Page 180 - Sedimentology and Stratigraphy

Tides 167

between the low and high tide, and the ebb tide
current, which flows in the opposite direction as the
$&
% water level returns to low tide. These are bipolar
currents acting in two opposite directions. Second,

the tidal flow varies in velocity in a cyclical manner.

At times of high and low tide, the water is still, but as

the tide turns, the water starts to move and increases
$ " in velocity up to a peak at the mid-tide point in each

% direction. Third, the strength of the flow is directly
related to the difference between the levels of the high
and low tides. As the tidal range varies according to
the series of cycles (see above) the velocity of the
current varies in the same pattern. The strongest
tidal currents occur when there are the highest spring
tides at the spring and autumn equinoxes.
The rotational pattern of the tidal wave within
$'
% amphidromic cells results in a flow of water that
follows a circular or elliptical pattern. These rotary
' (
tides can be important currents on shelves and in
epicontinental seas. During the course of the tidal
cycle the current varies in strength, but does not
Fig. 11.4 The North Sea of northwest Europe has a variable change direction and there may not be a period of
tidal range along different parts of the bordering coasts. slack water (Dalrymple 1992). These offshore tidal
Amphidromic points mark the centres of cells of rotary tides currents are important processes in the transport
that affect the shallow sea. and deposition of sediment on some shelf areas (14.3).

regimes, more pronounced if the range is mesotidal
11.2.4 Sedimentary structures generated
and are capable of carrying large quantities of sediment
by tidal currents
in macrotidal regimes. Nearshore tidal currents show
a number of features that produce recognisable char-
Bipolar cross-stratification
acteristics in sediment deposited by them (Fig. 11.5).
First, tidal currents regularly change direction from An analysis of current directions recorded by cross-
the flood tide current, which moves water onshore bedding in sands deposited by tidal currents may

,

*

Fig. 11.5 During the diurnal tidal
cycle the direction of flow reverses +

from ebb (offshore) to flood (onshore). *

*
*
The current velocity also varies from
peaks at the mid points of ebb and
flood flow, reducing to zero at high )
and low tide slack water before )*
accelerating again.