Chapter 2- The Diving Environment The Diving Environment 41 land, friction creates drag and slows water movement. Water is funneled between and around land masses. Therefore, tidal variances are formed around our planet. The variations are usually documented, however, and tables are available to assist with dive planning. A body of water in movement within a confined space, has a natural period of oscillation (back and forth movement like water sloshing in a basin) that depends on the dimensions of the container. The waters of the earth comprise numerous oscillating basins rather than a single oscillating body. The response of a basin of water to tide-producing forces determines the type of tide produced in the basin. Classification of Tides • Diurnal (daily) - One high and one low tide occurring every 24 hours and 50 minutes (time required for the moon to pass a fixed point on the earth twice)(figure 2-10). • Semi-diurnal (twice daily) - Two high and two low tides of approximately equal height every 24 hours and 50 minutes. A tidal change occurs approximately every six hours (figure 2-10). Flooding Tidal Current Open Water • Mixed - A combination of diurnal and semi - diurnal; the heights are unequal (figure 2-10). The height of tides varies considerably due to many factors, including shoreline configuration, time of month and year, and wind conditions. Tidal amplitude on an open shore is usually 0.5 to 1.5 m (2 to 5 ft), but the tidal range in harbors and estuaries can be very great (12 m 40 ft or more), resulting in extremely strong currents during ebb or flood. Further, the height of tides may vary over 9 m (30 ft) between two locations only a few hundred miles apart. A tidal current is periodic horizontal water movement associated with tides, reversing direction of flow as the tide changes. Water flowing toward shore or upstream with a rising tide is called a flood tide, and water flowing offshore or downstream with a falling tide is called an ebb tide (figure 2-11). Tidal currents can be very strong, especially through narrow areas. A narrow channel contiguous with a large body of water concentrates and amplifies tidal currents. On an open coast, where the direction of flow is not restricted, tidal currents flow continuously with the direction changing according to the tidal period (figure 2-12). Mixed Diurnal Semi-diurnal FIGURE 2-10. TYPES OF TIDE CURVES Channel Channel Ebbing Tidal Current Open Water FIGURE 2-11. FLOOD AND EBB TIDES PRODUCE VARYING CURRENTS IN CHANNELS, BAYS, AND HARBORS.
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