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REFRACTION DIAGRAM - A drawing showing positions of wave crests and/or 

 orthogonals in a given area for a specific deepwater wave period 

 and direction. (See Figure A-6.) 



RESONANCE - The phenomenon of amplification of a free wave or oscilla- 

 tion of a system by a forced wave or oscillation of exactly equal 

 period. The forced wave may arise from an impressed force upon 

 the system or from a boundary condition. 



RETARDATION - Tlie amount of time by which corresponding tidal phases 

 grow later day by day (about 50 minutes). 



RETROGRESSION OF A BEACH - (1) A continuing landward movement of the 

 shoreline. (2) A net landward movement of the shoreline over a 

 specified time. Also RECESSION. 



REVETMENT - A facing of stone, concrete, etc., built to protect a scarp, 

 embankment, or shore structure against erosion by wave action or 

 currents . 



REYNOLDS NUMBER - The dimensionless ratio of the inertial force to the 

 viscous force in fluid motion, 



R - ^ 

 Re - y 



where L is a characteristic length, v the kinematic viscosity, 

 and V a characteristic velocity. The Reynolds number is of im- 

 portance in the theory of hydrodynamic stability and the origin 

 of turbulence . 



RIA - A long, narrow inlet, with depth gradually diminishing inward. 



RIDGE, BEACH - A nearly continuous mound of beach material that has been 

 shaped up by wave or other action. Ridges may occur singly or as 

 a series of approximately parallel deposits. (See Figure A-7.) 

 British usage, fulls. 



RILL MARKS - Tiny drainage channels in a beach caused by the flow seaward 

 of water left in the sands of the upper part of the beach after the 

 retreat of the tide or after the dying down of storm waves. 



RIP - A body of water made rough by waves meeting an opposing current, 

 particularly a tidal current; often found where tidal currents are 

 converging and sinking. 



RIPARIAN - Pertaining to the banks of a body of water. 



RIPARIAN RIGHTS - The rights of a person owning land containing or border- 

 ing on a watercourse or other body of water in or to its banks, bed, 

 or waters . 



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