(upbeat music)
M1: When natural coastal defences
like beaches, sand dunes and wetlands
are not carefully managed, or when they simply can't cope
to prevent coastal flooding caused by storms
and sea level rise, hard engineering
is often a last resort to protect towns
and developments against coastal flooding.
(water crashes)
M2: Here, on the North Somerset coastline
catastrophic flooding in 1981 has led
to the development of a hard engineering approach.
This was paid for by the Environment Agency
at a cost of two million pounds in the 1990s.
(upbeat music)
M1: The first line of hard engineering defence
against the sea is often riprap or rock armour.
M2: So here we have an example of riprap:
a disorganised pile of large, resistant limestone boulders.
The idea of these boulders is that they provide
a physical barrier between the erosive power of the sea
and the easily eroded rocks behind.
So, the riprap is a rather random
and chaotic pile of boulders
simply dumped here to prevent the erosion of the cliff.
This may look like riprap but it's
actually called rock armour.
Rock armour is designed with this gradual slope,
which reduces the erosive power of the water
preventing processes such as hydraulic action
and abrasion attacking the coastline behind.
So the rock armour here is dealing with
the wave energy and the flooding is prevented
by the sea wall at the back.
M1: So the reason this rock armour's been placed here
is because this sea wall actually has
quite a vertical face to it, and we now know
that this kind of vertical face
can be a design flaw in a sea wall.
As waves hit the face of the sea wall
they're reflected back out onto the beach
and they scour and erode away the beach material
in front of the sea wall.
This can undermine the foundations of a sea wall
causing it to collapse, which is why
the Dawlish sea wall in South Devon
collapsed during the winter storms of February 2014.
So here, engineers have solved this problem
by placing rock armour in front of the sea wall,
and if we look further down the coast
we can also see how the problem can be solved
by changing the design of a sea wall.
(upbeat music)
So this is a wave return wall, or a recurved sea wall.
Now, when the tide is high, the sea level
comes right up to the base of the sea wall,
and when the waves are small, they'll
lap up the front of the sea wall, losing their energy
as the water shallows.
But during big winter storms, when we have
large destructive waves, the swash of the wave
flows up the sea wall, and the curve and the lip
of the sea wall flings it back out towards the sea.
The wave then comes splashing down on the sea surface,
and this prevents the backwash of the destructive wave
from eroding the material on the
beach in front of the sea wall,
which would undermine the structure of this sea defence.
This huge recurved sea wall stands over six metres
over the beach, protecting the whole town
of Burnham-On-Sea.
It was built following the storms in 1981
that destroyed the old sea wall
and caused catastrophic flooding in the town.
The stepped slope at the front of the sea wall
absorbs the wave energy, encouraging
the waves to break, and the curved top of the sea wall
deflects the waves back away from the town.
These holes in the front of the sea wall
also serve an important purpose.
If flood waters ever do overtop the sea wall,
that water can flow back out to the sea
through these carefully designed one-way
storm drains that allow the flood waters
to flow back out, but do not allow
the sea water to flow in.
M2: These metal cages filled with rocks
are a low cost alternative to sea walls called gabions.
Here, home owners have protected their properties
by building these lower cost ad-hoc defences.
(upbeat music)
M1: Hard engineering coastal defences like this
are very effective.
Even in the largest winter storms,
it's very unlikely that any waves
will get past this sea wall.
However, this kind of engineering damages
the natural landscape and habitat,
and it's also very expensive.
M2: With rock armour costing between one thousand
three hundred and fifty pounds and six thousand
pounds per metre, and sea walls costing
between seven hundred and five thousand four hundred
pounds per metre, depending on the complexity
of design and construction.
M1: So where possible, it's important
to carefully manage the natural protection
provided by beaches, sand dunes and wet lands
to avoid the need for this kind of hard engineering.
(upbeat music)