(lively music)
M2: Helping the water flow more quickly downstream
may help reduce flood risk here,
but it may increase flood risk downstream
where the river channel will struggle to cope
with the increased flow of water.
If you enable more water to be stored
within the river channel during flood events,
then the stream power or erosive energy
of the river will be increased,
and it may start to erode its bed and its banks.
That eroded material may then be transported downstream
by the river and deposited
in the river channel,
decreasing channel depth and channel capacity
and leading to an increase in flood risk.
Removing pools, riffles, gravel bars,
and vegetation and introducing unnatural,
hard engineering material to rivers
can destroy wildlife
and natural habitats along a river.
And these impacts can extend
for many kilometres upstream and downstream.
So, in recent years, these traditional methods
have been replaced by softer engineering.
(lively music)
M1: This kind of softer engineering
can include keeping hard engineering structures
to just one side of the river channel,
allowing floodplains like this on the other side
to flood naturally.
Also, maintaining the natural pool
and riffle morphology of the river channel,
which helps to keep river ecosystems healthy.
Rather than using hard engineering materials
like concrete and block stone
to protect the river bank,
more natural materials like willow
can be used to reinforce the bank instead.
If hard engineering and river channelization
has resulted in problems downstream,
this can often be solved by restoring
the natural river channel,
by reintroducing meanders, removing dams and weirs
and encouraging wetland formation
along the banks of the river
to slow the flow of water.
However, it can take decades, if not longer,
for the river to return to some kind
of pre-engineering state.
(lively music)
F1: Flood overfill basins like this one
beside the River Rheidol in Aberystwyth
is used to store excess water
during flood events.
This helps to prevent the flooding
of the surrounding buildings
and also helps to slow the flow of water
down the drainage basin.
This flood overflow basin
is set back about 100 metres
from the main river channel.
This provides a buffer zone of land,
which can hold water during flood events
even before the flood waters reach the overflow basin.
(lively music)
M1: Natural measures that slow the flow
of storm rainfall
can also help prevent flooding downstream.
Protected peat bogs, like this one
at Cors Caron, act like natural sponges.
All the ground beneath this walkway
and spread out in front of us
is completely saturated with water.
It's like a giant sponge that stops the water
from rushing down the drainage network.
The peat bog also provides a more natural solution
to slowing the flow of water
than a hard engineered reservoir dam.
Another form of natural flood management
involves planting trees, or afforestation.
Planting trees on flood plains
or in upper catchments like this
increases the interception of rain water
by vegetation and makes the ground surface rougher.
And all this slows the flow of water
on its way down towards the river channel.
(lively music)
F1: This woody debris is creating
a natural log jam.
It's acting to slow the flow of water in the river.
M1: Now, this is actually a very useful effect
that we can recreate artificially
by adding engineered log jams into river channels.
When engineers do this,
they're looking at the useful effects
of this natural process
and using it as inspiration
to find a softer engineering approach.
There have even been discussions
about reintroducing beavers to the UK.
Beavers build dams across rivers
that slow the flow of water,
a bit like this log jam,
and help create wetlands upstream.
In fact, many appropriate catchments for this
have been identified
and some beavers have already been introduced
during trials in Devon.
Soft engineering solutions generally cost less
than hard engineering.
Measures that use wood to protect the bed
and banks of the river, and to slow the flow of water,
can cost tens to hundreds of pounds per meter.
Whereas the hard engineering alternatives
can cost thousands of pounds per metre
or even millions in the case of major reservoir dams.
F1: Although this is an attractive approach,
especially the benefit to wildlife
through creating different habitats,
there is some debate
about whether this approach could really have an effect
on large-scale flooding.
(lively music)
It's not always possible
to create effective soft engineering solutions,
especially in densely populated urban areas
where using hard engineering
that can withstand severe flooding
is necessary to protect homes and infrastructure.
So, rivers can often be most effectively managed
by using a combination of hard
and soft engineering techniques
that work together
to reduce the risk of flooding.