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Wadis and Rivers

Wadis and Rivers. Types of flow Flash floods Channel form Channel initiation Arroyos Applied 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.

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Wadis and Rivers

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  1. Wadis and Rivers • Types of flow • Flash floods • Channel form • Channel initiation • Arroyos • Applied fluvial geomorphology

  2. Types of Flow • Ephemeral • flow with a short duration • several events each year • Perennial • a stream that flows continuously throughout the year

  3. Perennial Dryland Rivers • Exoreic; sources is sustained in drylands • Character is similar to humid rivers • Key difference is high specific yields

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

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

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

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

  8. 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

  9. A B C

  10. 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

  11. 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

  12. Sediment Loads • High suspended sediment load (SSC) • High amounts of scour • Rapid deposition

  13. 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 > semiarid > humid

  14. Scour Transport Deposition Seepage from GW Bank failure Vegetative growth weathering Chemical ppt Particulate movement Channel Processes

  15. 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

  16. 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

  17. 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

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

  19. Channel Initiation • Stability theory • Carson and Kirkby 1972 • Smith and Bretherton 1972 • Erosion thresholds • Horton 1945

  20. 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

  21. Erosion Thresholds • Tractive stress versus material resistance • Horton’s ‘belt of no erosion’

  22. Kirkby (1994) • Combined the two approaches • Progressive change in response from stability conditions to threshold behaviour • Semi-arid - humid environments

  23. 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

  24. Arroyo Hypothesis • Cutting is a result of force vs resitance • Flow velocity  •  discharge •  channel slope •  Flow depth • Resistance  •  surface roughness •  vegetation

  25. Applied Fluvial Geomorphology in Drylands • Irrigation • Soil and water conservation • Reservoir sedimentation • Flash flood hazard • Water quality • Water politics

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