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significant gravitational body forces of the form
[(
a
e
)/
a
]
g
per unit volume of effluent fluid, where
a
r
e
is ambient density and
e
is effluent fluid density. The
behavior of the plume thus depends upon the resultant of
the various buoyancy contributions due to temperature,
salinity, and suspended sediment concentration. For
example, negative buoyancy acts when sediment-laden
effluent jets of cool river water enter into marine basins at
delta fronts. The extent of influence of buoyancy on jet
behavi
o
r is
exp
ressed by the
densimetric Froude number
,
where
u
_
is the mean effluent velocity,
h
Inertial jet
(homopycnal)
r
a
Fr
is
the depth of the density interface from the surface of the
jet, and
u
/
gh
r
e
r
a
is the density ratio 1
(
e
/
a
). For values of
Fr
1, waves form at the effluent ambient interface;
these cause enhanced mixing, increased friction, and
greater deceleration of the buoyant fluid. The spreading
and expansion of a buoyant jet is best considered by refer-
ence to the production of superelevation of the effluent
arising from its buoyancy: the jet floats with its surface at
some small height (
Negatively buoyant
plume
(hyperpycnal)
Underflow
Fig. 6.55
Other kinds of coastal jets and plumes.
h
) above the ambient fluid.
In summary, three factors may influence the nature of
the sediment-laden freshwater jet itself: (i) the inertial and
turbulent diffusional interactions between the jet and the
ambient fluid; (ii) frictional drag exerted on the base of the
jet by the delta front slope; and (iii) any buoyant force due
to the jet's density contrast with the ambient fluid.
Jets dominated by their own inertia and by turbulent
diffusion are said to be
homopycnal
, with virtually the same
density for jet and ambient fluid. The majority of such jets
are dominated by turbulent effects. This is clear from a
simple calculation of an outlet Reynolds number of the
form
Re
o
for a considerable distance up the distributary channel.
Such jets are termed
hypopycnal
. As was discussed in the
context of estuary behavior, salt wedges are best developed
in deep channels with low tidal ranges. In large river deltas
like the Amazon the effluent jets remain dominant far onto
the shelf.
When the combined density of effluent jet water and
its suspended solids exceed that of the basin ambient
fluid (
where
u
o
is the mean cen-
terline outlet velocity,
h
o
and
b
o
are the depth and width of
the outlet, respectively, and
u
o
[
h
o
(
b
o
/2)]
0.5
/
1), the conditions are set for the jet to
underflow
in a state known as
hyperpycnal
(Fig. 6.55).
This is more likely to occur in lake waters since a sus-
pended load of at least or greater than 28 kg m
3
must
be present just to counteract the density of normal sea-
water. Perhaps the most spectacular underflowing delta
system is that of the Huang Hue, whose colossal sus-
pended load picked up on its passage through the central
China loess belt enables it to sink without trace in the
offshore region.
Waves and tides have a great effect on these simple jet
models of delta front dynamics. Wave power is substan-
tially reduced as waves pass from offshore areas over very
gently sloping nearshore zones; indeed some extremely
gentle slopes may cause almost complete dissipation of
wave energy. In coastal areas of high wave power relative
to river discharge, effluent jets may be completely dis-
rupted by wave reworking. The coastlines of such deltas
tend to be very much more linear in plan view than those
of more moderate to low wave power.
e
/
a
is the effluent kinematic vis-
cosity. Most deltas show outlet Reynolds numbers greater
than 3,000, indicating the dominance of turbulent mixing.
A turbulent jet will expand linearly with distance from the
outlet as the homopycnal jet expands laterally and verti-
cally. Delta fronts dominated by homopycnal flows are
commonest in lakes.
When the shoreface of the subaqueous delta slopes quite
gently and water depth is shallow relative to the magnitude
of the incoming effluent jet (Fig. 6.54), frictional effects
arising from bottom drag on the jet become very impor-
tant. Such plumes experience rapid seaward spreading,
deceleration, and hence deposition of bedload sediment.
Such friction-dominated jets quickly deposit sediment as a
distributary mouth bar
.
Low values of
Fr
1) suggest dominance by buoyant
forces whereby the outflow spreads as a narrow expanding
jet above a salt wedge (see Section 6.5.5) that may extend
(
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