Development of shingle beaches and structures - summary

The type of shingle beach or structure that develops in a particular location depends on four main environmental factors. There must be an available supply of sedimentary material at the same time as conditions of waves, winds and tidal currents are favourable for its movement. Since this coincidence is unpredictable, conditions without movement occurring may exist for considerable periods, interspersed by times of marked activity resulting in stable and mobile shingle habitats varying both in time and space (Randall 1988). The type of beach or shingle structure reflect both the past and prevailing conditions affecting sediment transport along the shore. The external structure is largely concerned with the size and position of the shingle material itself.

External structure

All shingle coasts contain a mixture of different sized sediments the primary sorting taking place on beaches. Some are well sorted and consist entirely of pebbles, while others are poorly sorted and may also contain sand and/or boulders. Most shingle shores become coarser towards the upper end of the beach, because backwash and gravity cannot move larger sized pebble once these have been thrown up by the advancing waves. Sediment grading along-shore also occurs due to selective transport of finer sediments in the down-drift direction as at Chesil Beach, where the shingle becomes progressively smaller from east to west. At other sites much more complex patterns result, especially where currents move on two directions. The eventual external ridge structures (of shingle structures) also vary dependent upon whether they have accreted vertically but been laterally stable, laterally migrating or developing on a seaward prograding plain. The depressions between ridge crests may be partly filled by washover and storm-tossed deposits, so that there is often a marked difference in average particle size and shape between ridge fulls (crests) and lows (Randall & Fuller 2001). However, on many coasts sediment grading has been found to be complex, related to seasonal variation in the longshore current regime and thus often difficult to interprete (Pye 2001).

Internal structure

Shingle coast micro-relief dynamics depend upon spring to neap tidal patterns and wind and wave conditions. The upper 50-80 cm of sediment is frequently remobilised forming berms and cusps that change from one tidal-cycle to another. More major changes occur seasonally as a result of spring to neap tidal fluctuations and especially at those times when storm-wave energy is higher. The internal sedimentary architecture of shingle landforms reflects the process regime and net evolutionary trends of the structure (Randall 1973). The internal structure, which develops once the beach becomes stabilised is extremely important to the type of vegetation that becomes established.

References

Pye, K. 2001. The nature and geomorphology of coastal shingle. In: Ecology & Geomorphology of Coastal Shingle, eds.Packham, J.R., Randall, R.E., Barnes, R.S.K. & Neal, A.Westbury Academic and Scientific Publishing, 2-22.

Randall, R.E. & Fuller, R.M. 2001. The Orford Shingles, Suffolk, UK: evolving solutions in coastline management. In: Ecology & Geomorphology of Coastal Shingle, eds.Packham, J.R., Randall, R.E., Barnes, R.S.K. & Neal, A.Westbury Academic and Scientific Publishing, 242-260.

Randall, R.E. 1973. Shingle Street, Suffolk: an analysis of a geomorphic cycle. Bulletin of the Geological Society of Norfolk, 24., 15-35.

Randall, R.E. 1988. The vegetation of Shingle Street, Suffolk in relation to its environment. Transactions of the Suffolk Naturalists' Society, 24., 41-58.