Wadis and Rivers

Types of flowFlash floodsChannel formChannel initiationArroyosApplied fluvial geomorphology

Types of Flow

Ephemeral flow with a short duration several events each year

Perennial a stream that flows continuously

throughout the year

Perennial Dryland Rivers

Exoreic; sources is sustained in drylands

Character is similar to humid riversKey difference is high specific yields

Ephemeral Flash Floods

Irregular and short durationHigh intensity rainfallUnsteady, non uniform flowHigh sediment loadsHigh transmission lossesDifficult to measure

For a given rainfall intensity, rains in drylands produce more runoff per

unit area than in temperate environments.

Characteristics

Short time to peakSharp peak (10-30 mins in)Short duration (1-5 hours)Less steep recession limbbore

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StageCum RainfallIntensity

Bores

Caused by:intense stormrapid translation of waterinteraction of wave and transmission

lossesvelocity increases d/sbore grows d/s

Channel Routing

Important because:Storm cells have limited spatial extentA single storm is unlikely to affect the

whole catchmentSuccessive storms wet different areas

of the catchmentCells migrate as they deliver the rain

A B C

Transmission Loss

Volume of channel discharge that infiltrates into channel bed

Due to coarse grained alluvial deposits

Economic significance of loss: loss of irrigation water problems for flood prediction and design effects ground water recharge

Magnitude of transmission loss related to: flow duration channel length and width antecedent moisture conditions peak discharge properties of alluvium patterns of flood waves sediment load

Sediment Loads

High suspended sediment load (SSC)High amounts of scourRapid deposition

Channel Form

Very variableWide with flat bedformsPoorly adjusted to imposed discharge Long profiles constant or convexPoor network integrationHigh drainage density

arid > semiarid > humid

Channel Processes

ScourTransport Deposition

Seepage from GWBank failureVegetative growthweatheringChemical pptParticulate

movement

Scour (Entrainment)

highly variablevertical and lateralrelated to competence of flowcomplicated by armouringinhibited in early stages of floods by

cemented clay layer

Transport

depends on available energyrelated to frictional lossesvegetation depends on

time of year of flood stem spouting ability number of seeds

roughness increase with flow

Deposition

aggradation = net depositionresults from loss of powerproduces fansdepositional features:

control transmission losses determine scour for low flows provide clues to sequence of flows

Piping

Subterranean channels Important form of subsurface water

and sediment dischargeIncreases with;

soluble salt content content of swelling clays surface runoff hydraulic gradient

Channel Initiation

Stability theory Carson and Kirkby 1972 Smith and Bretherton 1972

Erosion thresholds Horton 1945

Stability Theory

mathematical analysis of process lawsexamines conditions under which a

small perturbation will grow or shrinkunstable growth occurs if convergence

of flow allows more sediment to be removed than is brought in

occurs if sediment transport increases more than linearly with water discharge

Erosion Thresholds

Tractive stress versus material resistance

Horton’s ‘belt of no erosion’

Kirkby (1994)

Combined the two approachesProgressive change in response from

stability conditions to threshold behaviour

Semi-arid - humid environments

Arroyos

Trench with steep sides and rectangular cross section

Created by rapid incision into valley floor alluvium

Widespread in SW USA Main period of arroyo cutting 1865-

1915

Arroyo Hypothesis

Cutting is a result of force vs resitance Flow velocity

discharge channel slope Flow depth

Resistance surface roughness vegetation

Applied Fluvial Geomorphology in Drylands

IrrigationSoil and water conservationReservoir sedimentationFlash flood hazardWater qualityWater politics