Mean Sea Level by Moon and Sun Gravity

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2.6.1 Mean Sea Level by Moon and Sun Gravity

Sea level rate also influence by moon and sun gravity to earth, and the gravity forces in a year is constant as long as no changes in the position and material between moon and sun to the earth. Figure 2.9: a Forces Gravity Formation Evoke Spring Tide, b Forces Gravity Formation Evoke Neap Tide Spring tides are usually strong tides Figure 2.9a, when the formation of sun, moon and earth is in the line. These spring tides occur every 14-15 days during full and new moons. Meanwhile the neap tides Figure 2.9b occur when the gravitational forces of the Moon and the Sun are perpendicular to one another. The gravitational forces of the Moon and the Sun is constant so that the forces to lift the sea level in a year was constant too, but there is a anomaly condition called Proxigean Spring Tide, it very high tide occurs when the moon is both unusually close to the Earth and in the New Moon phase, this condition occurs at most once every 1.5 years. The stated pattern of tidal Figure 2.10 derived from WX Tide software, has proven that the moon-sun gravity forces was constant. 17 Figure 2.10: Tidal Level Patterns 2.7 Addressing Tidal Flood Hazard The sea level rise and land subsidence is the main factors the occurrence of tidal flood hazard and it cause loss material. Tidal flood is one problem of coastal city as the impact of the climate hazard which is influenced by the sea level rise. In Pantura Northern coastal of Java, Indonesia, the tidal flood occurred annually on almost all of region of the java coastal area, such as Jakarta, Subang, Cirebon, Semarang, and also Surabaya. Surabaya is the city which is located in coastal area has the second densest population city in Indonesia, has importance economic rationalities. A consequence of the rapid migration and economic development in Surabaya is the high demand necessity of the clean water. Excessive groundwater extraction for municipal and industrial use has lowered water tables in many urban areas in most coastal city in the world causing land subsidence and it will creating unusually high rates of relative sea level rise and salt water intrusion. This condition is the consequences of urbanization which is a net increase in total runoff due to the expansion of areas of impermeable pavements, buildings, and other covered surfaces, which reduce evapotranspiration and impede the infiltration of rainwater Douglas, 2001. Tobing. et al., 2006 noticed that in 18 Surabaya city the rate of land subsidence is ranging from 0.02-0.25 m per year due to compressibility and also the sediment soil type. The impact of tidal flood is the damage of the infrastructure, housing, transportation equipment, and also the economic loss. The loss of fish farmer resulted in crop failure about billions rupiahs due to tidal flood hazard, as well as corrosive material caused by salt water sea water Wuryanti. et al., 2002; Heryawan, 2009; Ritonga, 2009. 19 I I I . M E T H O D O L O G Y A N D M A T E R I A L S

3.1 Study Site