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

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(2) Detached Breakwaters
A primary consideration in the design of detached breakwaters for a beach project is the
desired plan form and beach width behind the breakwater. If a breakwater is close to the
shore and long with respect to the wavelength of the incident waves, sand will continue to
accumulate behind the breakwater until it connects with the breakwater; this is called a
tombolo (see Figure 14). If the breakwater is far from the shore and short with respect to
the wavelength of the incident waves, the shoreline will build seawards, but is prevented by
wave action and longshore currents from connecting with the breakwater. The shoreline
bulge that forms is termed a salient.
An important design parameter is the amplitude of the salient y s (see Figure 14), as it is a
measure of the amount of beach created by the breakwater. Hsu and Silvester (1990)
developed equations showing the relationship of y s /y B (ratio of salient amplitude to
breakwater distance from shoreline) and y B /L B (ratio of breakwater distance from shoreline to
breakwater length). Dean and Dalrymple (2002) prepared Figure 14 based on the formula
proposed by Hsu and Silvester. The curve yields some interesting results that if the
breakwater is located at a distance offshore greater than about 6 times its length, it will have
no effect on the shoreline. A tombolo may be formed for y B /L B less than 1. Therefore, the
curve may be unreliable for small values of y B /L B .
For multiple detached breakwaters, the gap width between breakwaters will also affect the
form of the shoreline within the breakwaters. Generally, the gap width should be larger than
the length of the breakwaters if no tombolo is to be formed, but should not be too large that
the interaction between breakwaters disappears or each breakwater will simply act as a single
offshore structure. More information on the gap width between breakwaters are given in
Dean and Dalrymple (2002).
(3) Artificial Headlands
Silvester and Hsu (CIRIA, 1996) developed design curves for the calculation of the final
equilibrium plan shape of a beach contained within headlands from observations or
measurements of natural crenulate bays which appear to be in equilibrium with the incident
wave climate. The method is based on the principle that waves dominating the longshore
transport for the stretch of coast are diffracted around the upcoast headland and refracted in
such a way as to arrive normal to the shoreline of the bay along its entire periphery.
Longshore transport is therefore reduced to a minimum and the shoreline remains stable with