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What does it mean?
• The hydrological cycle is the system which
describe the distribution and movement of
water between the earth and its atmosphere.
• The model involves the continual circulation
of water between oceans, the atmosphere,
vegetation and land
Components of drainage basin
Input Flow Store Output
Through fall interception Evaporation
Stem flow Soil water / soil
Precipitation Infiltration Surface water Transpiration
Percolation Ground water /
Through flow Channel storage River discharge
• The input is the water that is added to a
drainage basin in the form of precipitation.
• This can occur in a number of forms, such as a
rain, snow and hail.
• It can also occur at different times of the year
and in different amounts, intensities and
frequencies throughout the year.
• Evaporation is the process by which a liquid
is changed in to a gas.
• Solid into gas or gas into solid is sublimation
• Evaporation is the most important from
oceans and seas.
• It increases under warm, dry conditions and
decreases under cold, calm conditions.
Factors affecting evaporation include:
3 wind speed
Temperature is most important
1. amount of water available
2. vegetation cover and
• Transpiration is the process by which
water vapor escapes from a living plant,
principally the leaves, and enters the
• The combination effect of evaporation and
transpiration are normally referred to as
• River discharge refers to the movement of
water in channel such as stream and rivers.
• The water may enter the river as direct
channel precipitation or it may reach the
channel by surface runoff, groundwater flow,
or through flow.
• Interception refers to water that is caught and stored
• There are three main components :
Water that is retained by
plant surface s and that is
later evaporated away or
absorbed by the plant
water that either falls
through gaps in the
vegetation or that drops
from leaves or twigs
water that trickles along
twigs and branches and
finally down the main
• Soil water ( soil moisture) is the subsurface
water in soil and subsurface layers above the
water table. From here water may be:
• Transmitted downwards towards the water
• Transmitted upwards towards the soil surface
and the atmosphere.
• Surface water are a water which stored on
• There are a number of types of surface water.
These are :
• Ground water refers to subsurface water
that is stored under the surface in rocks.
• Ground water accounts for 96.5% of all fresh
water on earth.
• Channel storage refers to all water that is
stored in rivers, streams and other drainage
• Through fall refers to water either falls
through gaps in vegetation or that drops from
leaves or twigs.
• Stem flow refers to water that trickles along
twigs and branches and finally down the main
• Overland flow is a water that flows over the
• Overland flow occurs in two main ways:
• 1. when precipitation exceeds the infiltration
• 2. when the soil is saturated
• Infiltration is the process by which soaks into
or is absorbed by the soil.
• The infiltration capacity is the maximum rate
at which rain can be absorbed by a soil in a
• Infiltration is inversely related to overland flow
and is influenced by a variety of factors,
• Duration of rainfall
• Antecedent soil
• Vegetation cover
• Raindrop size and
• Slope angle
• Water moves slowly down wards from the
soil into the bed rock . This is known as
• Depending on the permeability of the
rock, this may be very slow or in some
• Through flow refers to water flowing
through the soil in natural pipes and
• Most groundwater is found within a few hindered
meters of the surface but has been found at
depths of up to 4 kilometers beneath the surface.
• Base flow refers to the part of a river’s discharge
that is provided by ground water seeping into the
bed of river.
• It is a relatively constant flow although it
increases slightly following a wet period.
• A hydrograph is a line graph showing
how water level in a river changes over time.
• There are two main types of hydrograph –
annual hydrographs (also known as river
regimes) and storm hydrographs (also
known as flood hydrographs).
• Annual hydrographs show variations in the
flow of a river over the course of a year,
whereas a storm hydrograph shows the
variation in the flow of a river for a period of
between 1 and 7 days.
• Stream flow occurs as a result of runoff,
groundwater springs and input from
• lakes and from melt water in mountainous
or sub-polar environments.
• character or regime of the resulting stream or
river is influenced by several variable factors:
• l the amount and nature of precipitation
• l the local rocks, especially porosity and
• l the shape or morphology of the drainage basin, its
area and slope
• l the amount and type of vegetation cover
• l the amount and type of soil cover
• Before the storm starts, the main supply of water
to the stream is through ground water flow or
• This is the main suppler of water to rivers. During
the storm, some water infiltrates into the soil
while some flows over the surface as overland
flow or runoff.
• This reaches the river quickly as quick flow, which
causes a rapid rise in the level of the river .
• The rising limb shows us how quickly the
flood waters begin to rise, whereas the
recessional limb is the speed with which the
water level in the river declines after the peal.
• The peak flow is the maximum discharge of
the river as a result of the storm , and
• The lag time is the time between the height of
the storm and the maximum flow in the river.
Influence on hydrograph
• Factors affecting storm hydrographs are:
A. precipitation type and
C. antecedent moisture
A.Drainage basin size and shape
C. Porosity and impermeability of rocks and soils
F. Vegetation type
Drainage basin characteristics
Water flows downhill through the river
channel. Because the flowing water has
Mass and Velocity. It has Energy and it
was this energy to do work.
• The three works that done in river
channel process :
of land forms
• Long profile is a line drawn from the
source of the river ( where it starts) to
the mouth of the river (where it meets
• The typical long profile is concave
steeper in the hills and gentler in the
• Erosion is the wearing away of the
surface of the earth.
• Rivers erode their channels as they
flow downhill towards the sea.
• Because rivers have
• 1. mass and
• 2. velocity energy
• The energy of rivers used to erode the rivers
channel in four main ways
• What are they?
Erosion when particles of sediments bump
Erosion by direct force of the flowing river
When river can dissolve rocks
Erosion by rivers load of sediment.
• There are a number of factors affecting rates
of erosion . These include:
1. Load- the heavier and sharper the load the
greater the potential for erosion.
2. Velocity – the greater the velocity the greater
the potential for erosion
3. Gradient – increased gradient increases the
rate of erosion
4. Geology – soft, unconsolidated rocks such as
sand and gravel are easily eroded
5. PH – rates of solution are increased when the
water is more acidic
6. Human impact – deforestation, dams and
bridges interfere with the natural flow of river
and frequently end up increasing the rate of
• Erosion by the river will provide loose
• This eroded material (plus other weathered
material that has moved downslope from the
upper valley sides) is carried by the river as its
• Rivers transport the load that is
supplied to the in four main ways:
When the dissolved load transported from
Large particles of bed load are rolled along
by the force of the flowing water
When sand, silt , and clay suspended in the
The small particles of bed load tend to hop
along the bed of the river
• The load of a river varies with discharge and
velocity. The capacity of a stream refers to the
largest amount of debris that a stream can
carry, while the competence refers to the
diameter of the largest particle that can be
• Deposition can take place whenever the river
loses its energy.
• There are a number of causes of deposition, such
A shallowness of gradient, which decrease
velocity and energy
A decrease in the volume of water in the channel
An increase in the friction between water and
Velocity and discharge
• The velocity and energy of a stream are
The gradient of the channel bed
The volume of water within the channel,
which is controlled by precipitation in the
The shape of the channel
Channel roughness, including friction
• Hjulstrom curves show what work a river will
do (erosion, transport, deposition)
• depending on its velocity and the size of
•There are three important features of
1. The smallest and largest particles require
high velocities to lift them
2. Higher velocities are required for
entrainment than for transport
3. When velocity falls below a certain
level those particles with a particular
settling or fall velocity are deposited.
Patterns of flow
• There are three main types of flow:
Patters flow/ type
flow Turbulent flow helicoidal
• A smooth, straight channel with a low velocity
• This allows water to flow in sheets, or
laminae, parallel to the channel bed.
• It is rare in reality and most commonly occurs
in the lower reaches.
• It is more common in ground water and in
glacier when one layer of ice moves over
• It occurs where there are higher velocities and
complex channel morphology such as a
meandering channel with alternating pools
• Bed roughness also increase turbulence.
• A horizontal turbulence often takes the form
of helicoidal flow or a corkscrewing motion.
• This is associated with the presence of
alternating pools and rifles in the channel bed
and where the river is carrying large amount
• The erosion and deposition by helicoidal flow
• There are three major types of channel
. These are:
S •Straight channel
• Straight channel are rare .
• Even when they do occur the thalweg moves
from side to side.
• This channel generally have a central ridge of
deposited material, due to the water flow
• Braiding channel occurs when the channel is
divided by islands or bars.
• Islands are vegetated and long-lived, whereas
bars are unvegetated, less stable and often
short term features .
• Braided channels are formed by various
• A steep channel gradient
• A large proportion of coarse material
• Easily erodible bank material
• Highly variable discharge
• Braiding tends to occur when a stream does
not have the capacity to transport its load in a
• It occurs when river discharge is very variable
and banks are easily erodible.
• Meandering is the normal behavior of fluids and
gases in motion.
• Meanders can occur on a variety of materials,
from ice to solid rock.
• The wave length of the meander is dependent
upon three main factors
• 1. channel width
• 2. discharge
• 3. nature of the bed and banks
Causes of meanders
• There is no simple explanation for the creation
for the creation of meanders, and a number of
factors are likely to be important.
Change over time
• There are a number of possibilities:
• Meanders may migrate downstream and
erode river cliffs
• They may migrate laterally (sideways) and
erode the floodplain
• They may become exaggerated and become
cut offs (oxbow lakes)
• Under special conditions, they may become
entrenched or ingrown.
POOLS AND RIFFLES
• Pools and riffles are formed by turbulence.
• Eddies cause the deposition of course
sediment (riffles) at high velocity point and
fine sediment (pools) at low velocity.
• Riffles are small ridges of material deposited
where the river velocity is reduced midstream
, in between pools (the deep parts of a
Waterfall and gorges
• Waterfalls occur where the river spills over a
sudden change in gradient, undercutting rocks
by hydraulic impact and abrasion, thereby
Reasons for change
• The reasons for this sudden change I gradient
along river are:
1. A band of resistant strata
2. A plateau edge
3. A fault scrap
4. A hanging valley
5. Coastal cliffs
• Gorges are formed by periods of river erosion.
• A rapid acceleration in down – cutting is also
associated when a river is rejuvenated, again
creating a gorge.
• Gorges may also be formed as a result of :
Glacial overflow channeling
The collapse of underground caverns in
carboniferous limestone area
Surface runoff over limestone during a periglacial
The treat of waterfalls
Levees and flood plain
Levees and flood plain deposits are formed
when a river bursts its banks over a long period
Deposition of heavy materials which can not
move a great distance near a channel edge. This
coarse deposits buildup to form embankments
• The finer material is carried further away to be
dropped on the floodplain, sometimes
creating back swamps. Repeated annual
flooding slowly builds up the floodplain.
• Old floodplains may be eroded the remnants
are known as terraces.
• At the edge of the terrace is a line of relatively
steep slopes known as river bluffs
Ox- bow lakes
• Ox- bow lakes are the result of both erosion
• Deltas are river sediments deposited when a
river enters a standing body of water such as a
lake, a lagoon, a sea or an ocean.
• They are a result of the interaction of fluvial
and marine processes.
• The material deposited as a delta can be divided
into three types.
• The character of any delta is influenced by the
complex interaction of several variables
The rate of river deposition
The rate of stabilization by vegetation growth
The presence (or absence) of long shore drift
Human activity ( deltas often form prime
farmland when drained)
Types of delta
• There are many types of delta, but the three
classic ones are:
Birds foot delta
• Deltas can also be formed inland. When a
river enters a lake it will deposit some or all of
its load, so forming a lacustrine delta.
• There are a number of human impact on
evaporation and evapotranspiration. These
• There has been an increase in evaporation due
to the construction of large dams.
• Water loss can be reduced :
by using chemical sprays on the water
By building sand –fill dams
By covering the dams with some form of
• It leads to a huge reduction in
evapotranspiration due to the lack of
• There may also be a slight increase in
evaporation because of higher temperatures
and increased surface storage.
Potential hydrological effect of
Urbanizing influence Potential hydrological response
Removal of trees and vegetation Decreased evapotranspiration and
Increased stream sedimentation
Initial construction of housing, streets and
Decreased infiltration and lowered ground
Increased storm flow
Decreased base flows during dry periods
Complete development of residential
commercial and industrial areas
Decreased porosity, reducing time of runoff
Increase peak discharges and compressing the
time distribution of the flow
Greatly increased volume of runoff and flood
Construction of storm drains and channel
Local relief from flooding; concentration of
floodwaters may aggravate flood problems
• Interception is determined by vegetation,
density and type.
• Most vegetation is not natural but represents
some disturbance by human activity.
Impact of Deforestation
A decline of surface
storage A decline in time lag
A reduction in
An increase in surface
Infiltration and soil water
• Human activity has a great impact on
infiltration and soil water
• Land use changes are important
• Urbanization creates an impermeable surface
with compacted soil. This reduces infiltration
and increases overland runoff and flood
Advantages of Water
• The advantage of dams are numerous, like:
• Flood and drought control:
• Hydro electric power
• Improved navigation
• Recreation and tourism
• The disadvantages are :
ground water change
Displacement of population
Drowing of archeological sites
• Deposition within the lake
• Channel erosion
• Erosion of delta
• Loss of nutrients
• Decreased fish catches
• Diseases have spread
• Floods are one of the most common of all
environmental hazards. This is because so many
people live in fertile river valleys and in low lying
• The recurrence interval refers to the regularity of
flood of a given size.
• Small floods may be expected to occur regularly.
• Larger floods occur less often but more
• The nature and scale of flooding varies greatly.
• Some environments are more at risk than
others. The most vulnerable include the
Vulnerable environments for
Low lying parts of active floodplains and river
Small basins subject to flash floods
Areas below unsafe dams
Low lying inland shorelines
Causes of flooding
• A flood is a high flow of water that overtops
the bank of river.
• The main causes of floods are climatic forces.
• The factors that influence the storm
hydrograph determine the response of the
basin to the storm. This factors include
topography, vegetation, soil type, rock type
and characteristics of the drainage basin.
• Urbanization increases the magnitude and
frequency of floods in at least three ways:
1. Creation of highly impermeable surfaces
2. Smooth surfaces served with dense network
of drains, gutters and underground sewers
increase drainage density
3. Natural river channels are often constricted
by bridge supports or riverside facilities,
reducing their carrying capacity
• Deforestation is also a cause of increased
flood runoff and decrease in channel capacity.
Forecasting and warning
• During the 1980s and 1990s, flood forecasting
and warning had become more accurate and
these are now among the most widely used
measures to reduce the problems caused by
• Despite advances in weather satellites and the
use of radar for forecasting.
• According to Unites Nations Environment
Programme's Publication Early Warning and
Assessment, there are a number of things that
could be done to improve flood warnings. These
Improved rainfall and snow pack estimates
Better gauging of rivers, collection of
meteorological information and mapping of
Better and current information about human
populations and infrastructure
Better sharing of information is needed
between forecasters, national agencies, relief
organizations and the general public.
• Loss-sharing adjustments include disaster aid
• Disaster aid refers to any aid, such as money,
equipment, staff and technical assistance, that
is given to a community following a disaster.
• Traditionally, floods have been managed by
methods of hard engineering. This largely
means dams, levees, wing dykes, and
straightened channels that are wider and
deeper than the ones they replace.
Hazard – resistant design
• Flood proofing includes any adjustment to
buildings and their contents that help reduce
losses. Some are temporary such as:
• Blocking up entrances
• Sealing doors and windows
• Removal of damageable goods to higher levels
• Use of sandbags
• Soft engineering generally refers to working
with natural processes and features rather
than attempts to control them. They include:
• whole catchments
• wetland conservation and
• river restoration
• Physical control of floods depend on two measures
1. Flood abetment : involves decreasing the amount of
runoff by reducing the flood peak in a drainage basin.
The ways are:
• Reseeding of sparsely vegetated areas to increase
• Treatment of slopes by contour ploughing or terracing
• Comprehensive protection of vegetation from wildfire,
overgrazing and clear cutting of forests..etc..
2. Flood diversion : refers to the practice of
allowing certain areas, such as wetlands and
floodplains, to be flooded to a greater extent.