On the Dynamics of “Almost Equilibrium” Beaches in Semi-sheltered Bays Along the Southern Coast of the Gulf of Finland - The Baltic Sea Basin - page 261

13.2.2 Forcing Factors
The effect of ice (which is usually present 60-80 days annually, Sooäär and Jaagus
2007 ) is mostly indirect and consists in either moving of boulders, damage to dune
forest, or reducing the wave loads during the ice season. For cliffed coasts and
exposed till and dune bluffs, the effect of frost heave and spring melt on the sand,
and layered cliffs contribute sediment to the littoral system. The tidal range is 0.01-
0.02 m in the area in question. The tidal currents are hardly distinguishable from
the other motions. Water level at the beaches is mainly controlled by hydrometeo-
rological factors. The range of its monthly mean variations is 0.2-0.3 m (Soomere
et al. 2008c ) but its short-term deviations from the long-term average are larger and
frequently reach several tens of centimetres. Water levels exceeding the long-term
mean more than 1 m are rare. The highest measured level at the Port of Tallinn is
1.52 m on 09.01.2005 (Suursaar et al. 2006 ) and the lowest is -0.95 m. Even larger
variations of the extreme water level occur in the eastern part of the Gulf of Finland,
for example, the historical highest water level has been 4.21 m in Sankt Petersburg
and 2.02 m in Narva-Jõesuu (Soomere et al. 2008c ) .
Coastal currents induced by large-scale circulation patterns are modest in the
whole Gulf of Finland (Alenius et al. 1998 ) . Their speed is typically 0.1-0.2 m/s and
only in exceptional cases exceeds 0.3 m/s. In the bayheads, such as the nearshore
of Pirita Beach, current speeds apparently are even smaller. Although there is some
evidence about a relatively stable pattern of coastal currents in Narva Bay (Andrejev
et al. 2004 ) , the current speed is usually modest there. Local currents are at times
also highly persistent in the coastal zone next to Pirita Beach (Erm et al. 2008 )
and may provide appreciable intensity of transport of finer fractions of sand that
are suspended in the water column even though the typical settling time of these
fractions is only a few minutes.
The magnitude of wave-induced bedload transport greatly exceeds that of the
current-induced transport even at relatively large depths (8-10 m) of open-sea areas
(see Soomere et al. 2007 and references therein). Therefore, wave action in the surf
zone evidently plays the decisive role in functioning of the beaches at the southern
coast of the Gulf of Finland, as is typical for beaches located in microtidal seas.
Exceptions form the mouths of relatively large rivers where seasonal variation of a
sill height is jointly governed by a similar variation of the magnitude of river outflow
(that erodes the sand bar) and the longshore wave-induced sediment transport (that
increases the sill height) (Laanearu et al. 2007 ) .
The wave climate of the Gulf of Finland matches that in the open Baltic Sea. It
is generally mild, with the annual mean significant wave height well below 1 m and
the typical wave periods usually not exceeding 7-8 s (Soomere 2005 ) . The average
and, in particular, the maximum wave heights in the gulf are much smaller than
those in the Baltic Proper (Soomere 2008 ) . Yet very rough seas with the significant
wave height >4 m occur in the open parts of the gulf approximately once in a decade
(Soomere et al. 2008c ) . The wave activity has a strong seasonal variation in the Gulf
of Finland with the highest wave loads usually occurring during the autumn and