Wave influence in the construction, shaping and destruction of river deltas: A review

Waves are an important agent in the construction, shaping and destruction of river deltas. Notwithstanding the commonality of waves in oceans and seas, wave influence on deltas varies considerably depending on the coastal morphology and nearshore bathymetry. Although there have been advances in understanding the way waves approaching a delta shape its shoreline, much still remains to be known of the interactions between waves and river deltas. Deltas are built essentially from sediments supplied by rivers. Sand-sized and coarser sediments may also be derived from nearby abandoned delta lobes or from older relict nearshore deposits, transported by wave reworking and longshore currents. Alternatively, delta erosion by waves can also release sediment that is redistributed alongshore or that accumulates offshore. The extent to which bedload is supplied to and sequestered in, or lost by, deltas through waves and longshore transport strongly depends on interactions between waves and fluvial discharge at the river mouth. These interactions and the mutual adjustments they engender are not only important in the overall balance between delta retreat, progradation or aggradation but also in processes such as avulsion and channel switching, as well as in the eventual survival of a delta in the face of sea-level rise. Where waves are important, fluvial liquid discharge is high, and sediment supply is rich in bedload, two important aspects are the blocking of waves and longshore currents by strong river discharge and the formation of bars at the river mouth. Field studies of the complex interactive processes prevailing where river flows encounter waves are, however, non-existent and numerical modelling, though promising, hampered by scale constraints and the difficulty of replicating them and generating mouth bars in the presence of longshore currents. This interaction influences the seaward extent of the delta mouth protuberance and its stability; this protuberance then forming the regional shoreline template to which waves and longshore currents adjust. Longshore currents can redistribute wave-reworked mouth bar deposits emplaced during strong river flow. Transport may be either divergent from the mouth or may be regionally unidirectional but wherein the symmetry of some deltas, probably rare, may be maintained by a strong river blocking effect on transport from the updrift flank. The mouth protuberance may be such as to foster transport reversal (counter-drift) at th