|
Sitges after the storms: contemplating the disappearing sand?
Many Catalan beaches are losing their sand. The November storms
have starkly revealed this problem and the long term solution is a complex one
involving a range of environmental, economic and social issues. The estimated
costs of 1.690 million ptas to restore the damaged beaches and promenades are
overshadowed by concerns regarding future sustainability, not just of the
beaches, but of the coastal fringe in general and the tourist industry in
particular. Both money and beach sediment are scarce resources in Catalunya, and
conflict between competing resorts all looking to bolster their share of the
tourist cake may be difficult to reconcile.
Sea levels are rising. Maximum wave heights have been steadily
increasing during the last decade, from 8.22 metres in 1991 to 9.92 metres on
9th November 2001. The best defence against coastal erosion and rising sea
levels is the humble beach, cheaper than traditional hard defences in both the
short and long term. But the creation of sustainable artificial beaches is more
complex than simply dumping sand or shingle on the shore and
the organisation responsible for these operations estimates annual sand loss
at 10% of the amount spread.
Beaches act as the natural sustainable defence system against
coastal erosion. The secret to their success lies in the fact they can adapt
their shape very quickly to changes in wave energy and also dissipate this
energy in minor adjustments of the position of each sand or shingle grain. The
beach is therefore able to maintain itself in a dynamic equilibrium with its
environment due to the mobility of its sediments. The beach tends to adopt
different profiles according to the season. Sand lost during winter storms tends
to be deposited off shore forming protective submarine banks, to be transported
back on to the beach during low-energy summer sea conditions. Material is lost
from the beach due to coastal currents (see below), but a constant source of new
sediments is normally supplied from river deposition and coastal erosion.
Alarmingly, the Catalan supply appears to be running out.
Rivers such as the Llobregat and Besòs now barely reach the sea due to
water abstraction. Where they do, damming, reforestation and increasing
urbanisation upstream all act to reduce sediment input. Marina, breakwater and
sea wall developments prevent the natural erosion of the coast and inhibit the
transport of sediments in a southerly and south-westerly direction along the
shore by coastal currents. The result is the same. The supply of sand is drying
up, witnessed by several resorts including Sitges.
The key to the transport of sediment in a south and
south-westerly direction along the Catalan coast is the dominant wave direction,
mainly from the east (charts 1 and 2).
This dominant easterly wave direction is linked to the distance
of open sea over which the wind can generate waves, (called the fetch). The
longest stretch of unbroken sea facing the Catalan coastline lies some 500
kilometres to the east and south east (see map 1).
This has been long-recognised by coastal engineers, and many sea defences are
aligned approximately north-south to protect against storm waves from the east.
The arrival of wave fronts on the coastline at an oblique angle
leads to sediment being carried up the beach at an angle approximately
perpendicular to the wave crest, but gravity will cause the material and the
backwash to take the steepest gradient seawards which in an oblique wave will be
a different course from that taken by the swash. Consequently, material may be
seen to drift south and south-westwards along the Catalan coast, a process
called longshore drift. (See diagram 1 ).
The interruption of the supply of new sediment carried by
longshore drift has been one of the factors leading to the loss of sand from the
beaches at Sitges. The construction of the Aiguadolç marina updrift in the late
1970s blocked sediment transfer to the main beaches in the south west. Hard
engineering schemes involving a number of breakwaters and eight rock islets were
then constructed to help protect and build beaches between Punta de la Torreta
and Punta de les Anquines. (See map). However,
the loss of sand is continuing, with the alarming total losses on some beaches (see
table 1), particularly from those beaches aligned south-east or lying within
bays protected by the islets.
The data shows a general decline to the south-west in beach
width, probably linked to the impact of the marina and breakwaters in preventing
the movement of sediment by longshore drift. The beaches which have suffered
least erosion (la Fragata, Anquines) are those protected by optimum breakwater
alignment to the easterly waves. The beaches most seriously affected by erosion,
(10, 12 and 13) are those within bays protected by the rock islets.
Photo 3: The effect of the rock islets: a negative one?
Photo 4: The November storms: the value of harbour walls
(Punta de les Anquines)
Photo 5: Sitges: the disappearing beaches
The rock islets absorb wave energy but interfere with the
pattern of waves as they enter the bays. The waves refract round the islets, (see
diagram 2) with wave trains crossing each other in the lee of the island.
This would leave some areas within the bays with waves approaching perpendicular
to the shore and others with waves approaching obliquely. Waves breaking at an
angle on some areas of the the bays would thus lead to the removal of sediment
by longshore drift (photo 3).
There are three projects proposed by the Sitges authorities
aimed at solving this problem of sand loss. They all involve the import of sand
from up-drift, from a coastal zone between Premià and Vilassar and the
replenishment of the beaches between the breakwaters of La Frageta and Les
Aquines. The eight artificial islands would also be eliminated. The most
expensive option is costed at 551 million ptas. Alternative proposals include
changing the orientation of some breakwaters (Manel Carbonell), constructing
submerged breakwaters (Lluís del Cerro) or beach replenishment from sea bed
dredging (Oriol Pascual).
While the Sitges authorities anxiously await a decision on
their proposals, Barcelona is planning the immediate construction of
semi-submerged breakwaters off the beaches of Bogatell and Mar Bella, those most
damaged in the recent storms. The costs are estimated at 525 and 375 million
ptas respectively. The need for urgency in restoring these beaches is because of
their great tourist value, a position taken by the Environment Minister, Jaume
Matas.
Hard engineering schemes may be the only short-term solution to
the loss of Catalan beaches, but they have a history of unexpected effects and
may have huge consequences to the areas down the coast. The consequences of
beach replenishment are equally uncertain. The grain size of the sediment is
critical in affecting beach permeability and the absorption of wave energy, as
the Weymouth, U.K. authorities discovered in 1996. A replenished beach
protecting the Preston coast road was transformed from relatively permeable to
relatively impermeable due to the smaller grain size of the imported material.
The result? Waves simply ran up the beach on to the road.
Perhaps the best solution, at least for the present, is a
shoreline management plan for the whole Catalan coast. This would offer a
strategic approach to shoreline management and attempt to co-ordinate activities
between coastal authorities and address the conflicts between competing
interests. The long-term approach may be to rethink the intense development of
the coastline, and work with the natural processes rather than against them. It
may be unrealistic, however to hope for the return of the salt marshes to the
Maresme!
| 
