Geography Reference
In-Depth Information
which to consider human disruptions to natural
processes. Sand mining (Earney 1990) removes
sediments directly, for building aggregates, soil
improvement and precious-mineral mining (e.g.
diamond mining on the Namibian coast).
However, the most pervasive influence at the coast
is the interference between onshore-offshore and
alongshore processes and nearshore structures.
Coastal morphodynamics tells us that beach
profiles expand and contract with changing wave
energy inputs. However, the emplacement of sea
walls prevents the natural expansion of the shore
profile to a fully dissipative state under storm
conditions. Large volumes of water on beaches
lead to a dominance of backwash, sediment down-
combing and landslide-like failure of saturated
beach sediments. The resultant narrowing and
lowering of beaches backed by structures by
comparison with profiles on adjacent non-
defended shores has been extensively documented
(e.g. Kraus and Pilkey 1988; Louisiana, USA:
Nakashima and Mossa 1991; Texas, USA: Morton
1988). As beach sediment loss continues, so walls
are threatened by undermining, so initiating a
cycle of progressively larger defences with bases to
lower and more seaward levels on the shore profile
(e.g. history of sea walls at Porthcawl, UK: Carter
1988). A further problem is that accelerated
erosion is often characteristic of the downdrift end
of a defence structure; the temptation, therefore, is
to extend the structure progressively downdrift,
ultimately until perhaps hundreds of kilometres of
coastline are so protected. Elsewhere in alongshore
directions, patterns of deposition and erosion
result from the interruption of longshore sediment
transport; these are well known (see Viles and
Spencer 1995 for review). Jetties (shore-normal
structures that extend seawards beyond the breaker
zone) induce updrift accumulation and downdrift
erosion. More shoreward, on the beach itself,
shore-normal groynes are used to conserve
remaining beach volumes or capture sediments
moving alongshore. Groyne fields can similarly
lead to problems of down-drift erosion, although
once 'full', longshore drift is re-established as
sediment bypasses the structure at either its
landward or seaward limits. Groyne fields are thus
an essential component in beach conservation on
many coasts (e.g. UK: Bray
et al
. 1992; Japan:
Walker and Mossa 1986). A rather different
interruption to longshore drift occurs with the
presence of offshore breakwaters: these reduce
wave energy, and thus longshore sediment
transport rates, leading to sediment accumulation
in their lee (and the need for dredging at port and
harbour installations).
The frequent consequence of this
humaninduced erosion is to attempt to alleviate
such problems by artificially re-establishing natural
sediment movements. Examples include the
pumping of sediments across inlets to maintain
downdrift sediment transport (e.g. Fort Worth,
Texas, USA) and the dredging and dumping of
offshore sediments to replenish beaches deprived
of sediment supply by updrift barriers (e.g. Santa
Barbara, California, USA). In recent decades,
beach nourishment schemes have become a
particularly favoured solution to beach volume
loss (see following section). However, such
activities must also be seen in the context of
sediment delivery to coastlines. Estimates for
contemporary fluvial inputs to the coastal zone
are not well known, but perhaps 10-15 per cent
of the total input of 10
16-17
t may contribute to
coastal aggradation. Regional variations are great,
partly controlled by plate tectonic controls on
drainage basin characteristics and partly by
climatic regime (Milliman and Meade 1983;
Milliman and Syvitski 1992). However, a further
set of controls has been, and continues to be,
human activity within feeder drainage basins. The
timing and style of catchment modification has
played an important role in determining sediment
supply and coastline response. Thus, for example,
in the Mediterranean, it appears that the sediment
pulse was initiated in pre-Classical times and
accelerated with Greco-Roman landscape
modification—thus northern Mediterranean
shores have been largely fossil for a thousand years
or more (Vita-Finzi 1964) and consequently
heavily managed to retain what sediment is
present. The sediment flush/sediment starvation
pulse in North America has been much more
recent (IGBP 1993). Even more recently, in Egypt,
