response to waves and currents acting within the geometric form of a 
given shoreline. This conceptual model is first considered in terms of 
unmodified natural beach phenomena, and is then examined to discern those 
features of the model that are of particular interest to the beach engineer. 
DEVELOPMENT OF A CONCEPTUAL BEACH MODEL 
Figure 1 shows a cross-section of a beach with its foreshore rising 
to the berm, a relatively flat backshore, and dunes in the hinterland. The 
beach deposit is a three-dimensional body of sediment that lies on some 
foundation, indicated as bedrock in the figure. There may also be some 
interfingering of beach and dune sand, as well as of beach sand and near- 
shore bottom silts, as indicated by the dashed beach limits in Figure 1. 
The elements from which the conceptual beach model is constructed are 
shown in the cross-section of Figure 2. The hinterland is arbitrarily 
separated from the beach proper, and may consist of dunes, a cliff or bank, 
or a lagoon such as occurs behind barrier beaches. The backshore portion 
of the beach is composed of dry sand at its surface. This sand is shifted 
about by blowing winds, and under suitable conditions the blown sand ac- 
cumulates as dunes piled up in part on and behind the backshore, as shown 
in Figure 1. Dune sand is generally finer than beach sand, and has some- 
what rounder particles, as a result of selective wind transportation in 
terms of grain size, shape, and density. Thus the dune sand is a deriva- 
tive deposit from the beach sand, and is properly considered as part of the 
hinterland. 
The foreshore and nearshore bottom in Figure 2 represent zones of 
active hydraulic forces along the shore. The crest of the berm marks the 
limit of maximum uprush of waves during any given state of the sea. More 
than one berm height may be preserved, some representing seasonal or storm 
uprush levels. The uprush-backwash zone, essentially coincident with the 
foreshore, lies between the berm and the plunge point where the waves 
break. This zone is subjected to a succession of highly turbulent up- 
rushing tongues of water that come momentarily to rest on the foreshore, 
and then return downslope as backwash less turbulent in its flow. The 
effect of this continuous to-and-fro motion is to sort out and arrange the 
foreshore material selectively according to its particle size, shape, and 
density; and to produce the geometrical form of the foreshore as expressed 
by slope, width, and height of berm. 
The plunge point is a relatively narrow zone of considerable activity, 
in which the breaking waves generate a high degree of turbulence that 
carries bottom material into suspension. Part of this material is swept 
beachward by the turbulent surge, and part of it, usually the finer material 
is moved seaward by turbulence diffusion. Normally the plunge zone is 
marked by a shallow trough in the nearshore bottom, composed of somewhat 
b 
