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The combination of the seasonality of wave fields and this interesting switch
between the two regimes has important consequences on the sediment transport in
the sill area. While wave-induced increase in the sill height dominates during the
autumn and winter season (when the river flow is modest and the lower layer is
usually arrested), the mostly one-layer, voluminous outflow causes intense seaward
transport of sediments in the sill area and a rapid reduction of the sill height dur-
ing the spring flood (Fig. 13.9 ) . A simple hydraulic model of such a two-layer flow
combined with a model of cross-shore transport adequately reproduced the seasonal
variation of the sill height, the magnitude of which is about 1 m (Laanearu et al.
2007 ) . As the properties of stratification of seawater play an important role in the
switch of the river flow between the one-layer and two-layer flows, any changes in
the large-scale hydrophysical properties (for example, an increase in the precipita-
tion and river discharge, or a decrease in the intensity of salt water inflow into the
Baltic Sea) may affect this balance.
13.6 Discussion and Conclusions
Many beaches along the northern coast of Estonia belong to an interesting class
of almost equilibrium, bayhead beaches located in an essentially non-tidal, highly
compartmentalised coastal landscape. They develop mostly under the influence of
wave action which is normally active in a relatively narrow nearshore band, but
occasionally (under meteorologically forced high water conditions) may reach a
certain sediment deficit. Their development is largely stabilised by the littoral drift
or finer sediment and gravel (usually eroded from till bluffs in the neighbourhood of
the beaches) towards the bayheads and by the postglacial uplift with a rate between
1 and 2.5 mm/year. Eolian transport and fluvial sediment supply have typically very
modest magnitude. As is typical for bayhead beaches, there is no lateral loss of
sediments towards the entrance of the bays. The combination of the listed features
suggests that such beaches, in general, develop quite slowly and may be in an almost
equilibrium stage even when the active sand mass is very limited.
Owing to a specific combination of the geometry of the coastline and dominant
wind directions in strong storms, wave conditions along such beaches are highly
intermittent. While the overall wave climate, estimated in terms of average wave
properties, is usually very mild and wave periods are comparatively small, at times
ferocious storms blowing from specific directions generate high and long waves that
directly attack the beach. The development of such beaches, therefore, is step-like:
many years of very slow development under low wave conditions approaching from
a fixed direction are interspersed with large changes occurring infrequently during
a strong storm.
Another class of beach in the area in question forms sections of the coast located
eastwards from the longitude of 27 E. The coast is almost straight from this longi-
tude, with a few headlands of moderate size around Kunda. This part of the coast
is significantly exposed to wave approach from the Gulf of Finland. Its dynam-
ics largely mimics that of an open ocean, high-energy coast and develops under
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