Planning Considerations - Beach Renourishment - page 116

The response of beaches to a rising sea level depends on a variety of factors,
including geology, oceanographic and nearshore processes (wave climate, storm
frequency), sediment supply and the intensity of hinterland development. Studies
of the effects of a rising sea level on beach-fringed coasts have shown that erosion
will be initiated or intensified as submergence proceeds, except where there is a
continuing natural or artificial supply of sediment to maintain beaches at progres-
sively higher levels. As the majority of beaches are already eroding (Bird 1985 ),
the implications of global climate change will only serve to exacerbate erosion.
The rate of beach erosion is expected to be two orders of magnitude greater than
the increase in sea level (Leatherman 2001 ).
Bruun ( 1962 , 1983 ) suggested that a sea level rise would cause sand to be
eroded from the top of the beach and deposited offshore. This concept is known
as the Bruun Rule, and is a widely used method of estimating the response of
beaches to a rise in sea level. It indicates that a beach profile will maintain its
shape as sea level rises, assuming it is able to migrate into an undeveloped hinter-
land. The concept is highly simplified and ignores the effects of longshore drift.
The coastline can be maintained by building sea walls and other protective
structures to prevent erosion and submergence, but as has been noted, these are
likely to cause further beach erosion, and in due course beaches (including those
that have been renourished) will disappear, leaving an artificial coastline. It will be
possible to maintain beaches by continuing renourishment as sea level rises, the
limiting factors being the availability and cost of suitable renourishment sediment
and the extent of hinterland submergence, which may have to be offset by building
sea walls or raising coastal lowlands by landfill.
In Sydney, Australia, AECOM ( 2010 ) estimated that for an assumed increase
of 0.1 m in sea level over the next decade, 9 million m 3 of sediment would be
required to reinstate and maintain beach amenity and provide some storm protec-
tion. Subsequent annual renourishment was estimated to require a further 3 mil-
lion m 3 . In South Australia the Coast Protection Board has considered whether
the Adelaide beaches (Sect. 4.3.4 , p. 67) could be maintained if sea level rises as
forecast, and found that renourishment would continue to be feasible, especially
if inland sources of sand were used, until the sea has risen 20-30 cm above its
present level, but thereafter it may be necessary to construct major sea walls and
accept that the beaches will disappear (Wynne 1984 ). Inevitably, there will be
future cost increases of beach renourishment projects, with more frequent and
more substantial filling. As Weggel ( 1986 ) remarked 'if projections of an increas-
ing rate of sea level rise are correct, it will become increasingly difficult to eco-
nomically justify future beach renourishment projects'.
Planning for projected sea-level rise increases should be based on credible sci-
ence, engineering and economics to ensure careful consideration of cost-effective
methods of sustaining the coast (Williams 2013 ). Coastal managers may have to
decide whether beach renourishment is a sustainable method of long-term coastal
management. Coastline retreat, by moving structures landward or elevating them
on pilings, or abandoning land and structures, may become necessary (Yohe et al.
1996 ).