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 267

increased considerably whereas strong erosional features are evident at its eastern
side (Fig. 13.7 ) . While it probably prevented the forming of the sand bar for some
time, the bar has re-emerged at the seaward end of the bulwark. The entire system
seems to have reached a more or less equilibrium stage in terms of long-term varia-
tions. This equilibrium at the river mouth is, however, highly dynamic in the sense
that considerable seasonal variations of the height of the sand bar may occur in the
system (Laanearu et al. 2007 ) .

13.4 Equilibrium Profiles and Transport Patterns

Although a particular beach profile may undergo substantial changes, an average of
the instantaneous profiles over a long period usually preserves a relatively constant
shape called the equilibrium beach profile (EBP, Dean 1991 ) . The temporal and
spatial resolution of available surveys at the southern coast of the Gulf of Finland is
too low for adequate estimate of properties of the EBPs. For that reason the relevant
studies (Soomere et al. 2007 , 2008b ) rely on theoretical estimates of the shape of
the EBP based upon the concept of uniform wave energy dissipation per unit water
volume in the surf zone (Dean and Dalrymple 2002 , Chap. 7 ).
The water depth h ( y ) along such profiles at a distance y from the waterline is

Ay 2 / 3 , where the profile scale factor A depends on the grain size of the
bottom sediments. As Narva-Jõesuu Beach is an example of large, high-energy
beach in the Gulf of Finland context, it is natural to assume that its grain size is
also largely homogeneous. This is largely the case also for Pirita Beach where in
2005-2007 bathymetry and sediment texture were mapped in the nearshore between
the waterline and the 11 m depth contour along an about 2.5-km-long section of the
coast (Soomere et al. 2007 ) . The average grain size in the nearshore of Pirita Beach
is close to 0.12 mm. Although the mean grain size does vary to some extent along
the beach, the corresponding variations of the factor A are fairly small: it is approxi-
mately 0.07-0.08 for the northern and about 0.063 for the southern part of the beach
(Soomere et al. 2008b ) . Therefore, it is adequate to use a fixed value of the fac-
tor A

h ( y ) =

= 0.07 that corresponds to the overall average grain size for Pirita (Dean et al.

2001 ) .
Several variations of the mean grain size and the content of the fractions should
occur naturally within the beaches in question. Since waves sort the sediments
and the finer fractions are gradually transported offshore, deeper areas usually host
the finest sediments and the coarser material is concentrated in the vicinity of the
breaker line and at the waterline (Dean and Dalrymple 2002 ) . Coarser-grained sand
is found along the waterline (where the maximum content of medium sand is up to
84%) and finer components in deeper areas of Pirita Beach indeed (Soomere et al.
2007 ) . The mean grain size along the waterline is much larger than in the rest of the
study area (Fig. 13.4 ) .
The proportion of coarser sand generally decreases offshore. This is evidently
related to the above-discussed highly intermittent nature of wave activity: the
breaker line is poorly defined and the relevant band of relatively coarse sand is