HYDROLOGYGEOMORPHOLOGY

AS Level Physical Geography 1

THE DRAINAGE SYSTEM

DEFINITIONS OF TERMS

Interception: Water caught and/ or stored by vegetationEvaporation: The process where liquid and solid turn into gasThroughfall: Water that falls through the gaps between the leavesStemflow: Water that trickles down the branches/ stems of the treeInfiltration: Water that soaked into/ is absorbed by the soilOverland Flow: Water that flows above the surface of the landThroughflow: Horizontal flow of water above the water table within the soil layerInterflow: Lateral movement of water in unsaturated zonePercolation: Vertical movement of water downward due to gravity from its earth surface to subsurfacePotential Evapotranspiration: The measure of the ability and atmosphere is able to remove water from the surface regardless of the water supply

The Hydrology Cycle• Closed Cycle• The cycle of water between the hydrosphere, lithosphere, atmosphere

and biosphere.• Hydrosphere: River, Ocean, Surface storage• Lithosphere: Groundwater, Soil Moisture storage• Atmosphere: Cloud, Precipitation• Biosphere: Plant

The Drainage Basin System• An Open System: Allows movement of energy and matter

across its border• Input: Precipitation (Energy: The Sun)• Output: Evapotranspiration, Runoff, Leakages

Storage in the System• Surface Storage• Groundwater Storage• Soil Moisture Storage• Vegetation Storage• Channel Storage

Factors that affect the Drainage Basin• Climate• Geology• Human Activity• Soil Type• Basin size• Vegetation

Precipitation• The conversion and transfer of moisture in the

atmosphere to the land• Rainfall, Snow, Dew, Frost, Hail• Elements that affect the drainage system1. Total Amount of Rainfall2. Intensity of Rainfall3. Type of Rainfall4. Variability5. Geographical Distribution

Interception• Water that is caught and stored by plant/ vegetation• 3 components:1. Interception Loss: Water that is retained by plant

surface or evaporated away or absorbed for plant use2. Throughfall: Water the drips to the ground from the

leaves/ twigs/ stem or fall through the gaps of the leaves

3. Stemflow: Water that trickles down along the stem/ twigs and branches – then the trunk.

• Smaller surface area – less interception• Density of vegetation – positive correlation with

interception

EVT………………………….PEVT• Process by which liquid or

solid turn into gas combined with the process by which water escape from plant

• Evaporation is higher in higher temperature

• Temperature• Wind speed• Humidity• Vegetation cover• Amount of water available• Color of the surface

• The water loss that would occur if there is unlimited water supply in the soil

• Concept of moisture availability

Infiltration• The process by which water soaks into or is absorbed by

the soil• Infiltration Capacity: The maximum rate by which rain can

be absorbed by the soil surface• This capacity decreases with time – soil becomes more

saturated• Type of soil e.g. Clay would be lower than sand• Antecedent Soil Moisture• Vegetation can slow down the rain reaching the surface• Infiltrated water is rich in chemical• Check page 3 for relationship between infiltration and

others

Soil Moisture• Water stored in the subsurface of the soil• Field capacity refers to the amount of water held in the

soil after excess water drains away• The range of moisture content in which permanent wilting

of plants occur = approximate rate of plant growth• Throughflow – water flowing through the soil in natural

pipe and percoline

Groundwater• The subsurface water – found at a few hundred metres

down• Percolation: Slow movement of water downward from the

soil to the bedrock – the speed depends on permeability of rock

• Zone that is permanently saturated: The Phreatic Zone• Upper layer of the phreatic = Water Table – usually higher

in winter due to increase precipitation• Aeration zone – seasonally wetted/ dried out• Base Flow - The part of the river discharge provided by

groundwater seeping into the bed of the river

Groundwater• Recharge: Refilling of water pores after they have been

extracted by human or dried up.• Aquifers: Permeable rocks that hold significant quantity of

water.• Aquifer – natural regulator – if not water will reach stream

too quickly – flooding – maintains stream flow – water from springs can become sources of stream

• Recharge – infiltration of precipitation, leakage from other aquifers, seepage from surface storage, artificial recharge (reservoir, irrigation)

RAINFALL – DISCHARGE SYSTEM

River Regime• The annual variation in the flow of the river – Annual

Hydrograph• Factors affecting the regime: The amount of precipitation,

the climate, the shape/ morphology of the basin, rock porosity/ permeability, amount/ type of soil cover, vegetation cover

• Winter may see more rainfall – low pressure system

General River Regime• Much lower in the summer – high EVT and more

vegetation hence more interception

The Water Budget• Equilibrium in the Drainage Basin between Inputs and

Outputs• P = Q + E (+/-) Change in storage• Precipitation > Evapotranspiration [Water Surplus]• Precipitation < Evapotranspiration [Water Deficit]

River Gradation

Water Budget• Precipitation = Evapotranspiration + run off + change in

soil water storage• Water budget may be refer to the amount of water stored• When precipitation is high and Evapotranspiration is low –

Ground waters surplus• In dry period – less precipitation – less water stored –

Groundwater utilization• More EVT than precipitation – Groundwater deficit• In winter groundwater recharge takes place

Discharge• Measured in cumecs (Speed times the cross sectional

area)• Q = A x V• Discharge = Cross-sectional Area x Velocity• Measures the amount of runoff – the ones not in the

storage

THE FLOOD HYDROGRAPH

Precipitation/ Hydrograph• Intensive rain – steep rising limb, high peak flow (intensity

surpasses the infiltration capacity)• Snow – at first, rising level drops as snow held at surface

when it melts – can produce a steep limb – much longer lag time – high peak flow [Frozen ground may play a part in infiltration]

Temperature/ EVT• Temperature affects the nature of precipitation• EVT rate affected – lower peak flow if high temperature?

However, warm air has more water – it will mean more rainfall eventually

Antecedent Soil Moisture• If ground is already saturated – less infiltration – higher

peak flow and quickflow – short time lag

The Drainage Basin• Smaller basin – responds quicker – shorter lag time• Circular basin – quicker than linear• Denser basin responds quicker – steeper recessional limb

The Drainage Basin• Shorter lag time in small basin – less distance for water to

travel to main channel• Shape – Any points on the watershed of a circular basin is

equidistant to the gauging station – faster lag time, higher peak flow

• Slope:1. How fast the water travels (lag time)2. How much time it has to infiltrate the soil

Extra reading• https://ih-igcse-geography.wikispaces.com/1.3.+

hydrographs

RIVER CHANNEL PROCESSES

Erosion• The breaking down of material by moving agents• Lateral Erosion: Makes the river widerMore frequent in upper course• Vertical erosion: Makes the river deeperMore frequent in lower courses

Erosion• Hydraulic Action: The force of the water removes rock

particles from the bed and backs• Abrasion/ Corrasion: Loads rub against the the bed/

banks – removing particles of rocks – strongest when large chunks of materials are carried – river flow is high and turbulent

Erosion• Attrition: The loads knock against each other• Cavitation: Air bubbles are trapped inside rocks – as they

implode – the force weakens the rock.• Corrosion: When rocks that contain substances such as

calcium carbonate e.g.. Chalk or limestone are dissolved in solution in the river

Transport• Dissolved materials are carried in solution• Small particles are carried near the surface – suspension• Saltation load – medium sized materials bounced along

the bed• Tracted load – rolled along the bed• Capacity: The largest amount of particles that can be

carried• Competence: The diameter of the largest particle a

stream can carry

Deposition• As the river loses energy – it can not transport the loads

anymore – the loads are deposited• Losing energy can be caused by lack of precipitation,

increase in abstraction(human uses) or increase its load – these reduce the discharge

• Flattening out of the gradient• Reaching a larger water body or the sea – that body will

reduce the gradient and absorb energy

The Hjulstorm Curve

RIVER CHANNEL LANDFORMS

Channel flows• Channel flow is affected by:• The gradient of the river• The amount of river discharge – simply the precipitation• The roughness of the bed/ bank• The shape of the channel

3 types of flow• Laminar flow: Low velocity,

straight channel allowing water to flow in sheets – rare – can be found in glaciers when glacial ice layer moves another

• Turbulent flow: High velocity, complex morphology eg. Meandering, pools, riffles – GORGE

• Helicoidal flow: Horizontal turbulence – pools and riffles – river carry large amount of materials – corkscrew motion

Potholes• Upper course landform• Required: Vertical erosion and

turbulent flow – load is large – Tracted – hence drill holes – load must be stronger than bed

• Cylindrical holes drilled into the bed

• Have different depth and diameter• When flowing water hit the loads –

forces over – downcutting on the bed – swirling Eddie current

• Pebbles trapped in it – the flow causes them to drill further – creating a smoother shape

V-Shaped Valley• Upper course landform• Erosion by river and even more so by weathering• Required: Greatest Gravitational energy – periods

of high discharge – erosion by abrasion with large bed loads

• Narrow valley with a deep V shaped bed• The deepening exposed the sides to weathering

and mass movement• Climate, vegetation and rock structure affects the

shape of the valley• Colder climate: Freeze-thaw weathering• Rainwater: Act as lubricant in mass movement• Vegetation can stop mass movement – increase

soil stability• Harder rocks – steeper valley – hard to weathered

Waterfall• Middle course and upper course – although it occur mainly

at the changes of lithology(rock type)• Rock change from resistant to less• River erodes the less resistant – creating a drop in the river

course the resistant rock is now higher up than the less• He water flows over the resistant rock – the fall is strong due

to gravitational power – eroding the less resistant rock• The splashing undercuts the less resistant rock on the side –

erosion through hydraulic and cavitation Leaving a cap rock unsupported

• This soon drop – causing the waterfall to retreat upstream• (Abrasion of dropped rocks/ screes deepen the plunge pool)• A gorge of recession is formed

Rapids• Upper Course• Required: Steep gradient – increase in turbulence and

speed – hard resistant rock – a sudden drop in the river gradient

Meanders• Middle Course• Required: An increase in the river’s sinuosity, alternating riffles

and pools• (Sinuosity is the ratio between channel length and displacement

– 1 = straight line – more than 1 - normal)• Riffles – shallow, pool – deep• At the pool, channel is more efficient• At riffle, less efficient• Erosion on one side, deposition on the end• Creates an illusion of a bending river• Outer bend – more erosion• Increases the turbulence – Helicoidal flow – this increases

erosion and deposition process

Ox-bow Lake• The deposition/ erosion creates a

larger bend – more further away from the straight line

• The neck of the river narrows down• At high discharge water might flow

over the neck• Creating a new course• When return to normal discharge,

river continues to flow there• Deposition cuts off the original bend

– leaving it in an ox-bow shape lake fed mainly by precipitation

• Soon the lake dries up – meander scar

Braided Channel• Middle – lower courses• Required: Lots of sedimentary

loads, Steep gradient, discharge of river change regularly

• As the volume load exceeds river capacity or discharge is not enough – river deposits its loads

• Forming EYOTS• River is divided into many

channels

Floodplain• Middle – Lower Courses• Required: Flat land, reduced velocity of river, fine

sediments, high discharge• During flood, river’s efficiency reduces due to high friction

– causing it to deposit the load• The load deposited on land is called alluvium• Sinuosity and meanders determine the size of the

floodplain

Levees• Natural embankment produced during a flood

• The heaviest, largest load deposited first close to the

bank• The fine materials

deposited further – looks like it tapered off

• The mound build up over repeated flood

Delta• Mouth of the river• A body of water with lower velocity – relatively calm – low

tidal range – deposited sediments must not be quickly washed away

• When meeting still water – river’s velocity drop – deposition

• Bottomset bed: Clay (clay combines with salt – flocculation – clay becomes heavier - sinks) and other fine settlement – this bed stretches far out as fine sediments are easily transported

Delta• Foreset bed: On top of the Bottomset – Coarser

sediments – doesn’t go as far as Bottomset – dips, taper off

• Topset: Coarser sediments simply deposited on top – horizontally bedded

THE HUMAN IMPACT

Human effects on precipitation• Cloud seeding• Introduction of Silver Iodide, Dry ice or nitrate into the air

to create precipitation• Increasing rain in one area can reduce rain in another

area• Precipitation can also be increased by pollution in the air

Human Effects on Evapotranspiration• This is not as big of an impact• Dams – construction of large

catchment area – expose large surface area – allowing for high level of evapotranspiration

Prevention: Adding chemicals, sand filled dam – covering with plastic – e.g. Aswan dam lose 1/3 of the water• Urbanization – the construction of

urban area can increase surface storage but also reduce vegetation in the process of deforestation

Deforestation• Decrease Evapotranspiration- Transpiration reduced due to less leaves- Evaporation reduced due to less interception loss• Less interception – leads to more surface runoff, higher

peak discharge and shorter lag time• More stream sedimentation – no root to hold the soil –

weathering/ erosion

Construction of houses• Decreased infiltration• Less ground water – lower water table – ground water

deficit• Higher storm flow/ peak discharge• Less base flow in dry period

Complete residential Development• Lower porosity of ground – due to cementation etc.• Increase peak discharge• Higher flood damage potential- Construction of storm drains etc.1. Local relief from flooding2. Aggravate flood issues down stream

Interception• Vegetation – density, type• Type: Certain agricultural areas – plants with larger leaves

intercept more• Throughfall/ Stemflow is slow and tends to end up as

surface storage – deforestation will reduce this

Infiltration/ Soil Water• Human’s land use has great impact on this• Urbanization – impermeable surface – less interception –

more compact surface – more overland flow• Grazing – compaction and ponding of the soil – less

infiltration• Ploughing – increases infiltration – loosen up the soil• Waterlogging can occur if drainage is poor• When water table is close to the top, evaporation is quick.

This leaves behind immovable salt crust• Construction of dam = more surface storage• A decrease in surface storage – quick draining

Effects of Dams• More water Storage• Decreased flood peak – except when it bursts• Less flow in the river• Decreased in sediment yield of river – all is kept in the

dam• Can cause, in long term, earthquake• More flooding• Changes in temperature and water salinity due to

evaporation increase• Large dams can affect the climate

Adv.………………DAM.……….DISADV

• Control of Flooding and drought e.g.. Egypt (1972-73) and Lesotho Highland project

• Irrigation e.g. Aswan Dam• Hydro-Electric Power e.g.

Netherland• Tourism and recreation –

Bhumipol Dam – Bangkok – Singapore’s catchment area

• Salinization – reduction of crop yield e.g. Aswan

• Effects on the river• Groundwater affected – secondary

salinization• Displacement of population• Drowning of archaeological site• Seismic stress: Aswan dam caused

the November 1981 earthquake• Channel erosion increases• Deposition increases• Erosion further down e.g. the Delta• Retaining of sediments – loss of

nutrients• Less fishes• Spreading of diseases

Changes in Ground Water - Texas• Irrigation: started in 1930s• Equilibrium was reached – recharge = discharge• Centre-Pivot Irrigation Scheme• Rapid uses of ground water• Water table to lower by 50 meter• Narrower Aquifers

Changes in groundwater• In industrial areas – less groundwater are being retrieved• Waterlogging issues• Leakage from ancient pipelines- Increase in springs, river flows- Flow from dry-springs- Flooding of surface water- Pollution can quickly spread to groundwater- Basements of houses flooding- Leaking into tunnels- Walls are less stable- Foundations are affected – chemicals attack- Clay swells

Groundwater recharge• Water spreading- There must be enough surface storage- Aquifers where permeability is there• Pumping into wells when ground are impermeable• Case Study: hydrology in the Aral Sea

Flood• Flood easily affects people the most – people live in flat

land close to river or the coast• Recurrence interval: the frequency of occurrence of floods

of a certain size• England/ Australia: Less than 2%• USA: 10%• Asia: Faced the most flood• Occur in estuaries or low-lying floodplain e.g. Bangladesh

– 110 million people face risks of flood every year.• Smaller basin – flash flood• Semi/ Arid area – flash flood• Short intense flood in steep regions can kill

Flood• Areas close to dams – e.g. China’s Sichuan earthquake

caused dam-bursting – flood• Inland shorelines – e.g. The Great Lake USA• MEDC- HIGH LOSES – LESS DEATH

• LEDC- LOW LOSES – HIGH DEATH – MAY INCREASE DUE TO MIGRATION

Flood Control• Since the end of WW2 – changes in the idea of hazard

responses• Less about physically controlling the damage• More about reducing the vulnerability

Causes of flood1. In Western areas – Low pressure system2. In Asia – Monsoon season brings rainfall3. Snowy regions – Rapid melting of snow• Damage potential of flood increases with velocity of water

– fast water comes with rocks and debris• Urban areas – create impermeable surface, dense

network of drain – short lag time

Forecasting and Warning of Flood• 1980s, 1990s – flood warning has became more accurate1. Estimate rainfall, snowpack2. Studies of river – meteorological check-ups3. Information on human environment – to produce a risk

assessment model4. Better sharing of information between experts5. Sharing of information between countries regarding

international drainage basin6. Sharing of advanced technologies

Amelioration/ Flood Prevention – Loss-Sharing

• Loss Sharing- Disaster Aids – aids from agencies – food, medicines,

technical aids etc.- Insurance

Hard Engineering/ Soft Engineering

• Dams• Artificial Levees – concrete • Wind Dykes – narrows the

channel and allow water to pass through an area in risk of flooding more quickly – causes flood down stream

• Channel Straightening –cuts off meanders to let water through faster

• Diversion Spillways

• Floodplain zoning – reduce risk of flood in important areas – limit development

• Afforestation/ Reforestation

• Wetland Restoration – creating areas of large surface storage – reduce runoff – decrease the areas available for farming

Flood Management in the Future• Climate change – could result in more frequent flooding• Urbanization = higher economic lost when flooding• Some propose to give up farmlands/ villages to reduce

risk of being flooded• Some propose redesigning of houses• Some propose better sewage system

Problems with River management (Urban areas)

• Levees too close to river = no storage space for excess water

• Hard engineering and defenses expensive• Draining water upstream = more flood downstream• High levees may overtop – causing river bed to rise• Straightened courses (to reduce river channel length) –

can result in higher velocity• Flashflood in culverted river if blocked by debris

Websites• https://www.austintexas.gov/sites/default/files/files/Water/

CER/river_process_may_2013s.pdf