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572 SEDIMENTARY ENVIRONMENTS/Shoreline and Shoreface Deposits Figure Key features related to water waves approaching a smooth dipping coast; L is the wave length, h is the wave height, and a is the wave amplitude Figure Typical summer and winter beach profiles During the summer months, fair weather waves move sediment onshore and build up a berm Winter storms erode the beach and deposit the sediment in the shoreface and offshore transition zone the water depth becomes less than the wave base and the waves interact with the seafloor Waves hitting the coast may be classified as reflective, dissipative, or intermediate forms Reflective waves strike very steep coastlines (with features such as cliffs or a seawall) and the majority of the energy is reflected directly back into the water body Purely dissipative waves gradually lose their energy through frictional interaction with a very gently dipping seafloor and not break at all A majority of the waves that affect active shorelines are intermediate between these two forms In intermediate cases, interaction with the seafloor slows the waves and pushes them up; the waves steepen until they eventually collapse or break If the shoreline is steep, the waves will break as plunging forms, whereas if it inclines more gently, the waves will tend to surge in a landward direction Steeper shorefaces with plunging waves tend to be simple systems in which the waves break directly on the beach Most shorefaces dip gently and have a complex topography, including numerous bar forms over which the waves may break before reaching the shoreline, to reform and then break again These systems include complex sets of plunging and surging waves During fair weather periods, waves tend to move sediment in an onshore direction because water loss due to percolation into permeable beach sediments reduces the efficiency of the backflow Large storms tend to erode the beach and to deposit sediment offshore, or in the case of barrier systems, in the lagoon behind the beach During the summer months, when few storms occur, there is a net onshore migration of sediment; during the winter storm period, the beach is frequently eroded and does not have time to be repaired Consequently, typical summer and winter profiles for beaches may be defined (Figure 3) The preserved rock record is a complex mixture of the depositional products of the repeated seasonal changes If waves approach the shoreline perpendicularly, there is no net movement of sediment along the coast However, in many cases, waves from the open ocean approach the shoreline obliquely This results in an oblique onshore movement of sediment with the incoming wave The returning swash tends to be more perpendicular to the shore, which in turn leads to migration of sediment parallel to the shore; this is termed ‘longshore sediment transport’, or longshore drift (Figure 4) Transport of sediment by longshore drift is a key factor in moving volumes of sediment and in controlling the morphology of wave-dominated shoreline systems The sedimentary structures produced by waves show an increase in bedform asymmetry towards the shoreline, passing from oscillatory, symmetrical wave ripples near the wave base into combinedflow wave ripples as the water becomes shallower (Figure 5) In the shallowest part of the system, shear between the water mass and the seabed can
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