Definition: Cyclic changes in the beach profile resulting from alternating periods of erosion and deposition

• Erosion, which flattens sandy beaches is associated with storms that produce
  • shorter period waves
  • Greater wave heights and steepness (H/L is high)
  
  The Effectiveness of a storm is determined by
  • Size and intensity (fetch and wind speed) and low pressure
  • speed (duration)
  • tidal phase
  • timing between storms (controls prior saturated conditions and local elevation of gw table)
  • position of the coast relative to storm track/path of storm

  Summary of factors influencing erosion and deposition

  • Groundwater conditions (high pore water pressure increases erosion)
  • Changes in beach face permeability (grain size, sorting, degree of saturation)
  • Sediment characteristics: Critical wave steepness values required for erosion are typically greater for coarse clastic beaches
    • Settling velocity of sediment (largely determined by grain size)
  • The onshore/offshore movement of a sediment grain is ultimately influenced by the velocity asymmetry in shallow water waves, wave height (which influences the height of suspension), and particle settling velocity relative to wave period.

• Deposition occurs by vertical accretion onto the berm, and by welding of bars to the beach face. Deposition increases the berm width and steepens the beach face.
  • Occurs when wave steepness (H/L) is low and wave periods (T) are longer
• The rate of ridge (bar) migration is determined by the following factors:
  • Average wave height
    • larger swell result in bar faster migration
  • slope of beach and offshore area
    • Plume Island --steep beach and fast rate of migration
    • Crane Beach --flat beach and slower rate of migration
  • grain size : fine grain--slower migration because the beach is flatter
  • tidal range: controls the length of time waves can work at a single level

• Stage 1: Early post-storm (up to 3-4 days after storm - may be greater along meso/macrotidal beaches)
  • flat to concave upward (dissipative beach)
  • beach is smooth, fine grained and featureless
  • severest storms produce a scarp in the foredune ridge
• Stage 2: Early acretional or constructional profile (2 days to 6 week)
  • small berms, beach cusps, and ridge and runnel systems developed
• Stage 3: Late accretional or maturity (6 weeks or more after storm)
  • Landward migrating ridge and runnel systems weld on to backbeach
  • broad, convex berm developed (largely reflective)
  • On some beaches welding may be incomplete and giant ridges (1.2m high) lie
  • between the low-tide terrace and backshore

  Not all beaches may actually behave in this manor. (See morphodynamics outline)

• beaches are erosional during the winter
  • affected by northwesterly storm swell produced in the north Pacific by winter storms
  • erosional profile (shallow profile) = winter profile
  • During the summer wave climate is characterized by waves with smaller wave heights and longer periods
  • accretion occurs
  • depositional profile (steep profile) = summer profile

     

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• the beach profile is less seasonal
  • northeasters result in the greatest amount of erosion (northeasters occur when low pressure systems over the SW and SE move up the Atlantic coast)
  • these storms bring lost of ppt, winds up to 40-60 mph, and produce storm surges of 1/2 to 4 feet
  • frequencies of 5 to 7 per year
  • erosional profile = storm profile 
• Because of differences in wave climate and beach composition, not all beaches experience the same cycles or have the same erosional or depositional morphologies described above. (See also, discussion of beach morphologies by Wright et. al., in Davis p. 434-435, and morphodynamics outline)

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Lindley Hanson/email /Gls214

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

updated 11/6/03