Constructive margins
� Occurs where two plates diverge, or move away, from each other and new crust is created on a boundary. This process is known as sea floor spreading and occurs on the Mid-Atlantic Ridge.
� As the plates diverge, molten rock or magma rises from the mantle
Destructive margins
� Where continental and oceanic plates converge
� Continental plates surround the Pacific Ocean, which extends over five oceanic plates.
� At the Nazca Plate, made up of oceanic crust, which cannot override continental crust, is forced to dip downwards at
Collision Zones/ margins
� Formation of fold mountains (orogenesis)
� Fold Mountains occur where oceanic crust is subducted by continental crust.
� A second, though less frequent, occurrence is when two plates composed of continental crust move together.
� The Indian subcontinent
Conservative margins
� The main effects of a conservative plate boundary are earthquakes, which can be fairly violent and frequent.
� Two plates move parallel to each other, without creating or destroying any crustal rocks.
� As they move past each other they often get stuck,
Moho Discontinuity
Shockwaves begin to travel faster here - indicating a change in structure. The Moho Discontinuity is the junction of the Earth's crust and the mantle where seismic waves are modified.
Oceanic Crust
Ocean Crust (Sima)
Thickness 6-10Km on average
Age of Rocks Very young, mainly under 200 million years
Weight of Rocks Heavier, with an average density of 3.0
Nature of Rocks Dark in colour; many contain silica and magnesium; few types, mainly basalt
Continental Crust
Continental Crust (Sial)
Thickness 35-40 km on average, reaching 60-70 km under mountain chains
Age of Rocks Very old, mainly over 1500 million years
Weight of Rocks Lighter, with an average density of 2.6
Nature of Rocks Light in colour; many contain sil
Lithosphere
The lithosphere is the oceanic crust, continental crust and upper mantle.
Convection Currents
The plates move due to convection currents in the mantle. These are hot currents of molten rock that slowly move within the mantle and cause the plates above them to move, usually by as little as a few centimeters each year.
Weathering
The disintegration and decomposition of rock in situ (in their place of origin). There are two types of weathering: Physical or chemical
Denudation
Erosion that lowers the landscape in known as denudation.
Physical Weathering
The disintegration of rocks into smaller pieces caused by physical processes without any change to the chemical compound of the rock. It occurs on bare rock that lacks vegetation. Physical weathering usually produces sand.
Chemical Weathering
The decomposition of rock caused by a chemical change in the rock. It produces changed substances and soluble, and usually forms clay. It is more likely to occur in areas in warm moist climates where there is associated vegetation on rocks. It tends to at
Freeze thaw shattering
Occurs in rocks that contain crevices and joints (e.g. joints formed in granite as it cooled, bedding planes found in sedimentary rocks, and pore spaces in porous rocks), where there is limited vegetation cover and where temperature fluctuates around 0 0C
Salt crystallisation
If water entering the pore spaces or joints in rocks is slightly saline then, as it evaporates, salt crystals are likely to form. As the crystals become larger, they exert stresses upon the rock, causing it to disintegrate. This process occurs in deserts
Spheroidal Weathering
In jointed rock, the weathering and heating/cooling takes place along all joints so this temperature change produces rounded boulders.
Exfoliation
Occurs in hot arid and desert climates where diurnal ranges can range up to 500C (below zero to 40). It also occurs in places of high altitudes in low latitudes. These rocks are usually heated via conduction. Because the outer layers of the rock warm up f
Pressure Release
Many rocks have developed under considerable pressure. The confining pressure increases the strength of the rocks. If these rocks are exposed to the atmosphere, then there will be a substantial release of pressure. The release of pressure weakens the rock
Wetting and drying
Affects less resistant rocks such as clays. The clay is porous and has the ability to absorb. When these rocks are wet they expand and when dry is contracts. Over time they disintegrate the rocks.
Biological Weathering
When tree roots penetrate and widen weaknesses in the rock until blocks of rocks become separated.
Hydrolysis
Hydrogen in water reacts with minerals in the rock; there is a combination of H+ and OH- ions in the water and ions of mineral (combines rather than dissolves the mineral). It affects mostly granite (igneous rock - crystallised magma underground), which i
Carbonation - solution
Rainwater contains carbon dioxide in solution, which produces carbonic acid (H2CO3). The weak acid reacts with rocks that are composed of calcium carbonate, such as limestone/ chalk and rocks that have calcareous rock. The limestone dissolves and is remov
Oxidation
This occurs when rocks are exposed to oxygen in the air or water. An example of this is when iron rusts. The rock or soil, which may have been blue or grey, is discoloured into a reddish-brown colour - in a process called rusting. Oxidation causes rocks t
Hydration
Certain rocks, especially those containing salt minerals, are capable of absorbing water into their structure, causing them to swell (about 0.5%) and to become vulnerable to future breakdown. This process is most active following successive periods of wet
Chelation
Organic Weathering/ Chelation: It requires a bio agent e.g. plants (chelates/ organic acid) and animal excretion. The decomposition of minerals in the rock leads to the crumbling of rock. Humic acid, derived from the decomposition of vegetation (humus), c
Factors that influence weathering: Climate
� Greatest rates of weathering are found in equatorial regions, because it's humid all year round. Chemical weathering needs water and is the active weathering agent in weathering.
� For every 10 degrees rise in temp. weathering rates increase by 2 1/2 ti
Factors that influence weathering: Vegetation
� Weathering via humic and organic acids (chelates)
�Equatorial regions/ low latitudes: Dense vegetation in Tropical Rainforests. Constant leaf fall leads to rapid decomposition. Humus and organic acids with constantly infiltrating rainwater. This acceler
Factors that influence weathering: Geology (Rock type/ Structure)
�Lithology
� Structure is to do with joints, fractures, horizontal bedding planes, cracks and openings.
� Water can penetrate via joints and the opening provides a greater surface area for weathering to take place.This can cause chemical weatheirng via hy
Factors that influence weathering: Rock type is the composition and texture
Composition:
� This Hydrolysis can cause granular disintegration in granite. the breakdown into particles due to differential hydrolysis e.g. Kaolinite clay
� In limestone (a calcareous rch rock that have both vertical and horizontal joints. Rainwater ent
Factors that influence weathering: Relief
� Altitude affects climate and vegetation and thus the type of weatheirng.
� Higher altitudes carbonation occurs.
� The steeper the slope the less the infiltration the greater the overland flow because there is no time for the water to infiltrate because
Factors that influence weathering: Time
� Ice ages reduce length of time weathering has been operating at current rates e.g. the last ice age concluded 12,000 years ago in the northern hemisphere e.g. the UK where weathering in this temperate latitude has produced relatively shallow regolith wh
Factors that influence weathering: Human Activities
� Mining
open cast
A deep pit
lager surface area
Collect water - increasing chem weather
unstable slopes
Toxic waste washed into rivers - water pollution thus increasing chemical weathering
� Air Pollution
Carbon dioxide, carbon monoxide, nitrous oxide, s
Peltier Diagram - Types of weathering in different climates
This is a diagram showing the relationship between climate and the rate/ amount of weathering.
Physical Weathering
� Frost shattering is important in a climate where temperatures fluctuate at around 0oC but if a climate is too cold, or too warm, or too dr
Regolith Weathering
Plants decompose and produce humus and adds an organic component known as regolith in the soil. Deep regolith is caused by weathering. High rates of weathering occur in the equatorial/ tropics area due to the lack of seasons and that it rains all year rou
Limestone
Limestone is a rock containing at least 80% calcium carbonate and is formed primarily over four geological periods.
Carboniferous limestone:
� Hard, grey, crystalline and well-jointed
� Contains fossils such as coral, crinoids and brachiopods
� The rock m
Granite
� Granite was formed when magma was intruded into the Earth's crust.
� Having been formed at a depth and under pressure, the rate of cooling was slow and this enabled large crystals of quartz, mica and feldspar to form. As the granite continued to cool, i
Mass Movement
Is the movement of weathered material down a slope under gravity.
The slope is a system with inputs, outputs and flows. It has exogenetic factors (external factors) and endogenetic factors (internal factors).
The slope as an open system.
Classifications of mass movements
The types of processes can be classified in a number of different ways:
- Speed of movement
- Water content
- Type of movement: flows, slides, slumps
- Material
The mechanism of mass movement
Rock particles on slopes are held on the slope by friction in a state of dynamic equilibrium. Their steady state (not moving) represents a balance between the internal (within/ between the particles known as internal or shear strength) and external forces
What keeps a slope in place
Vegetation (Binds soil thus stabilising slopes)
Friction (will vary with the weight of the particle and slope angle, and can be overcome with the help of water)
Cohesive force (An act to bind the particles of the slope and prevalent is water-less clay)
Pi
What factors lead to increasing stress and decreasing shear resistance
What factors lead to increasing stress and decreasing shear resistance:
Water
Weathering and the type of material can reduce resistance.
Shear Stress can be increased by:
Stress can be increased by:
Steepening of a slope
Undercutting of a slope
Addition of a mass of regolith
Dumping of mining waste
Sliding from higher up the slope
Vibrational shock and earthquakes.
Factors that contribute to increased shear stress:
� Remov
Types of mass movements
- Falls
- Heaves
- Flows
- Slides
Rockfalls
- Sub-aerial weathering e.g. physical weathering by changes in temperature wetting/ drying ? block disintegration at the top of the slope
- Rockfalls occur on slopes exceeding 400.
- Blocks at the top become loose and fall vertically down the free face (9
Heaves
- Slowest
- Effects and movement - imperceptible and hardly visible
- Most widespread - most occurring
- Associated with relatively fine materials e.g. silt, unconsolidated material
Frost Heaves
- Occurs at high lats. And altitudes in mid lats.
- Water freezes during winter and produces a lens of ice beneath the particles because the particles conduct the cold. The cold, ice expands by 9% and pushes outwards.
Soil Creep
- Mass movement process
- Occurs on slopes of about 50 and produces terracettes
- The impact on the slope can be direct (microfeatures include terracettes (tiny ripples under the grass due to a accumulation of soil in very small ridges)).
- The impact can
Solifluction
- This process meaning 'soil flow' is a slightly faster movement averaging between 5cm to 1m per year.
- Often takes place under periglacial conditions where vegetation cover is limited.
- During winter both the bedrock and regolith are frozen. In summer,
Flow:
- Fast, need well lubricated material
- The material behaves like a viscous fluid
- Material size - large boulders - small grains
- The debris avalanche (large boulders) is the fastest of the flows
- Some other types of flows include earthflows and debris
Slides
- Occurs on a slide/ slip plane or what is known as a failure surface, which is lubricated by rain water which had infiltrated along this major line of weakness.
- Slides may be rotational or translational (planar).
- In the planar slide, the weathered ro
Example of a (accelerated by man) Landslide in a MEDC: Abbotsford Landslide, Dunedin, NZ 1979 - Causes
Cause:
� 1978 families noticed cracks appearing in their homes.
� 1979 workmen discovered that a leaking water main had been pulled apart. Geologists discovered that water had made layers of clay on the hill soft, and the sandstone above it was sliding on
Example of a (accelerated by man) Landslide in a MEDC: Abbotsford Landslide, Dunedin, NZ 1979 - The landslide
The landslide:
� On July 27th the slide began to accelerate.
� Early warning system was put in place by Civil defense and a civil emergency was declared on the 6th of August 1979. This was not thought to have been necessary, as geologists believed the slo
Example of a (accelerated by man) Landslide in a MEDC: Abbotsford Landslide, Dunedin, NZ 1979 - Impacts
Impacts:
� Nobody was killed but 69 homes were destroyed or damaged and 200 people were displaced. The total cost from the destruction of the homes, infrastructure and relief organization amounted to �7 million ($10-13 million NZ today). In total 18 ha wa
Case Study on a Physical Landslide in a LEDC: Vargas State, Venezuela - 1999 - Causes
Causes:
� First two weeks of December 1999 saw an unusually high amount of precipitation (40-50% above normal rainfalls).
� Political corruption - allowing shanty-towns to be built on steep slopes surrounding Caracas. The slopes around the region were cha
Case Study on a Physical Landslide in a LEDC: Vargas State, Venezuela - 1999 - The Landslide
Landslide:
� 15-16th December the slopes of the 2000m Mt Avila began to pour forth rock and mud burying 300 km stretch of the central coast.
� Rains triggered mudslides, landslides and flash floods in between the mountains and the Caribbean Sea.
� Search
Case Study on a Physical Landslide in a LEDC: Vargas State, Venezuela - 1999 - The Impacts
Impacts:
� Rains triggered mudslides, landslides and flash floods which claimed the lives of 10,000 -50,000 (unknown accurately as most people were buried under mud or swept to sea) in between the mountains and the Caribbean Sea.
� 150,000 were left homel
Impacts of Human Activity on Slopes
Undercutting
� In order to build a horizontal base plus reasonable access for roads, a cut-and-fill technique is used and a small level terrace with an over-steepened slope at both ends.
� The steep slope now devoid of vegetation and soil, is potentially
Drainage Basin
A Drainage basin is an area of land drained by a river and its tributaries.
A Drainage basin is known as an open system.
Watershed
A ridge of high land beyond which any precipitation will drain into its adjacent basins marks its boundary.
Inputs
Precipitation
- Snow
- Sleet
- Hail
- Fog
- Dew
- Rain
Outputs
Where water is lost in the system.
Evapotransipiration
Evaporation
Transpiration
Flows
Throughfall
Stemflow
Infiltration
Overland flow (Hortonian and saturated)
Percolation
Throughflow
Baseflow
Channel flow
Stores
Interception
Surface storage
Soil storage
Vegetation storage
Groundwater storage
Channel storage
Evaporation
It's the physical process by which moisture is lost directly into the atmosphere from water surfaces, excluding vegetation and the soil, caused by the effects of air movement and the sun's heat. It is the energy lost through the process of water vapor tur
Evapotranspiration
Transpiration is a biological process which water is lost from a plant through the minute pores in its leaves. Transpiration rates are affected by the time of year, amount of vegetation, the availability of moisture and length of the growing season. Poten
Interception
Interception: Interception is the catching and storing of incoming precipitation through the canopy. It is greater in woodland areas than grass. In light precipitation the water may never reach the ground and could be lost in the system. It is estimated t
Stemflow
If rainfall persists, water may reach the ground. A way in which it reaches the ground is through stem flow. This is through water flowing down the trunk.
Throughfall
If rainfall persists, water may reach the ground. A way in which it reaches the ground is through fall. This is through water dropping off the leaves.
Infiltration
Infiltration is when water flows from the surface through the soil in a vertical (downwards) direction. Soil will gradually admit water from the surface, if the supply rate is moderate, allowing it slowly to infiltrate vertically through the pores in the
Overland flow
In most environments, overland flow (this is water running off or across the surface and eventually develop in rills, channels and eventually rivers (surface runoff)) is rare except in urban areas (impermeable) and during heavy rains. When rainfall is gre
Percolation
As water reaches the underlying soil or rock layers, which tend to be more compact, its progress is slowed. This constant movement is known as percolation (the vertical movement of water from soil to rock) and creates groundwater storage.
Water table
Water eventually collects above an impermeable rock layer, or may fill all pore spaces, creating a zone of saturation. The upper boundary of the saturated material (the upper surface of the groundwater) is known as the water table (the level of water belo
Baseflow
Water may then be slowly transferred laterally through rock as base flow (or groundwater flow).
Recharge
Recharge: Percolating water adds to the groundwater store raising the water table. Recharge is the replacement of water during drier months.
Spring
The point at which subsurface water emerges at the surface or the point at which the water table reaches the surface.
Aquifer
A water-bearing porous rock
Porous vs. Non porous rocks
Porous: Water is held in pores between rock/soil particles. Porous rocks have the ability to hold water. They do not allow water to pass through them e.g. clay.
Non-Porous: Does not have pores and cannot hold water within the rock
Transportation
Load is either transported through suspension, solution or bed load (traction & saltation). For sediment to move resisting forces have to overcome, competent velocity has to be achieved (this is the lowest velocity at which particles of a particular size
Traction
Traction occurs when the largest cobbles (100-1000mm) and boulders (bed load) roll or slide along the bed. The largest of these may only be moved during times of extreme flood (high discharge).
Saltation
Bed load is either moved through saltation or traction. Saltation occurs when pebbles (1-100mm), sand (0.1-1mm) and gravel are temporarily lifted by the current and bounced along the bed in a hopping motion.
Solution
If the bedrock of the river is readily soluble, it is constantly dissolved in flowing water and removed in solution. Except in limestone areas, the material in solution forms only a relatively small proportion of total load.
Suspension
Very fine particles of clay and silt (0.001-0.1mm) are dislodged and carried by turbulence in a fast-flowing river. The greater the turbulence and velocity, the larger the quantity and size of particles which can be picked up. The material held in suspens
Deposition
When velocity of a river begins to fall, it has less energy and no longer has the competence (maximum size of material being transported) or the capacity (the total load actually transported) to carry its load. So starting with the largest particles, mate
Sedimentation
Sediment in a river comes from a variety of sources. It may be from outside the river (exogenetic) that includes, mass movement, rill and gully erosion and sheet wash. Or from within the rivers channel itself (endogenetic) that could be material from the
Competence
Competence is the maximum size of material a river can transport
Capacity
Capacity is the total load actually transported
Erosion in a river
Vertical erosion: This form of erosion deepens channels, aided by weathering mass movement and soil creep. Characteristics of a channel undergoing vertical erosion include large bed load comprising coarse hard particles. Potholes and deep narrow gorges ar
Abrasion
Smaller material, carried in suspension, rubs against the riverbanks and wears it away.
Attrition
When bed load is moved downstream, boulders collide with other material and the impact break the rock into smaller pieces. In time, angular rocks become increasingly rounded in appearance.
Corrosion/ Solution
This occurs continuously and is independent of river discharge or velocity. When acids in the river dissolve rocks, which form the river's bed/ bank. It is related to the chemical composition of the water e.g. the concentration of carbonic acid and humic
Hydraulic Action
The sheer force of the turbulent current hits riverbanks, pushes water unto cracks. The air in the cracks is compressed, pressure is increased and over time the back will collapse.
Cavitation
Cavitation is a rare form of hydraulic action and the sudden and violent implosion of gas bubbles caused by this process shatters banks extremely rapidly. The resultant shockwaves hit and slowly weaken the banks. This is the slowest and least effective pr
Corrasion
Corrasion occurs when the river picks up material and rubs it along its bed and banks, wearing them away by abrasion. This process is most effective during times of flood and is the major method by which the river erodes both vertically and horizontally.
Volume of a river
Most streams and rivers obtain their water from rainfall and other forms of precipitation. This precipitation evaporates, soaks in or contributes to the run-off or drainage of the land surface. As rivers flow from high (source areas) to low areas (the mou
Velocity
Velocity is more or less constant along the length of a river. It is true that steeper slopes do encourage higher velocities, but the larger channels of the lower course exert relatively less friction than the small channels of the upper course, causing a
Load
Small streams carry a greater quantity of fine material than coarse material. Conversely, large rivers with more available energy carry larger/ coarser material.
Energy of a river
Volume of water carried + velocity of this water = energy of the river
Discharge
Discharge: Discharge is defined as the volume of water passing a particular point in the river in a unit of time, expressed as m3/s-1, or cumecs. Q=A x V (A= cross sectional area (width x depth) and V = velocity)
Flow patterns: Tubulent flow
Irregularities in the riverbed cause eddying. This causes the river to appear rough with a white foamy surface.
Flow patterns: Laminar flow
Laminar: Occurs because the riverbed is completely smooth. The water flows in parallel lines.
Flow Patterns: Helicoidal flow
Associated with a meandering river in a corkscrew motion. The thalweg goes down one side of the channel, up the other side and across the channel.
Braided channel
Braided: If a stream has a high proportion of bed load in relation to its discharge, it deposits much of its load as sand and gravel bars/ islands/ eyots (which shift as discharge changes and are stabilized by roots on vegetation) in the streambed. These
Straight channels
Straight: Straight channels can exist under natural circumstances for short distances usually at fault lines, rock joints or steep gradients. However most straight channels with parallel linear banks are artificial channels.
Meandering channels
The most common channel pattern in humid climates display broad sweeping river bends known as meanders. Over time these sinuous meandering channels also wander from side to side across their low-gradient floodplains widening the valley by lateral erosion
Sinuosity
Sinuosity = actual channel length / straight-line distance.
Potholes
Circular depressions, sometimes found on the riverbed. They are found on rivers flowing across bedrock. It is usually located in the source zone. Potholes can cause eddying of the water and turbulent flow. These depressions can cause the load of the river
Waterfalls
Waterfalls: Waterfalls form when a river, after flowing over relatively hard rock (bed rock), meets a band of less resistant rock or where it flows over the edge of a plateau. As the water approaches the brink of the falls, velocity increases because the
Rapids
Rapids: Occurs where layers of hard rock and soft rock are very thin, and so obvious break of slope develops as a waterfall.
V-Shaped Valleys
V-Shaped valleys: Any spare energy possessed by a river near to its source will be used to transport large boulders along its bed. These results in the river cutting rapidly downwards, a process called vertical erosion. Vertical erosion leads to the devel
Thalweg
Thalweg: This is the line of fastest flow in a stream and is usually exaggerated variation of the stream channel shape that crosses to the outside of each meander at the point of inflection. Because erosion is greatest where the stream flow is fastest, th
Pools and Riffles
Riffle and pool sequence: River channels have irregularities in the bed, which cause the thalweg to shift from the middle. These are known as 'pools' and 'riffles'. In a flowing stream, a riffle-pool sequence (also known as a pool-riffle sequence) develop
Point Bars
Point bars: On a meander, material deposited on the convex inside of the bend may take the form of a curving point bar. Material is deposited here where velocity is at its lowest round a bend.
Ox Bow Lakes
Ox Bow lakes: Continual erosion on the outside bends, results in the neck of the meander getting narrower until the river undercuts through the neck and shortens the coarse. The current will take the path of least resistance, giving it renewed energy. The
Flood Plains
Flood Plains:
It's a flat wide expanse of alluvium covering the valley floor formed due to deposition when the river is in overbankful. As the river floods, the river slows down, loses energy and consequently deposits its large (capacity) load of small ma
Levees
Levees: When a river overflows its banks, the increase in friction produced by the contact with the floodplain causes material to be deposited. The coarsest material is dropped first to form a small, natural embankment (levee) alongside the channel. Durin
River Terraces
River Terraces: They are the remnants of former floodplains which, following vertical erosion caused by rejuvenation, have been high and dry above the maximum level of present day flood plains. If a river cuts rapidly into its floodplain, a pair of terrac
Deltas
Deltas: Deltas are usually composed of fine sediment, which is deposited when a river loses energy and competence as it flows into an area of slow-moving water such as a lake or the sea. When the river meets the sea the meeting produces an electric charge
Alluvial Fans
Alluvial Fans: In order to form, alluvial fans require a flat or a gently sloping plain near the foot of a hill or plateau, where a stream carrying sediment emerges abruptly from a mountain front and spreads out. As the stream reaches the flat plan, known
Zones of a river: Source zone
Source zone:
� Steep gradient
� Little discharge
� Rapid velocity
� Limited amount of load
� Large sized load e.g. boulders
� Channel is bedrock
� Small narrow channel
� Dominant process is erosion
� Rapids, waterfalls, and potholes form here
Zones of a river: Transfer Zone
Transfer Zone:
� Gentler gradient
� Increase in discharge
� Velocities vary
� Increase in amount of load
� Decrease in size of load
� Channel may be bedrock and alluvium
� Dominant process is transportation
� Meanders, ox bow lakes, and flood plains form
Zones of a river: Accumulation Zone
Accumulation zone:
� Very gentle gradient
� Increase in discharge - because the addition of water from tributaries and the catchment and subsurface storage of water.
� Slight increase in velocity
� Increase in the amount of load
� Increase in the size of
Hydrograph
A Hydrograph is a graph showing the discharge of a river at a given point (a gauging station) over a period of time. A storm hydrograph shows how a river responds to a given input of rainfall (a particular storm). When a storm begins, discharge does not i
Measurements on a hydrograph
Velocity is the speed of the river. It is measured in meters per second
Volume is the amount of water in the river system. It is the cross sectional area of the river's channel measured in square meters
Discharge is the velocity of the river times its vol
Lag time
The Lag time is the length of time between peak rainfall and peak discharge
Peak Discharge
Peak Discharge is the highest discharge on the hydrograph.
Overbank full
The line of flooding occurs when the river overflows its banks. The river is thus said to be overbank full.
The rising Limb
The Rising limb is the increase in discharge from the beginning of the rainfall to the point of peak discharge
Recession limb
The Recession Limb is the discharge from the peak to the base flow level in the river at the end of a storm after overland and through flow has occurred.
Baseflow
Base Flow is the water that flows laterally through the rock. It is the slowest of flows.
Basin Watershed
Basin watershed: A drainage basins boundaries, marked by a ridge of higher land is called a watershed. A watershed separates one drainage basin from the neighboring one.
Drainage Density
Density: The total length of all the streams, and rivers in a drainage basin divided by the total area of the drainage basin. The greater the density, the greater the risk of flood.
Factors influencing Hydrographs
- Climate (amount & intensity of rainfall) (Temperature (affects evaporation rates))
- Rock type (Permeable: allows water in) (Impermeable: Will produce through flow and overland flow) (Porous: Holds water but may not transmit water storage - fewer flows
Flashy Hydrographs reasons
Flashy Hydrographs reasons:
� Impermeable rock
� Steep Hydrograph
� Steep slope
� Small round drainage basin
� No forest Catchment
� High density drainage basin
Delayed Hydrographs
Delayed hydrographs:
� Flat, slightly curved hydrograph
� Lower flatter later peaked hydrographs
� Sometimes known as attenuated hydrographs
� Light rain over a prolonged period
� Porous and permeable rock
� Relief possibly gentle
� Large drainage basin
�
Unusual hydrograph
Unusual Hydrograph
� Response to two periods of rainfall (each has a relatively short lag time)
� Could be different rock types in one drainage basin
� Clover leaf drainage basin
How factors affect hydrograph shape:
How factors affect hydrograph shape:
� Shape
o Circular basins produce water at the gauging station at the same time from over the whole catchment area. The rivers have about the same distance to travel. (Flashy Hydrographs).
o Elongated basins, the tribu
The nature of the storm hydrograph is the result of:
The nature of the storm hydrograph is the result of:
� The nature and amount of the input of precipitation
� The amounts of water already dry in the system. This is known as antecedent moisture and is the result of the rain that has previously fallen. If
Hydrograph Case study: River Ribble
Hydrograph Case study: River Ribble
The combination of catchment factors that influence the discharge of the river. The river is 121km, and is in a drainage basin that has an area of 2182m2. It rises in the hills known as the Pennines, which are up to 442
A flood
A river is in flood when it overtops its banks. This is known as overbankful discharge. This water will spread out across the flood plain.
Causes of floods
Causes of floods:
1. Physical Causes
a. Precipitation
b. Snow melt
2. Human Causes - also influence flooding, however they don't really cause flooding and just intensify floods.
a. Urbanization
b. Deforestation
c. Dam/ reservoir failure
d. Agricultural pr
Case Study Physical and human intensification of floods: Mississippi River
Case Study Physical and human intensification of floods: Mississippi River
� Huge river system - 3800 Km long - 1400 million m3 per day discharge
� Every year there is either flooding or severe drought
� Engineering of the channel to control floods, but t
Hard Engineering
Hard Engineering
This is the changing and modifying of the river
1. Straighten and widen the channel. This will lead to a shorter channel width with a greater capacity to accommodate surface runoff. Water is controlled via sluice gates.
a. Advantages: The
Soft engineering
Soft engineering
1. Afforestation
a. Increased interception therefore the rain takes longer to reach the channel (flatter hydrographs, longer lag times, lower peak).
2. River restoration
a. Conversation measures to the channel bank/ flood plains
b. This i
Droughts
Droughts
� The hydrological cycle accounts for 1% of the total water on the planet
� The hydrological cycle is a closed system because water is neither added nor lost
� Over the last 300 years the world's population has increased x7 and demand for water h
Weather
The hour-by-hour state of the atmosphere. It is short term and can be localized in relatively small areas.
Climate
It's the average weather conditions of a place taken over 30 years. It is expected, rather than actual conditions.
Troposphere
Contains almost all the moisture in the atmosphere, and is where 'weather occurs'. As altitude increases within the troposphere, temperature decreases.
Stratosphere
Where most of ultraviolet radiation from the sun is absorbed.
Radiation
Solar energy travels in the form of radiant energy or radiation.
Wavelength
Radiation can be distinguished by its wavelength. This wavelength depends on the temperature of the radiating body. The hotter the body, the shorter this wavelength is. The sun is around 6000oC and therefore has significantly short wavelengths.
Solar radiation
Solar radiation can pass through a clear and cloudless atmosphere with little or no interruption towards the Earth's surface. This radiation is absorbed or reflected by the Earth's surface. The sun does not heat the Earth directly - the warmed Earth surfa
Convection
The vertical movement of a parcel of air, which is at a different temperature than its surroundings. Any portion of the atmosphere at a different temperature from the air around it will tend to remain separate rather than mix with its surroundings - 'The
Clouds' roles in radiation exchanges
Plays an important role in regulating the Earth's radiation exchanges, and reflecting the sun's radiation back into space. The influence depends on the cloud type and its height. In low clouds e.g. Stratus Clouds most shortwave radiation (incoming solar r
Long wave radiation
The Earth gives out long wave radiation, which is absorbed and delayed before it returns to space (Without this delay and absorption the atmosphere would be too cold for humanity to exist). However this form of radiation cannot pass as easily through the
Shortwave radiation budget
Shortwave radiation budget:
� In the stratosphere 3 units of radiation (ultraviolet) is absorbed by the ozone. This prevents too much UV entering the Earth's surface.
� 25 units of solar radiation pass directly through the atmosphere to the Earth's surfac
Sublimation
The process whereby a solid is converted into vapour with no liquid state intervening. This process can be responsible for the formation of ice crystals.
Deposition
The change in state from vapour to ice, which are directly from air containing water vapour. It is usually associated with hoar frost.
Radiation Cooling (Horizontal air movements)
Occurs on calm cool evenings, when the ground looses heat rapidly through terrestrial radiation and the air in contact with it is then cooled by convection. If the air is moist, vapour will condense to from radiation fog and if it is under 00C it will for
Advection Cooling
Warm moist air moving over cooler land or ocean surfaces. Advection fogs are formed when warm air from over the land drifts over cold offshore ocean currents.
Orographic/ frontal uplift
Warm moist air is forced to rise as it crosses a mountain barrier or meets cold air at a front.
Convective/ adiabatic cooling
Air warmed during the daytime rises in pockets of thermals. As this air expands it uses energy so the temperature in these pockets drops and sinks back to the ground.
Humidity
Measure of water vapour content in the atmosphere
Absolute humidity
Mass of water vapour in a given volume of air. This is measured in g/m3.
Specific Humidity
This is measured in g/kg. The mass of the water vapour in the air per 1kg of air.
Relative humidity
Amount of water vapour in the air at any temperature expressed as % of the maximum amount of vapour the air can hold.
Humidity depends on:
Temperature of the air and the limit of water the air can hold.
Energy budget variation with latitude
Energy budget variation with latitude
� Occurs in 100 strips
� In terms of radiation received and emitted by each strip, those between 0-400 gain more than they lose, while from 40-900 more is lost than gained.
� A transfer mechanism must work to maintain
How is energy transferred around the world?
1) General circulation
� Tri-cellular model
o Hadley Cell
� Moves anticlockwise in the northern hemisphere in convection currents
� Moves between 0-30 degrees and is associated with cumulonimbus cloud formations
o Ferrell Cell
� Associated with warm south
Factors affecting Isolation - Long-term factors:
� Height above sea-level
o The atmosphere is not warmed directly by sun but by heat radiated from the Earth surface and is distributed via convection and conduction.
o As height increases, it produces a decreasing surface area e.g. mountains, from which t
Short-term factors affecting isolation:
Short-term factors:
� Seasonal changes
o Summer (due to the Earth's tilt) the sun is overhead at the tropics, the hemisphere experiencing 'summer' will receive maximum insolation.
o In spring/ autumn when the sun is directly over the equator, insolation i
6 factor model (Daytime budget)
�Incoming solar radiation
Main energy input
Strongly influenced by amount and type of cloud
Dependent on the angle of the sun
�Sensible heat transfer
Convective transfer
The movement of parcels of air to or from the point at which the enrgy budget is asse
Energy available at suface
Energy available at surface = Solar radiation receipt - [reflected solar radiation + surface absorption + latent heat + Sensible heat transfer + Long wave radiation]
4 factor model (Night time)
�Latent heat (condensation)
�Sensible heat transfer
Warm air at a given point contribute energy and keep temps up.
If cold air moves in energy levels will fall
�Subsurface Supply
�Long wave radiation
Weather phenomena associated with local energy budgets: Mist and Fog
� Droplets of water in the atmosphere, which are so small they fall only very slowly to the surface under gravity. This phenomenon is known as mist.
� Fog droplets are denser than mist droplets; therefore visibility is lower than mist.
� Mist visibility i
Dew
� Condensation on a surface
� Occurs when air in contact with the surface has dropped sufficiently.
� It may occur when more moisture is introduced into the air from onshore breezes.
Temperature Inversions
Temperature Inversions
� Air is warmed and becomes less dense than surrounding air, so it rises.
� The parcel of air will cool at a fixed rate. Nothing changes that rate.
� The surrounding air presents more of a problem since temperature at any given heig
Katabatic Winds
A wind that carries high-density air from a higher elevation down a slope under the force of gravity. It is very cold. The 'barber wind' that blows over Greymouth is an example of a katabatic wind.
Anabatic winds
These winds are winds that blow up a steep slope or mountainside, driven by heating of the slope through isolation.
Environmental Lapse Rate:
The decrease in temperature is usually expected with an increase in height in the troposphere. The ELR is 6.50C per 1000m but varies with local air conditions. It may vary due to height (the ELR is lower the lower to the ground), different surfaces and di
Adiabatic Lapse rate:
Describes what happens when a parcel of air rises and the decrease in pressure is accompanied by an associated increase in volume and a decrease in temperature.
� If upward the movement of air does not lead to condensation, energy used by expansion will c
Stability
Stability: Linked with anticyclonic weather, high pressure systems and calm dry conditions. A sate of stability is achieved when a rising parcel of unsaturated air cools more rapidly than air surrounding it. If there is nothing to force the parcel of air
Instability
This is linked with thunderstorms and the formation of cumulonimbus clouds. Instability arises on warm days when localized heating of the ground warms adjacent air by conduction, creating higher lapse rates. The resultant parcel of rising unsaturated air-
Conditional Instability
It occurs when the ELR is lower than the DALR but higher than the SALR. Rising pockets of air cooling at the DALR, become cooler than surrounding air, and should sink down to the ground. However it may be forced to rise because of a rise in relief. This m
Weather Phenomena - Resultant weather
Clouds
� Form when air cools to dew point and vapour condenses into water droplets and/or ice crystals.
� There are many different types of clouds and they form under different conditions.
Snow
� Forms under similar conditions to rain except dew point is below 00C
� Sublimation occurs
� Ice crystals form if hydroscopic nuclei is present
Sleet
� Mixture of ice and snow formed when upper air temperature is below 0, allowing snowflakes to form, and the lower air temperature is around 2-40C, which allows there partial melting.
Hail
� Frozen water droplets which exceed 5mm in diameter
� Forms in cumulonimbus clouds from an uplift of convection currents
� Quite common in warmer climates
Global warming and the Greenhouse effect
The greenhouse effect
It is the process by which certain gases absorb outgoing long-wave radiation from the earth, and return some back to Earth. In all greenhouse gases such as CO2, Methane, CFC's, nitrous oxides and water vapour, raise the Earth's tempe
Impacts of global warming
Impacts of Global warming
� Climatic
o Increased storm activity
� Tornadoes in the mid west
� Hurricanes
� Typhoons
� Cyclones
o Temperature increases
� With no action, temperatures will increase by 2.50C in the next 50 years.
o Reduced rainfall
� Leads t