Rifqi Muhammad Harrys
Faculty of Earth Science and Technology, Institut Teknologi Bandung, Indonesia
rifqi.m.h@gmail.com
Coasts and estuaries are boundaries between land and sea. The development of both of them are affected by interaction between the land properties (e.g. land water hydrodynamics, land sediment/soil) and sea properties (e.g. seabed sediment, ocean hydrodynamics).
There are two kinds of coasts by the ocean hydrodynamics developing them, tide-dominated coast and wave-dominated coast. Tide-dominated coast is developed mostly by tide and wave-dominated coast is developed mostly by wave. Wave bring more energy than tide, so the finer sediment is suspended within the water column, while the courser sediment still remain. This behavior has great effects to sediment transport, which is affecting the development of the coasts. In other word, the beach with finer sediment (e.g. mud, clay) referred to tide-dominated coast and the beach with coarser sediment (e.g. sand, pebble, rock) referred to wave-dominated coast. The energy brought by sea water not only affecting the sediment transport, but also affecting the steepness of the coasts. Because of the erosion and deposition, the more energy it (sea water) brought landward, the steeper the coasts and vice versa. Erosion is when the sediment leaving its place, and deposition is when the sediment coming to its new place.
There are three kinds of sediment transport occurred in coasts. They are longshore transport, cross shore transport, and river supply. The differences between them is at how it generated mainly. Longshore transport is generated mainly by wave. A longshore current is established which flows along the shoreline at an oblique angle. Cross shore transport is generated mainly by tide. The tide occurred generating tidal current which flows landward perpendicular to the shoreline. The river supply transporting sediment along the river current coming by the estuary to the coast.
Estuary is an area where a river meet a sea. According to the water circulation and salinity distribution, estuaries are classified into three kinds. They are salt wedge estuaries, partially mixed estuaries, and well mixed estuaries. Salt wedge estuaries develop where river water flows into seas with low tidal range (less than 2 meters). The sharp salinity differences between overlying freshwater (land water) and underlying seawater develop a salt wedge between these two kinds of water. Partially mixed estuaries develop where rivers discharge into seas with moderate tidal range (between 2 and 4 meters). Where rivers discharge into seas with high tidal range (more than 4 meters), the tidal current creating a turbulence so that the freshwater and the seawater well-mixed and developing well mixed estuaries.
References
The Open University. (1999). Waves, Tides, and Shallow-Water Processes. Oxford: Butterworth-Heinemann.
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