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Coastal Reservoir

What is a Coastal Reservoir?

A coastal reservoir is a freshwater reservoir located within the ocean, with an impermeable barrier separating the two water bodies. The reservoir may be located close to or within a river mouth, and may be constructed as a solid dam, a soft dam or any combination thereof.  The water within the reservoir can be utilized for various domestic, industrial or agricultural purposes.

The First Generation Coastal Reservoir

Following growing awareness of water resource challenges, the first generation of coastal reservoirs were developed to utilize rainfall runoff, with the following issues:

  • A solid and very expensive dam or barrage is used to separate the seawater and freshwater, based on construction methods and engineering design of traditional inland dams.  In fact, a coastal reservoir depth is generally less than 10m, with a water level similar to the depth of the sea water column, thus the pressure difference is significantly smaller than that of inland dams. Over-designed barrages have led to inflated costs, and with the advent of new technology, coastal reservoirs are becoming significantly more affordable.
  • Barrages constructed at river mouths collect almost all rainfall runoff from the catchment, but also collect pollutants such as domestic waste overflow, or industrial and agricultural chemicals. As a result, there is an accelerated growth of algae and other organisms that thrive on the nutrients in the coastal reservoir, and cause water quality deterioration. Modern technology allows for more flexible placement of coastal reservoirs, and allows for selective entry of flow and more flushing options.
  • The alignment of first generation reservoir barrages is perpendicular to the flow direction. This causes changes in the river flow may interfere with environmental and ecological concerns, such as fish migration routes, navigation channels and flooding. Modern coastal reservoirs are capable of circumventing this issue by adapting the curvature and positioning of the barrages to integrate with natural ocean wave movements.

Recognizing that the previous designs of coastal reservoirs were suboptimal, IACRR has embarked on an international mission to promote and improve on existing technology.

The Second Generation Coastal Reservoir

Every year, about 40,000km3of rainfall runoff is lost to the sea. The second generation of coastal reservoirs can utilize a portion of this vast resource in a manner that is environmentally friendly, aesthetically pleasing, structurally stable and cost-effective, because:

  1. The coastal reservoir can select good quality river water to store, thus its water quality is comparable with those inland dams;
  2. The storage capacity matches the water demand exactly (i.e., not too big, or too small);
  3. The river mouth generally is not fully closed, thus fish and ships can move towards upstream, without interruption.

Figure shows Coastal reservoirs without sealing river mouth.

The general comparison of different reservoirs is shown below: 

Table 1. Difference between Inland Reservoirs and Coastal Reservoirs.

Parameter Inland Reservoir 1st generation Coastal Reservoir 2nd generation Coastal Reservoir
Water quality Good (virgin catchment) Poor (collect and store all contaminants) Good (collects only clean water)
Water level Variable water level, above sea level Variable water level near sea level Almost constant water level near sea level
Dam alignment 90o with flow direction 90o with flow direction Small angle with flow direction
Dam-site Limited (Require narrow width or Gorge) Limited (only inside a river mouth) Unlimited (inside/outside

river mouth)

Dam design High pressure, concrete, earth/rock Low pressure but with wave/tidal surge, concrete, earth/rock. Low pressure but with wave/tidal surge, concrete, earth/rock with/without soft dam.
Dam length Short Short Long
Environmental impacts High Medium (obstruction to floodwater, fish, navigation) Low
Seepage By pressure difference By  density difference By  density difference
Pollutant Land lased Land-based + seawater Land-based + seawater
Emigrant cost High No No
Water supply By gravity By pump By pump
Water from % of catchment. 10~50% 100% 100%