Beach Renourishment Principles - Beach Renourishment - page 80

4.4.3 Cross-Shore Profile and Determination of Volume
Similar considerations apply to the shape in profile. The profile of a beach ren-
ourishment is a geometric shape typically consisting of a dry beach berm and a
foreshore slope. Cost constraints, environmental issues and sponsor preferences
commonly influence the steepness of the slope and the length of the dry berm
(USACE 2002 ), both determining total volume. As discussed earlier, profile design
can improve beach stability and reduce the amount of adjustment following renour-
ishment. In the Dutch method (Sect. 4.4.1 , p. 79) the volume of sediment required
is calculated using data on historical rates of erosion, and the sediment is then
placed between − 1 m below low tide line and the dune foot on the sandy beaches.
When considering the renourishment of the whole profile, the aim is to estab-
lish a relatively stable 'equilibrium profile'. Beaches with concave profiles are
more stable than those with straight, convex or irregular profiles, and once con-
cave profiles are attained they become relatively (but not absolutely) stable. The
gradient of the concave profiles varies with grain size and preceding wave con-
ditions. Subsequent oscillations occur with episodes of storm wave erosion and
fine weather accretion: the profile following storm wave erosion is different from
a calm weather profile. As long as the beach profile oscillates between these limits
the beach can be considered relatively stable, but as has been noted (Sect. 1 , p. 2)
beach erosion is prevalent and most renourished beaches need further inputs of
beach sediment (Sect. 4.5 , p. 84).
In the United States much attention has been given to theoretical equilibrium
profiles (Campbell and Benedet 2006 ). It is thought that beaches tend towards an
equilibrium profile and that a newly renourished beach will adjust over time to
achieve this equilibrium (Dean 1977 , 1991 ). An equilibrium beach slope can be
predicted, given the grain size of renourishment sediment; conversely, an appro-
priate grain size can be chosen to provide a required equilibrium beach slope.
The latter was demonstrated by Firman et al. ( 2011 ) where a particular profile
was required to avoid renourishment sediment encroaching on adjacent offshore
seagrass habitat. At Colwyn Bay in the United Kingdom, grain size was chosen
to create the most stable beach profile and beach width in response to storms
(Oliveira et al. 2011 ). Testing of a number of options demonstrated that using sedi-
ment with a mean grain size diameter of 0.45 mm was more stable than a mean
grain size diameter of 0.25 mm.
Depth of closure is defined as the depth (and so distance offshore) beyond
which no significant profile fluctuation takes place due to coastal processes (wave
and current action) (Hallermeier 1981 ). A renourished beach can extend out
to the depth of closure, but beyond this sediment deposited will not contribute
to the maintenance of the beach. This concept is of little use on oceanic coasts:
on the New South Wales coast the depth of closure is typically 22 m (at about
1.2 km offshore) (Neilson 1994 ), but at Salina Bay in Malta the depth of closure
was estimated to be 5 m (approximately 100 m offshore), and was used to limit