King
of the hill
The fi rst of a new type of Dutch fl ood defence system was installed in Somerset last year. The
system consists of a grid of precast concrete blocks with an unusual design that reduces wave
run-up, which enables fl ood defences to be built lower, potentially saving an immense sum on
construction costs
Having now been used in
Netherlands for more than ve
years, the Hillblock concept
was invented by the late
Dutch entrepreneur Hans Hill. His original
idea in about 2003 applied the principle of
a structure containing pockets to absorb
the energy of waves in water. But it was an
entirely different environment: swimming
pools. Noticing that the wake generated
by other swimmers was
slowing down his lap times,
he designed a oat for the lane ropes.
A conversation with a professor at the
University of Delft, Marcel Stive, led to him
considering applying the idea to the larger
market of ood defences.
That topic is something of a Dutch
national preoccupation, following the
devastating oods in 1953 that killed
1,800. As half of the country’s ground lies
within a metre of sea level, it is protected
by a system of dykes and levees. They
are generally built from a sand or clay
core covered with geotextile to improve
resistance to breakthrough, as well as
10-15 cm of aggregate. Then the landward
side is usually planted with grass,
and the sea face protected by rock-
or concrete-based protection called
revetment. These might consist of
basalt blocks or rock armour, which
consists of a rough irregular surface
of cobble-sized or larger stones,
secured in place with dry grouting –
gravel or pebbles poured over the
top of the surface that lodge in the
gaps and cracks between them
and tend to lock them in place.
Storms threaten these
defences because they send large waves
crashing against the dykes. Two aspects
are particularly destructive. First, large
waves might go over the top the dyke,
potentially weakening the dyke from the
rear side, as happened in 1953. Second,
the scouring force acting on the seaward
surface that erodes dunes and pulls out
revetment blocks increases with wave
height. Returning sand that washes
away from beaches is a big industry in
the Netherlands, reports Aroen Mughal,
Hillblock chief technology of cer.
The Hillblock design counters both
threats. Shaped a bit like a champagne
cork, the assembled two-piece blocks
feature a rounded bulbous head tapering
to a narrow waist, and then bowing out to
form the top of a foot, which tapers slightly
again at the base. When the blocks are
installed in a grid, gaps of up to 10cm
at the surface channel seawater into a
kind of subterranean chamber, sapping
the wave of energy and reducing run-up.
Mughal estimates that the effect reduces
wave runup by 30%, saving 1m-2m of
dyke height, depending on its design.
On a common 1:3 slope, every metre of
height requires three metres of width,
so the blocks can save a huge amount
of earthwork. The maximum slope angle
allowed is 1:1; above that, the blocks
risk falling out due to gravity. In the
Netherlands, these blocks are allowed to
be used in areas experiencing storm wave
height up to 5m.
Hillblocks are installed dry on top of
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