RIVER
DRAINAGE
BASINS
A RIVER SYSTEM ACTS LIKE A
SYSTEM OF DOWNPIPES AND
GUTTERING ON A HOUSE - IT
ALLOWS THE MOVEMENT OF
RAINWATER INTO THE SEA
A TYPICAL RIVER DRAINAGE BASIN. THIS IS THE CATCHMENT AREA FOR ALL
THE RAIN THAT FALLS WITHIN THE WATERSHED
Watershed
(higher ridge
of land, the
boundary
between one
drainage
basin and
another)
Confluence
(a place
where two
rivers meet)
Source
(the place
where a
river begins
-a river
system will
have
numerous
sources,
such as
springs)
Tributary stream (a smaller river that flows
into a larger river)
All rain falling within a
drainage basin ends up in
the river system.
How does it get there?
OVERGROUND
Raindrops might fall directly into a
river or stream -
Water runs downhill
over the ground
towards a river or
stream -
CHANNEL FLOW
SURFACE RUNOFF
Raindrops
INFILTRATE
soil and rock.
The water then
moves
underground
towards a
stream or river
UNDERGROUND
(uppermost level
of saturated
ground)
So, which route to the river is quicker
- surface runoff or infiltration / groundwater flow?
ANSWER:
Rain travelling as surface runoff
will get to rivers and streams much more quickly, reaching
a river or stream with little delay,within a short time-span.
Water that infiltrates and travels underground travels
much more slowly, finally reaching a river or stream
bit by bit over a much longer period of time.
Which situation is a river more likely to be able to cope
with, without flooding?
1.
After how many hours does rainfall peak?
2.
What is peak river discharge in Scenario 1? (in
cumecs)
3.
What is peak river discharge in Scenario 2 (in
cumecs)
4.
What is the lag time in Scenario 1?
5.
What is the lag time in Scenario 2?
Scenario 1
Peak discharge
overland flowhigh
Scenario 2
Peak discharge
groundwater
flow - lower
Peak rainfall
A FLOOD HYDROGRAPH
So:
Surface runoff
Shorter lag time
Higher river peak discharge
More likelihood of FLOODING
The rainfall itself very heavy, in big
droplets (less likely
to infiltrate)
Water table already
very high (little
scope for infiltration
if ground is already
saturated)
Steep slopes in
drainage basin (water
flows downhill rapidly
-less time for
infiltration to occur
Non-permeable rock
within drainage basin
(water can’t infiltrate
easily)
What conditions
will make surface runoff more likely?
Urbanisation within the
drainage basin - large areas
concreted or tarmaced over,
making surface nonpermeable. (Storm drains are
built in, of course, but these
carry water very quickly to
the river, making lag time
even shorter)
are needed to see this picture.
TIFF (Uncompressed) decompressor
QuickTime™ and a
Few trees (tree roots break
the soil, providing little gaps
for water to infiltrate).
Trees then take up and
store water from
underground
Storm drains are
connected to rivers
and streams by an
underground network
of pipes, called the
storm sewer system,
which helps to
prevent flooding from
rain.
Which of
these
conditions
applied to the
River Severn
drainage basin
in July 2007?
The rainstorms
over the weekend
20th-23rd July
saw between 4-5
inches of rainfall
- as much as
would normally be
expected for the
whole of July.
There had been
some very wet
weather in the
Severn drainage
basin in June221% of previous
average
Rainfall was
intense with large
drops.
Ground in some
areas was
saturated
River Severn flow was
already exceptionally
high in June
Although still mainly rural,
considerable urbanisation has
occurred within the Severn
catchment over the past 50 years
including the expansion of
existing cities, towns and villages,
for example the significant
expansion of Redditch and
Telford.
(Environment Agency - ‘A Flood Management Strategy For
the River Severn Corridor’)
More intense use of agricultural land during the
twentieth century has resulted in larger fields, more
efficient drainage systems and larger and heavier
machinery causing deeper compaction of soils. This
has resulted in water being moved through
agricultural land faster and the rate of run-off being
higher. This, combined with loss of woodlands and
rough vegetation (which absorb and retain water), to
farmland or urban development, may mean that
volumes of water in rivers are higher and water levels
rise faster. Several groups in the UK are currently
investigating the effect of land and soil management
practices on soil erosion and run-off. (Environment Agency - ‘A Flood
Management Strategy For the River Severn Corridor’
The Upper reaches of the Severn drainage basin are in
the Welsh uplands - slopes here are steep, favouring
surface runoff. Rain water getting into the river quickly
here, will flow fast within the river channel towards the
lower reaches of the drainage basin - to places such as
Tewkesbury and Gloucester
The rainfall itself very heavy, in big
droplets (less likely
to infiltrate)
Water table already
very high (little
scope for infiltration
if ground is already
saturated)
Steep slopes in
drainage basin (water
flows downhill rapidly
-less time for
infiltration to occur
Non-permeable rock
within drainage basin
(water can’t infiltrate
easily)
What conditions
will make surface runoff more likely?
Urbanisation within the
drainage basin - large areas
concreted or tarmaced over,
making surface nonpermeable. (Storm drains are
built in, of course, but these
carry water very quickly to
the river, making lag time
even shorter)
?
are needed to see this picture.
TIFF (Uncompressed) decompressor
QuickTime™ and a
Few trees (tree roots break
the soil, providing little gaps
for water to infiltrate).
Trees then take up and
store water from
underground
?
It is important that we
have some understanding of
drainage basin conditions
and storm hydrographs
(both natural and man
made) if we are to
understand what can be
done to reduce future
flood risks