682 BAKBER AND TUCKER [CHAP. 19 



be no breaking at all, the waves being reflected back towards the open sea. The 

 style of breaking or reflection depends in some way, not fully understood, upon 

 the steepness (ratio of height to wavelength) of the waves in deep water and 

 upon the slope of the beach. Tides are long low waves and may be said to be 

 reflected from all coasts unless the bore which can develop over extensive 

 littoral flats is to be thought of as a breaking tide. Low swell on the other hand 

 may need a beach slope of perhaps 1 : 5 to reflect it without breaking. Experi- 

 mental or theoretical studies of breaking waves are discussed by Stoker (1957, 

 page 351), but the topic is a difficult one. As a rough guide to plunging breakers, 

 it may be said that they usually develop where the undisturbed depth is a little 

 greater than the height of the breaker. Spilling breakers can start in much 

 deeper water. 



Though it was not mentioned in section 1 of this Chapter, it is known (Lamb, 

 1945, Art. 250) that steep waves produce some overall forward transport of 

 water. When waves steepen as they pass into shallow water and reach the surf 

 zone, they drive increasing quantities of water forward towards the shore. This 

 water may tend to return seaward at special places, determined perhaps by 

 the contours of the covered beach. Such "rips" carry water back through the 

 surf and are a danger to bathers, but the current usually disappears just sea- 

 ward of the surf zone (Shepard, Emery and LaFond, 1941). Groups of high 

 waves can bring in extra amounts of water and the fluctuating depth of the 

 water near the beach is often quite evident. The seaward flow of this water, 

 during intervals when the surf is low, can produce at intervals of two to five 

 minutes low surges that are transmitted out to sea and can be observed in 

 deeper water by sensitive instruments (Munk, 1949; Tucker, 1950). These 

 surges have been called the "beat of the surf". 



Beach material is continually in motion where the water is shallow. Sand 

 ripples develop everywhere on the submerged beach as a result of the backward 

 and forward motion of the water, but tend to be erased as the beach becomes 

 exposed (Bagnold, 1947). Where long swell penetrates into shaflow water, the 

 waves induce a forward transport of water, most especially in a thin layer next 

 to the sea-bed (Longuet-Higgins, 1953). Such waves, therefore, drive material 

 shoreward and build up the beach. The reverse is the case with short steep 

 waves built up by a local wind. Their forward transport takes place mainly in 

 the surface and the compensating drift near the sea-bed tends to carry beach 

 material out seaward and the beach is eroded. 



Waves that approach a shore obliquely tend to carry material along it and 

 it is commonly found that sandy beaches arrange themselves so that their 

 contours are at right angles to the mean direction of the incoming waves. On a 

 coastline inclined to the prevailing waves or swell, beach material is continually 

 in progress along the coast. Groynes do not permanently arrest this drift but 

 they serve to build up the beach level until sand can pass the groynes. The 

 danger in introducing any large barrier which would arrest the drift of sand is 

 that it may starve beaches lying further along the coast and lead to erosion. 

 The problem in designing many harbours is that sand must be allowed to cross 



