fluctuations.  As  depicted,  diagonal  deployment, in lieu of perpendicular, has been generally found more effective in flowing streams. Procedures to contain spills on land vary with the amount and type of oil spilled, the type of soil and the terrain. Less viscous oil and more porous soil allow greater and more rapid penetration and lateral migration in the soil. Where feasible, ab- sorbent materials should be applied as soon as possible. Larger spills may require containment devices such as interceptor trenches or collecting pools from which the oil may be pumped. Spill containment by the use of hose spray can be an effective method in confined areas. This technique is immediately available to ships’ forces and provides the earliest form of containment. Phase  IV—Recovery,  Mitigation, and Disposal This phase of an oil spill involves those ac- tions taken to recover spilled oil from the affected environment as well as the monitoring activity associated  with  determination  of  the  effectiveness of the cleanup operation. It includes those actions taken to mitigate damage cause by the spilled oil, and  to  dispose  of  the  recovered  oil  in  an  en- vironmentally   acceptable   manner. REMOVAL.—Removal of spilled oil and oil derivatives may be accomplished several ways, including: 1.   Allowing   evaporation   to   take   place (gasoline and JP-4). 2. Use of physical removal methods such as manual collection or collection by mechanical equipment, such as skimmers. 3. Removal by fostering biodegradation. 4. Removal by burning. 5. Removal by dispersion (emulsification). 6. Pumping of oil in land spills. Because of effects which are detrimental to the environment, method 4 is not recommended, or practiced, by the Navy unless there is a direct threat to human life and property. Because of the lengthy reaction time involved, and because of the possibility  of  toxic  by  products,  method  3  is  not practiced  nor  recommended  as  a  desirable  Navy practice. However, it may occur, and can con- stitute a final polishing action if all the oil is not removed by physical means. In  addition,  gelling  agents  (chemicals  which convert the spill to a semisolid mass) or sorbent materials such as straw, polyester plastic shavings, or polyurethane foam may be used to help the subsequent manual or mechanical removal of a spill. PHYSICAL  REMOVAL  METHODS.—The Navy  prefers  physical-mechanical  methods  of removal, and has designated the types of skim- mers for use with Navy spills in various locations. 1. Small Skimmers. The small unit which is designed for use in congested harbor areas is based on the weir principle. The weir depth of these skimmers is controlled by adjusting the flow rate of  the  attached  pump.  As  the  flow  rate  is increased,  the  fluid  is  removed  from  the  rear buoyancy chamber, tipping the unit clockwise, and thereby increasing the weir depth. Decreas- ing the flow rate allows the buoyancy chamber to  fill,  tipping  the  unit  counterclockwise,  and thereby reducing the weir depth. This unit is most effective in a stationary mode where it is posi- tioned and the oil directed to it. 2. Medium Skimmers. The  medium skimmer selected by the Navy is an “endless” belt unit. It  is  operable  from  a  pier  via  handheld  controls. The principle of operation is shown in figure 8-4. The rotating belt submerges the oil and directs it to the collection well where it concentrates and from which it eventually is pumped to a temporary storage. This principle is entitled the dynamic in- clined plane (DIP t m ). 3.  Large  Skimmers.  The  large  skimmer selected for use by the Navy is a larger version of  the  medium  skimmer  (DIP^tm).  This  unit  is vessel-mounted for use in protected open waters, and is quite effective even in choppy water in that it overruns and submerges the oil layer before col- lecting it. A rotating belt directs the oil to the col- lection well. 4. Suction Based Skimmers. Other commer- cially available units for oil removal are based on suction, either taken directly off the surface of the water or by the development of a submerged vortex. Since these units are highly susceptible to wave action and clogging, they work best in calm, ENGINEMAN  1  &  C 8-8