Tidal is a phenomenon of the periodic rise and fall of sea level caused by the attractive force of astronomical objects, especially the sun, earth, and moon. The effect of other celestial objects can be ignored because of greater distance or smaller size (Dronkers, 1964).
During the spring tides, when the position of the sun is in line with the earth-moon axis, the maximum tide occurs at the point on the earth’s surface that is on the axis of the relative position of the earth, moon, and sun. This condition occurs during the new moon and full moon (Poerbandono and Djunarsjah, 2005).
During the neap tides, which is when the position of the sun is perpendicular to the earth-moon axis, there is minimum tide at a point on the earth’s surface that is perpendicular to the earth-moon axis. This condition occurs at a quarter of the early moon and a quarter of the late moon. The tidal phenomenon in such a position is called the neap tide or dead tide. The tidal range (highest water and lowest water distance) during spring is greater than during neap. (Poerbandono and Djunarsjah, 2005).
Theory of Equilibrium Tides
The equilibrium tide theory is an assumption where the earth is considered entirely covered by sea water. The equilibrium tide theory states that the rise and fall of the sea level is proportional to the tidal generating force. The tidal generating force is the resultant of centrifugal force and the force of attraction of celestial bodies (moon and sun), so that there are two bulges of sea water mass on earth. In this theory, the earth is considered ideal.
Component of Tidal
The observed tides are a superposition or summation of tidal components due to the attractive force of the moon and other celestial bodies and due to the rotation of the earth. The tidal components are sinusoidal waves that have different amplitudes, phases, and periods. Mathematically, the tidal equation can be written as:
Tidal components can be divided into main tidal components which are divided into semidiurnal, diurnal, and long-period components and shallow water components. The following are the components of tides.
The Formzahl number (F)
The Formzahl value is the ratio between the sum of the amplitudes of the singular harmonic constants K1 and O1 to the sum of the amplitudes of the multiple harmonic constants M2 and S2. The Formzahl value can be used to determine the type or types of tides. The following is the formula for calculating the Formzahl value.
Type of Tide
The type of tide is determined by the frequency of high and low tides each day. This is due to the different response of each location to the tidal generating force. So, there are different types of tides along the coast. According to Kahar (2008), there are four types of tides for clarification, namely:
- Diurnal tide is a situation that one day there is one high tide and one low tide, the average tidal period is 24 hours 50 minutes.
- Semidiurnal tide is a situation that one day there are two tides and two low tides with almost the same height and tides occur sequentially and regularly. The average tidal period is 12 hours and 25 minutes.
- Mixed tide prevailing diurnal is a situation that in one day there is one tide and one low tide, but sometimes for a while there are two tides and two low tides with very different height and period.
- Mixed tide prevailing semi diurnal is a situation in which there are two tides and two low tides in one day, but sometimes temporarily there is one high tide and one low tide with different height and period.
Tide on Bathymetry
Processing of bathymetry measurement data must consider various corrections, such as measured depth, transducer depth (echosounder), tidal correction, ship draft correction, barcheck correction, sound velocity correction, pitch, roll, yaw correction, etc.
In this case, one of the objectives of this tidal observation is to determine the depth chart datum and determine the correction of the results.
Tidal measurements as vertical control of the bathymetry survey, carried out to determine the Chart Datum to provide corrections for the reduction of bathymetry study results.
Chart datum is a reference point or reference fields used in marine navigation map and tidal forecasting, which is generally connected to the low water level.
The chart datum value is obtained from the calculation of the lowest low tide water, so that the determination of chart datum is closely related to tidal events at a particular location.
Here are the terms for water level position at sea:
The Measurement Tool of Tidal
Tide data recording is done by placing the pressure sensor on a wooden board that has been equipped with a tide gauge staff, the placement of the tide observation station is selected at an appropriate location in the survey area, the location is protected from breaking waves and does not occur dry during low tide. Data recording was done automatically using Valeport’s tide gauge logger system which can be set to various data recording intervals. The data recorded is the time (date and time) and water level against the position of the pressure sensor above the zero level of the tide gauge staff.
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