Wadis and Rivers

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Wadis and Rivers

• Types of flow
• Flash floods
• Channel form
• Channel initiation
• Arroyos
• Applied fluvial geomorphology

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Types of Flow

• Ephemeral
• flow with a short duration
• several events each year
• Perennial
• a stream that flows continuously throughout the
  year

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Perennial Dryland Rivers

• Exoreic sources is sustained in drylands
• Character is similar to humid rivers
• Key difference is high specific yields

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Ephemeral Flash Floods

• Irregular and short duration
• High intensity rainfall
• Unsteady, non uniform flow
• High sediment loads
• High transmission losses
• Difficult to measure

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• For a given rainfall intensity, rains
• in drylands produce more runoff per
• unit area than in temperate environments.

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Characteristics

• Short time to peak
• Sharp peak (10-30 mins in)
• Short duration (1-5 hours)
• Less steep recession limb
• bore

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Bores

• Caused by
• intense storm
• rapid translation of water
• interaction of wave and transmission losses
• velocity increases d/s
• bore grows d/s

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Channel Routing

• Important because
• Storm cells have limited spatial extent
• A single storm is unlikely to affect the whole
  catchment
• Successive storms wet different areas of the
  catchment
• Cells migrate as they deliver the rain

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A
B
C
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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

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• 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

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Sediment Loads

• High suspended sediment load (SSC)
• High amounts of scour
• Rapid deposition

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Channel Form

• Very variable
• Wide with flat bedforms
• Poorly adjusted to imposed discharge
• Long profiles constant or convex
• Poor network integration
• High drainage density
• arid gt semiarid gt humid

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Channel Processes

• Scour
• Transport
• Deposition

• Seepage from GW
• Bank failure
• Vegetative growth
• weathering
• Chemical ppt
• Particulate movement

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Scour (Entrainment)

• highly variable
• vertical and lateral
• related to competence of flow
• complicated by armouring
• inhibited in early stages of floods by cemented
  clay layer

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Transport

• depends on available energy
• related to frictional losses
• vegetation depends on
• time of year of flood
• stem spouting ability
• number of seeds
• roughness increase with flow

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Deposition

• aggradation net deposition
• results from loss of power
• produces fans
• depositional features
• control transmission losses
• determine scour for low flows
• provide clues to sequence of flows

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Piping

• Subterranean channels
• Important form of subsurface water and sediment
  discharge
• Increases with
• soluble salt content
• content of swelling clays
• surface runoff
• hydraulic gradient

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Channel Initiation

• Stability theory
• Carson and Kirkby 1972
• Smith and Bretherton 1972
• Erosion thresholds
• Horton 1945

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Stability Theory

• mathematical analysis of process laws
• examines conditions under which a small
  perturbation will grow or shrink
• unstable 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

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Erosion Thresholds

• Tractive stress versus material resistance
• Hortons belt of no erosion

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Kirkby (1994)

• Combined the two approaches
• Progressive change in response from stability
  conditions to threshold behaviour
• Semi-arid - humid environments

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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

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Arroyo Hypothesis

• Cutting is a result of force vs resitance
• Flow velocity ?
• ? discharge
• ? channel slope
• ? Flow depth
• Resistance ?
• ? surface roughness
• ? vegetation

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Applied Fluvial Geomorphology in Drylands

• Irrigation
• Soil and water conservation
• Reservoir sedimentation
• Flash flood hazard
• Water quality
• Water politics