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4.3 OPEN INTAKES 83
are the costliest type of intake but they allow collecting the largest volume of water per unit surface of coastal beach area and, therefore, they are more attractive choice for larger desali- nation plants. As indicated previously, infiltration gallery has been used for a 50,000 m3/day (13.2 MGD) SWRO desalination plant in Japan but has not found wide application because the cost of the intake is over 50% of the cost of the desalination plant. Usually, the construc- tion cost of open intakes and other types of subsurface intakes is 10%e30% of the total con- struction cost of the desalination plant.
4.3 OPEN INTAKES
Based on the location of their inlet structure, open intakes are typically classified as onshore and offshore. The inlet structure of onshore intakes is constructed on the banks of the source water body while the inlet structure of offshore intakes is typically located several hundred to several thousand meters away from the shore.
4.3.1 Onshore Open Intakes
To date, onshore intakes have found application mainly for very large thermal or hybrid seawater desalination plants. Such intakes typically consist of large and deep intake canal ending into a concrete forebay structure equipped with coarse bar screens followed by fine screens and intake pump station.
Depending on the coastal conditions, onshore intakes could be installed: on a sandy coast with low gradient; on a rocky coast; or in natural or artificial enclosure (i.e., ship turning basin, marina, industrial port or lagoon). Of the three coastal environments, rocky bottom conditions are the most favorable for construction of onshore intakes. The main factors asso- ciated with the feasibility and water quality of such intakes are: wind and swell regimes, water level variations, tidal regime, bathymetry, and coastal currents.
Onshore intakes on sandy coast with low gradient are usually constructed with a long entrance canal that is designed to protect the intake from littoral sediment transport by prevailing near-shore currents, winds, swells, and tides. The intake canal is constructed with jetties, which are oriented such that they create a protective shield of the intake against the prevailing current and prevent the littoral drift form carrying sediments into the intake area. Without the jetty the canal would fill up with sand and would need to be dredged frequently. The jetties are constructed of stone rock blocks and usually extend to elevation of 2e3 m (6.6e10 ft) above the mean water level. To avoid fish entrapment, the canal is designed for average velocity at mean water level of 0.3 m/s (1 fps) and at 0.15 m/s (0.5 fps) at high water level. The canal ends into an onshore inlet structure with trash racks.
Rocky-coast onshore intakes typically are concrete or metal structures, which are open directly to the water body. Depending on the site-specific conditions, such intakes may be designed with jetty protection. If possible, it is preferable to design the water entrance at a depth or at least 2.0 m (6.6 ft) below the low tide level and to protect the entrance with wave-breaking jetties. The main difference between rocky bottom and sandy bottom shore- line conditions is that wave action typically does not cause significant stir up of sediments and elevated source water turbidity.