15
5.
Oyster reef
Instead of using hard structures
like breakwaters made from rock
or concrete blocks, we can also introduce natural
elements like shellfish that form reefs and mitigate
erosion. Three large synthetic oyster reefs were built
in 2010 in the Eastern Scheldt. The reefs are non-
galvanised steel cages filled with the shells of dead
oysters. It has been found that new oysters colonise
the reefs quickly and that the amount of sludge behind
the reef increases. The iron cage soon rusts away
and the oysters then provide the required stability.
Laboratory measurements have demonstrated that
the structures damp waves in shallow water. Similar
techniques have been used in the United States to
restore oyster reefs on a large scale.
6.
Forebanks
River forebanks with
vegetation break waves. Fields
with willows in front of dikes
mitigate wave impacts and
so the dike does not need
to be as high. Studies have
shown that a strip of willows
one hundred metres wide can
reduce the height of one-
metre-high waves by 80%.
Dikes protected in this way
can be much lower and they
can be covered in clay rather
than rock. So introducing
vegetation to forebanks
can prevent expensive dike
upgrades and also enhance
the natural and recreational
value of these areas.
7.
Sand Motor
Sand replenishment can be combined with natural forces to maintain a sandy
coastline and protect the hinterland from flooding. 2011 saw the launch of the pilot
study the ‘Sand Motor’ between the Hook of Holland and The Hague. It will look at the
efficacy of mega-replenishment. This sustainable approach requires less sand and
gives the ecosystemmore time (approximately 30 years) to recover. The Sand Motor
is a flat sand bank containing a dune lake. The excess sand is gradually distributed
by the currents and tides, and the lake provides additional natural diversity. That
maintains the natural coastal defences, and provides more openings for nature and
recreation. If the mega-replenishment principle works, it can also be used in other
locations.
8.
Mangroves
Mangrove forests are natural coastal defences and
valuable ecosystems. Even so, they are rapidly
being lost worldwide, particularly to make room
for fish ponds. Traditional hard structures are being
built to offset the resulting coastal erosion and
to improve coastal protection. However, on muddy
terrain, these structures are neither sustainable nor
effective because they subside so quickly. As a result, a range
of pilot projects have been launched to restore mangrove
forests. Coastal erosion can be prevented by, for example,
building berm structures from natural materials in front of the
coast, catching sediment and restoring the natural conditions
in which mangrove forests can grow again, and protecting the
coast once again in a natural way. The valuable ecosystem
can also recover.
PHOTO: DIRK HOL
PHOTO: RIJKSWATERSTAAT
WANT TO KNOWMORE?
For more applications and cost/benefit analyses, see 'Eco-
engineering in the Netherlands. Soft interventions with a
solid impact', published by Deltares and Rijkswaterstaat:
http://www.deltares.nl/en/expertise/919492
/
ecology/1457099
DELTARES, JANUARY 2014