Geoscience Reference
In-Depth Information
Wind/wave action
and morphology
River action
Wind (speed, duration,
direction and fetch)
River
3
Erosion
Transportation
2
surface currents
1
Resuspension
0
Bioproduction
3
2
1
Delta
sedimentation
wave base
under currents
0 1 2 3
'pelagic' sedimentation
Accumulation
River plume
sedimentation
Slope > 5%
Transportation
Turbidity
currents
Turbidity
currents
Rate of deposition ( v )
Transportation
Accumulation
wave base
Accumulation
d
v
Bottom dynamics
v
D T-A
d
Distance from river mouth ( x )
Coarse Fine
Grain size ( d )
1. Erosion (winnowing) - coarse deposits
2. Transportation - coarse and fine deposits
3. Accumulation - fine deposits (= "pelagic" sedimentation)
4. Turbidific sedimentation - fine deposits
1. Delta sedimentation - coarse deposits
2. River plume sedimentation - coarse and fine deposits
3. Turbidific sedimentation - fine and coarse deposits
Fig. 4.8 Illustration of major sedimentological and bottom-dynamics processes in lakes. (Modified from Håkanson & Jansson 1983.)
in lakes (Håkanson & Jansson 1983): (i) an
energy factor related to the effective fetch and
the wave base (see the ETA diagram in Fig. 4.6);
(ii) a form factor related to the percentage of
the lake bed above the wave base (see Fig. 4.9);
and (iii) a lake slope factor related to the
fact that slope-induced transportation (turbidity
currents) may appear on bottoms inclining more
than 4 -5%.
One approach to calculate the areas where
resuspension occurs (the ET areas) is based on
the wave base and the form of the lake (the form
factor Vd
form of the lake, which in turn is calculated
from the form factor (
volume development,
Vd ). An equation expressing A WB , as a function
of Vd , the area of the lake (Area in m 2 ) and the
maximum depth of the lake ( D max in m), is also
given in Fig. 4.9. The ET areas are generally
larger than 15% of the lake area because there
is always a shore zone dominated by wind/wave
activities, at least in all lakes larger than approx-
imately 1 ha. One can generally also in shallow
lakes find sheltered areas and deep holes with
more or less continuous sedimentation, i.e.
areas that actually function as A areas, so the
upper boundary limit for ET is often set at 99%
of the lake area.
=
the maximum
depth, as illustrated in Fig. 4.9). The wave
base (WB), which is set equal to the depth, D TA ,
separating T areas and A areas is given by:
=
D m / D max ; D max =
4.2.2.3 Post-depositional processes
D TA =
(45.7 ·
Area)/(21.4
+√
Area)
4.2.2.3.1 Bioturbation Bioturbation is the mixing
of the deposited materials from the movement
(eating, digging and foraging activities) of the
bottom fauna (zoobenthos). The sediment clas-
sification scheme in Table 4.4 focuses on the
An evident boundary condition for this approach
is that if D TA >
D max .
The area above the wave base ( A WB =
D max then D TA =
Area A ) may be calculated from the hypsographic
Area
 
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