The form of the model in Figure 7 is more flexible than that in 
Figure 5, in that it can separate the influence of different energy 
sources that may operate successively on essentially the same materials. 
Thus, the response elements of the foreshore processes produce a set of 
backshore controls whose responses under wind work are sand dunes. 
In all models of the form shown in Figure 7 at least one major feed- 
back loop is present, although additional feedback loops extend from the 
response block to the central energy block. 
Two implications of the model in Figure 7 deserve comment. The first 
is that the blocks can be expressed as matrices of numbers, and the model 
may be manipulated by the procedures of matrix algebra, conveniently 
treated with high-speed computers. For this sort of analysis the central 
matrix is a transformation matrix, as pointed out by Griffiths (1962) in 
his discussion of a general petrogenetic model. 
A second feature of the conceptual beach model is that it permits 
examination of those aspects of beach processes that can be controlled by 
the engineer. The engineer enters from outside the natural beach model, 
and thus introduces an element different either from data interlock or 
feedback. 
SHORE PROCESSES AND THE BEACH ENGINEER 
The role of the engineer may be seen by referring back to the process- 
response models in Figures 5 and 7. Though the engineer cannot directly 
control the height, period, or angle of approach of the waves, he can 
design structures such as offshore breakwaters to modify the pattern of 
energy flow to the beach, as is illustrated by Halil (1963) - Figures 13 
and 15. 
The engineer can exert direct control on the material factor of the 
process elements. This may be accomplished by introducing imported sand 
onto a beach in the form of stock piles or a sand fill placed along the 
foreshore. By this introduction of selected material, the engineer 
strongly influences such response elements as the foreshore slope and 
average grain size of the resulting modified beach deposit, as well as 
the width of the backshore. 
Another important way in which the beach engineer can modify the 
response side of the beach model is by building structures that introduce 
special boundary conditions into the geometry of the shoreline. Among 
the commonest of these is the groin, built normal to the shoreline. 
Introduction of a groin produces an accumulation of sand (if beach drift 
is present) on the upbeach side. Entrapment of sand on the upbeach side 
of the groin lessens the amount of sand moving downbeach from the groin. 
During this time of downbeach impoverishment, erosion may occur immediately 
downbeach of the groin. If this is severe, the structure may be flanked, 
