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(Soomere et al. 2008b ) . The difference is particularly large (up to several orders
of magnitude) when the sediment layer is not continuous (as it is northwards of
Pirita Beach) or has a limited thickness. For this reason, the calculated rates must
be used with caution. For example, the difference between the estimates for differ-
ent sections of the beach carries the key information about their vulnerability with
respect to changes of sediment transport processes. Another key quantity is the ratio
of net and bulk transport rates that characterises the intensity of sediment transit at
a particular site.
13.5.1 Sediment Balance at Pirita Beach
Detailed calculations of the potential transport rate for Pirita Beach have been per-
formed by Soomere et al. ( 2008b ) based on numerical simulation of the local wave
properties and the CERC ( Coastal Engineering Research Center ) method. The lat-
ter is based on the assumption that the potential immersed weight transport rate
I t is proportional to the rate of beaching of wave energy flux (wave power) per
unit of the coastline P t . The latter quantity depends on the wave height, period,
and approach angle. The relevant expression I t
KP t is usually referred to as the
CERC formula. The non-dimensional proportionality coefficient, K , is frequently
expressed as a certain function of the wave approach angle, the maximum orbital
velocity in breaking waves, and the sediment fall velocity in the surf zone, the latter
dependence implicitly expressing the properties of sediments (Coastal Engineering
Manual 2002 , part III-1). Such model set-ups have been widely used in the southern
Baltic Sea conditions (Kuhrts et al. 2004 , Fröhle and Dimke 2007 ) .
The calculations revealed that the potential transport rate (consequently, also the
overall functioning of the sedimentary system) at Pirita is almost independent of
the grain size for a fairly wide range (from 0.063 to 0.2 mm) of the mean size.
Longshore sediment motions at Pirita are thus almost entirely governed by the match
of the wave propagation direction and the geometry of the coast. This feature sug-
gests that potential changes of the transport patterns when the grain size is modified
(for example, through beach refill) are fairly modest.
The calculated transport rate patterns generally coincide with different geomor-
phic features such as sections of intense sediment transit, areas of erosion and
accretion, the presence and orientation of sand bars, or sections with extensive
retreat of the dune toe during strong storms. The discrepancies between numeri-
cally simulated transport patterns and the appearance of the beach are fairly minor
and become evident only in limited sections. This match shows that the quality
and resolution of the wave and sediment transport models in use, the quality of the
information about the granulometry, and the quality of the atmospheric forcing are
sufficient for resolving the basic features of functioning of typical beaches along the
North Estonian coast.
The performed simulations made it possible to derive an estimate of the anthro-
pogenic changes of the magnitude of the littoral drift from the North to Pirita Beach.
The beach is fed by the flux of relatively fine sediments from the North (say, with
=
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