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 269

(24 evenly spaced directions, grid step of about 1/4 nautical miles, 24 frequencies
from 0.042 to 0.41 Hz with an increment of 1.1 for Narva Bay, 42 frequencies up
to 2.08 Hz for Tallinn Bay) allow description of wave properties in the coastal zone,
up to depth of about 5 m and as close to the coast as about 200-300 m (Soomere
2005 ) .
The model was forced with wind data from Kalbådagrund (Fig. 13.2 , 59 ◦ 59 ◦ N,
25 ◦ 36 ◦ E). This is the only measurement site in the Gulf of Finland that correctly
represents marine wind conditions. The presence of ice is ignored. The computed
annual mean parameters of wind waves are, therefore, somewhat overestimated and
represent average wave properties during the years with no extensive ice cover. The
model was used for calculation of long-term statistics for Tallinn Bay (Soomere
et al. 2007 , 2008b ) and for time series of wave properties in 2002 for Narva Bay
(Laanearu et al. 2007 ) .
Detailed calculations have been performed for sections with a length of about
0.5 km relating to the nearshore off Pirita for 1981-2002. The threshold for the
significant wave height occurring with a probability of 0.137% varies between 1.45
and 1.58 m along the beach. The typical peak period T s in such storms is about
7 s. Expression (1) gives reasonable values of 2.36-2.57 m for the closure depth
that match the bathymetric survey data (Soomere et al. 2007 ) . These values are
apparently typical for many bay beaches along the northern coast of Estonia. Only a
few more exposed sections may be subject to larger wave loads (in terms of both the
threshold of the significant wave height that occurs 12 h a year and the peak period in
such wave conditions) and host equilibrium profiles extending to somewhat greater
depths. For the case of Pirita, given the approximate value of the scale factor A =

0.07, the width of the equilibrium profile is expected to be about 250 m and its mean
slope approximately 1:100.

13.5 Applications for “Almost Equilibrium” Beaches

One of the basic properties characterising the beaches is the magnitude of sediment
transport. Its properties and spatial patterns of longshore transport can be relatively
easily calculated for the almost equilibrium beaches under consideration. It is con-
venient to estimate the intensity of alongshore sediment transport in terms of its
potential rate Q t (Coastal Engineering Manual 2002 ) . An equivalent measure is the
potential immersed weight transport rate

I t = (ρ s − ρ) g ( 1 − p ) Q t , (2)

which accounts for voids between sediment particles and the specific weight of the
sediment components. Here

ρ s and ρ are the densities of sediment particles and

9.81 m/s 2 is the acceleration due to gravity; and p is the
porosity coefficient. Both these measures express the volume of sediments carried
through a cross-section of the beach in ideal conditions within a unit of time. The
factual magnitude of the transport is much less along the North Estonian beaches

seawater, respectively; g =