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OVERLAND FLOW

Splash Erosion : Dispersal of fine particles

due to impact of rain drops Unconcentrated (Sheet) Wash : Thin layer of

water moving over slopes during rainfall.

Concentrated Wash : Flow of water on slopeduring rainfall in small & narrow channels(Rills) or broad & deep channels (Gullys).

Overland flow can detach & transport finegrained sediments (silt & clay sizes) onslopes.

Most overland flow reaches the foots ofslopes in valleys where streams are found.

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STREAMS & RIVERS

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STREAMS & RIVERS

Bodies of water that flow from highland tolowland areas along well defined channelslocated in valleys;

only difference being that streams carry

small volumes of water. Most flow to the sea or oceans, except for

those in inter-montane basins that oftenflow into lakes.

Found in all areas of the world, thoughhaving more active roles as agents oferosion & deposition in areas with humid

climates, particularly in the humid tropics.

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PERENIAL - Water in channels throughoutthe year. Groundwater table close to groundsurface and contributes to water flow duringdrier periods.

TEMPORARY - Water in channels duringperiods of rainfall. Groundwater table at

great depths and does not contribute towater flow. Also in cold climates, wherewater flow during summer.

INTERMITTENT - Water in channels during

periods of rainfall and some time after.Groundwater table relatively close to groundsurface - contributes to water flow followingtheir rise as a result of rainwater infiltration.

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DRAINAGE BASIN : Area influenced by a river& its tributary streams. Limited by drainage

divide.

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DRAINAGE BASIN : Area influenced by ariver & its tributary streams. Limited by the

drainage divide.

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LONGITUDINAL PROFILE

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STREAM CHANNEL

WIDTH (W) : Width of channel between river

banks

measured along the surface. DEPTH (D) : Difference in height between

surface of water and channel floor or bed.Depth usually variable along cross-sectionbetween river banks.

CROSS-SECTIONAL AREA (A) : Area ofcross-section between river banks -

measured perpendicular to the banks. WETTED PERIMETER (P) : That part of

the cross-section that is in direct contactwith the water.

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STREAM CHANNEL

CHANNEL GRADIENT ( ) : Angle betweenthe hoizontal and the water surface. Different parameters are dependent upon the

volume of water flowing in the channelstream.

Hydraulic Radius (r) = A/P Influencesvelocity of water.

Shape Ratio = D/W Due to frictional drag, maximum stream

velocity is at center of channel near thesurface and minimum near the channel floor(bed) and sides (banks).

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Variations inHydraulic

Radius

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STREAM DISCHARGE

Flow of water through a stream or riverchannel. Measured in cubic meters persecond.

Q = A x Vm = Wm X Dm x Vm

where Q is the discharge, A is thecross-sectional area, Vm is the meanvelocity, Wm is the average width andDm is the average depth of flow.

Stream discharge varies over both timeand space.

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Discharge normally increases downstreamas more water enters the stream channel

from overland flow and groundwater. Discharge also varies temporally because of

variations in inputs as precipitation (rainfall)and snow melt.

When discharge increases, there is arecorresponding changes in velocity, channeldepth & width within the stream system.

Of the three variables that change within thestream system with an increase in discharge,velocity is the least responsive.

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STREAM VELOCITY

Dynamics of stream flow primarily influenced by

friction, channel topography & channel shape.Three types flow can be observed:

LAMINAR FLOW Water flows as parallelmolecular streams and not altered in its direction.

TURBULENT FLOW - Water flows as discreteeddies & vortices due to channel topography andfriction.

HELICAL FLOW - Spiral flow in a stream due tochannel shape. Meandering channels cause this

type of flow. In three dimensions, there is a complex pattern

as the line of maximum velocity (THALWEG)moves from side to side within a channel.

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Line of maximumvelocity (THALWEG)

moves from side

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STREAM LOAD

Various materials are transported by streamsas they flow from highland areas towardslow-lying terrain.

Materials (or Sediments) brought intostreams not only by overland flow, but alsoby erosion of the channel sides (or banks)and floors (or stream beds).

Quantity of sediments varies temporally dueto changes in discharge.

Normally, as discharge and velocity increase,the amount of sediment being carried by thestream rises correspondingly.

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STREAM LOAD

When discharge increases, more sediment is

eroded from stream bed and stream banks. Streams generally transport three types of

material: BED LOAD Sand & pebbles which move

along stream bed without being permanentlysuspended in the flowing water SUSPENDED LOAD Silts & clays in

suspension DISSOLVED LOAD Material in solution. Absolute quantities and the relative

proportions of these types of stream loadvary from one stream to another, and within asingle stream from one time to another.

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STREAM LOAD

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STREAM EROSION

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DETACHMENT, TRANSPORT & DEPOSITIONOF PARTICLES BY STREAMS

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CRITICAL VELOCITY (cm/sec) FOR EROSION,TRANSPORTATION & DEPOSITION

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LONGITUDINAL PROFILE- Variations in gradient & velocity- Graded profile Equilibrium between erosion& deposition

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CHANNEL CHANGES ALONG PROFILE

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DELTA

(RIVERMOUTH)

Deposition ofsediments

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INCREASED DISCHARGE FLOODSCoarse sediments deposited close to channel

(Levees), while finer sediment depositedfurther away (Back-swamp deposits)

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DEVELOPMENT OFLEVEES

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SUMMARY

Streams drain land areas & transport

sediment from highland to lowland areas. In highland areas, down-cutting of channels

(linear erosion) is dominant, whilst inlowland areas, lateral movement of river

channels is dominant. Variations in velocity due to environmental

factors (especially rainfall), give rise todifferent sites of erosion or deposition (i.e.

different sizes of particles, etc). At river mouths, deposition predominates

giving rise to deltas.