NAUI Master Scuba Diver 42 The Diving Environment At each reversal of current, a short period of little or no current exists, called slack water. During flow in each direction, the speed will vary from zero at the time of slack water to maximum strength about midway between the slack periods. Divers should use local tide tables. Personal evaluations of water movement must be made in order to determine slack water times which often present more favorable diving conditions. Tide tables and related information are contained in various forms in many navigational publications and in newspapers. Tidal current tables, issued annually, list daily predictions of tides. In some channels or straits, you will be limited to 15-20 minutes of easy diving time during slack water. Precautions are essential when diving in such areas. Careful pre-planning is necessary to be able to take advantage of this narrow window of diving opportunity. You should not attempt to swim against a strong current. If caught in a current, inflate your buoyancy compensator and swim perpendicular to the current toward shore, or signal and wait to be picked up by a boat. Make sure that someone ashore knows your dive plan and expected time of return, so that a timely search or rescue effort can be started if necessary. Seiches When the surface of a large, partially enclosed body of water, such as one of the Great Lakes, a bay, or a harbor, is disturbed, long waves may be established which rhythmically oscillate as they reflect from opposite ends of the basin. These waves, called seiches (pronounced “say-chez”), have a period that depends on the size and depth of the basin. Many people are unaware of seiches due to their very low wave height and extremely long wavelength. In large lakes, seiches may result from differential atmospheric pressure changes, but more frequently are caused by winds. For example, a strong wind blowing for several hours along the axis of a large lake will drive the surface water toward the leeward (downwind) end of the lake, raising the water surface there as much as 3 m (10 ft), while lowering the level at the windward end of the lake. This oscillation, which diminishes rapidly in amplitude, has a period that may exceed 12-14 hours. In bays or harbors that are open to the ocean, seiches nearly always result from a long-period wave train. When the water in the bay is sent in motion by the initial wave, seiches continue at the natural period of oscillation for that harbor or bay. Seiches can affect diving by reducing visibility and by rapidly changing the water level at entry and exit points. CURRENTS In addition to tidal currents, there are several other types of currents with which you should be familiar. Longshore Current Waves approaching shore at an angle cause a current system that flows parallel to shore. These currents, which can achieve a velocity too great to swim against when the surf is large, are known as longshore currents (figure 2-13). The speed of the current is usually less than one Slack Time Most Current Slack Ebb Meters per second Meters TIDES TIDAL CURRENTS FIGURE 2-12. TIDAL HEIGHT VS. TIME
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