The oceanography of the Gulf of Thailand
The Gulf of Thailand is characterized as an estuary of drowned river valley with the seabeds which was once emerged above sea surface. On the sea bottom, there are ancient deep channels connected to present rivers such as Mae Klong River, Bang Pakong River, Chantaburi River etc. The fresh water from four major rivers, Mae Klong, Thachin, Choapraya, and Bangpakong, all flow into the upper Gulf of Thailand, meanwhile, the gulf is also supported by minor rivers from the east and west parts of Thailand and is the basin of deposited sediments from those rivers. Based on the surveys of the Hydrographic Department, Royal Thai Navy, it’s indicated that sandy mud or mud is appeared the center of the gulf while some parts in the Western Gulf of Thailand are covered by sandy mud, muddy sand, and sand. The details on the oceanography of Thailand’s coast (Royal Thai Naval Academy,B.E. 2550) are as follows:
Depth of the seabeds
A pan-like shape seabed of the Gulf of Thailand with the deepest point of about 80 meters has the main deep channel of over 50 meter deep laying on the middle of the gulf. The upper Gulf of Thailand has an inverted U-shape with the area of approximately 100 x 100 square kilometers, the deepest point of the upper Gulf of Thailand is about 40 meters on the east part while the west part is more shallower about 15 meter. The Gulf of Thailand is split from the South China Sea by two underwater ridges. The first ridge stretches south-eastward from Khota Bharu for 160 kilometers with the average depth of 50 meters. The second ridge stretches south-westward from Cape Camou for about 100 kilometers with a sill depth of about 67 meters. Moreover, there is an underwater sill that acts as a hydraulic controller of current flow in the lower Gulf of Thailand.
The ocean current that generated by the sea surface wind because the surface wind can produce a movable layer of water mass which is called Ekman layer; (50 meters of the Ekman layers are generally found in oceans and 30 – 40 meters found in the Gulf of Thailand), and the movement of the water mass is called Ekman transport. Theoretically, in the Northern Hemisphere, the layer of water mass from the surface is moved at an angle of 45 degrees to the right of the surface wind and the angle is increasing in the deeper layer until they reach at the bottom of Ekman layer, the net transportation of water mass is at an angle of 90 degrees to the right of the surface wind. That means at this point the direction of wind is totally opposite of the movement of water mass.
The water current generated by rivers, which produce gravitational circulation, that is, the freshwater discharged from the rivers is suspended in the upper layer because of the lower density of freshwater, while the higher density of saline water is at the lower layer. The mixing of both fresh and saline water is influenced by currents and waves. Generally, the salinity in the Gulf of Thailand is extremely affected by freshwater discharge but the impact of salinity on variation of current circulation in the Gulf of Thailand is not significant because the annual volumes of the freshwater discharged into the gulf is very small comparing with the total quantity of water mass in the Gulf of Thailand .
However, the difference of water density causes oceanic circulation in both horizontal and vertical direction. According to the study of Sverdrup, Johnson, and Fleming (1942 cited in Thai Royal Naval Academy, B.E. 2550), it was found that oceanic current in The Northern Hemisphere will flow perpendicularly to the slope of the density of water surface. The lower density is on the right-hand side of the observer who is facing toward the flow direction.
Diurnal tide cycle is mostly found in the Gulf of Thailand where only one high and one low tide each day occurred. Since the Gulf of Thailand is shallow with rough seabeds, the movement of tidal waves is not consistent; when the waves approach the shorelines, they will then move back to the sea again and interfere to other waves. As the results, only single low and high per day is remained. Some locations experience a mixed tide where two uneven tides a day, or sometimes one high and one low each day are occurred. The high and low tide reported by the Tidal Monitoring Station of Hydrographic Department, Royal Thai Navy at Ko Prab, Surat Thani Province were about 2.9 and 0.32 meters, respectively. The tidal range was 2.61 meters (Hydrographic Department, B.E. 2556).
The generation of waves in the Gulf of Thailand is influenced by monsoon; the northeastern monsoon produces the larger waves in the western Gulf of Thailand, and the southwestern monsoon produces the larger wave in the eastern Gulf of Thailand. The waves in the upper gulf are not too large because the southwestern monsoon is mild and occurred in a short period of time. Generally, the waves in the Gulf of Thailand are relative small with the average height of 1 – 2 meters. Wave period should be considered to address the impact of waves on shorelines, for example, the impacts of a small wave with long period are higher than such impacts of a large wave with short period.