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Tsunami Dynamics
As a tsunami approaches shore, the height of its waves increases significantly due to the shallower waters and the impact of seafloor and shoreline topography. The extent of a tsunami̵7;s modification of a coastline depends on its own energy and the particular configuration and geological character of that coastline. Low-lying coasts of yielding material, mangrove swamps or sandy beaches, for example, are typically most affected, at least where fully exposed to the brunt of the waves.
Erosion and Deposition
A powerful tsunami can profoundly impact the shape of a sandy coastline. Waves may cleave or entirely dismantle barrier islands, sand spits and bars. Beach ridges can be destroyed or lowered, and gaps between them enlarged or newly formed. The oncoming surge or ̶0;run-up̶1; of tsunami waves as well as the retreating backwash can scour with erosive power, but tsunamis also deposit sediment as the water loses energy. Waves may drape sand and silt from shoreline beaches and the nearshore seafloor well inland.
Tsunami Landforms
The breaching of beach ridges often creates a distinctive post-tsunami topography on sandy shores. Remnants of heavily eroded dune crests remain as sculpted pedestals separated by washed-out swales. Leeward of these breaches, weakening waves commonly form miniature deltas of sand often called ̶0;tsunami-scour fans.̶1; Where enough breaches have occurred, these fans may coalesce into broad sand sheets behind the ravaged beach. Where they fill depressions or coastal wetlands, these sheets may become relatively stabilized and form a distinctive hummocky landscape. A dry sand sheet exposed to wind, however, may also evolve into a tract of parabolic dunes.
Beach Recovery
Sandy coastlines are inherently dynamic landscapes regularly sculpted by wind and waves. Where enough sediment is available for the rebuilding of sandbars and beaches, these landforms may appear fairly quickly after the coastal erosion of a tsunami. Given such a supply of sand, prevailing winds can also rebuild beach ridges and dunes in short order. In these situations, tsunami landforms such as scour fans and pedestals may be obscured or destroyed within a few decades. However, beach recovery may take much longer if a tsunami temporarily removes significant quantities of sand from the nearshore and coastal system by depositing it far inland or washing it well offshore. In some cases, there may actually be more material available for deposition after a tsunami if the event was associated with a local earthquake: Such a tremor can shed much sediment into rivers, which wash it out into coastal waters where it becomes entrained in nearshore wave processes.