Chapter 2- The Diving Environment The Diving Environment 45 recognized diver distress signal. Boat Diving The following procedures are recommended for diving when currents are anticipated or present (figure 2-15): • Always begin the dive against a current and not with it unless trained, prepared, and equipped to make a drift dive. Stay close to the bottom, pull yourself along, and avoid unnecessary kicking, which can lead to overexertion and fatigue. Turn around with at least one-half of your air supply remaining and return with the current. • Descents should be made down a weighted descent line or down the anchor line. Free descents in currents should be avoided. • A trail line at least 30 m (100 ft) in length should be extended from the dive boat for divers to use to pull themselves back to the vessel hand over hand if they surface downstream. • A qualified operator should remain on the boat at all times to assist anyone swept downstream. Note that the maximum swimming speed of a fully equipped physically fit scuba diver is about 0.5 m/sec (1 knot), and even that may be sustainable for only a brief period, depending on the diver’s fitness. Swimming against a 0.5 m/sec (1 knot) current is, therefore, unwise because it will quickly lead to exhaustion. Diving by drifting with a current is a popular activity in some areas, but training and supervised experience are paramount for safety. An orientation to drift diving is presented later in this book. River Currents Currents are also caused by gravity, demonstrated when water flows downhill as in rivers. Such currents can be extremely strong and hazardous. Sediment and other matter or bubbles on the surface can reduce visibility in rivers to zero. Currents in rivers are swiftest in the middle, at the surface, and on the outside edges of bends. The speed of the current decreases with depth and with proximity to the shore. The swift waters of rivers and the varying bottom Current FIGURE 2-15. CURRENT DIVING Trail Line conditions can produce rapids, whirlpools, sucking holes, waterfalls, eddy currents, and other unique and hazardous forms of water movement, particularly around and below dams and weirs. Special training, procedures, and equipment are required for diving in swift flowing rivers. THERMAL CHANGES Water temperatures can vary with season, with depth, and with changing wind and weather conditions. The common thermal changes in fresh and salt water will be described below. Upwelling During conditions of continuous, strong offshore, sideshore, or side-onshore winds along a coast, warm surface water can be blown offshore and replaced by colder, nutrient-laden water from beneath. This flow of colder water, brought vertically to the surface from the depths, is known as upwelling (figure 2-16). Certain areas with offshore drop-offs and seasonal winds are prone to have upwellings. An upwelling initially results in colder, clearer water with increased amounts of nutrients, which can lead to plankton blooms. Good diving conditions prevail for a few days until the excess nutrients begin to foster plankton growth which will reduce water visibility. Thermal Stratification During summer months, the surface waters of lakes and quarries are warmed by the sun and form a layer of
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