Backwash marks

The sand on a beach displays several interesting features, as seen in the upper color insert on page C-l. Ripples are the marks made in sand by waves that rush onto the shore. Rills are small, branched depressions in the sand that drain water back toward the ocean. Diamond-shaped deposits of silt are backwash marks, places where the shells of animals interfere with the normal backwash of water. Regularly spaced, crescent-shaped depressions along the sand are called cusps. No one knows for sure how cusps form, but many believe them to be due to irregularities along the beach that are enlarged by swash, the water that runs off the beach after a wave breaks. 

The process centers on a fixed-bed downflow reactor that allows catalyst replacement without causing any interruption in the operation of the unit (Figure 9-28). Feedstock is introduced to the process via a filter (backwash, automatic) after which hydrogen and recycle gas are added to the feedstock stream which is then heated to reactor temperature by means of feed-effluent heat exchangers whereupon the feed stream passes down through the reactor in trickle flow. Sulfur removal is excellent (Table 9-18), and substantial reductions in the vanadium content and asphaltenes content are also noted. In addition, a marked increase occurs in the API gravity, and the viscosity is reduced considerably. 

The swash can carry materials, such as driftwood, seashells, and seaweed, which are often left on the beach as a line marking the extent of high tide. On a falling tide the backwash may form a series of channels. 

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