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 oscillation 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. The 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 shore- 

 line. (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, 



V 



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

 a characteristic velocity. The Reynolds number is of importance 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 by wave or other action. Ridges may occur singly or as a series of 

 approximately parallel deposits. British usage, FULL. (See Figure A-7.) 



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. 



RIP CURRENT. A strong surface current flowing seaward from the shore. It 

 usually appears as a visible band of agitated water and is the return 

 movement of water piled up on the shore by incoming waves and wind. With 

 the seaward movement concentrated in a limited band its velocity is 

 somewhat accentuated. A rip consists of three parts: the FEEDER CURRENTS 

 flowing parallel to the shore inside the breakers; the NECK, where the 

 feeder currents converge and flow through the breakers in a narrow band or 

 "rip"; and the HEAD, where the current widens and slackens outside the 

 breaker line. A rip current is often miscalled a rip tide. Also RIP 

 SURF. See NEARSHORE CURRENT SYSTEM. (See Figure A-7.) 



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