EA Position On Dredging
Uploaded by
The GuardianEA Position On Dredging
Uploaded by
The GuardianRiver Dredging
and Flood Defence
To Dredge or Not to Dredge?
River maintenance
The Environment Agency undertakes
maintenance within rivers so water
can flow freely and to reduce flood
risk.
• This maintenance includes...
Checking structures
• Maintaining weirs, sluices, culverts and other hydraulic
structures that regulate the flow
Weirs Sluices
Culverts
Clearing small debris
• Removing recent accumulations of debris which create
‘unnatural’ obstructions to flow
Floating Leaves
Rubbish
screen
Clearing large debris
• Removing large obstructions which restrict flow
Trees Rubbish
Clearing natural debris
• Clearing natural blockages such as shrubs and weeds
Before After
What is dredging?
• Dredging is the process
of removing silt from the
bottom and sides of the
river channel
Grab
Why are rivers dredged?
• River dredging is primarily undertaken to
improve navigation, by increasing depths and
straightening channels
• Another purpose is to improve land drainage,
particularly by creating artificial channels
• Commercial dredging (where permitted) is also
undertaken to acquire sand and gravels for
construction sites
Rivers in flood
• A ‘floodplain’ is defined as all land adjacent to a river or
stream, over which water flows during a flood
Floodplai
n
Channel
Natural river channels
• The river channel is not large enough to contain extreme
floods, even after dredging
• Dredging of river channels does NOT prevent flooding
during extreme river flows
Floodplain
Extreme
flood level
Channel
What happens during a flood?
When normal flows are in When extreme flows are in
the channel, water level the floodplain, water level
rises rapidly due to the rises more slowly as there is
small volume a large volume to fill
Floodplain
Channel
River section in summer flow
• Normal summer flows - the water
level is within the channel
Channel
River section in winter flow
• Normal winter flows - the water level can
rise rapidly. It can reach the top of the
river channel, on average, every two
years
Channel
River section in extreme flow
• For extreme flows the
water level can enter the
floodplain. It happens,
on average, at intervals
of more than two years.
The water then rises
slowly during the flood. Floodplain
Click here for more detail
Simulation
Agency flooding Information
The extent of floodplains are shown on “Indicative
Floodplain Maps” . These are available on the
Environment Agency website:
www.environment-agency.gov.uk
(‘What's in your backyard’)
Click here for more detail
Example
What is silt?
• Silt is sediment consisting of small sized particles of
rock. It comes from the breakdown of rocks and gravel in
the upper river sections, from overland flow across soils
in the fields, and from outfalls draining urban areas
• Silt is naturally transported in a river like ‘passengers’ on
a train
• During a flood, when the river flow is fast, the silt is
moved within the water
• After a flood, when the river flow slows down, the silt is
deposited on the river bed
Before an extreme flood
• Water level is within the channel
and the river bed is silted
During an extreme flood
• Heavy rainfall erodes silt from land
upstream from the river by overland flow
• Silt from urban areas is drained into the
river after a heavy rainstorm
• The strong flow of the river erodes silt from
the river bed
The extreme flood
• While river levels are
high and flows are
fast, the silt is
transported mainly in
the water and the
channel bed is
‘flushed’ free of silt
After an extreme flood
• As the river level falls, the flow in the river
slows down and has insufficient power to
carry the silt ‘passengers’
After an extreme flood
• Most of the silt settles on the river bed,
particularly in stagnant areas such as at the
approaches to bridges and culverts
After an extreme flood
• Silt is also deposited in slower flow areas such as
channel bends
Siltation
To Dredge or Not to Dredge?
First issue: Major change to the river
• The natural formation of floodplains is a geological
response to extreme flooding
• The concept of dredging to prevent extreme flooding is
equivalent to trying to squeeze the volume of water held
by a floodplain within the volume of water held in the
river channel
• Since the floodplain volume is usually many times larger
than the channel volume, the concept becomes a major
engineering project and a major environmental change
To Dredge or Not to Dredge?
• To contain a flood within
a normal river channel
would mean dredging the
river channel several
metres deep
• The river channel would
also have to be widened
by several metres
To Dredge or Not to Dredge?
Second issue: Cost to the taxpayer
• Dredging is expensive
• It must be repeated after every extreme flood, as
the river silts up again
To Dredge or Not to Dredge?
Third issue: Impact on the environment
• Removing river bank vegetation such as trees
and shrubs decreases bank stability and
increases erosion and siltation
• Removing gravel from river beds by dredging
leads to the loss of spawning grounds for fish,
and can cause loss of some species
• Removing river bank soils disturbs the habitat of
river bank fauna such as otters and water voles
To Dredge or Not to Dredge?
Fourth issue: The need to
protect river structures
• Dredging in the way of bridges, weirs, culverts,
river walls and other structures will undermine
their foundations
To Dredge or Not to Dredge?
• If the river channels are
dredged and structures
are not realigned,
‘Pinch Points’ at
structures would occur.
This would increase the
risk of flooding at the
structure.
To Dredge or Not to Dredge?
Fifth issue: Restrictions on silt disposal
• The land levels of a
floodplain must not be
raised as they are part
of the river’s natural
flood storage area
• Any contaminated silt must be sent to a licensed
tip for safe disposal
To Dredge or Not to Dredge?
Summary
• Silting is a natural process
• During major floods the river cannot be
contained within the normal river channel
• Dredging can be effective for improving land
drainage but is rarely able to prevent flooding
Further information?