Depositional current structures: ripples, large ripples (dunes), plane beds, and antidunes.

Other structures generally not preserved in layers include rills (dendritic channels-marks region of gw discharge), rhomboids (associated with upper flow regime plane beds), current crescents (formed by diversion of flow around an obstacle)

Observed Bedforms are a function of:

• flow type of most recent current: unidirectional or oscillatory (waves)
  • Most currents affecting the foreshore are formed by translatory waves. Therefore bedforms are most commonly related to unidirectional flow.
  • exception: wave ripples formed by small waves during high tide may be preserved in a trough (runnel) if the trough was not later occupied by a current.
• flow velocity and depth (flow regime: Froude#=v²/(gd))
• duration of current relative to time need for equilibrium to be obtained. (e.g. non-equilibrium bed forms: 2-D current ripples, planed-off ripples, etc.)

• grain size: certain bedforms such as small ripples don’t form in coarse sediments. Beach gravels may only exhibit plane beds.
  

Typical distribution of bedforms

• Beach face: plane beds
• Bar: -stoss side and bar crest: plane beds and antidunes¹
  • lee side: planar avalanche facies formed by grain flow down the lee side
• Trough: wave (sym) or current ripples (asym), planed-off ripples
• Berm: plane beds
• Low tide terrace: all of the above may be observed at one time or another

Significance

The following may be interpreted from the vertical sequence of strata observed in a trench dug across the beach.

• Mechanism of beach growth
  • welding of ridge and runnel systems and/or
  • accretion by swash deposition
• Stages of seaward growth

¹ antidune laminae dip gently up-flow. Laminae may be noticeably coarser or contain a concentration of heavy minerals

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Lindley Hanson/email /Gls214

Department of Geological Sciences, Salem State College, Salem, MA