The present invention relates to an artificial sea-mount which is constructed on the seabed to prompt vertical mixing of seawater near the compensation depth.
There has recently been provided a technique of artificially constructing a sea-mount shaped structure on the seabed and making, using the natural energy of a tidal current or ocean current, nutrient salts abundantly contained in seawater below the compensation depth upwell to the surface sea zone (compensation depth) where phytoplankton grows under sunlight. The structure is known to produce an effect of enriching the waters near the artificial sea-mount and causing phytoplankton serving food for fishes and shellfishes to proliferate. In this specification, this structure will be referred to as an artificial sea-mount. Such an artificial sea-mount is constructed by piling up many block bodies made of a recycled material such as coal ash, concrete block bodies, artificial block bodies using discarded materials, or natural block bodies made of stone or the like (to be referred to as block bodies hereinafter).
To obtain a desired upwelling effect by the artificial sea-mount, it is necessary to build a larger-scale artificial sea-mount in the deep sea area. However, since constriction is burdensome, a demand has arisen for an artificial sea-mount that is expected to produce an optimum upwelling effect near the compensation depth, i.e., attain vertical mixing of seawater in a scale as small as possible.
An artificial sea-mount of this type is proposed in U.S. Pat. No. 5,267,812 (reference 1). Reference 1 describes an example of an artificial sea-mount which has a ridge portion formed by linearly connecting, in the horizontal direction, the vertices of a pair of cones arranged at a predetermined height so as to traverse a tidal current. There is also described another example of an artificial sea-mount whose ridge portion that connects the vertices of a pair of cones is lower in height than the vertices. In both artificial sea-mounts, a current that ascends the slanting surface and peels off from the horizontal linear portion of the ridge of the artificial sea-mount makes vortices with a horizontal axis, and simultaneously, a current that goes round the side surfaces of the artificial sea-mount and peels off from there makes vortices with a vertical axis. The two kinds of vortices combine in the countercurrent area formed behind the artificial sea-mount, thereby generating a large upwelling vortex intermittently.
Recent field observations and analyses revealed that in the deep sea area with strong density stratification, the wavelength and wave height of generated internal waves change depending on the water depth, current state, stratification state, and height and shape of an artificial sea-mount. Additionally, it was found that since the direction and velocity of current change over time due to the influence of tidal ebb and flow and the like, the wavelength and wave height of internal waves change temporally. “Density stratification” means a continuous density layer structure formed by surface water which has a low density because of a high temperature and low salinity concentration and deep water which has a low temperature and high salinity concentration.
In general, seawater is difficult to vertically mix in the density stratification state. However, it was found that internal waves generated by a current colliding with an undersea structure or the like cause vertical mixing even near the surface layer. Thus required is an efficient artificial sea-mount which induces vertical mixing in the deep sea area where the water mass density greatly changes between the surface layer and the bottom layer, and water conditions including the density and the vertical distribution of nutrient salts change variously.
To efficiently generate horizontal vortices under water conditions having no or weak density stratification, the artificial sea-mount of reference 1 forms a horizontal ridge portion which runs linearly in a direction almost perpendicular to the current, thereby advantageously inducing an upwelling vortex. However, according to the present inventor's examinations, no remarkable upwelling vortex could be observed in the deep sea area with strong density stratification. It was found that when the density stratification is strong, heavy seawater of the bottom layer is made to readily ride across a structure to efficiently generate internal waves, allowing to actively promote vertical mixing.