beach site or brought in by currents and tides. The average grain dia- 
meter of the foreshore sand, for example, is controlled by the particular 
combination of process elements that have recently occurred or are going 
on at some given time. 
Although the same attributes of the shore and beach materials are 
listed on the process and response sides of the model in Figure 5, those 
on the left represent initial properties, whereas those on the right are 
response properties. Thus, by winnowing action on the original material, 
or by inflow of new materials along the shore, the response properties 
of the beach material may be significantly different from the initial 
materials. 
A feature common to all elements in the conceptual model, though not 
explicitly stated in Figure 5, is the pattern of areal variation shown by 
the grain size, the angle of foreshore slope, and by other attributes, over 
the beach area as a whole, as was developed in Figure 4. Beach firmness, 
selected as an example for that map, represents a complexly interlocked 
response related to grain size, moisture content, and porosity or degree 
of packing of the grains. 
Until relatively few years ago, limitations both in instrumentation 
and in methods of data analysis have hindered quantitative expression of 
conceptual beach models. As more quantitative data become available, 
additional implications of these models can be examined in terms of data 
interlock and feedback on natural beaches. As previous paragraphs indi- 
cate, close relations occur among the process elements themselves, in that 
the geometry of the beach site as expressed by nearshore bottom slope, 
influences the pattern of energy distribution on the shore. This is an 
example of interlock in that these two process elements are not wholly 
independent. A beach deposit, formed by wave and current energy, may 
involve changes in the configuration of the bottom slope with time. These 
changes in turn modify the pattern of wave approach, so that a response 
element may exert a feedback control on one or more process elements. 
These complexities are relatively common in process-response models, and 
a generalized feedback arrow is included in Figure 5. 
Figure 6 shows a few data-interlock and feedback relationships that 
occur in natural beach processes. These have a strong bearing on engin- 
eering design. For example, a shore structure may produce unexpected 
results, if data interlock and feedback are not taken into consideration. 
As Figure 6 shows, wave energy exerts a controlling influence on the grain 
size of particles on the beach; it influences the foreshore slope and 
nearshore bottom slope; and (if waves approach at an angle) it controls 
in part the velocity and direction of the shore current. The shore cur- 
rent influences average grain size, and may also modify the nearshore 
bottom slope. Hence, the dashed arrow along the top of Figure 6, from 
bottom slope to wave energy, represents feedback. 
