PROBABLE MAXIMUM HURRICANE - A hypo-hurricane that might result 

 from the most severe combination of hurricane parameters that 

 is considered reasonably possible in the region involved, if 

 the hurricane should approach the point under study along a 

 critical path and at optimum rate of movement. This estimate 

 is substantially more severe than the SPH criteria. 



DESIGN HURRICANE - A representation of a hurricane with specified 

 characteristics that would produce hurricane surge hydrographs 

 and coincident wave effects at various key locations along a 

 proposed project alinement. It governs the project design 

 after economics and other factors have been duly considered. 

 The design hurricane may be more or less severe than the SPH, 

 depending on economics, risk, and local considerations. 



IMPERMEABLE GROIN - A groin through which sand cannot pass. 



INDIAN SPRING LOW WATER - The approximate level of the mean of lower low 

 waters at spring tides, used principally in the Indian Ocean and 

 along the east coast of Asia. Also INDIAN TIDE PLANE. 



INDIAN TIDE PLANE - The datum of INDIAN SPRING LOW WATER. 



INLET - (1) A short, narrow waterway connecting a bay, lagoon, or similar 

 body of water with a large parent body of water. (2) An arm of the 

 sea (or other body of water), that is long compared to its width, and 

 may extend a considerable distance inland. See also TIDAL INLET. 



INLET GORGE - Generally, the deepest region of an inlet channel. 



INSHORE (ZONE) - In beach terminology, the zone of variable width extend- 

 ing from the low water line through the breaker zone. SHOREFACE. 

 (See Figure A-1 .) 



INSHORE CURRENT - Any current in or landward of the breaker zone. 



INSULAR SHELF - The zone surrounding an island extending from the low 

 water line to the depth (usually about 100 fathoms) where there is 

 a marked or rather steep descent toward the great depths. 



INTERNAL WAVES - Waves that occur within a fluid whose density changes 



with depth, either abruptly at a sharp surface of discontinuity (an 

 interface) or gradually. Their amplitude is greatest at the density 

 discontinuity or, in the case of a gradual density change, somewhere 

 in the interior of the fluid and not at the free upper surface where 

 the surface waves have their maximum amplitude. 



IRROTATIONAL WAVE - A wave with fluid particles that do not revolve around 

 an axis through their centers, although the particles themselves may 

 travel in circular or nearly circular orbits. Irrotational waves may 

 be progressive, standing, oscillatory, or translatory. For example, 

 the Airy, Stokes, cnoidal and solitary wave theories describe irro- 

 tational waves. See TROCHOIDAL WAVE. 



A-18 



