External 6 Overland

7/30/2019 External 6 Overland

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Hydrological Cycle



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DISTRIBUTION OF WATER IN THE HYDROSPHERE

Reservoir TotalWater

(%)

TotalFresh

Water (%)

Total FreshWater

(Unfrozen) (%)

Oceans & Seas 97.54 - -

Snow & Ice 1.81 73.9 -

Groundwater 0.63 25.7 98.4

Lakes & Rivers -

Saline

0.007 - -

Lakes & Rivers -Fresh

0.009 0.36 1.40

Atmosphere 0.001 0.04 0.20



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SPLASHEROSION

• Only effective when therain falls with sufficient

intensity.• Soil particles detached

and moved shortdistances – effectssolely on-site.

• Although largequantities of soil may

be moved, it is allmerely redistributedback over the surface ofthe soil.



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SPLASH EROSION

• On steep slopes, there will be a slight netdownslope movement of splashed soil.Thus a more descriptive term might be'rainsplash redistribution'.

• As rainsplash requires high rainfallintensities, it is most effective underconvective rainstorms in equatorial regions.

• Rainsplash is relatively ineffective where

rain falls with a low intensity (e.g. becauserainfall is of frontal origin), as in USA ornorth Europe.



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SURFACE RUNOFF

• Rainfall sometimes does not infiltrate (soakinto) the ground, but flows downhill (onsloping surfaces) under the influence ofgravity : Surface Runoff or Overland Flow.

• Runoff occurs due to two reasons:1. If rain arrives too quickly to infiltrate (i.e.

high intensity) - Infiltration Excess Runoff, or

Hortonian Runoff.2. If the soil is already fully saturated (or

frozen) – Saturation Excess Runoff.



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SURFACE RUNOFF

• Runoff moves downhill first as a thindiffuse film of water which has lost virtuallyall the kinetic energy it possessed asfalling rain.

• This runoff (or Sheet Wash) moves onlyslowly and is generally incapable ofdetaching or transporting soil particles.

• The microtopography (i.e. small-scalepattern of irregularities) of the soil’ssurface tends to cause this overland flowto concentrate in closed depressions,which slowly fill: this is known as‘detention storage’ or ‘ponding’.



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Sheet Erosion



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SURFACE RUNOFF

• Both flowing water and water in detention

storage, protect the soil from raindrop impact,so that rainsplash erosion decreases overtime within a storm, as the depth of surfacewater increases.

• If rain continues, increasing depth of waterwill eventually overtop the microtopographicdepressions.

• Overland flow that is released in this way islikely to flow downhill more quickly and in

greater quantities (i.e. possess more flowpower as a result of its kinetic energy), and somay be able to begin transporting and evendetaching soil particles. Where it does so, thesoil’s surface will be lowered slightly.



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RILLS & GULLEYS

• Lowered areas form preferential flow pathsfor subsequent flow, and these flow pathsare in turn eroded further.

• Eventually, this positive feedback results insmall, well-defined linear concentrations of overland flow (‘microrills’ or ‘traces’).

• In many cases, individual microrillsbecome ineffective over time due to

sedimentation.• A subset, however, grow further to becomerills; and a smaller subset may go on todevelop into gullies.



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RILL & GULLEY EROSION

• Process of ‘competition’ between microrillsand rills leads to self-organized formationof networks of erosional channels, whichform efficient pathways for the removal of water from hillslopes.

• It is in such erosional channels that water erosion also operates most effectively todetach and remove soil by its kinetic

energy.• In most situations erosion by concentratedflow is the main agent of erosion by water.



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RILL & GULLEY EROSION

• As erosional channels increase in size (i.e.

larger rills & gullies), processes asgravitational collapse of channel walls andheads increase in importance.

• Runoff & sediment from rills & gullies maybe moved into ditches, stream and rivers,and so transported well away from the pointof origin.

• Sediment may also be deposited within therill or gully, or beyond the rill or gully’s

confines in a depositional fan, at locationswhere the gradient slackens.• Sediment may be stored for a variable

period of time, until a subsequent erosionevent is of sufficient size to re-erode thestored sediment.



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RILL EROSION



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GULLY EROSION



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OVERLAND FLOW

• Flow-dominated erosional channels

separated by interrill areas where dominantprocesses are rainsplash and diffuseoverland flow.

• Boundaries between rill and interrill areas areboth ill-defined and constantly shifting.

• Subsurface flow sometimes, may beimportant in determining where channelerosion begins and develop (e.g. at base ofslopes, and in areas of very deep soils such

as tropical saprolites).• Meltwater from thawing snow operates in a

broadly similar way to rain-derived overlandflow, detaching and transporting unfrozen

soil in areas of concentrated flow.



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BADLANDS

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