IDENTIFY ENVIRONMENTAL AND OPERATIONAL HAZARDS

CHAPTER 6 — Operational Planning Change A 6-15 These currents may run as fast as two knots and may extend as far as one-half mile from shore. Rip currents, not usually identified in published tables, can vary significantly from day to day in force and location. Surface Current Generated by Wind. Wind-generated surface currents are temporary and depend on the force, duration, and fetch of the wind. If the wind has been blowing steadily for some time, this current should be taken into consideration especially when planning surface swims and scuba dives. 6-5.3.5.1 Equipment Requirements for Working in Currents. A diver wearing a surface- supplied outfit, such as the MK 21 SSDS with heavy weights, can usually work in currents up to 1.5 knots without undue difficulty. A diver supplied with an addi- tional weighted belt may be able to accomplish useful work in currents as strong as 2.5 knots. A scuba diver is severely handicapped by currents greater than 1.0 knot. If planning an operation in an area of strong current, it may be necessary to schedule work during periods of slack water to minimize the tidal effect. 6-6 IDENTIFY ENVIRONMENTAL AND OPERATIONAL HAZARDS Underwater environmental conditions have a major influence on the selection of divers, diving technique, and the equipment to be used. In addition to environ- mental hazards, a diver may be exposed to operational hazards that are not unique to the diving environment. This section outlines the environmental and operational hazards that may impact an operation. 6-6.1 Underwater Visibility. Underwater visibility varies with depth and turbidity. Hori- zontal visibility is usually quite good in tropical waters; a diver may be able to see more than 100 feet at a depth of 180 fsw. Horizontal visibility is almost always less than vertical visibility. Visibility is poorest in harbor areas because of river silt, sewage, and industrial wastes flowing into the harbor. Agitation of the bottom caused by strong currents and the passage of large ships can also affect visibility. The degree of underwater visibility influences selection of dive technique and can greatly increase the time required for a diver to complete a given task. For example, a diving team preparing for harbor operations should plan for extremely limited visibility, possibly resulting in an increase in bottom time, a longer period on station for the diving unit, and a need for additional divers on the team. 6-6.2 Temperature. Figure 6-11 illustrates how water temperature can affect a diver’s performance, and is intended as a planning guide. A diver’s physical condition, amount of body fat, and thermal protection equipment determine how long expo- sure to extreme temperatures can be endured safely. In cold water, ability to concentrate and work efficiently will decrease rapidly. Even in water of moderate temperature (60–70°F, 15.5–21.5°C), the loss of body to the water can quickly bring on diver exhaustion. 6-6.3 Warm Water Diving. Warm water diving is defined as those diving operations that occur in water temperatures exceeding 88° F. During recent studies at the Navy Experimental Diving Unit, physiological limits have been developed for diving