hydrologic nrotes

8/13/2019 Hydrologic Nrotes

1/23

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. Midlatitude areas normally have moderate cyclonic or frontal rainfall. Polar regions have lowprecipitation 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 summerseasons.

Orographic rainfall: air mass rising above a land barrier, such as mountains, withmoisture deposited on the windward side, with the leeward side having much less

Frontal: warm air mass rises after encountering a colder, denser mass. Warm frontshave less turbulence and precipitation, while cold fronts have heavier storms

Snow, sleet and hail are less common forms at higher latitudes1.2 Intensity of Precipitation

Humid temperate: low intensity of about 0.5-4 mm/hr. Warm fronts, light rain overa prolonged period


2/23

Tropical: high intensity, up to 100-150 mm/hr. High temperatures, rapid evaporationlead 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 downbranches and trunks as stemflow, both delivering water to litter layer. Some isstored as litter interception while rest infiltrates the soil.

Only part of total rainfall reaches soil while rest is lost as interception loss2.2 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 water3.2 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 watersurface and atmosphere. Higher temperature, lower relative humidity, increasedvapour 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


3/23

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 wetareas 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 freelydrained soil can store

When Potential Evapotranspiration > Precipitation, soil moisture withdrawal occurs,reducing moisture below field capacity. Occasionally, a moisture deficit developswhen actual evapotranspiration falls below potential evapotranspiration

When water is extracted by plants, water is drawn from finer pores and nearersurface of soil particles. When matric force exceeds the ability of plants to absorbwater, 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 rechargeoccurs, 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 ismore favourable to plant growth

Sandy soil has very low capillary action due to having very little surface area on eachsoil particle. Many small pores increases pore volume, allowing for greatergravitational draining

Clayey soil is platy and has high surface area, increasing capillary action, and clayparticles further expand with more water contact. However, this plate structurereduces 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 direction2.1 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

capacity2.1.1 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


4/23

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

infiltration2.1.5 Terracing

Increasing time water in retained on slopes, increasing infiltration2.1.6 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 concrete2.2 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/hr3. Throughflow

Lateral, downslope flow of water underground, eventually emerging as small springs orseepages, contributing to surface runoff

More irregular and slower than overland flow, takes very long to reach rivers, due toflow through small pores fissures

Generated with decreasing permeability with increasing soil depth due to lowerpermeability of underlying parent bedrock, possibility of containing a clay pan due towashing down of fine materials by water, compaction due to weight of soil above

Water is forced to drain laterally downslope, occasionally forming underground pipes inthe soil so flow is concentrated along well defined percolines, increasing speed ofthroughflow

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 Flow4.1.1 Sheet Flow


5/23

Sheet flow/unconcentrated wash is not confined to channels. Occurs onupper 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 devegetation4.2 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 tooccur at capacity rate. Excess water accumulates on soil surface, initiallyoccupying small irregularities called depression storage

Depression storage quickly overflows to form sheet of water down theslope. 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 andlesser 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 increasein 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 areaswhere capacity is low, and agricultural lands where soil has beencompacted 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 lesspermeable layers, generating Throughflow

Soil at base of flow becomes saturated, saturated zone giving rise to higher watertable, extending upslope

Forms overland flow by return flow and direct precipitation onto saturated ground


6/23

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 storage2. Type of River Channels

Perennial channels are occupied by flowing water throughout the year, most common inhumid tropics, where water table intersects the channel all year round

Intermittent channels are seasonally occupied by water, found in areas with strongseasonal 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 occupiedafter a storm, due to water table being very far down, and it takes time for water toinfiltrate. 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 ofoverland 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, riverdischarge 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 stopped3.2 Factors Influencing the Forms of Storm Hydrographs

Differences in rate of increase of discharge and recession3.2.1 Location of Rainstorm

If storm is located at upper part of basin, peak discharge generated takestime to pass down main channel. Gauging stations located downstream willhave longer lag times, as well as a dampened or lesser pronounced peakprogressively


7/23

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 flow3.2.3 Basin Size, Shape and Relief

Bigger basins have longer lag times due to longer distance of flow. Peak maybe higher if more precipitation is captured

Longer basins have longer lag time and lower peak, as same amount ofdischarge is spread over a longer time

Steeper-sided valleys of basins will have higher peaks and shorter lag timesdue 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 times3.2.5 Basin Geology

Permeable rocks and soil give hydrographs with low peaks and long lagtimes, 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 lagtimes 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 glacier4. Annual Hydrographs

River regime, the fluctuations of rivers discharge over a year, is climate dependent dueto seasonal fluctuations

Britain difference in discharge from winter to summer, reflecting differences in

precipitation amount and evapotranspiration loss. For River Tees, in late summerdischarge 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 notinteract with channel flow. River Wye, made of permeable carboniferous limestone, hasincreased infiltration and percolation, having more baseflow, with groundwaterinteracting to regulate the stream flow, slowing response of river to rainfall, reducingshort term fluctuations


8/23

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 (trappedgas bubbles in volcanic rocks)

Permeability is capacity of a rock to permit ready transmission of water into and throughrock. Primary is natural pore spaces, while secondary is through fractures

Permeability depends on size of voids, while porosity is total volume of voids. Shale ishighly 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 Table2.1.1 Surface Topography

Water tables have gradients similar to surface relief. In flat areas, the tablewill be relatively flat, but in hilly areas, it rises and falls with the land, due toreplenishing of water by precipitation. If rainwater stopped, water table willbe 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 perchedwater 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 rises2.2.1 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 andprecipitation recharge of aquifers, lowering water table. In winter,precipitation exceeds evapotranspiration, recharging the water table, evenintersecting valley floors, producing the intermittent streams

Zone of intermittent saturation within this zone the water table rises andfalls 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 deeperin arid areas


9/23

Saudi Arabia limestone and sandstone aquifers contain water far belowground level. Not recharged by present day rainfall fossil groundwateraccumulated during pluvial periods of the Quaternary. If extracted, water isnot replaced water table falls

Sahel long term changes in water table results from extraction. Since norecharge 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 Pollution4.1 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 recharge4.2 Groundwater Pollution

Increase in population, urbanisation and industrialisation can pollute surface andunderground 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 intowells, and must be abandoned

Fresh groundwater floats on sea water due to being less dense lens with convexfaces

Depth of fresh water below sea level is 40 times elevation of water table above sealevel

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 byevapotranspiration (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 = highevapotranspiration. Runoff is high, perennial streams. Storages are also plentiful.


10/23

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. Lowwater storages on surface, in soil and underground. Minimal runoff due to very lowwater 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 duringmonsoons

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 ofintense 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 reduceinfiltration capacity, increasing stormflow in relation to baseflow.

2. Flood Prediction and Flood Forecasting2.1 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


11/23

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 reliable2.2 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 alongthe 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 Q pk = 0.278 CIA

Q pk is peak rate of discharge (m 3/s), C is rational runoff coefficient, I israinfall intensity (mm) and A is drainage area (km 2)

C is the index of soil type, topography, roughness, vegetation and basin landuse

System works ideally for catchments of less than 0.8km 2. Works best forurban and suburban areas with high runoff, somewhat steep channels,limited channel storage and no lakes

Assumes that generation process is Horton flow with whole catchmentcontributing

Assumes uniform precipitation over entire basin, precipitation does not

vary with time/space, there is little catchment storage, does not vary withstorm intensity or antecedent soil moisture

3. Case Studies3.1 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 occursduring monsoon at beginning and end of year

Topography Bukit Timah Granite and Jurong Formation are flood prone due tohaving steep sided valley walls concentrating flood water on low valley floors

Recent Developments rapid urbanisation, impermeable surfaces and lined drainchannels have increased to 48.6% in 1988, removing vegetation and reducinginterception storage and infiltration capacity, leading to more overland flow andhigher flood propensity. Storm drains carry rainwater efficiently, making channelsexceed 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 largepart of Bangladesh


12/23

Deforestation Severe deforestation across catchments, especially in Nepal, isaccelerating runoff and increasing erosion rates. Deforestation makes sedimentsupply to channels increase as rainsplash on bare slopes washes off soil, loadincreases, silting up channels, raising channel beds, worsening floods, but degree towhich it worsens is debated

Coincidence of flood peaks The timing of rains and snowmelt vary between thethree catchment basins, but in 1988 the peaks coincided, worsening flooding

4. Management of Floods4.1 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

hazards4.1.2 Secondary and Tertiary Effects

Secondary: long term as a result of primary effects, Tertiary: very long termchanges. 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, transportdisruptions

Disease water borne diseases such as cholera, made worse by deadbodies festering

Tertiary: Location of river channels may change as result, leaving oldchannels dry. Sediment may destroy farm land. Loss of jobs. Insurance ratesmay increase, corruption from misuse of funds, destruction of wildlifehabitat

4.2 Prediction4.2.1 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 statisticalmethod calculating probability

Long term changes may also be taking place i.e. not ceteris paribus, due tomodification of drainage basin such as deforestation, urbanisation andagriculture

Requires long periods of data accuracy of small sample questionable

4.2.2 Forecasting and Limitations Many unrealistic assumptions of the rational runoff method


13/23

Regardless, forecasts can allow flood warnings to be issued in time, butflash 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 life4.3.4 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 difficult4.3.5 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, loweringlevel of water in the river, reducing chance of levee failure

4.3.6 Non-structural Approach

Floodplain zoning laws restricting construction and habitation offloodplains. Can be zoned for agricultural or recreational use


14/23

Building codes structures allowed within floodplain should be able towithstand velocity of waters, high enough to reduce risk of water damage

Buyout programs cost effective for governments to buy rights to landrather than pay reconstruction costs every time river floods

Mortgage limitations refuse to loan to those who want to build houses orbusinesses in prone areas

Catchment management holistic system of managing different land useswithin catchment, to assess flood risk, improve water quality and land usein 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 andrescue. Curfew was imposed, National Guard brought in.

Evacuation and shelter. Many evacuated, leaving poor and old behind possibleinequity 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 andwater supplies

Long term commitment. Immediate global assistance dwindled as media coveragegets lesser, hampering the process of recovery. Should have long term assistance byrelief 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 andunreliability 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 riversand 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 drainsresources from neighbouring states. Contrasts in water endowments betweenneighbouring countries

Potential for armed conflict, since no international law officially governs such situations2. Reasons for Water Conflict

Water is a limited resource where demand exceeds supply. Supply is usually limitedbecause the region is arid (e.g. Nile, Jordon and Colorado) and there is no other majorsurface water such as lakes and rivers. Demand is high due to water needed for various


15/23

purposes such as agriculture (where potential evapotranspiration is very high in aridregions) and industries that is water intensive.

Rivers are often transboundary, inevitably leading to conflicts between upstream anddownstream states regarding water quality and quantity. Downstream countries willsuffer if there is pollution downstream since water quality usually deteriorates with

increase distance downstream if waste is discharged into rivers untreated orinsufficiently treated (e.g. Danube). Downstream countries will also be deprived of theirdue share of water i f there is excessive withdrawal upstream. Some downstreamcountries 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 isalso inevitable. The unequal sharing can be the result of Agreements/Treaties in favourof one party (e.g. Egypt/Nile) or the military might of a party (e.g. Jordan/Israel).

3. Case Studies3.1 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. Fortyyears 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 waterfrom the two main rivers feeding the sea to farm cotton, but inefficiencies led tofailure 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 diebefore 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 in1970s, but stopped due to guerrilla attacks

The Blue Nile supplies more than 80% of Nile water. Fertile silt is brought down byits waters. Diplomatic agreements formed by Britain, Egypt and Ethiopia in 1902made 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 fundthe Aswan Dam. Gamal Nasser nationalised the Suez Canal, leading to the Suez Crisis

to regain British sovereignty.


16/23

Aswan Dam allowed electrification of Egypt and desert was cultivated. Agreement In1959 gave Egypt the right to more than two-thirds of Nile water, and Sudanaccepted on the condition that they were allowed to share all the water. ButUganda, Kenya and Ethiopia are still being sidelined possible future clashes

3.4 River Jordan 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 Plans 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 ofPalestinians 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 stil l 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 isincreasing, 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-water-

use plants for landscaping3.6 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 wasbuilt 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 maycause possible floods downstream


17/23

Channel Morphology

The study of channel pattern and geometry at points along a river channel, inc luding tributaries. Riversability to perform geomorphological work (erosion and transportation) is determined by energy.

1. Generation and Dissipation of River Energy1.1 Energy Generation

Stored potential energy as a result of high position sun evaporates water enablingit to be deposited at higher level. Energy converted to kinetic energy, allowing riversto 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 rivers1.2 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 fluvialprocesses (carrying capacity)

Turbulence is important in created upward motion to lift and support sediment toaid in erosion and transportation

2. Factors Affecting River Discharge/Energy River discharge (Q) in m 3/s = Cross sectional area (A) in m 2 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 erosionand transportation of load

Arid regions with permeable channels volume decrease downstream due to highevaporation and seepage. Decrease in energy downstream, less efficient

2.2 Velocity and River Energy Mannings 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


18/23

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 fine4. Urbanisation and Effects on Stream Velocity

Urban drainage systems are straight, smooth and semi-circular to increase R and reducen, 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 channels1.1 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, creatingsmooth 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 erosion1.1.3 Attrition

Wearing away of suspended and bedload as fragments collide against eachother

Particles become more rounded and decrease in calibre downstream1.1.4 Solution

Occurs in dissolvable rocks, such as limestone, due to carbonic acids inrainwater, 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 Erosion1.2.1 Vertical Downcutting

Characteristic of fast rivers with a lot of coarse bedload. High velocity offlow abrades and potholes the channel floor, lowering the river bed,forming a rock-walled gorge


19/23

Rate of downcutting may increase if there is river rejuvenation (rise in landor 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 ofconcave 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 erosion2. 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 greatestproportion of total load, increasing towards the river mouth. Size and amount ofload 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 velocity2.2 Hjulstrom Curve

Particles 0.5mm in size have the lowest competent velocity i.e. the velocity which aparticle of a certain size requires to be eroded or entrained. Smaller particles likeclay are cohesive and bonded, requiring higher velocity. Larger particles have highercompetent velocities due to being heavier.

Greater the particle size, the greater the velocity required to transport it, so biggerparticles have a higher settling velocity (velocity at which a particle is deposited)

Velocity maintaining particles in suspension is less than the velocity required toentrain 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 coarseparticles 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


20/23

Sediment tend to be rounder and of finer calibre downstream due to attrition andgentler 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 discharge3.1 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 andenergy sharply decrease, depositing load, which may result in an alluvial fan

a cone-shaped mass of alluvium with apex at the point between highlandsand 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). Concavebanks retreat while convex banks advance, accumulating alluvium. Floodplain 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, depositingsilt and clay on the floodplains

May form natural levees, as deposition occurs starting with the largestparticles. Coarsest particles will be deposited just beyond the banks, canbuild up over repeated flooding

Channel Plan Forms

Generally, there are straight, meandering and braided channels straight channels are rare, onlyoccurring when a river flows down steep slopes or when it is strongly influenced by joints or faults.

1. River Meanders1.1 Sinuosity Ratio

Ratio between distance along centre line and distance of entire channel i.e. how farthe channel deviates from a straight line. A river is meandering only when the ratioexceeds 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 amplitude1.3 Reasons for Meander Development

Maybe the stream needs to lose energy due to surplus energy, so meandering is amethod 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


21/23

Riffles are regularly spaced bars of coarser sediment on the river bed, where theriver is shallower and more symmetrical

Pools occur between the riffles where sediment is finer, the river is deeper andmore asymmetrical

The spacing of the pool-and-riffle sequence is related to size of channel distancefrom one riffle to the next is roughly 5-7 times channel width

Riffles tend to slope alternately towards opposite sides so that the thalweg (linetracing deepest water of greatest velocity) winds between the riffles, deflectingbetween 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 inerosion and friction causes sediment to be deposited at the convex bank to formpoint bars

When river becomes too sinuous, cutting of meander necks results in oxbow lakes1.5 Meander Movement

Extension, translation, rotation, enlargement, lateral movement, complex change2. Braided Rivers

Main characteristic is subdivision of water flow along anabranches separated by mid-channel bars. Highly active but still rather stable. Individual channels may beabandoned, 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 aninefficient channel with high width-depth ratio and larger wetted perimeter

River flow is unstable or seasonal. Fluctuating discharge is necessary for theformation of mid-channel bars by allowing time for erosion and deposition. Braidingis therefore more common in semi-arid or temperate regions prone to irregulardownpours or seasonal melting

Braided rivers tend to have coarser bedload, which are deposited during lowdischarge to form mid-channel bars. In colder regions, for example, freeze-thawweathering supplies coarse debris to rivers

Low elongated unvegetated bars of sand and gravel and vegetated islands abovewater 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 energyincrease. Banks are also eroded, widening the channel

During lower discharge, energy decreases and the river will deposit load to form midchannel bars. Coarse bedload forms the core, whereby sediment accumulates andthe mid channel bars grow

Midchannel bars further constrict water flow around them, localising river flow toincrease velocity, eroding banks further. As discharge falls and banks widen, waterlevel decreases to expose the bars

Some mid channel bars will be washed away, but some will be colonised byvegetation, turning them more stable as plants help to trap sediment, eventually

becoming islands. Other looser bars may be eroded during next high dischargeseason


22/23

3. Comparison of Meandering and Braided Channels

Braided River Meandering RiverStream Power andFlow Velocity

Higher on average, during floods Lower on average

Sediment amountand Size

Greater and larger coarser, morebedload, due to being mainly inupper courses with steep slopes andfreeze thaw

Lesser and smaller finer. Middleand lower courses with channelsmade of finer material

Proportion of Bed toSuspended Load

More bedload coarse material fromupper course used to form midchannel bars

More suspended load load is offiner calibre, along with high energydownstream

Width-depth Ratio Higher wide and shallow due tobank instability (incoherent material)and constant lateral erosion

Lower balanced, made of morecohesive material and withstandmore lateral erosion

Channel Gradient Higher steeper due to being inupper courses. R is low, S increases

to compensate for inefficiency

Lower due to being in middle lowercourses, gentler

Channel Stability Generally more stable mid channelbars experience erosion, but overallchannel remains same

Overall less stable meandersconstantly change, oxbow lakes etc.

Drainage Basin Analysis

Useful techniques for analysing drainage systems. Quantitative analysis enables relationships betweendifferent aspects of drainage pattern of the same basin to be formulated as general laws

1. Stream Order Analysis1.1 Strahlers Method

Smallest tributaries are first order streams. When two streams of the same ordermeet, they increase by one order. When a stream of lower order joins one of higherorder, there is no change in order

The trunk stream of the basin is therefore the highest order1.2 Strengths and Weaknesses of Strahlers 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 discharge1.3 Law of Stream Number

Law of stream number: inverse geometric relationship between stream order andstream number: it is likely that there are many first order streams andlogarithmically 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


23/23

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 besufficient to cope with normal discharge from precipitation

2.1 Drainage Density Calculation Dd is expressed in km/km 2, 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/km 2 on permeablesandstone, to about 500km/km 2 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 useful2.3 Factors Controlling Drainage Density

Time originally, drainage network may be open and spaced (lower Dd), but overtime 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 percolatesdownwards

Annual Precipitation/Rainfall Intensity Higher annual precipitation and highintensity 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

hydrologic nrotes

Documents