PORT WORKS DESIGN MANUAL PART 5 Guide to Design of ...

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and erosion portions of the shore near the structure will tend to adjust their alignments
parallel to an equilibrium orientation.
It should be noted that the orientation of the beach alignment may vary with time due to
variation of wave directions. A dynamic equilibrium of the shoreline orientation exists in
which its mean position over a long period of time remains unchanged.
2.4.2 Beach Profile
Cross-shore transport takes place mainly as a result of the change in wave conditions and
water levels. Beach profile changes continuously in response to these changes. By
averaging these profiles over a long period, a mean profile or equilibrium beach profile can
be defined. A simplified description of the equilibrium beach profile is given by
Bruun (1954) :
d = Ay 2 / 3
where
d = Water depth at distance y from the shoreline (m).
y = Distance from the shoreline (m).
A = Dimensional factor having unit of length to the one-third power (m 1/3 ), mainly
depending on the stability characteristics of the bed material.
For information, A can be expressed as (Dean, 1991) :
A = 0.21D 0.48
where
D = Grain size in mm.
The equation was derived empirically as an appropriate representation of natural beach
profiles averaged over a long time span. It does not exhibit bars and troughs. It simply
represents a best-fit description of a profile passing through such features. There are other
forms of expression for more complicated cross-shore profiles; further details can be found in
CUR (1987).
In reality, the water level and sediment size at a given site can vary and therefore the
equilibrium beach profile changes accordingly. Figure 5 shows how the equilibrium profile
changes if one of these factors changes. A dynamic equilibrium exists in which the profiles
are more or less the same over a long period of time.