite complex two-way
movements of water and sediment bodies which migrate with river and tidal pulses. Sand
moves as bed load and is deposited most commonly in non-turbid outer and lower parts
of the system where currents are at their strongest. Tidal sandflats exhibiting large-scale
megaripple
and
sand wave
bedforms attest to their scour (Plate 17.3). Mud is moved and
deposited from turbid suspension in low-energy inner and upper estuarine environments.
A
halophyte
(salt-tolerant) vegetation succession responds to, and in turn assists, this
zonation. Pioneer algal mats help to arrest and stabilize mud particles. This permits
grasses, sedges and rushes to colonize the upper intertidal zone, with its progressively
shorter and fewer periods of
