Oceanography of the coast along Andaman sea
The coasts of the southern provinces along Andaman Sea are submerged shorelines, caused by the tremendous deformation of coasts since 65 million years ago during Tertiary Age. The submergence of the earth crusts produces the steep, rough and indent shorelines, including many bays and islands such as Kraburi estury in Ranong Province, Ko Phuket, Ko Tarutao, Ko Lanta, Ko Libong, Ko Prathong, and Ko Yao Yai. Some coastal areas, from Ao Phang Nga to Trang Province, are covered with mangroves while coastal erosions are found in some shorelines such as Ao Chalong, Ao Phuket, Ao Rawai, and Ao Maproa. The oceanography of the coast along Andaman Sea (OSM Andaman, B.E. 2555) is shown as follows
Depth of seabed
The depth of the seabeds of southern provinces along Andaman Sea can be divided into 2 areas based on the depth contour map; the areas have a variety of coral reefs in terms of their types and abundance. The seabeds along the coasts of Ranong Province, the western coast of Phang Nga and Phuket Province are highly steep with the average depth of 1,000 meters. Andaman basin is the deepest part in Thai waters with the depth of 3,000 meters, covered by sand and muddy sand. Whilst, the seabeds along the southern coast of Phang Nga, the eastern coast of Phuket, Krabi and Trang Province are lower in steepness. The depth at the continental shelf is not exceed 300 meters
The currents in Andaman sea along the southern provinces influenced by monsoons, leading to the various directions of the currents that can be classified into 2 characteristics: the currents with uncertain direction or turbulence found in the southern coast of Ranong and western coast of Phuket, and the current with certain direction found in the northern coast of Ranong, the western and eastern coast of Phuket. It can be concluded as follows:
The currents of the southern coast of Ranong Province and western coast of Phuket have uncertain flow direction; this current pattern is dynamic due to the characteristics of shorelines and geography of the coastlines, the combination of various current patterns. Generally, the turbulence patters are mostly found in open seas.
The currents at the northern coast of Ranong, southern and eastern coasts of Phuket are characterized by tidal current which is influenced by tides. During the high tide, the water mass is moved eastward from the south of Ko Mathew to Ranong estuary and another current is flowed eastward from the south of Ko Phuket to Ao Phang Nga. The reversed flows of these currents are also found during the low tide.
These currents influence on the coastal environment in many ways such as the turbidity of the coastal water, habitats for aquatic animals, etc., and they also differently influence on each coastal areas. Moreover, the certain direction of current in the southern and eastern coast of Ko Phuket and Ranong estuary can be periodically occurred because of the influence of the currents in Phuket and Rayong coast, enhancing the rapid sedimentation which supports mangrove growth and their existence. On the other hand, uncertain direction of currents causes a delay of the sedimentation that affects to the abundance of mangroves in the western coasts
A semidiurnal tide is occurred along the coasts of Andaman Sea where two almost equal high tides and two low tides are found each day. The high and low tide reported by the Tidal Monitoring Station of Hydrographic Department, Royal Thai Navy at Ko Ta Phao Noi, Phuket Province were about 3.60 and 0.38 meters, respectively. The tidal range was 3.22 meters (Hydrographic Department, B.E. 2556).
Surface waves along the coasts of Andaman sea are found for a long time period; within each year, wave period is up to six months each year. Wind and waves are moved into the west coast with the average medium speed of about 7.20 kilometers per hour and the maximum speed of approximately 69 kilometers per hour causing the high waves of 0.43 meters and 4.15 meters high, respectively; and this surface wave can increase coastal erosion. However, the influence of the wave in the eastern coast of Phuket is insignificant, leading to the high sedimentation rate within this area. For example, the extending of the mudflats at Sa Pan Hin tha generated by sedimentation leads the seabed of this area gets shallower.