Decompression sickness: a review.
Work in compressed air is a dangerous activity, whether it is carried on under water in a conventional diving suit, in self-contained breathing apparatus or in a diving bell, or in relatively dry conditions in a caisson or an underwater tunnel. Surprisingly large numbers of men have worked at one time or another in compressed air in tunnels or caissons and it is the dangers which arise to the health of these men that are considered here. When a tunnel is driven through water-logged strata or through porous ground under a river, the open end of the tunnel must be sealed off and air pumped in to balance the hydrostatic pressure, should there be any danger of water flooding in and bringing with it unmanageable quantities of silt or sand. A pier of a bridge may be constructed in deep water by means of a caisson (Fr. caisse, a box), which is a compressed air chamber in which a gang of men excavate foundations. In both tunnels and caissons men come and go through an air lock and must be compressed on entering and decompressed at the end of the work period. Although working conditions in these circumstances have much improved over the last 6o years, our understanding of the reactions of the human body to atmospheric pressures greater than the normal and of the pathogenesis of decompression sickness is still insufficient to prevent illness completely. In engineering practice it is customary for working pressure to be expressed as gauge pressure in pounds per square inch (p.s.i.g.) which is the pressure over and above the normal atmospheric pressure of I4-7 p.S.i. (I 03 kg./cm.2). For certain calculations absolute pressure (gauge pressure plus atmospheric pressure) is used, and for clarity it must always be stated whether gauge or absolute pressure is referred to. In naval practice it is usual to think in terms of feet of sea water, each 33 feet of depth corresponding to about one atmosphere or approximately I5 p.s.i. Sometimes the unit of pressure used is the atmosphere (atm.). On the
Decompression Sickness During the Sinking of a Caisson
Most of the studies of decompression sickness in industry have been made during the construction of tunnels.
Velocity moments in alongshore bottom stress parameterizations
The time-averaged alongshore bottom stress is an important component of nearshore circulation models. In a widely accepted formulation the bottom stress is proportional to , the time average of the product of the instantaneous velocity magnitude |u→| and the instantaneous alongshore velocity component υ. Both mean and fluctuating (owing to random, directionally spread waves) velocities contribute to . Direct estimation of requires a more detailed specification of the velocity field than is usually available, so the term is parameterized. Here direct estimates of based on time series of near-bottom currents observed between the shoreline and 8-m water depth are used to test the accuracy of parameterizations. Common parameterizations that are linear in the mean alongshore current significantly underestimate for moderately strong alongshore currents, resulting in overestimation of a drag coefficient determined by fitting modeled (with a linearized bottom stress) to observed alongshore currents. A parameterization based on a joint-Gaussian velocity field with the observed velocity statistics gives excellent overall agreement with the directly estimated and allows analytic investigation of the statistical properties of the velocity field that govern . Except for the weakest flows, depends strongly on the mean alongshore current and the total velocity variance but depends only weakly on the mean wave angle, wave directional spread, and mean cross-shore current. Several other nonlinear parameterizations of are shown to be more accurate than the linear parameterizations and are adequate for many modeling purposes.