Hydrology notes - Singapore A Level Geography

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Hydrologic Processes, Hazards and Management
The Hydrological Cycle
Amount of fresh water available for human use is only 2.8% of total supply, and most of it is locked in ice
sheets and glaciers, accounting for the water stress of people.
1. The Global Hydrological Cycle
 Flows and exchanges of water between atmosphere, biosphere, hydrosphere and
lithosphere
 Water evapotranspirated from oceans, seas, rivers, soil, vegetation etc. transfers water
to atmosphere
 Water vapour condenses to form rain clouds to precipitate, transferring water to other
parts of the hydrological cycle
 Over land, precipitation exceeds evaporation, and over oceans, evaporation exceeds
precipitation
 Net gain for land, net loss for oceans, due to advection of water vapour over oceans to
land
 Surplus water on land flows as streamflow/runoff into oceans
2. The Basin Hydrological System
 Used in studying hydrology of rivers and drainage basins
 Inputs: precipitation, rain and snow
 Storages: precipitation in basin stored in storages: interception, surface, soil moisture
and groundwater storage. Slows down movement of water.
 Flows: link storages together: stemflow and leaf drip, infiltration, percolation, overland
flow, throughflow and baseflow
 Outputs: water which leaves the basin as evapotranspiration or streamflow
Precipitation, Interception and Evapotranspiration
1. Precipitation
 Provides initial input of water into the system. Distribution varies with climatic region
 Tropical region has high precipitation due to high temperature, humidity and air
instability. Subtropical areas have low annual precipitation due to subsiding air. Mid
latitude areas normally have moderate cyclonic or frontal rainfall. Polar regions have low
precipitation due to lowered water vapour capacity, low temperatures and subsidence
1.1 Types of Precipitation
 Rain is the most common. Convectional rainfall is the result of displacement of
warm air upward in a convectional system, common in tropical regions and summer
seasons.
 Orographic rainfall: air mass rising above a land barrier, such as mountains, with
moisture deposited on the windward side, with the leeward side having much less
 Frontal: warm air mass rises after encountering a colder, denser mass. Warm fronts
have less turbulence and precipitation, while cold fronts have heavier storms
 Snow, sleet and hail are less common forms at higher latitudes
1.2 Intensity of Precipitation
 Humid temperate: low intensity of about 0.5-4 mm/hr. Warm fronts, light rain over
a prolonged period
 Tropical: high intensity, up to 100-150 mm/hr. High temperatures, rapid evaporation
lead to high humidity. Unstable air causes large clouds to form
2. Interception
 Precipitation trapped on vegetation and other surfaces before reaching ground.
Interception loss is intercepted precipitation evaporated to atmosphere
2.1 Types of Interception, Throughfall and Stemflow
 Dense vegetation can act as interception storage, such as canopy interception.
Throughfall such as leaf drip penetrates gaps in canopy. Water can run down
branches and trunks as stemflow, both delivering water to litter layer. Some is
stored as litter interception while rest infiltrates the soil.
 Only part of total rainfall reaches soil while rest is lost as interception loss
2.2 Factors Affecting Interception
 Interception depends on rainfall characteristics and vegetation
 High intensity and short duration of rain results in less interception storage. Pine
forests can intercept 94% of low intensity but only 15% of high intensity
 Denser the foliage, greater interception storage especially in tropical forests
 Brazilian forest – only 60% of water ever reaches ground
3. Evapotranspiration
 Major output of water from drainage basins
 Evaporation from precipitation accumulated on surfaces, soil and interception
 Transpiration from plants
3.1 Potential vs. Actual Evapotranspiration
 Potential evapotranspiration is the maximum rate at which evapotranspiration can
take place i.e. if there is enough water
 Actual evapotranspiration is the measured rate of evapotranspiration, which can be
below the potential rate when there is not enough water
3.2 Factors Affecting the Rate of Evapotranspiration
 Temperature: higher temperature, more energy to evaporate, can hold more air
 Relative Humidity: ratio between amount of water vapour in the air at a given
temperature and maximum vapour the air can hold. Lower the relative humidity,
greater rate of evapotranspiration
 Temperature and relative humidity influence the vapour pressure between water
surface and atmosphere. Higher temperature, lower relative humidity, increased
vapour pressure gradient, greater rate of evapotranspiration
 Wind speed: positive relation with evapotranspiration, mixing saturated with
unsaturated air
 Vegetation cover: more vegetation = greater evapotranspiration. Large tree can
transpire several hundreds of litres a day
 Soil texture: affects field capacity and wilting point, determining the water available
for evapotranspiration
Soil Moisture Storage, Infiltration, Throughflow and Overland Flow
1. Soil Moisture Storage
 Soil comprises of mineral and organic particles, and is porous. Size of pores depends on
the size and shape of particles
 Pores serve as narrow passages, capillaries, to allow for rain water to pass through
 Water can be stored as capillary water, adhering to soil particles by soil tension
1.1 Forces that Retain Soil Moisture
 Soil tension is caused by matric force, adhesion between water molecules and soil
particles, cohesion between water molecules
 Matric force is strongest at the surface, retaining capillary water. Water beyond 0.06
mm from soil particle is drained by gravity
 Water moves from areas of low matric force to areas of high force – i.e. from wet
areas to drier areas, via capillary movement
1.2 Seasonal Soil Moisture Variations
 Wet season: beginning of year, Precipitation > Potential Evapotranspiration, thus
there is a water surplus in the soil
 Upon precipitation, soil attains saturation capacity where moisture content is equal
to porosity of soil. Gravitational water is drained away from bigger pore spaces,
leaving capillary water – this is field capacity, the maximum amount of water freely
drained soil can store
 When Potential Evapotranspiration > Precipitation, soil moisture withdrawal occurs,
reducing moisture below field capacity. Occasionally, a moisture deficit develops
when actual evapotranspiration falls below potential evapotranspiration
 When water is extracted by plants, water is drawn from finer pores and nearer
surface of soil particles. When matric force exceeds the ability of plants to absorb
water, hygroscopic water unavailable to plants remains. This is wilting point.
 Available water capacity is soil moisture between field capacity and wilting point.
This is available for plants.
 When Precipitation > Potential Evapotranspiration again, soil moisture recharge
occurs, until field capacity is reached
1.3 Soil Texture and Available Water Capacity
 Water availability of soil varies with texture of soil. Soil with more available water is
more favourable to plant growth
 Sandy soil has very low capillary action due to having very little surface area on each
soil particle. Many small pores increases pore volume, allowing for greater
gravitational draining
 Clayey soil is platy and has high surface area, increasing capillary action, and clay
particles further expand with more water contact. However, this plate structure
reduces pore size, limiting infiltration largely and reducing moisture amount
2. Infiltration
 Seeping of water into soil, dependent on gravity and capillary action. Gravity moves
water vertically down, capillary moves from wet to dry in any direction
2.1 Factors Affecting Infiltration
 Infiltration capacity: maximum rate a soil in a given condition can absorb water
 Infiltration rate is the actual rate of infiltration, dependent on nature of rainfall and
capacity
2.1.1 Rainfall Characteristics
 Varying amounts, duration and size of rain drops
 Light rain, small drops and short duration will be largely intercepted by
surface vegetation, minimising infiltration
 Heavy storm, large drops, high intensity rain minimises infiltration by
compacting the soil due to impact
 Highest where rain is steady, vegetation breaking up drops into smaller size
2.1.2
Soil Texture
 Determined by the constituent particles of the soil
 Coarse texture results in large pore spaces – soil is porous and permeable
 Fine, clayey soils have small, numerous pores, less permeable
 Gravity flow is limited by pore size – flow resistance increases as diameter
of pore increases
 Water is trapped in pores by surface tension
2.1.3 Vegetation
 Plants and soil fauna churn through soil, providing passages for soil
movement
 Causes soil structure to form aggregates – loose, friable crumb structure
increasing pore space
 Protect soil from packing of rainsplash action, preventing crusting
2.1.4 Compaction
 Perhaps by machines or animals. Forms platy aggregates in soil, impeding
infiltration
2.1.5 Terracing
 Increasing time water in retained on slopes, increasing infiltration
2.1.6 Antecedent Soil Moisture
 Water from previous rains still in soil. Can impede passage of fresh rain.
2.1.7 Urbanisation
 Replacement of vegetation by asphalt and concrete
2.2 Variation in the Rate of Infiltration over Time
 Beginning of rain, infiltrates at rapid rate unless soil is saturated or hardened
 Over time, rate is reduced due to reduction in storage capacity, depending on rate
of loss of water at the base of the soil
 Also, capillary action reduced due to filling of pores, impact of raindrops breaking
and compacting soil, clay minerals swell reducing pore size.
 Rate settles after a period (10-20 min) and becomes about constant at median 25
mm/hr
3. Throughflow
 Lateral, downslope flow of water underground, eventually emerging as small springs or
seepages, contributing to surface runoff
 More irregular and slower than overland flow, takes very long to reach rivers, due to
flow through small pores fissures
 Generated with decreasing permeability with increasing soil depth – due to lower
permeability of underlying parent bedrock, possibility of containing a clay pan due to
washing down of fine materials by water, compaction due to weight of soil above
 Water is forced to drain laterally downslope, occasionally forming underground pipes in
the soil so flow is concentrated along well defined percolines, increasing speed of
throughflow
4. Overland Flow
 Occurs when rain is unable to infiltrate into soil, flowing over land surface
 Temporary – only active during and slightly after rainstorms. Most responsible for soil
erosion.
4.1 Forms of Overland Flow
4.1.1 Sheet Flow
 Sheet flow/unconcentrated wash is not confined to channels. Occurs on
upper part of slope where surface is smooth
 Sheet erosion – soil removed in uniform thin layers
 Accumulates at base of slope to form thickening colluvium/slope wash
4.1.2 Rills and Gullies
 Concentrated wash occurs when rainfall is channelled along surface
depressions and irregularities
 Occurs on lower slope which is steeper
 Small channels incised into slope surface form rills, leading to rilling and rill
erosion, developing channels
 Rills can integrate into larger gullies over time, as erosion is accelerated
with devegetation
4.2 Generation of Hortonian Overland Flow
4.2.1 Condition for Generation of Hortonian Overland Flow
 Occurs when rainfall intensity exceeds infiltration capacity
 If intensity is low (temperate frontal rain), surface water infiltrates easily.
Infiltration rate = rainfall intensity
 High intensity (thunderstorms, humid tropics) rain causes infiltration to
occur at capacity rate. Excess water accumulates on soil surface, initially
occupying small irregularities called depression storage
 Depression storage quickly overflows to form sheet of water down the
slope. Water stored on hillside is surface detention
4.2.2 Variation of Hortonian Overland Flow on Slope
 Amount and velocity of Horton flow varies in downslope direction
 Amount increases downslope due to accumulation of surface water
 Velocity of flow increases downslope due to increased slope gradient and
lesser friction as flow depth increases
4.2.3 Variation of Hortonian Overland Flow with Time
 Infiltration capacity decreases with time and becomes constant after a
while, so if rainfall intensity remains constant, Horton flow should increase
in time and then remain stable
4.2.4 Limitations of its Applications
 Limited as Horton flow is rarely generated under natural conditions e.g.
Britain, since temperate conditions mean low intensity rainfall
 Model works well in semi-arid areas where intensity is high and vegetation
is sparse, urban areas where capacity is almost zero, devegetated areas
where capacity is low, and agricultural lands where soil has been
compacted or removed to expose less permeable sub-soil
4.3 Generation of Saturation Overland Flow
 Common occurrence in temperate regions
 Occurs when ground gets saturated – with rain falling onto slope, downward
movement of water through soil may be impeded due to presence of less
permeable layers, generating Throughflow
 Soil at base of flow becomes saturated, saturated zone giving rise to higher water
table, extending upslope
 Forms overland flow by return flow and direct precipitation onto saturated ground
Channel Flow and Hydrographs
1. Sources of Channel Flow
 Channel flow and overland flow form surface runoff
 Discharge which makes up channel flow is channel storage, as water is stored
temporarily within these channels
 Sources of channel flow: direct precipitation, or channel precipitation forms a small part
 Overland flow, throughflow contribute as well
 During non-rain periods, continuous flow of water is provided as baseflow from
groundwater storage
2. Type of River Channels
 Perennial channels are occupied by flowing water throughout the year, most common in
humid tropics, where water table intersects the channel all year round
 Intermittent channels are seasonally occupied by water, found in areas with strong
seasonal contrasts, like chalk valleys in England. In winter, water table rises to surface,
but falls and dries in summer
 Ephemeral channels are dry for most of the time, normally in arid regions, only occupied
after a storm, due to water table being very far down, and it takes time for water to
infiltrate. Discharge decreases with distance from source.
3. Storm Hydrographs
 River discharge is plotted against time.
 Annual hydrographs show long term/seasonal changes in discharge
 Storm hydrographs illustrate short term fluctuations
3.1 Features of a Typical Storm Hydrograph
 Channel precipitation gives initial rise of discharge, followed by overland flow,
inducing the rising limb, which are concave, and steepness indicates proportion of
overland flow and response speed to rainfall
 Peak discharge occurs when river reaches highest level
 Lag time is interval between peak of rainfall intensity and peak of channel discharge,
since it takes time for water to flow to gauging station
 Reflects time needed for rain to generate overland flow until it eventually reaches
station. Thus river may peak some time after the rain peaks
 Shorter lag tend to have higher peak and more prone to flooding as rainwater is
concentrated in river over shorter time
 Double peaks may result from overland flow, and then throughflow
 Recession limb is when discharge is decreasing and river level is falling, river
discharge returning to baseflow. Gentler and generally concave
 Stormflow/quickflow is part of discharge from overland and throughflow
 Baseflow is discharge contributed by groundwater, very slow to respond to storm
compared to stormflow. Maintains river flow after rain has stopped
3.2 Factors Influencing the Forms of Storm Hydrographs
 Differences in rate of increase of discharge and recession
3.2.1 Location of Rainstorm
 If storm is located at upper part of basin, peak discharge generated takes
time to pass down main channel. Gauging stations located downstream will
have longer lag times, as well as a dampened or lesser pronounced peak
progressively
3.2.2
Nature of Precipitation
 Intense rainfall leads to higher proportion of stormflow, saturating soil,
reducing lag time and increasing peak, because of Horton flow
3.2.3 Basin Size, Shape and Relief
 Bigger basins have longer lag times due to longer distance of flow. Peak may
be higher if more precipitation is captured
 Longer basins have longer lag time and lower peak, as same amount of
discharge is spread over a longer time
 Steeper-sided valleys of basins will have higher peaks and shorter lag times
due to faster flows
3.2.4 Effects of Vegetation
 Vegetation intercepts rainfall, storing water on its leaves as interception
storage, reducing total discharge
 Plant roots reduce throughflow, reducing peak
 Vegetation increases capacity and rate of infiltration, so more throughflow
occurs, reducing peaks and extending lag times
3.2.5 Basin Geology
 Permeable rocks and soil give hydrographs with low peaks and long lag
times, such as chalk subsoil having high porosity increasing infiltration
3.2.6 Urbanisation
 Infiltration capacity decreased greatly due to artificial surfaces, increasing
volume and rate of Horton flow. Smooth surface makes the flow very fast,
conveying water to channelized, hydraulically efficient streams
 Accumulation of storm water downstream much faster, leading to short lag
times and very high peaks, worsening floods
3.3 Hydrograph of Glacial Melt Water
 During summer in regions like Alps, surface melting peaks during early afternoon
and minimum at dawn
 Hydrographs of streams draining from glaciers show daily peaks.
 Lag time reflects time for meltwater to flow off ice surface or through tunnels within
and beneath glacier
4. Annual Hydrographs
 River regime, the fluctuations of river’s discharge over a year, is climate dependent due
to seasonal fluctuations
 Britain – difference in discharge from winter to summer, reflecting differences in
precipitation amount and evapotranspiration loss. For River Tees, in late summer
discharge is lowest due to low soil moisture and groundwater flow. In spring,
evapotranspiration is low and snowmelts from Pennine moorlands release water
 The Volga, USSR, has high discharge between March and June due to snowmelt
 River Derwent has impermeable shale-sandstone, leading to much less baseflow and
flashy hydrographs – short lag times with high peaks. Groundwater storage does not
interact with channel flow. River Wye, made of permeable carboniferous limestone, has
increased infiltration and percolation, having more baseflow, with groundwater
interacting to regulate the stream flow, slowing response of river to rainfall, reducing
short term fluctuations
Groundwater Storage
1. Aquifers and Aquicludes
 Whether a rock is an aquifer or aquiclude depends on amount of groundwater stored In
the rock, depending on porosity and permeability of the rock
 Aquifers are rock formations which are porous and permeable, while aquicludes are not.
 Porosity is percentage of rock consisting of voids, which can be pore spaces, fractures of
joints, solution cavities (like limestone, carbonation and solution) and vesicles (trapped
gas bubbles in volcanic rocks)
 Permeability is capacity of a rock to permit ready transmission of water into and through
rock. Primary is natural pore spaces, while secondary is through fractures
 Permeability depends on size of voids, while porosity is total volume of voids. Shale is
highly porous, but impermeable due to small pores
2. Groundwater Storage and the Water Table
 Groundwater storage occurs when water can percolate downwards
 Water table divides saturated rocks from unsaturated rocks
 Vadose (zone of aeration) air and water fills openings in soil and rock (field capacity)
 Phreatic (zone of saturation), all spaces are filled by groundwater (saturation capacity)
 Water table, vadose and phreatic zones fluctuate with changing seasons
2.1 Factors Affecting the Forms of Water Table
2.1.1 Surface Topography
 Water tables have gradients similar to surface relief. In flat areas, the table
will be relatively flat, but in hilly areas, it rises and falls with the land, due to
replenishing of water by precipitation. If rainwater stopped, water table will
be pulled down flat to around valley level
2.1.2 Geological Structure
 Sometimes, pockets of groundwater are stored above main water table,
due to alternate layers of aquifers and aquicludes, giving rise to perched
water tables
2.2 Fluctuations in the Height of the Water Table
 Determined by input and output of water into and out of groundwater storage
 When precipitation exceeds evapotranspiration, precipitation recharge occurs
(input), when exceeds baseflow and springflow (output), the water table rises
2.2.1 Seasonal Water Table Fluctuations
 Short term fluctuations occur in areas with strong seasonal climatic contrast
 In Britain, there is more rainfall in winter than summer. In summer,
potential evapotranspiration is very high, ceasing percolation and
precipitation recharge of aquifers, lowering water table. In winter,
precipitation exceeds evapotranspiration, recharging the water table, even
intersecting valley floors, producing the intermittent streams
 Zone of intermittent saturation – within this zone the water table rises and
falls in response to climatic conditions. Smaller in humid regions –
fluctuation is less
2.2.2 Long Term Water Table Fluctuations
 Water table reflects precipitation amount and forms underground
reservoirs of rainwater, closer to surface in humid regions but much deeper
in arid areas
 Saudi Arabia – limestone and sandstone aquifers contain water far below
ground level. Not recharged by present day rainfall – fossil groundwater
accumulated during pluvial periods of the Quaternary. If extracted, water is
not replaced – water table falls
 Sahel – long term changes in water table results from extraction. Since no
recharge occurs, water table is lowered, forming cones of depression
3. Groundwater and Channel Flow
 Groundwater affects channel flow as the level of the water table determines whether
baseflow occurs, whether stream conditions are effluent or influent
 When water table is high, groundwater moves into river channels as baseflow (effluent).
When it is low, beneath river bed level, flow seeps underground (influent)
 Humid regions: permanently effluent due to high precipitation, perennial streams,
constant baseflow.
 Temperate regions: seasonal contrasts, both effluent and influent, intermittent streams
 Arid regions: water table far below surface, permanently influent, ephemeral streams
4. Problems Associated with Groundwater Utilisation and Pollution
4.1 Ground Subsidence
 Subsidence/sinking of land as a result of reduction of groundwater storage
 Central Valley of California, Mexico City, Venice and Bangkok
 Southern California: artificially replacing water by diverting rivers over permeable
deposits, groundwater recharge
4.2 Groundwater Pollution
 Increase in population, urbanisation and industrialisation can pollute surface and
underground water
 Wastes from industries, landfills. Percolating rainwater picks up ions, carrying
leachate down to water table, polluting groundwater storage (e.g. chemical waste)
4.3 Salt Water Intrusion
 Sustained groundwater withdrawal in coastal zones eventually draw salt water into
wells, and must be abandoned
 Fresh groundwater floats on sea water due to being less dense – lens with convex
faces
 Depth of fresh water below sea level is 40 times elevation of water table above sea
level
 Eventually, elevation of salt water is high enough to be drawn into wells,
contaminating freshwater supply
Water Balance
Balance between water inputs into river basin as precipitation (P), and water outflow by
evapotranspiration (E), stream flow (R) and change in water storage (S). P = E + R ± S
1. Spatial Variations in the Water Balance
 Water balance varies greatly between climatic regions
1.1 Water Balance of Singapore
 High rainfall in excess of 2700 mm/year. Potential evapotranspiration is high due to
constantly high temperature, but input of precipitation is always larger, so vast
water surpluses throughout, no water deficiency. Large biomass = high
evapotranspiration. Runoff is high, perennial streams. Storages are also plentiful.
1.2 Water Balance of Sudan
 Very low precipitation of less than 18 mm/year. Potential evapotranspiration is very
high due to very high daytime temperature – large water deficiency prevails. Low
water storages on surface, in soil and underground. Minimal runoff due to very low
water table. Streams are ephemeral.
2. Temporal Variations in the Water Balance
 Considerable fluctuations within a year – in places with distinct seasons
2.1 Water Balance of Britain
 Winter precipitation greater than summer. Potential evapotranspiration, negligible
in winter, is enhanced during summer due to increased temperature and vegetation
 Winter: high P (+P), low E (-E) = high runoff (+R), recharge of storages (+S)
 Summer: low P (-P), high E (+E) = low runoff (-R), water deficiency (-S)
 Intermittent streams
Flood Management
1. Causes of River Floods and Flood Intensifying Conditions
 Floods may be caused by vast input of water into river channels, exceeding bankfull
discharge and overflowing occurs
 Conditions of basin and channels may increase or decrease flood propensity
1.1 Causes of River Floods
 Excessive rain – high intensity/long duration. Monsoons, tropical cyclones,
prolonger rainfall increases input e.g. Pakistan floodplains, Indus River during
monsoons
 Rapid snowmelt – often alongside rainfall, in late spring and early summer.
Bangladesh flooding due to Himalayas snow melt
 Volcanic action – cause rapid snowmelt. Iceland glacier melted.
 Landslides – rock can damn upstream, building up water and causing flooding:
Gansu Province in China, Bailong River got dammed by boulders as a result of
intense rains. Town of Zhouqu flooded.
 Dam Failure – St Francis Dam failure in 1928 flooded the San Francisquito Canyon,
killing 500
1.2 Flood Intensifying Conditions
 Many factors combine to determine the flood propensity of an area.
 Basin conditions: area/shape, climate, geology, soil type, vegetation
 Channel conditions: slope, storage, shape, roughness, load, flood control works
 Manmade characteristics affect nature and intensity since man can alter basin and
channel characteristics: deforestation, urbanisation and cultivation can reduce
infiltration capacity, increasing stormflow in relation to baseflow.
2. Flood Prediction and Flood Forecasting
2.1 Flood Prediction
 Whether a flood of a particular magnitude will occur during a specific time span
 Foretells the likelihood of a flood occurring
2.1.1 Flood Recurrence Intervals
 Statistical probability based on past floods. Long records are required.
 Maximum river discharge is identified for each year. Peak discharge for each
year is ranked according to discharge volume. Recurrence Interval = (n+1)/R,
n is number of years records exist for, R is rank of a discharge
2.1.2
Interpreting Flood Frequency Graph
 Recurrence interval plotted against flood discharge produces flood
frequency graph
 Predicts chance of a flood of a certain discharge happening in a year
 Vague estimates based on past records – changing circumstances make it a
lot less reliable
2.2 Flood Forecasting
 Shorter time intervals undertaken when a rainstorm occurs
 With past data of basin, channel flow and measurement of precipitation distribution,
amount and intensity, it can be calculated when the flood will reach a point along
the channel and how high it will be
2.2.1 Rational Runoff Method
 Predicts runoff rates by assuming that stormflow discharge is a fixed
proportion of rainfall intensity
 Qpk = 0.278 CIA
 Qpk is peak rate of discharge (m3/s), C is rational runoff coefficient, I is
rainfall intensity (mm) and A is drainage area (km2)
 C is the index of soil type, topography, roughness, vegetation and basin land
use
 System works ideally for catchments of less than 0.8km2. Works best for
urban and suburban areas with high runoff, somewhat steep channels,
limited channel storage and no lakes
 Assumes that generation process is Horton flow with whole catchment
contributing
 Assumes uniform precipitation over entire basin, precipitation does not
vary with time/space, there is little catchment storage, does not vary with
storm intensity or antecedent soil moisture
3. Case Studies
3.1 Flooding in Singapore
 Small scale, micro flooding
 Rainfall – high rainfall about 2550 mm/year. Intense rainfall causes a lot of
stormflow to concentrate in river channels in a short time, flooding. Normally occurs
during monsoon at beginning and end of year
 Topography – Bukit Timah Granite and Jurong Formation are flood prone due to
having steep sided valley walls concentrating flood water on low valley floors
 Recent Developments – rapid urbanisation, impermeable surfaces and lined drain
channels have increased to 48.6% in 1988, removing vegetation and reducing
interception storage and infiltration capacity, leading to more overland flow and
higher flood propensity. Storm drains carry rainwater efficiently, making channels
exceed bankfull discharge
3.2 Flooding in Bangladesh
 Big, macro scale flooding
 Bangladesh is in the lower flood plain delta formed where the Ganges, Brahmaputra
and Meghna converge
 May and June – snowmelt in Himalayas increases discharge greatly. By July, this
reaches Bangladesh, coinciding with the summer monsoon rains, inundating a large
part of Bangladesh
 Deforestation – Severe deforestation across catchments, especially in Nepal, is
accelerating runoff and increasing erosion rates. Deforestation makes sediment
supply to channels increase as rainsplash on bare slopes washes off soil, load
increases, silting up channels, raising channel beds, worsening floods, but degree to
which it worsens is debated
 Coincidence of flood peaks – The timing of rains and snowmelt vary between the
three catchment basins, but in 1988 the peaks coincided, worsening flooding
4. Management of Floods
4.1 Effects of Floods
4.1.1 Primary Effects
 Occur due to contact with water, primary hazards
 High discharge, high velocity, larger load, including rocks, sediment, cars,
houses and bridges
 Massive erosion, undermining bridge structures, levees, undercutting e.g.
Canyon Gorge, USA, 2004
 Water damage by flooding homes, property damage e.g. Shorewood,
Washington, 2010
 Deposited sediment covers everything with mud
 Flooding farmland, affecting crops and livestock
 Drowning
 Concentrate rubbish, debris, toxic pollutants which can cause secondary
hazards
4.1.2 Secondary and Tertiary Effects
 Secondary: long term as a result of primary effects, Tertiary: very long term
changes. Includes disrupting services, health problems, changes in position
of river channels
 Disruption – shortages of food and cleaning supplies, leading to starvation.
Drinking water may be polluted. Gas and electricity disruption, transport
disruptions
 Disease – water borne diseases such as cholera, made worse by dead
bodies festering
 Tertiary: Location of river channels may change as result, leaving old
channels dry. Sediment may destroy farm land. Loss of jobs. Insurance rates
may increase, corruption from misuse of funds, destruction of wildlife
habitat
4.2 Prediction
4.2.1 Recurrence Intervals and Limitations
 Useful in calculating probability of floods
 However, in reality do not occur at regular time intervals. e.g. Red River in
North Dakota had two 250 year floods within 110 years. Only a statistical
method calculating probability
 Long term changes may also be taking place i.e. not ceteris paribus, due to
modification of drainage basin such as deforestation, urbanisation and
agriculture
 Requires long periods of data – accuracy of small sample questionable
4.2.2 Forecasting and Limitations
 Many unrealistic assumptions of the rational runoff method
 Regardless, forecasts can allow flood warnings to be issued in time, but
flash floods are an issue since it requires time.
4.2.3 Hazard Mapping
 Determine areas susceptible to flooding when bankfull discharge exceeded
 Historical data + topographic maps to show what area has a how large
chance of being flooded, e.g. 10-year flood and so on
 Scale models often constructed as well. Can be used to decide interest rates
for houses, as well as insurance rates.
4.3 Mitigating River Floods
 Roughly split into engineering vs. non-structural
 Structural solutions are expensive, give false sense of security
4.3.1 Natural Levees
 Broad, low ridges of fine alluvium built along both sides of stream channel,
built up by natural events over a long period
 Heightened artificially by earth dykes to protect property in floodplain
4.3.2 Artificial Levees
 Slopes are steeper than natural levees. Built by piling earth on level surface,
broadbased and tapered top. e.g. Mississippi River, Sacramento, Danube.
 Can worsen flooding by depositing sediment on floor, which would
otherwise have been deposited onto floodplains, so floor is built up,
channel capacity decreases.
 Increase height of levees or dredge up the ground.
 May fail, like the Mississippi in 1993
4.3.3 Dams
 Flood control damns store floodwater, releasing it slowly, spreading out the
flood over a longer time. Can also be used for irrigation, hydroelectricity,
recreation
 Can cause silting behind dam
 Barrier to migration of aquatic life
 Inundation and loss of land space behind dam
 Dam failure (Teton Dam, Idaho. St. Francis Dam in the San Francisquito)
 Thermal stratification – heating of top, stagnant surface, changing
environmental conditions for aquatic life
4.3.4 Channelisation
 Enlarges cross sectional area, allowing more discharge to be held.
Straightening and shortening the stream increases gradient and velocity.
Smoothness increase velocity, leading to more efficient transport of water,
such as Mississippi
 Preventing river from re-meandering is difficult
4.3.5 Floodways
 Areas that act as an outlet to a stream during flooding
 Land between Mississippi and Lake Ponchartrain is used as a floodway
when River peaks. Spillway is opened to allow for water to drain, lowering
level of water in the river, reducing chance of levee failure
4.3.6 Non-structural Approach
 Floodplain zoning – laws restricting construction and habitation of
floodplains. Can be zoned for agricultural or recreational use
 Building codes – structures allowed within floodplain should be able to
withstand velocity of waters, high enough to reduce risk of water damage
 Buyout programs – cost effective for governments to buy rights to land
rather than pay reconstruction costs every time river floods
 Mortgage limitations – refuse to loan to those who want to build houses or
businesses in prone areas
 Catchment management – holistic system of managing different land uses
within catchment, to assess flood risk, improve water quality and land use
in catchment (UK, 8 catchment plans covering England and Wales)
4.4 Responses
 Phases of response efforts: search and rescue, immediate relief, reconstruction and
recovery, long term redevelopment
 Issues occurring during emergency response: Civil disturbance. Looting and violence
was widespread after Hurricane Katrina, while police was busy with search and
rescue. Curfew was imposed, National Guard brought in.
 Evacuation and shelter. Many evacuated, leaving poor and old behind – possible
inequity damage. Conditions in shelter was squalid and provisions were insufficient
 Health effects. Prolonged flooding could have led to dehydration, food poisoning,
hepatitis A, cholera, tuberculosis, typhoid fever, due to contamination of food and
water supplies
 Long term commitment. Immediate global assistance dwindled as media coverage
gets lesser, hampering the process of recovery. Should have long term assistance by
relief organisations.
Catchment Management
1. Introduction to Catchment Management
 Extensive manipulation of water by humans recently – dams, groundwater schemes,
sewage disposal and irrigation
 13% of river flow is controlled by mankind
 Key issues: provision of enough water to meet demands of a growing population, impact
of water developments on the environment, and problems with climate change and
unreliability of water sources
 Water is a limited resource: critical shortage of water in USA by national water survey.
Water pollution pose problems for quantity and quality of water supplies
 Conflicts of interest: transboundary rivers and catchments. International collaboration
(Canada and USA for Columbia River) is possible, but there are often conflicts over rivers
and groundwater
 Politicising of water resources – possible source of tension in Middle East
 Scenarios: international drainage basins where upstream states have control over
resources vital for downstream countries. International aquifers, where pumping drains
resources from neighbouring states. Contrasts in water endowments between
neighbouring countries
 Potential for armed conflict, since no international law officially governs such situations
2. Reasons for Water Conflict
 Water is a limited resource where demand exceeds supply. Supply is usually limited
because the region is arid (e.g. Nile, Jordon and Colorado) and there is no other major
surface water such as lakes and rivers. Demand is high due to water needed for various
purposes such as agriculture (where potential evapotranspiration is very high in arid
regions) and industries that is water intensive.
 Rivers are often transboundary, inevitably leading to conflicts between upstream and
downstream states regarding water quality and quantity. Downstream countries will
suffer if there is pollution downstream since water quality usually deteriorates with
increase distance downstream if waste is discharged into rivers untreated or
insufficiently treated (e.g. Danube). Downstream countries will also be deprived of their
“due share” of water if there is excessive withdrawal upstream. Some downstream
countries do not get any water at all when all the water was removed upstream (e.g.
Mexico and Colorado).
 When there is “unfair” distribution or sharing of the limited water resource conflicts is
also inevitable. The unequal sharing can be the result of Agreements/Treaties in favour
of one party (e.g. Egypt/Nile) or the military might of a party (e.g. Jordan/Israel).
3. Case Studies
3.1 Problems of Major Rivers
 Yellow River in China
 Hoover Damn in Colorado: upstream vs. downstream
 Danube: transboundary through Bulgaria, Romania, Yugoslavia, Hungary,
Czechoslovakia, Austria, Germany, Switzerland. Anyone can pollute the river. Forty
years of multinational talks about cleaning the river up
 Nile River: the dam and Lake Nasser. The unreplenished delta is sinking.
3.2 The Aral Sea
 Inland lake bisecting Kazakhstan and Uzbekistan. Nikita Khrushchev diverted water
from the two main rivers feeding the sea to farm cotton, but inefficiencies led to
failure of watertightness of the channels, water logging the ground and making it
salty
 80% of water was lost to evaporation and seepage. The Aral Sea shrank, increasing
salinity. Today, half has been lost, affecting cotton farming and commercial fishing.
Wind blows contaminated, salty dust, affecting health problems. 1 in 10 babies die
before turning one. Malformation and anaemia problems.
3.3 River Nile
 The Nile is 6600km long and flows through 10 countries. Two tributaries, the Blue
Nile from Lake Tana in Ethiopia, and the White Nile from Lake Victoria in Uganda
 The British aimed to control the Nile. After securing Lake Victoria, a dam was built
with cooperation of Britain, Uganda and Kenya in 1954, generating electricity and
controlling flow of water
 Britain annexed Sudan in 1898 in a conflict over Nile water to alter the course of the
river in southern Sudan by avoiding the Sudd swamplands. Excavation started in
1970s, but stopped due to guerrilla attacks
 The Blue Nile supplies more than 80% of Nile water. Fertile silt is brought down by
its waters. Diplomatic agreements formed by Britain, Egypt and Ethiopia in 1902
made Ethiopia promise not to take any water from the Blue Nile – unfeasible.
Whether this agreement should still remain binding is disputed.
 Egypt recognised the USSR and China, leading to Britain and US refusing to help fund
the Aswan Dam. Gamal Nasser nationalised the Suez Canal, leading to the Suez Crisis
to regain British sovereignty.
 Aswan Dam allowed electrification of Egypt and desert was cultivated. Agreement In
1959 gave Egypt the right to more than two-thirds of Nile water, and Sudan
accepted on the condition that they were allowed to share all the water. But
Uganda, Kenya and Ethiopia are still being sidelined – possible future clashes
3.4 River Jordan
 ¼ of the Arab world has no surface water. Population could increase by 34 mil in
next 30 years, and water resources are shared between Arab and non-Arab nations
 Jordan and Yarmuk Rivers were tapped by Israel to water the desert. National Water
Carrier system of canals transport water from Sea of Galilee to Negev Desert
 Jordan only covers 200km – very limited amount of water
 Israel uses a large amount of water. 1964 Arab Summit proposed to stifle Israel
 Syria constructs the Headwaters Diversion Plan to prevent Jordan River reaching
Israel. Israel responded by attacking the Plan’s sites
 Six Day War gave Israel control over Gaza Strip, West Bank and Golan Heights, and
the Yarkon-Taninim Aquifer
 Imbalance of water resources between countries: Israel has 8 times that of
Palestinians living in the West Bank and the Gaza Strip. Farmers lack good water –
groundwater drops 15-20cm each year, resulting in saltwater intrusion, spoiling crop
 1994 peace treaty with Jordan ensuring more equitable distribution
 Palestinians’ water still controlled by Israel – require permission for drilling in West
Bank, and wells cannot be deeper.
 Kibbutz – deserts bloom. 70% of water used for agriculture, 20% of electricity used
to pump water from Sea of Galilee. Underground aquifers drained in decades.
3.5 River Colorado and Las Vegas
 Phoenix, Arizona – an average family uses over 1 million litres a day
 $4 billion conduit – Central Arizona Project – transports water from Colorado River,
after the damming by the Hoover Dam
 Regulated to the max – amount permissible is under treaty, but demand is
increasing, thus turning to groundwater, which is depleting quickly
 Deprives Mexico of its water as well.
 How will reductions be affected? Agriculture? Who is afforded priority?
 Programmes to change attitudes: school awareness programmes, using low-wateruse plants for landscaping
3.6 Three Gorges Dam
 Controls the Yangtze River in China. Costs 28 billion dollars.
 In August 1998, China was devastated by floods from the Yangtze. The dam was
built to control such floods by controlling the release of water
 Many risks and hard work involved in building the dam – dynamite, excavation
 Main Yangtze is polluted – carbon emissions from coal and acid rain. Leading cause
of death is heart disease caused by such pollution
 Dam generates 18.2 million kilowatts of electricity
 Flooding upstream allows for goods to be shipped directly to Chongqing, increasing
shipping volume by 5 times
 Communities along river bank have to be evacuated – about a million people, such
as people from Fongdu and Fuling – sense of community lost
 Fertile banks will be flooded. Silting may cause flooding upstream, dam failure may
cause possible floods downstream
Channel Morphology
The study of channel pattern and geometry at points along a river channel, including tributaries. River’s
ability to perform geomorphological work (erosion and transportation) is determined by energy.
1. Generation and Dissipation of River Energy
1.1 Energy Generation
 Stored potential energy as a result of high position – sun evaporates water enabling
it to be deposited at higher level. Energy converted to kinetic energy, allowing rivers
to erode and transport load
 Amount of kinetic energy determined by volume and velocity of flowing water
 Energy possessed determined by discharge (volume x velocity)
 Velocity variation is more important – x2 velocity = x4 increase in energy, so large
rivers are exponentially more powerful than smaller rivers
1.2 Energy Dissipation
 Energy used up when river erodes channel, transports load and experiences friction
(both along river surfaces and between threads of water, turbulent flow of eddies)
 About 95% of energy is used to overcome friction, leaving the rest for fluvial
processes (carrying capacity)
 Turbulence is important in created upward motion to lift and support sediment to
aid in erosion and transportation
2. Factors Affecting River Discharge/Energy
 River discharge (Q) in m3/s = Cross sectional area (A) in m2 x River velocity (V) m/s
2.1 Volume of Water and River Energy
 Increase in amount of water = higher discharge = more efficient river
 Humid tropical and temperate regions – volume of water increase downstream due
to tributary contribution, leading to more energy downstream: increased erosion
and transportation of load
 Arid regions with permeable channels – volume decrease downstream due to high
evaporation and seepage. Decrease in energy downstream, less efficient
2.2 Velocity and River Energy – Manning’s Equation
 V = R2/3 x S1/2 / n
 Velocity (V), Channel Slope (S), Hydraulic Radius (R), roughness coefficient (n)
2.2.1 Channel Slope
 Change in gradient of river will affect amount of energy – steeper the
gradient, higher the velocity (^R = ^V)
2.2.2 Coefficient of Roughness
 Higher the n, lower the velocity, due to increased friction
 Downstream, river is smoother as it is more likely to be made of
clay/silt/sand instead of rocks and boulders (largely due to erosion of load)
2.2.3 Hydraulic Radius
 Ratio between area of cross-section and length of wetted perimeter
 Higher hydraulic radius means less water in contact with bed and banks,
decreasing friction increasing energy
 Channels made of silt and clay are deeper and narrower than coarser
materials as they are cohesive, promoting bank stability
 Shape of ideal channel is semicircle, to reduce restrictions on stream
velocity
3. Downstream Variation in Stream Velocity
 On average:
 S decreases downstream due to being in the lower course = lower V
 R increases downstream due to increase in channel width and depth = higher V
 n decreases downstream due to smoother materials = higher V
 On average, velocity increases/remains constant downstream despite gradient drop
 Water flows more efficiently in larger channels, more energy available to transport load
(carrying capacity increased), so lower gradient is fine
4. Urbanisation and Effects on Stream Velocity
 Urban drainage systems are straight, smooth and semi-circular to increase R and reduce
n, leading to very high flow velocity to rapidly clear water
Fluvial Processes
1. River Erosion
 Allows a river to deepen, widen and lengthen its channels. Erosion processes vary in
different parts of the channel and in different types of channels
1.1 Erosion Processes
1.1.1 Abrasion/Corrasion
 Common in upstream regions and rock-cut channels
 Coarse, angular fragments of rock are dragged and rolled along channel
floor especially during floods, rubbing and wearing away rock outcrops
 Responsible for downcutting, deepening channels
 In rivers with strong eddy motions, pothole drilling can occur, where
pebbles are trapped in hollows, generating localised erosion, creating
smooth depressions (potholes) in the bedrock
1.1.2 Hydraulic Action
 More common in middle-lower course and in alluvial channels (semi
coherent sand/clay/silt)
 Sheer force of flowing water dislodges particles of unconsolidated material
 Bank collapse (at concave banks of meanders), lateral erosion is more
significant
 Cavitation may occur when extreme turbulence occurs. Bubbles in water
collapse, resultant shock waves weaken river banks and lateral erosion
1.1.3 Attrition
 Wearing away of suspended and bedload as fragments collide against each
other
 Particles become more rounded and decrease in calibre downstream
1.1.4 Solution
 Occurs in dissolvable rocks, such as limestone, due to carbonic acids in
rainwater, along with humic acids from plants
 Wide range of rocks susceptible. Independent of river discharge/velocity
 Dissolved load in rivers come mainly from ions in groundwater
1.2 Components of River Erosion
1.2.1 Vertical Downcutting
 Characteristic of fast rivers with a lot of coarse bedload. High velocity of
flow abrades and potholes the channel floor, lowering the river bed,
forming a rock-walled gorge
 Rate of downcutting may increase if there is river rejuvenation (rise in land
or fall in sea level), causing downcutting to the base level of erosion,
forming deep gorges or deep, narrow V shaped valleys
1.2.2 Lateral Erosion
 Occurs when river meanders. When river swings and attacks concave banks,
erosion is concentrated where velocity is the highest, resulting in retreat of
concave banks
1.2.3 Headward Erosion
 Active at the head of the river or where the river is locally steep
 First case: like rivers emerging from underground streams in limestone
areas, erosion will extend valley headwards
 Second case: like in waterfalls, where lateral erosion occurs at the bottom.
The oversteepened bank collapses, resulting in headward erosion
2. River Transport
2.1 Transportation Processes
 Bedload is transported by either traction or saltation
 Large rock fragments roll along the stream bed, called traction. Most important at
source of stream, where steep valleys deliver coarse debris to river channels
 Smaller rocks may be transported by saltation, bouncing along the bed of the river
due to turbulence
 Suspended load is transported by suspension, where particles are small enough to
be constantly held up by turbulence. Suspended load normally forms the greatest
proportion of total load, increasing towards the river mouth. Size and amount of
load able to be suspended increases with increasing velocity
 Dissolved load is transported in solution, which largely comes from underground
 Proportion of bed and suspended load fluctuates with velocity
2.2 Hjulstrom Curve
 Particles 0.5mm in size have the lowest competent velocity i.e. the velocity which a
particle of a certain size requires to be eroded or entrained. Smaller particles like
clay are cohesive and bonded, requiring higher velocity. Larger particles have higher
competent velocities due to being heavier.
 Greater the particle size, the greater the velocity required to transport it, so bigger
particles have a higher settling velocity (velocity at which a particle is deposited)
 Velocity maintaining particles in suspension is less than the velocity required to
entrain them. For fine clays, competent velocity is high but settling velocity is almost
zero – a very large drop in velocity is required to deposit it. The difference for coarse
particles is smaller – a smaller drop in velocity is sufficient to deposit
2.3 Velocity and River Transportation
 River velocity affects river capacity and river competence
 River capacity: total volume of sediment a river is able to carry. Varies with the third
power of river velocity
 River competence: the heaviest load a river is able to carry. Varies with the sixth
power of river velocity
 Nature of sediment load transported is also affected by geology and climate
2.4 Downstream Changes in Sediment Load
 Amount of sediment increases downstream due to contribution by tributaries,
erosion of channels and continual feeding of sediment from valley sides
 Sediment tend to be rounder and of finer calibre downstream due to attrition and
gentler finer calibre valley side slopes
3. River Deposition
 Occurs when rivers competence or capacity is lowered, either when there is an input of
load causing river to overload, or when there is a sudden loss of energy either due to
decreased velocity or discharge
3.1 Features Associated with Depostion
3.1.1 Alluvial Fans
 Upland with steep valleys, tributaries flowing in valleys flow along very
steep gradients, carrying lots of load. Upon reaching the plain, velocity and
energy sharply decrease, depositing load, which may result in an alluvial fan
– a cone-shaped mass of alluvium with apex at the point between highlands
and the plain
3.1.2 Point Bars and Flood Plains (Lateral Accretion)
 In meanders, lateral erosion occurs along concave banks. Some sediment is
transported to convex banks to form point bars (helicoidal flow). Concave
banks retreat while convex banks advance, accumulating alluvium. Flood
plain can be created when point bars undergo lateral accretion
3.1.3 Flood Plains (Vertical Accretion)
 When river overflows its banks, the floodwater containing sediment
decreases greatly in velocity due to increased wetted perimeter, depositing
silt and clay on the floodplains
 May form natural levees, as deposition occurs starting with the largest
particles. Coarsest particles will be deposited just beyond the banks, can
build up over repeated flooding
Channel Plan Forms
Generally, there are straight, meandering and braided channels – straight channels are rare, only
occurring when a river flows down steep slopes or when it is strongly influenced by joints or faults.
1. River Meanders
1.1 Sinuosity Ratio
 Ratio between distance along centre line and distance of entire channel i.e. how far
the channel deviates from a straight line. A river is meandering only when the ratio
exceeds 1:1.5
1.2 Geometric Features of Meanders
 Meanders are usually symmetrical and forms are relatively consistent. Wavelength
of a meander is about 7-10 times channel width
 Features of a meander: pools and riffles, point bars, river bluffs, cross over point,
meander wavelength and meander amplitude
1.3 Reasons for Meander Development
 Maybe the stream needs to lose energy due to surplus energy, so meandering is a
method of expending energy to do work
1.4 Meander Formation
 Meanders develop due to constant erosion and deposition, and seem to begin with
development of pools and riffles in channels
 Riffles are regularly spaced bars of coarser sediment on the river bed, where the
river is shallower and more symmetrical
 Pools occur between the riffles where sediment is finer, the river is deeper and
more asymmetrical
 The spacing of the pool-and-riffle sequence is related to size of channel – distance
from one riffle to the next is roughly 5-7 times channel width
 Riffles tend to slope alternately towards opposite sides so that the thalweg (line
tracing deepest water of greatest velocity) winds between the riffles, deflecting
between alternate banks
 Where deflection occurs, concentrated bank erosion occurs due to hydraulic action,
resulting in a retreating concave bank and retreating river bluffs
 Helicoidal flow drags sediment across the river bed to the other side. Energy lost in
erosion and friction causes sediment to be deposited at the convex bank to form
point bars
 When river becomes too sinuous, cutting of meander necks results in oxbow lakes
1.5 Meander Movement
 Extension, translation, rotation, enlargement, lateral movement, complex change
2. Braided Rivers
 Main characteristic is subdivision of water flow along anabranches separated by midchannel bars. Highly active but still rather stable. Individual channels may be
abandoned, buried or eroded but the overall pattern remains
2.1 Main Features of a Braided Channel
 Banks are often made of incoherent materials such as sand and gravel, thus
experiencing largely lateral erosion and widening the channel, resulting in an
inefficient channel with high width-depth ratio and larger wetted perimeter
 River flow is unstable or seasonal. Fluctuating discharge is necessary for the
formation of mid-channel bars by allowing time for erosion and deposition. Braiding
is therefore more common in semi-arid or temperate regions prone to irregular
downpours or seasonal melting
 Braided rivers tend to have coarser bedload, which are deposited during low
discharge to form mid-channel bars. In colder regions, for example, freeze-thaw
weathering supplies coarse debris to rivers
 Low elongated unvegetated bars of sand and gravel and vegetated islands above
water level, which are more stable, permanent and withstand erosion better
2.2 Formation of Braided Channels
 During high discharge, large amounts of sediment are entrained due to energy
increase. Banks are also eroded, widening the channel
 During lower discharge, energy decreases and the river will deposit load to form mid
channel bars. Coarse bedload forms the core, whereby sediment accumulates and
the mid channel bars grow
 Midchannel bars further constrict water flow around them, localising river flow to
increase velocity, eroding banks further. As discharge falls and banks widen, water
level decreases to expose the bars
 Some mid channel bars will be washed away, but some will be colonised by
vegetation, turning them more stable as plants help to trap sediment, eventually
becoming islands. Other looser bars may be eroded during next high discharge
season
3. Comparison of Meandering and Braided Channels
Stream Power and
Flow Velocity
Sediment amount
and Size
Proportion of Bed to
Suspended Load
Width-depth Ratio
Channel Gradient
Channel Stability
Braided River
Higher on average, during floods
Meandering River
Lower on average
Greater and larger – coarser, more
bedload, due to being mainly in
upper courses with steep slopes and
freeze thaw
More bedload – coarse material from
upper course used to form mid
channel bars
Higher – wide and shallow due to
bank instability (incoherent material)
and constant lateral erosion
Higher – steeper due to being in
upper courses. R is low, S increases
to compensate for inefficiency
Generally more stable – mid channel
bars experience erosion, but overall
channel remains same
Lesser and smaller – finer. Middle
and lower courses with channels
made of finer material
More suspended load – load is of
finer calibre, along with high energy
downstream
Lower – balanced, made of more
cohesive material and withstand
more lateral erosion
Lower – due to being in middle lower
courses, gentler
Overall less stable – meanders
constantly change, oxbow lakes etc.
Drainage Basin Analysis
Useful techniques for analysing drainage systems. Quantitative analysis enables relationships between
different aspects of drainage pattern of the same basin to be formulated as general laws
1. Stream Order Analysis
1.1 Strahler’s Method
 Smallest tributaries are first order streams. When two streams of the same order
meet, they increase by one order. When a stream of lower order joins one of higher
order, there is no change in order
 The trunk stream of the basin is therefore the highest order
1.2 Strengths and Weaknesses of Strahler’s Method
 Simple and easily applied – widely used nowadays
 Order number does not reflect relationship with size and capacity, which is a
limitation since stream ordering should provide a scale and indicate discharge
1.3 Law of Stream Number
 Law of stream number: inverse geometric relationship between stream order and
stream number: it is likely that there are many first order streams and
logarithmically fewer higher order streams
 Law of stream length: higher order streams are likely to be longer
 Law of basin areas: higher the stream order, greater the mean drainage basin area
1.4 Bifurcation Ratio
 Dividing number of streams in one order by the number in the next order, then
taking the average of all figures
 Ratio will be low for branching rivers and high for simpler patterns
 For low ratios, a sharper peak is likely, for high ratios, a gentler peak is typical.
However, link between bifurcation ratio and lag time is not concrete
2. Drainage Density
 Measure of the frequency and spacing of stream within a basin, reflecting to some
extent the amount of runoff a basin generates since channel capacity needs to be
sufficient to cope with normal discharge from precipitation
2.1 Drainage Density Calculation
 Dd is expressed in km/km2, total stream length over total basin area
 Allows comparisons to be made, such as between wet and arid regions, or between
permeable and impermeable basins. Normally from 5km/km2 on permeable
sandstone, to about 500km/km2 on unvegetated clay
2.2 Problems Associated with Drainage Density Calculation
 With distinct wet and dry seasonal areas, surface drainage may be intermittent
streams. Dd for wet and dry seasons will be different – higher for wet.
 In areas with permeable rocks like limestone, calculated Dd will be low because
underground streams are not taken into account – valley density may be useful
2.3 Factors Controlling Drainage Density
 Time – originally, drainage network may be open and spaced (lower Dd), but over
time creation of tributaries leads to higher Dd over the same area
 Rock Type – impermeable rocks tend to have greater overland flow, increasing Dd.
On the other hand, permeable rocks have lower Dd since most water percolates
downwards
 Annual Precipitation/Rainfall Intensity – Higher annual precipitation and high
intensity may result in more discharge and overland flow, increasing Dd
 Vegetation – denser vegetation results in greater infiltration, reducing Dd
 Relief – steeper slopes generated more runoff, increasing Dd
 Infiltration Capacity – permeable soil has lower Dd
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