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RIVER DRAINAGE BASINS

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The rainfall itself - very heavy, in big droplets (less likely to infiltrate) ... Non-permeable rock within drainage basin (water can't infiltrate easily) ... – PowerPoint PPT presentation

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Title: RIVER DRAINAGE BASINS


1
RIVER DRAINAGE BASINS
2
A RIVER SYSTEM ACTS LIKE A SYSTEM OF DOWNPIPES
AND GUTTERING ON A HOUSE - IT ALLOWS THE MOVEMENT
OF RAINWATER INTO THE SEA
3
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)
4
(No Transcript)
5
All rain falling within a drainage basin ends up
in the river system. How does it get there?
6
OVERGROUND
Water runs downhill over the ground towards a
river or stream - SURFACE RUNOFF
Raindrops might fall directly into a river or
stream - CHANNEL FLOW
7

Raindrops INFILTRATE soil and rock. The water
then moves underground towards a stream or river
UNDERGROUND
(uppermost level of saturated ground)
8
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?
9
Scenario 1 Peak discharge overland flow- high
  • After how many hours does rainfall peak?
  • What is peak river discharge in Scenario 1? (in
    cumecs)
  • What is peak river discharge in Scenario 2 (in
    cumecs)
  • What is the lag time in Scenario 1?
  • What is the lag time in Scenario 2?

Scenario 2 Peak discharge groundwater flow - lower
Peak rainfall
A FLOOD HYDROGRAPH
10
So Surface runoff Shorter lag time Higher
river peak discharge More likelihood of FLOODING
11
Non-permeable rock within drainage basin (water
cant infiltrate easily)
The rainfall itself - very heavy, in big droplets
(less likely to infiltrate)
Urbanisation within the drainage basin - large
areas concreted or tarmaced over, making surface
non-permeable. (Storm drains are built in, of
course, but these carry water very quickly to the
river, making lag time even shorter)
Water table already very high (little scope for
infiltration if ground is already saturated)
What conditions will make surface runoff more
likely?
Steep slopes in drainage basin (water flows
downhill rapidly -less time for infiltration to
occur
Few trees (tree roots break the soil, providing
little gaps for water to infiltrate). Trees then
take up and store water from underground
12
Storm Drains
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.
13
Which of these conditions applied to the River
Severn drainage basin in July 2007?
14
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. Rainfall was intense with large drops.
There had been some very wet weather in the
Severn drainage basin in June- 221 of previous
average Ground in some areas was saturated
15
River Severn flow was already exceptionally high
in June
16
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)
17
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
18
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
19
Non-permeable rock within drainage basin (water
cant infiltrate easily)
The rainfall itself - very heavy, in big droplets
(less likely to infiltrate)
Urbanisation within the drainage basin - large
areas concreted or tarmaced over, making surface
non-permeable. (Storm drains are built in, of
course, but these carry water very quickly to the
river, making lag time even shorter)
Water table already very high (little scope for
infiltration if ground is already saturated)
What conditions will make surface runoff more
likely?
?
Steep slopes in drainage basin (water flows
downhill rapidly -less time for infiltration to
occur
Few trees (tree roots break the soil, providing
little gaps for water to infiltrate). Trees then
take up and store water from underground
?
20
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
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