Geoscience Reference
In-Depth Information
sand dunes was widespread and alternating
periods of occupation and abandonment have
been documented in many dune systems, which
probably relate to alternating periods of stabil-
ity and instability in which humans themselves
may have played a role (Gilbertson et al. 1996).
Wilson & Braley (1997) described the nine-
teenth century engulfment of houses in Donegal,
Ireland by blown sand as a result of overgrazing
in adjacent dunes.
Navigation posed the next major human threat
to natural coastal morphodynamics. Tidal inlets
provide strategic links between inland systems
and the open sea. Although inlets may be suffi-
ciently deep, adjacent tidal deltas pose hazards
to navigation. Dredging of channels and ebb
deltas was the initial response to this hazard.
Such alteration of the sediment budget causes
readjustments, and, inevitably, the deltas reform
and require further dredging. The next phase of
control was the construction of jetties to fix the
location of tidal inlets. In most cases, however,
maintenance dredging is still required.
Increased levels of human utilization and
occupation of the coast have been accompanied
by developments in engineering that have pro-
moted ever increasing interference with natural
coastal functioning. Human interventions fall
into three main categories.
1
Planned modification
- where there is deliberate
planned construction of harbours, reclamation
of coastal lands, sea wall construction, etc. This
is almost always an engineered approach and
is often planned around a relatively short-term
time-scale (decades). Generally, little attention is
given to long-term morphodynamics at the site.
2
Accidental modification
- often a knock-on
effect from (1), where further along the coast
there is a direct impact on the general wave pat-
terns and sediment transport pathways from,
for example, an engineered structure. In many
cases, simple ignorance of coastal processes
leads to a direct modification of the coast. For
example, the removal of sediment from beaches
can have a beach-lowering effect and therefore
an increase in beach vulnerability from storm-
wave attack where less energy is dissipated and
backshore erosion takes place.
3
Reactive modification -
in response to planned
or accidental modification. A process of progres-
sive coastal modification can take place when
attempts to solve sedimentary problems using
engineering solutions produce further sediment-
ary problems. These are then addressed by further
engineering. Examples occur when longshore
supply of sediment is halted by stabilization of
eroding cliffs that threaten human infrastructure.
Beaches, deprived of sediment are then 'stabilized'
using groynes, which in turn starve other areas
of the coast of sediment. Such situations produce
progressive down-drift intervention.
The above types of intervention take place
through four main groups of human activit-
ies, comprising coastal engineering, agricultural
activity, the extractive industry and recreational
activities. Each one of these will be addressed in
turn but in many cases a combination of two of
more of these activities may be present at the
coastal site and there may be feedback between
them as levels of activity change over time.
8.5.1 Engineering works
Off-site engineering has the potential to affect
sediment supply to the coast. In cases where
fluvial sediment supply is important, construc-
tion of impoundments, flow reduction and
sediment abstraction from rivers can reduce the
supply and have an impact on the coastal sedi-
ment system. At the Nile delta the reduction in
sediment supply (through impoundments) has
led to enhanced subsidence and relative sea-level
rise (Stanley & Warne 1998; see Chapter 7). The
sandy barrier islands at the leading edge of the
delta are subject to both impacts. In California,
impoundment of numerous small rivers has led
to severe erosion on many beaches as the sandy
sediment supply has been reduced (Sherman
et al. 2002). Another form of off-site impact
may occur through modification of back-barrier
areas. Reclamation of the coastal fringe, for
example, can reduce the tidal prism and in turn
may induce changes in the tidal current regime
and the adjacent coast (Case Study 8.2). Coastal
engineers typically attempt to modify coastal
processes in order to protect property/structures
