wadis and rivers ztypes of flow zflash floods zchannel form zchannel initiation zarroyos zapplied...
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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
0
0.2
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1.2
29 Day in September 1997
Sta
ge (
m)
40
60
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Acc
um
ula
ted R
ain
fall
(mm
)In
tensi
ty (
mm
/h)
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
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
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
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