Geoscience Reference
In-Depth Information
Dune scarp
1. Before the wall
Beach narrowing
2. Wall constructed.
Development proceeds as buyers believe
property protected by the wall
Steepening of
offshore slope
Beach gone
Offshore slope has
steepened
3. Two to forty years later
High-rise development
Fig. 8.16
Impacts of seawalls. Seawalls fix the landward
margin of the beach, thereby inhibiting its ability to respond
to storms or sea-level rise. They also may be constructed on
the beach, thus causing effective loss of the active beach.
Active loss occurs as waves reflect off the seawall and erode
the beach, particularly during storms. Over the medium
term, seawalls cause beaches to narrow as waves are
reflected and sediment is lost. The beach may disappear and
the seawall itself may be undermined as the shoreface
steepens. (After Pilkey et al. 1998.)
Original oceanfront house destroyed in
large storm. Bigger, 'better' seawall built
As slope increases,
wave size increases.
A higher wall is needed
4. Ten to sixty years later
interference with the shore processes will result
in new problems.
(Dean 1974). Beach nourishment often goes hand
in hand with navigational channel maintenance
where dredge spoil provides suitable sediment.
A variety of emplacement modes have been
employed to nourish beaches. They include:
single direct emplacement from offshore dredgers,
followed by mechanical profiling; trickle feed-
ing, where sediment is introduced from a single
source at small volumes; recycling of sediment
from down-drift sinks to up-drift source areas;
and emplacement in the nearshore for natural
onshore transport (French 2001).
Nourishment has been seen as a panacea
for erosion control. Once emplaced, nourished
beaches adjust to natural dynamics and mor-
phology by dispersing sediment and producing
8.5.1.2 Soft engineering
Beach nourishment is a relatively recent tech-
nique of shore protection and following some
early twentieth century emplacements (Bird 1996)
widespread application began after the 'Ash
Wednesday Storm' of 1962 on the eastern USA.
Beach nourishment tackles erosion by replacing
lost sediment and relies in part on natural wave
action dispersing introduced material along the
shoreline and achieving equilibrium with ambi-
ent dynamics. Ideally therefore, the introduced
sand should replicate the native sand texture
