A body of water has a natural period of oscillation that is dependent upon its dimensions. No ocean appears to be a single oscillating body, each one is made up of a number of oscillating basins. Basins are acted upon by the tide-producing forces. some respond more readily to daily or diurnal forces, others to semidiurnal forces, and others respond equally to both. Tides at a given place are classified as semidiurnal, diurnal, or mixed.
Semidiurnal. This type of tide, there are two high and two low,waters each tidal day with small change in the consecutive high and low water heights.
A second reference plane based on a selected highwater average is used as a basis for the measurement of charted heights and vertical clearances of objects above the water. If the selected low water average is mean low water, and the selected high-water average is mean high water, then the difference between these two planes is called the mean range of the tide.
It is important to remember that the water level is occasionally below the reference plane. The depth of water can be less than the charted depth. This is indicated by a minus sign ( - ) placed before the height of tide as shown in the Tide Tables. The depth of water is equal to the algebraic sum of the charted depth and the height of tide, so that when there is a negative tide, the numerical value of the height of tide is subtracted from the charted depth to find the depth of water. In man" coastal areas, the actual height of tide at any time can be influenced by winds from a particular direction, especially if the winds are strong and for several days. Mean Low Water is the average of all low tides. Tide Tables lists the time and height of the tide at each high water and low water in chronological order for each day of the year at a number of places which are designated as reference stations;
All times stated in the Tide Tables are standard times; each page of Table 1 indicates the central meridian of the time zone used. Adjustment must be made for the use of daylight time or any other deviation from standard time at the locality concerned..
Because the lunar or tidal day is a little more than 24 hours in length (an average of about 24h 50m), the time between successive high or low tides is a little more than 12 hours. When a high (or low) tide occurs just before midnight, the next high (or low) tide occurs about noon of the following day, and the next one occurs just after midnight. Under these conditions, three consecutive high (or low) tides may occur on three different dates, although the total interval may be no more than the average period of a lunar day, 24h 50m. This means that on the middle of the three days, there is one high or low water.
