Coastal zones in many countries suffer from strong offshore direct rip currents. Rip currents regularly
lead to hazardous situations, and at some beaches swimming is prohibited for a considerable time of the
year especially during summer storm. Also, the swimming may be prohibited at beaches where the wave
height is too high. Saski et al (1975) concluded that breaker heights smaller than 0.6m and current
velocities smaller than 0.2m/s are considered as comfortable swimming conditions, but it is hard to swim
against a rip current of 0.5m/s and breaker height greater than 2.0m even for good swimmers. So, there is
an essential need to construct suitable coastal structure in order to secure safe conditions for swimmers.
There are several plans and investigations for improvement of the swimming conditions. The use of
coastal structures is the tool in many cases and, hence, the impact of their construction becomes of great
interest. The impact of marine structures on shoreline changes and water quality under various waves,
current and site conditions is of great interest to engineers and scientists. The present study has been
recommended the use of perched beach as a possible alternative for safe swimming conditions. So, water
quality within perched beach basin must be considered, and it is particularly important for health and
environmental quality, especially in warmer climates where biological processes are accelerated.
Successful control of water quality is usually dependent upon periodic exchange of the basin water with
the sea water of the open sea. RMA model was applied to investigate various configurations of the
perched beach including submergence ratio of the breakwater, groin with/without gap, the gap
width/location and emerged/submerged groins. These configurations have been compared from the point
of view of flushing rates to develop general guidelines for the design of similar constructions.
It has been found that the perched beach could be a reliable solution for protecting swimmers along the
coast with minimum impact on the shoreline while preserving acceptable water quality within the
perched beach. Good flushing rates can be achieved if the crest level of the submerged breakwater and
groins is at least 0.5m below M.S.L. and the groin at the up drift side has a gap located near its offshore
end, while the groin at the down drift side has a gap close to the shoreline. To allow water flow along the
shoreline, it is recommended to have the up drift gap wider than the down drift one. |
