Beach Renourishment Principles - Beach Renourishment - page 71

just offshore. It has been suggested that it may be more useful to renourish the
whole profile, including backshore dunes and the nearshore sea floor, and not just
the upper beach terrace, in order to attain a more stable transverse configuration
(Bruun 1990 ). Shore profile renourishment is preferred to upper beach renour-
ishment because the latter leaves unnaturally steep seaward edges, which can be
reflective and a cause of nearshore scour, and are also subject to erosion and re-
shaping, with often rapid initial losses. Profile renourishment reduces these losses,
and has lower construction and maintenance costs. It also permits the use of a
wider range of grain sizes, subject to sorting by wave processes into appropriate
zones on the profile.
The aim is to establish a relatively stable 'equilibrium profile'. Beaches with
concave profiles are more stable than those with straight, convex or irregular
profiles, and once concave profiles are attained they become relatively (but not
absolutely) stable. The gradient of the concave profiles varies with grain size and
preceding wave conditions. Subsequent oscillations occur with episodes of storm
wave erosion and fine weather accretion. 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.
In the Netherlands the value of backshore dunes as a reservoir of sand and a
barrier to storm waves and marine flooding has long been realised. On the Atlantic
coast of the United States, Kana and Stevens ( 1990 ) discussed techniques of beach
and dune profile restoration following erosion by a hurricane.
Bruun ( 1990 ) noted that shore profile renourishment required dredging and
dumping equipment of the kind used in the Netherlands (Stive et al. 1991 ). A
transverse profile can be maintained by backpassing (Sect. 4.3.6 , p. 70), using per-
manent offshore dredging and pumping stations, but this could lead to frequent
disruption of sea floor plant and animal communities and fish habitats, and some
would consider such offshore structures obtrusive.
Renourishment of the whole shore profile was carried out after the failure of sev-
eral projects that dealt only with the upper beach terrace at Ocean City, New Jersey.
Detailed investigations prepared the way for a project, which used 4.6 million m 3
of sediment to shape a beach 30 m wide, with a concave shore profile on the sea-
ward side, as well as a backshore dune (Fulford and Grosskopf 1989 ; Anders and
Hansen 1990 ). Subsequent changes were monitored, and in January 1992 a major
storm removed most of the beach and part of the dune (Houston 1995 ), but there
was little property damage in the resort, and it seemed likely that a protective beach
and dune could be maintained if the beach profile were renourished after each
storm. Renourishment of the whole of the shore profile, including the nearshore
zone and backshore dunes, was thus seen as a more effective way of establishing
and maintaining a protective beach than simply dumping sand to form an upper
beach terrace.
At Miami the beach renourished in 1975 included a low backshore dune, a flat
terrace and a gentle seaward slope, a landform association that has remained fairly
stable, proving remarkably resilient even during successive hurricanes (Finkl 1981 ).