Drainage Basins

Grade 12 Geomorphology. SA Curriculum. Drainage Basins.

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The Drainage Basin System
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What is a drainage basin?
How does the drainage basin system operate?
Factors affecting run-off and infiltration in a drainage
basin.
Drainage Basin Management.
Stream Order.
Laminar and Turbulent flow.

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Catchment
The area from
which water drains
into a particular
drainage basin.
Tributary
A river which joins a larger river.
Confluence
The point at which two rivers join.
Watershed
The boundary
dividing one
drainage basin
from another- a
ridge of high
land.
What is a drainage basin?
Source
The upland area
where the river
begins.
Mouth
Where the river
flows into the sea,
or sometimes a
lake.

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Drainage Basin
Watershed
Interfluve
Water table
Groundwater
Confluence
Consequent
stream
Subsequent
stream
Drainage Basin Terminologies
An area of land drained by a river and it’s
tributaries
The boundary between 2 drainage basins
High lying area between 2 stream in the same
drainage basin
Level at which saturation occurs in the soil
Water stored in rocks following percolation
Where two tributaries join
Main stream
Main tributary

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Do you know your drainage basin terminology?
Click here for online
test
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test' wrkC5h2ss?566iiiLc Drai rn gLk Lwn6
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The drainage basin of the Afon Caerfanell, a tributary of
the River Usk, South Wales.
the watershed can
be clearly seen on
this photograph
Watershed

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The confluence of two rivers

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Watershed
Tributary
Match the label to the correct letter
Confluence
a
b
c
Source
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Source: http://www.boardworks.co.uk/

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Sketch the river showing its catchment
Add the following labels – Watershed, Confluence, Tributary' wrkC5h2ss?566iiiLc Drai rn gLk Lwn 6
Source: http://www.boardworks.co.uk/

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2 1
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What is a drainage basin?
How does the drainage basin system operate?
Factors affecting run-off and infiltration in a drainage
basin.
Drainage Basin Management.
Stream Ordering.
Laminar and Turbulent flow.
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The Drainage Basin System

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All rain falling within a drainage
basin ends up in the river
system.
How does it get there?

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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

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UNDERGROUND
(uppermost
level of
saturated
ground)
Raindrops
INFILTRATE soil
and rock. The
water then moves
underground
towards a stream
or river

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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?
Which route to the river is quicker
- surface runoff or infiltration / groundwater flow?

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How does the drainage basin system operate?
A drainage basin system is a part of the world’s
hydrological (water) cycle:
The hydrological cycle is a closed system.
The drainage basin is an open system.
You should be able to explain why once you have seen
the following slides!
Water Cycle
animation

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The water cycle – more detailed
http://www.managingwholes.co
m/flash/watercycle.htmo krwWchHss(c,,iii)* Drai rn g)w )kn ,
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The Water Cycle

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The water cycle – definitions!o krwWchHss(c,,iii)* Drai rn g)w )kn ,
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The drainage basino krwWchHss(c,,iii)* Drai rn g)w )kn ,
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river
evapotranspiration
precipitation
throughflow
surface runoff
groundwater flow
water table
interception
percolation
infiltration
Drainage basin system

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Drainage basin termso krwWchHss(c,,iii)* Drai rn g)w )kn ,
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Drainage basin system
inputs
precipitation
outputs
river runoff
evaporation
transpiration
stores and flows
interception
surface runoff
surface water storage
soil water storage
groundwater storage
infiltration
groundwater flow
throughflow
percolationo krwWchHss(c,,iii)* Drai rn g)w )kn,
Source: http://www.boardworks.co.uk/

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Page 114
Click for video
Click for video

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Drainage basin systemo krwWchHss(c,,iii)* Drai rn g)w )kn ,
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What is a Drainage Basin?
How does the drainage basin system operate?
Factors affecting run-off and infiltration in a Drainage
Basin.
Laminar and Turbulent flow.
Drainage Basin Management.
Stream order.
What is a flood hydrograph?
What factors influence the shape of a flood hydrograph?
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The Drainage Basin System

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Low density (Coarse texture)
 Medium density (Medium texture)
 High density (Fine texture)
 Very high density (Superfine texture)
Factors affecting drainage density are:
gradient, vegetation, rock type, soil type,
evaporation, amount of rainfall, type of rainfall and
man.
How do the above factors influence drainage
density?
Factors influencing Drainage Density

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Factors affecting Drainage Density

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Drainage Density and Texture
Schuters pg 145

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Factors affecting Drainage Density
Which rock will cause the most overland flow and why?
Which rock might slow down the passage of water the
most?
What is:
a)Porosity
b)Permeability?

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Factors affecting
Drainage Density
Which one might
be pervious?
Which one would
slow down the
passage of water
the most?
Which one would
be most likely to
flood?

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A number of factors will effect how quickly water gets
to a river.
If water returns to a river too quickly it may result in
flooding.
Look at the following slides (35-39)
How do you think they affect how quickly water
returns to the river?
Explain how each of these factors will affect drainage
density. Insert you answer in a textbox.

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•Reduce run-off in the upper areas of the
river by ensuring that the slopes are
covered with vegetation.
Flood Controls
 Use terraces and contour ploughing in

farmlands to help control surface run-
off.
Build dams to control the flow of water
through an area.
Build levees that can withstand the
pressure and high water levels.
Build larger channels and divert water
away from certain areas.
Don’t build
settlements in areas
below the floodline.
Flooding in Australia Coastal floodingPowerpoint

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Activity 4: Causes and impacts of floods
Pg.119
(5)
(3)
(3)
(5)

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Factors Influencing Infiltration – Schuters Pg 144

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Activity 1: Direct and indirect run-off
Oxford
Pg.116

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What is a drainage basin?
How does the drainage basin system operate?
Factors affecting run-off and infiltration in a drainage
basin.
Drainage Basin Management.
Stream Order.
Laminar and Turbulent flow.
What is a flood hydrograph?
What factors influence the shape of a flood hydrograph?

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Mgeni Drainage Basin
Schuters pg 182 and 183

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What is a Drainage Basin?
How does the drainage basin system operate?
Factors affecting run-off and infiltration in a Drainage
Basin.
Drainage Basin Management.
Stream Order.
Laminar and Turbulent flow.
What is a flood hydrograph?
What factors influence the shape of a flood hydrograph?
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Stream order
•A method of classifying or ordering the
hierarchy of natural channels.
•Strahler (1957) is the most widely used
system.
•See GIS stream
ordering
www.usda.gov/stream_restoration/chap1.html

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Law of Stream Numbers and Law of Basin areas.
There is a fairly fixed regularity with which the area of a drainage basin increases from one
stream order to the next this is known as the law of basin areas. [ The fixed regularity is 4,7
times]
The law of stream numbers states that the higher the stream order the fewer the number of
tributaries e.g. there will always be more stream order ones than two’s in a given drainage
basin.

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Calculation of area according to the squares method
Count the number of squares covered
by + half
Count the number of WHOLE squares
that fall within the outline
Number of squares fully
covered = 18
Number of squares covered
by +half = 7
Total number of square covered =25
Area of one square = LXB
= 1cm x 1cm
= ,5 km X ,5 Km = ,25 square Km
1 cm
1cm
Area of 25 squares
= ,25 X ,25 = 6,25 square Km

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Calculation of Drainage Density
Number of squares
fully covered = 42
Stream order at X = 4
Number of squares covered
by more
than half = 13
Total number of
Squares = 55
Area of 1 square
= L X B
1 cm
1 cm
= 1cm X 1cm
= ,5 km X ,5 km
= ,25 square km
Area of 55 squares
= 55 x ,25 sq. km
= 14 sq. km

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What is a drainage basin?
How does the drainage basin system operate?
Factors affecting run-off and infiltration in a drainage
basin.
Drainage Basin Management.
Stream Ordering.
Laminar and Turbulent Flow.
L
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 River-bed is level and even
 Water flows in layers
 Water in contact with the bed and banks flows slowly due
to friction
 Successive layers achieve greater speeds. Fastest
speeds occur in the middle just below the surface
 Water travels faster, but there is less erosion since
the lower layers protect the river channel.
 River-bed is uneven and steep
 Water continually changes levels and flows with a
‘bubbling’ action
 Lots of erosion takes place, because the sand
particles are lifted by water.
Laminar Flow Turbulent Flow

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What do you know about drainage basins?( vrwcChossMC//iiijb Drai rn gjw jvn /
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