In terms of foreshore slope as a response element, the diagram 
indicates that this is influenced by wave energy, by shore currents, by 
the average size of the grains present on the foreshore, and very likely 
by the nearshore bottom slope. A response produced by the simultaneous 
effect of several factors (some of which may themselves be response 
elements), raises an important question in the evaluation of the relative 
effects played by each of the contributing factors. The engineer who 
replenishes a beach with imported sand needs to ask himself whether the 
mean grain size of the imported sand is the most important slope control 
in the area concerned, or whether other factors such as wave energy and 
shore currents are more important. 
Until relatively recently, quantitative field data for analysis of 
these interlocking situations were not available; moreover, until the 
high-speed computer became generally available these complex regression 
problems could not be handled without a severe drain on man-hours avail- 
able for hand computation. 
It is evident from earlier remarks that the continued refinement of 
quantitative beach models involves analysis of field interrelations be- 
tween process and response elements, as well as scale model experiments 
involving these same elements treated perhaps on a more analytical level. 
These interrelations can be studied in terms of a large variety of con- 
ceptual, physical, mathematical, and statistical models of beach behavior, 
In part the identification of an optimum model requires a search for under- 
lying principles of data interlock and feedback, such that these may ulti- 
mately be expressed as functional relationships. The concept of energy 
acting upon material furnishes a basic approach, as is illustrated in the 
following paragraphs. 
Figure 7 represents a rearrangement of elements in the process- 
response model of Figure 5, to indicate an alternative way of expressing 
the model as it applies to the foreshore, with explicit treatment of the 
energy factor. The model is now divided into three parts. On the left 
are two elements that may be called shore controls, which include the 
shore geometry and shore material as shown on the left-hand side of 
Figure 5, The right-hand block, showing the beach responses, contains 
the elements of foreshore beach geometry and beach material from the 
right-hand side of Figure 5. The central block in Figure 7 contains the 
energy factors - the operative geological agents - that produce the fore- 
shore effects indicated by the main arrow pointing to the right. 
In this variant of the model the energy factors - waves, tides, and 
currents - are those acting on the foreshore. A corresponding model for 
the backshore and dunes would have the geometry and materials of the back- 
shore in the left-hand block, with a new central energy block having the 
term "wind work", and with a new response block on the right representing 
the geometric form and material composition of the hinterland dunes. 
