Responses to Beach Erosion - Beach Renourishment - page 29

of the structure by wave reflection (Sect. 2.14 , p. 24), or by reducing a previous
supply of sediment from the eroding coastline. Long-term monitoring of coastal
change around structures built in response to beach erosion has frequently been
shown to produce adverse environmental effects adjacent to, and on shores further
away (Hamm et al. 2002 ; Bruun 1995 ). Exceptions to this have been discussed and
illustrated by Basco et al. ( 1997 ) and Rakha and Kamphuis ( 1997 ).
Primitive sea walls are often banks of earth or other locally available material,
particularly where it can be extracted from an adjacent excavation, often a parallel
ditch. Subsequent reinforcement may be necessary in response to damage of such
banks by storm waves, and may be followed by stages in the evolution of larger,
more competent and costly solid structures.
Sea walls and boulder ramparts are generally introduced to prevent wave attack
on an eroding coast, usually a receding cliff, an undermined and slumping bluff,
or a truncated dune, in each case fronted by a beach that has not been sufficiently
high and wide to prevent waves reaching the back of the shore.
Modern sea walls are usually large stone or concrete walls designed to with-
stand the force of the breaking waves, which are reflected seaward, and may
scour away the beach. An alternative is to build boulder ramparts, also known as
revetments or riprap, some of which are irregular heaps of rocky debris, others
more carefully fitted blocks arranged on a seaward slope, usually on a mattress
of sand or gravel. Tetrapods, made of reinforced concrete, are shaped to interlock
and remain in position on the shore during phases of strong wave action. Boulder
ramparts and tetrapods are less reflective than solid sea walls, and the expecta-
tion is that waves will break into crevices, producing swash and backwash that do
not cause erosion on their seaward sides. In some places, rocky debris has been
dumped to protect an earlier sea wall from undermining and disintegration by
wave attack, or in the hope that a boulder apron would prove less reflective than a
solid sea wall, and permit some recovery of a lost or diminished beach.
Sea walls and boulder ramparts are subject to damage by the impact of waves
during storms (Fig. 3.1 ), and by scour due to the hurling of sand and gravel against
Fig. 3.1 Storm-damaged sea
wall at Black Rock, Victoria,
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