Pressure equalisation modules (PEM) are vertical pipes that are placed in rows forming matrix along the costline (above figure). The ussage of this system is to stimulate the accretion of sand on certain beaches and slow down the erosion process on other beaches. Cross-shore sand tongue develops in front of each row of the PEM systems to create the same effect as groynes in aresting some of the transported long shore sand, thus building up the sand between the rows. The pipes act as pressure equalizer that equalise the pressure in the groundwater zone and increased circulation of seawater in the coastal profile will take place thus promoting sedimentation.
The PEM system is simple in design but is based on relatively difficult phenomena under the principle of soil drainage. The swash zone is the zone is the area of the beach whare waves crash and run up along the beach slope. The seawater then returns down the beach face but at the same time permeates into the profiles. These mechanism will put the PEM system to provides a faster passage for the water within the vertical profile of the swash zone to drain. During the high tide, the seawater extends further up the beach which means pressure is built up in the layer underneath where there is fresh water tongue originating from the hinterland. The drainage passage created by the PEM system thus equalises the pressure within the upper and the lower profiles.
When the water level is low on the coast during the period from low tide to high tide, the water circulation in the swash zone increases the depositing of materials on the foreshore, thereby building up the beach from the sediment transported along the coast, so that this PEM system is the one of the suitable system in the slowdown of the erosion rate of the beach as well as to protect the beach from severe erosion.
(adapted from JURUTERA magazine August 2009)
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