Geoscience Reference
In-Depth Information
(a) Open water setting (b) Nearshore setting
(1) Wind motion over the sea-surface
generates sea waves
(5) Shallowing leads to an
increase in wave height.
Wave break when wave
height equals water depth
Wave length (L)
Crest
Trough
Wave
height (H)
(4) As waves approach
the shore water shallows
below wave base and
water motion leads to
interaction with sea-floor
sediments
(3) Orbital water
motion decreases
with depth
(2) Sea waves induce
an oscillatory motion
within water particles
Effective wave base (D)
(D = L/2)
Fig. 1.9
Schematic diagram illustrating wave motion in (a) open-water settings, and (b) as waves approach the shoreline.
low slope angles and are thought to be a major
cause of sediment distribution across contin-
ental shelf (see Chapter 10) and slope settings.
In low-density flows, most of the fine sediment
is transported in suspension. This mechanism
of transport is common at the distal ends of
turbidite deposits where the finest sediment has
remained in suspension, and along the distal
margins of deltas where fine suspended sediment
settles out along the delta front (see Chapter 7).
Settlement of fine grained suspended sediment is
enhanced where mixing of fresh and saltwater
occurs. Under these conditions, even slight in-
creases in salinity (
(a)
D2
D1
Turbidite
current
High density currents D1 > D2
(b)
D1
D2
Slow settling
of sediment
in deeper
water
1) will promote the aggrega-
tion of fine clay particles. This process is known
as flocculation and leads to an increase in grain
size and thus in grain settling velocity. Floccula-
tion is a common process in estuarine environ-
ments (see Chapters 7 and 9) and in salt marshes,
and may lead to the development of zones of
high turbidity and fine sediment deposition.
The largest proportion of the contaminant
load in sediment systems is transported by the
particulate matter. For example, Gibbs (1977)
suggested that up to 90% of the metal load is
transported by sediments in rivers, but this can
vary from metal to metal. Similar observations
have been made for organic contaminants, such
as chlorinated organic compounds. This particu-
late portion of the contaminant load comprises
contaminant-rich grains (e.g. metal sulphide
grains from tailings effluent) or contaminant
>
Low density currents D1 < D2
Fig. 1.10
Schematic diagrams illustrating differences in density
flows as fluid enters a standing body of water. (a) High density
flows occur where the entering fluid has a higher density than
the standing water body. (b) Low density flows occur when the
situation is reversed. (Adapted from Selley 1994.)
and create a density current (Fig. 1.10a; see
Chapter 4). Where the fluid entering the body of
water has a lower density, for example where
freshwater enters the sea, flow will typically occur
as a plume across the water surface (Fig. 1.10b).
In the former case, a specific type of density cur-
rent, known as a turbidity current, is commonly
generated. These are capable of transporting
very large volumes of sediment across even very
