ESPL Aeolian processes lecture 3

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  • Created on: 29-05-22 23:51
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  • ESPL aeolian processes LECTURE 3
    • RIPPLES
      • Ripples form on most dry bare sandy surfaces Align locally to the flow direction 
      •  smallest aeolian bedform and form on most dry, bare, sandy surfaces (with 4 exceptions) align at right angles to flow direction
        • Do NOT ‘grow’ into dunes, short-lived and have short geomorphic ‘memories’ (10 mins for equilibrium)
          • Dunes and ripples exist on different planes of landforms Equilibrium on spacing and height 
        • ripples DON'T form where...
          • 1) where sand is very coarse 2) at v high u* values (but rare in nature)
            • U* values are important too Higher wind speeds= less 
              • riffles with longer wave lengths Ripples are a microscale landform and so don’t require significant U* vales to form
            • 3) in places where grainfall occurs      4) where sand avalanches
              • the steep downwind lee-side of dunes (lack saltation)
                • Need saltation as a dominant process Steeper slopes don’t have saltation as the main mechanism and so ripples don’t form 
          • Bedforms can form together at different scales
            • •Ripples are NOT dunes •Ripples form ON dunes •Part of a ‘hierarchy’ of bedforms… -Bedform scale -Superimposition of bedforms
      • wave length = 1-25 cm (but can be more)   h = 0.5-2 cm (but can be more)  asymmetric cross-section  convex stoss slope 8-10°  lee slope 20-30°  crest = relatively coarse
        • wavelength and h are closely related to grain size and u*
        • Ripples in coarse sand have greater wavelength than fine sand
      • Wavelength increases and become more variable Size ranges Coalescing of riffles 
      • BAGNOLD
        • •Bagnold (1941) proposed the impact/ballistic mechanism theory 
          • •Ripples formed by saltation, their spacing linked to saltation hop/path length
            • Rectation and grain splashing leads to the initial formation of the ripple 
        • •Ripples are formed by saltation •But Bagnold theory is too simple… 
          • Saltation causes ripples, but l ? hop length Reptation (splash) is important
    • DUNES
      • •is a subaerial body of sand shaped by the wind •a depositional landform 
      • interacts with airflow when a certain size •exhibits “morphodynamic” feedback/behaviour
      • initiation
      • Development
  • Duran et al. (2014)
    • Not just the initial impacts but the splashing that forms the ripples 

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