1. Hydrological Cycle
 Water stored in biosphere, lithosphere, atmosphere and hydrosphere
 Inputs, outputs, flows, storages
2. Precipitation, Interception and Evapotranspiration
Precipitation
 Differs by latitude: tropics – high, subtropical – low, mid-latitudes – cyclonic / frontal,
polar – low because lower temperature holds less moisture
 Influences run-off and evapotranspiration
 Types
o Convectional: displacement of warm air upwards in convectional system
o Orographic: meets barrier (land mass) and must rise above it, deposits
mostly on windward compared to leeward
o Cyclonic: warm air mass rises after encountering cooler, denser air mass
 Warm front – drizzles, cooler front – heavier showers
o Snow: water vapour frozen directly into solid, minute ice crystals forming
around nuclei
o Sleet / hail / frost
 Intensity affects nature of channel flow: 0.5 – 4mm/h vs. 100-150mm/h
o Higher intensity, more flow
Interception
 Types: canopy, throughflow, stemflow, litter
 Factors
o Types of rainfall: short and heavy vs. prolonged drizzle: pine trees intercept
15% vs. 94%
o Type of vegetation: tropical: 40%, temperate: 30%, savanna: full leaf then
more interception but varies seasonally
Evapotranspiration
 Potential: at field capacity vs. actual: below field capacity
 Factors
o Temperature
o Relative humidity
 Low RH, vapour pressure gradient high, evapotranspiration increases
 Results in higher RH, vapour pressure gradient falls, temperature
increases
 Results in lower RH, vapour pressure gradient increases,
evapotranspiration increases
o Wind: replace surface layer with unsaturated layer of air – facilitates mixing
of saturated and unsaturated air molecules
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o Vegetation; soil texture: determines wilting point and field capacity which in
turn determines water capacity
3. Soil Moisture Storage, Infiltration and Throughflow
Soil moisture storage
 Pores form narrow passages where water flows through – water not removed by
throughflow and percolation become capillary water due to capillarity of water
(water tends to stick to solid particles)
 Seasonal variations
cm of water (monthly means)
Precipitation
Water deficit
Soil moisture
recharge
Soil moisture
withdrawal
Potential
evapotranspiration
(Winter)

Field (Summer) Wilting
capacity
point
time (month)
o When precipitation > potential evapotranspiration, soil reaches saturation
capacity – gravitational water is drained, leaving capillary water – soil
reaches field capacity
o When precipitation < potential evapotranspiration, water drawn from soil
and is drawn from increasingly thinner pores, leaving hygroscopic water –
soil reaches wilting capacity which is maintained till precipitation > potential
evapotranspiration again
Large water capacity – greater difference between field capacity and wilting point –
more favourable for soil
Infiltration
 Water drawn into soil by gravity and capillary action
 Factors
o Rainfall: amount, duration, size – affects ease of entry into soil
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
o Soil texture: coarse vs. fine-grained: water encounters more flow resistance
as diameter of pores decreases because it sticks to grains instead of flowing
through
o Vegetation
 Decaying vegetation assists infiltration
 Affects soil structure: changes soil to crumb-like structure (loose and
friable structure allowing rapid infiltration and drainage)
 Rain splash action: reduces chances of raindrops sealing natural soil
openings
o Compaction by tractors / trampling by cattle changes soil into platy structure
which impedes downward movement of water
o Terracing increases amount of time water is retained on slopes
o Antecedent soil moisture: rain water from previous rainfall
o Urbanization: replacement of vegetation with concrete
Rate of infiltration decreases over time due to
o Less storage capacity: depends on rate of water loss
o Filling of thin pores reducing capillary action
o Impact of rain breaks up soil aggregates to fill pores
o Wet clay swells in size and decreases size of pores
Throughflow
 Generated by lower permeability of soil at greater depths
o Occurs because permeability of soil is greater than the underlying rock
o Clay pan (less permeable region) is formed below because finer particles are
washed down by percolating water to fill pores
o Soils at greater depth experience more compaction due to weight of soil
above: restricts downward flow of water and hence water moves laterally
(throughflow)
 Sometimes throughflow can be a flow along well-defined sub-surface seepage lines
(percolines) like tunnels / pipes where soil particles are washed away by sub-surface
flow
4. Overland Flow
Forms
 Sheet wash
o Upper part of slope with smooth surface experiences sheet erosion
o Downslope experiences slope wash causing debris accumulated in thickening
layers
 Rills and gullies
o Flows along depressions downslope cause small channels to be incised,
forming rills which are innumerable, closely-spaced channels
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o Gullies are the large channels formed due to erosion and devegetation
Horton overland flow
 When rainfall intensity > infiltration capacity, excess water is stored in depressions –
surface detention
 Variations on slope
o Amount increases downslope due to accumulation
o Velocity increases due to steeper gradient + less friction between water and
slope
 Variation over time: increases if rainfall intensity does not fall because infiltration
capacity decreases with time
 Limitations
o Rarely generated under humid temperate conditions where rainfall
intensity > infiltration capacity by a wide margin
o Works for semi-arid environments, urban areas, devegetated areas, places
where soil is trampled by cattle
Saturation overland flow
 Ground saturated – rise in water table because rainfall impeded from flowing
downwards due to impermeable B-horizon
 Rain falls directly on saturated soil – cannot be absorbed – causing overland flow
 Migration of water through soil downslope as throughflow will seep out as return
flow
5. Channel Flow
 Sources: channel precipitation, overland flow, throughflow, baseflow
 Types: perennial, intermittent, ephemeral: determined by baseflow
Storm hydrographs
 Features
Q/m3s-1 Peak discharge
Lag time
Recession
Rising
limb
limb Storm flow
Base flow
Time/h
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o Initial rise in discharge due to channel precipitation
o Rising limb due to overland flow
o Lag time because need time for water from rainfall to travel to gauging
station and time for overland flow to be generated
 Shorter lag time means more prone to flooding due to increase in
discharge spread over a shorter time interval
o Double peak: overland flow + throughflow
Factors
o Location of rainstorm: upper part of basin – longer lag time and less
pronounced peak
o Nature of precipitation: heavy – shorter lag time and higher peak
o Basin characteristics
 Size: bigger – longer lag time (need more time to reach gauging
station) and higher peak (more water captured)
 Shape: elongated vs. circular (shorter lag time and lower peak)
 Relief: steeper – shorter lag time and higher peak
o Vegetation
 Interception reduces total discharge
 Plant roots reduce throughflow – lower peak discharge
 Increase capacity and infiltration rate – increase proportion of
throughflow and baseflow – longer lag time and lower peak
o Basin geology: more permeable rocks and soil increase infiltration
o Urbanization: increase velocity and amount of discharge
Hydrograph of melting glaciers: melt in early afternoon where temperature highest,
peak discharge late afternoon causing short-term variations
Annual hydrograph
o Seasonal variations
 Climate: eg. Britain: least in late summer, most in spring due to
amount of evapotranspiration and water varies
 Basin geology eg. River Derwent: impermeable shale-sandstone vs.
River Wye: permeable carboniferous limestone
 Flow regulation
6. Groundwater Storage
Porosity and permeability
 Aquifers: water-bearing rock formations high in porosity and permeability vs.
aquicludes: non-porous and non-permeable
 Porosity: % of total volume consisting of voids
o Factors
 Spaces between mineral grains eg. sand and gravel high porosity but
may be cemented by smaller minerals
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
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Fractures
Solution cavities eg. limestone where solution activities form holes
and pits that can be enlarged into caves as water flows over
 Vesicles: basalt and volcanic rocks found on top layer of lava flow –
very high porosity due to trapped air bubbles
Permeability: capacity of rocks to transmit fluids (size of pores)
o Primary: passage of water through pores
o Secondary: passage of water through fractures
Groundwater storage and water table
 Groundwater result of percolation
 Water table: boundary separating unsaturated rocks above from saturated rocks
below
o Zone of aeration: air and water fill openings
o Zone of saturation: fractures, groundwater
 Factors affecting water table
o Surface topography
 Shape of water follows shape of relief – greater depth at hills than at
valleys because gravitational pull downwards
 If rain ceases, water level slowly subside to height of valleys
o Geological structure: perched water table due to alternating layers of
aquiclude and aquifer
 Fluctuations in water table: determined by amount of input and output
o Seasonal
 Zone of intermittent saturation (between minimum and maximum
point of saturation)
 Eg. Britain: more rain in winter than in summer. April-October:
precipitation < evapotranspiration but this changes after October –
precipitation recharge
o Long term
 Eg. deserts – water table lies at great depths – fossil groundwater
from pluvial periods
 Water table getting lower because people use the aquifer by building
wells to draw water, forming cones of depression
Groundwater and channel flow
 Effluent: water table higher than channel: seepage into channel
 Influent: seepage from channel into ground
 Problems associated with groundwater utilization and pollution
o Ground subsidence due to over-pumping eg. valley of California, Mexico City.
In Southern California, artificially divert rivers over permeable deposits to
recharge groundwater
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o Groundwater pollution: bury waste in unsaturated region subjected to
reaction with percolating water leads to contamination of groundwater
o Salt-water intrusion: depth of freshwater underground 40x that of
freshwater above ground. Normally freshwater floats on denser salt water.
But excessive pumping lowers water table – bottom of freshwater zone will
rise 40x – eventual salinization of water
7. Water Balance
 Balance between water inputs (precipitation), water outflow (evapotranspiration
and stream flow), change in water storage
 P=E+RS
 Spatial variations
o Singapore: precipitation > potential evapotranspiration all year round,
especially beginning of year with NE monsoon – water surplus. Though high
potential evapotranspiration, even higher precipitation
o Sudan – arid region: precipitation < potential evapotranspiration due to high
temperature – water deficit
 Temporal variations: Britain: winter – surplus, summer – deficit
8. Flood Management
Causes: climatological vs. non-climatological
 Excessive rainfall: eg. UK regular winter floods due to series of depressions – heavy
rainfall – overland flow due to already saturated ground
 Rapid snowmelt in spring / early summer eg. Bangladesh floods due to snowmelt in
Himalayas
 Volcanic action induces snowmelt
 Landslides: displacement of water – overflow banks eg. rockslide in Vaiont reservoir
in Northern Italy inundated the Piave Valley and the town of Longarone
 Dam failures: eg. failure of St. Francis dam – San Francis Quito Canyon flooded
Flood-intensifying conditions: basin conditions + channel conditions
Flood prediction and forecasting
 Flood prediction: likelihood of occurrence
o Recurrence intervals – flood frequency graph
o Limitations
 Talking about probability only, not certainty
 Basin / channel conditions may change with time – need to update
 Short records – inaccurate – may miss extreme floods
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Flood forecasting: severity of flood
o Rational runoff
 Peak rate of runoff Qpk= 0.278CIA, C=rational run off coefficient,
I=rainfall intensity, A=drainage area
 Assumes Horton overland flow
 Most ideal for area of 200 acres / urbanized area with high run off
rates
Flooding in Singapore
 Nature of rainfall: high and intense especially during monsoon – saturate soil quickly
– overland flow
 Topography: Bukit Timah Granite and Jurong Formation – steep-sided valleys
concentrate floodwaters on low valley floors
 Recent development: urbanization – concretization – reduces infiltration capacity
and efficient storm drainage – increase flood propensity
 Flood management programs to curb but can never evade floods
Flooding in Bangladesh
 May to June: snowmelt from Himalayas to reach Bangladesh in July
 Worst hit: 60% island inundated
 Deforestation in mountain catchment areas of Nepal
 Coincidence of flood peaks from 3 rivers in 1988: Brahmaputra, Ganges, Meghna
Effects of floods
 Primary: direct contact with flood waters
o High velocity – carry heavy load that can injure people
o Can cause massive erosion – undermine structures
o Suspended load which is deposited when flood retreats, covering buildings
with a layer of wind
o Farmland loss
o Drowning
o Furniture / equipment damaged by water especially US homes as they are
made of timber / plastic
 Secondary: disruption of essential services and health hazards and psychological
impact
 Tertiary: change in river channels, loss of jobs, corruption
Prediction: recurrence interval, hazard mapping, warnings
Mitigating
 Levee / dams but if fail, aggravate situation. Failure of levees in Mississippi in 1993.
Failure of Teton Dam in Idaho.
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Channelization
o Enlarge cross-sectional area. Straighten channels using artificial cut-offs –
shorten channels – steeper gradient and velocity enables discharge to
dissipate quickly
o But like in Mississippi, difficult to work against river’s natural tendency to
meander
Floodways: outlet of flow eg. Lake Ponchartrain. Normally used for recreation.
Non-structural
o Expensive and false of security
o Flood-plain zoning: monitoring land use in floodplains
o Building codes
o Buy out programs: to relieve burden on government funds
o Mortgage limitations
9. Channel Morphology
Generation and dissipation of river energy
 Generation: discharge = volume x velocity
 Dissipation
o Erosion, transportation (5%)
o Frictional drag (95%) along river and banks
 Adjacent threads of water flowing at different velocities eg. turbulent
flow
 Water interchanged in eddies – local changes in velocity – loss in
energy
Factors affecting river energy
 Volume of water
o Humid tropics: volume increases downstream due to tributaries – more
efficient river downstream
o Arid regions: volume decreases downstream due to evaporation – convex
profile
 Velocity – Manning’s equation V=1.49R2/3S 1/2/ n
o R: hydraulic radius: ratio of cross-sectional area to length of wetted
perimeter
 More contact with bed and banks, more friction
o S: channel slope
o n: coefficient of roughness
 Smoother downstream because bed made up of silt / sand / clay
o 2x velocity leads to 4x discharge
o Downstream: average stream velocity increases / remains constant. Increase
in R and decrease in n compensated by decrease in S
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10. Fluvial Processes
Erosion processes
 Abrasion: coarse and angular fragments dragged across riverbed, rubbing and
wearing away exposed rock outcrops
o Occurs upstream because lots of large load, forming rock-cut channels due to
down-cutting
rock-cut
channel
Solid rock

o Pothole drilling: localized erosion in eddies forms shallow depression. Any
load that gets trapped will be swirled round to form potholes
Hydraulic action: sheer force of water to dislodge particles – lateral erosion
o Occurs at lower/ middle courses, forming alluvial channels
alluvial channel
alluvium



solid rock
o Cavitation: collapse of bubbles of water – shock waves hit and slowly weaken
bank
Attrition: breakdown of the load itself due to collision – more rounded downstream
Solution: dissolve constituents eg. limestone. Water / humic acid
Components
o Vertical down-cutting – gorges as neighbouring potholes merge – lowering of
riverbed. River rejuvenation – deep v-shaped valleys / gorges
o Lateral erosion
 Erosion concentrated at / below water surface where the thalweg is
 Usually when river meanders
 Collapse of upper face of banks – retreat of concave banks
o Headward erosion
 Head of river eg. limestone terrain: emergence of springs
 Profile of river locally steep – could result in waterfall / collapse of
overhang – retreat upstream
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River transport
 Processes
o Traction: rolling of larger load across riverbed. Usually at source of river
where there are large load and steep gradient
o Saltation: ‘bouncing’ of smaller load. Lifted due to turbulence and land a
distance downstream
o Suspension: smaller particles eg. silt / clay small enough to be held by
turbulence. Greatest part of load transported. Occurs near river mouth.
Greater turbulence and velocity, larger load can remain in suspension.
o Solution: dissolved load
 Hjulstrom curve

o 0.5mm diameter – sand – lowest competent velocity: minimum velocity
required to move particles loosely resting on the riverbed
 Larger particles – higher competent velocity due to weight
 Smaller particles – higher competent velocity due to high
cohesiveness and electrical bonding
o Positive relationship between speed and particle size
 Larger particles have a higher settling velocity: velocity at which
particles becomes too heavy to be transported and are hence
deposited
o Less velocity to transport than to erode
 Need very huge fall in velocity for smaller particles to be deposited
even if they are eroded upstream vs. larger particles
Velocity
o River’s capacity: ability to transport volume of load proportional to
discharge3
o River’s competence: ability to transport weight and size of load proportional
to discharge6
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o Affected by geology and climate
River deposition
 Sudden input of load – overloaded eg. landslide
 Loss of energy: river broadens, n increases. Low precipitation, discharge falls.
 Land forms
o Alluvial fan: where valley meets plain – sudden drop in gradient – loss of
energy. Deposit load in a fan-like shape. Sorting of alluvium with coarser
ones downstream of the apex due to further fall in velocity
o Point bars / floodplains (lateral accretion): erosion of banks – load deposit on
point bars – continued lateral accretion – floodplains for meandering rivers
o Floodplains (vertical accretion)
 Floodwater overflows banks – sudden drop in river competence and
capacity – deposit coarser loads on margins of bank (levees) and
accumulation of silt over floodplain
11. Channel Plan Forms
River meanders
 Sinuosity ratio: ratio between distance of centre line of valley and distance along
channel. Meander if exceed 1:1.5
 Geometric features

Formation: erosion and deposition processes
o Erosion of concave bank: thalweg diverted against – impact of hydraulic
action greatest – concentrate erosion
o Load dragged across river bed to convex side but loss in river energy due to
friction and previous erosion – deposit load – point bars
o Helicoidal flow – increase meander amplitude and sinuosity – ox-bow lakes
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o Migration of meanders
Braided channels
 Features

Formation
o High discharge – lots of bed load – erosion of channel banks
o Low discharge – coarser load starts being deposited to form nuclei of bars.
Flow disrupted, velocity decreases downstream, finer particles settle on
nuclei
o Further decrease in discharge – expose bars
o Some bars will be washed away by the next high discharge but some will be
stable and vegetated – assists trapping of more sediment
o Braided channels markedly unstable. Kosi River, India receives load from the
Himalayas: shifts 112km in 228 years. Catastrophic erosion of new channels
and abandonment of old ones
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12. Drainage Basin Analysis
Stream order analysis
 Strahler’s method: does not reflect relationship between channel size and capacity
 Law of stream number
o Number inversely proportional to order
o Length proportional to order
o Size of drainage basin proportional to order
 Bifurcation ratio: dividing number of streams in one order by the number of streams
in the next highest order, higher ratio – more prone to flooding
Drainage density
 Total length of stream / total basin area
 Limitations: intermittent streams, limestone terrain: dry valleys, underground flows
 Factors (those influencing infiltration and overland flows)
o Time for erosion / migrate headwards
o Rock type, relief, infiltration capacity of soil, total annual precipitation /
rainfall intensity, vegetation
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