AU2004210782A1 - Method of preventing water pollution and water-blocking fence - Google Patents

Description

FPO4-0044-00 DESCRIPTION METHOD FOR PREVENTING WATER QUALITY CONTAMINATION, AND WATER FLOW-BLOCKING FENCE Technical Field 5 [0001] The present invention relates to a method for preventing water quality contamination in order to maintain low levels of generated and/or proliferated algae in water withdrawn from reservoirs, and to a water flow-blocking fence used for the method. "Reservoir" refers to a water reservoir intended for public, industrial or agricultural 10 water use, in the form of a dam reservoir, tidal pool, marsh, pond or the like. Background Art [0002] The increase in demand for running water has required designs for more effective utilization of water resources by dam 15 development, and this has led to a greater degree of dependence on reservoirs for public water (dam reservoirs, tidal pools, marshes, ponds, etc.). Such designs for effective utilization of water resources by dam development have resulted in more water detention basins susceptible to eutrophization, thus leading to a greater of number of environments 20 conducive to generation and/or proliferation of algae. Contamination of public water sources by algae also causes various other problems such as increased pH, generation of odors and greater turbidity. [0003] For these reasons, from the standpoint of preventing problems in the water purification process such as poor flocculation 25 and amplified numbers of organisms or residual odors in filtered water, it has been necessary to adopt such measures as increasing the amount 1 FPO4-0044-00 of flocculants, disinfectants, pH regulating acids and active carbon added, and this has produced a need for introduction of new facilities for addition of pH regulating acids or active carbon at water purification plants lacking them. 5 [0004] In the case of industrial water as well, one of the problems encountered is a lower cooling capacity caused by adhesion of algae onto cooling apparatuses. [0005] These problems are minimized somewhat by lake water circulating techniques which attempt to reduce the absolute volume of 10 algae in water reservoirs (see Sato, A., Magara, Y., ed. "Josuido ni Okeru Soruishogai - Anzen de Ryoshitsu na Suidosui wo Motomete -" [Impairment of Waterworks by Algae - Toward Safer and Higher Quality Public Water], 5/15/1996). However, the results have been less than satisfactory, and hence techniques for further reduction of algae 15 contents in public water sources have been a desired goal. [0006] Against this background there has also been proposed a method of installing, either on the surface or interior of a reservoir, light-shielding structures covering 1/3 to 2/3 of the water volume based on the total water volume of the reservoir, as a means of inhibiting 20 generation and/or proliferation of reservoir algae, i.e. phytoplankton (see Japanese Unexamined Patent Publication HEI No. 8-243538). Disclosure of the Invention [0007] However, this prior art method of installing light-shielding structures has a problem that is impractical for reservoirs with large 25 volumes, because of the excessive amounts of the light-shielding structures that must be installed. 2 FPO4-0044-00 [0008] In light of the circumstances described above, it is an object of the present invention to provide a method for effectively preventing algae contamination of the water quality in water area flowing into water withdrawal channels, as well as a water flow 5 blocking fence used for the method. [0009] In order to solve the aforestated problem, the method for preventing water quality contamination of this invention is a method wherein water quality contamination is prevented by using light shielding structures to shield a portion of the water surface of a 10 reservoir having at least one inlet channel and at least one withdrawal channel, in order to inhibit generation and/or proliferation of algae, the method being characterized by installing in the reservoir a water area dividing fence which inhibits migration of algae, so as to partition off a water quality contamination-prevented water area having at least one 15 inlet channel and at least one withdrawal channel from the other water areas, and by anchoring or floating the light-shielding. structures on at least a section of the water surface of the water quality contamination prevented water area for shielding of light thereto. [0010] According to this method, the water quality 20 contamination-prevented water area is partitioned off from the other water areas by the water area-dividing fence which inhibits migration of algae, so that algae does not flow from the other water areas into the water quality contamination-prevented water area. Since the water quality contamination-prevented water area has an inlet channel and a 25 withdrawal channel, water flows easily from the inlet channel to the withdrawal channel in the water quality contamination-prevented water 3 FPO4-0044-00 area. Thus, algae does not flow from the other water areas as easily as when the water quality contamination-prevented water area has no inlet channel. [0011] Furthermore, since partitioning from the other water areas 5 is accomplished by a water area-dividing fence and the light-shielding structures are anchored or float only in the water quality contamination-prevented water area in which inflow of algae is inhibited to prevent generation and/or proliferation- of algae, as explained above, the area to be shielded from light is limited, and the 10 content of algae in the water supply which is withdrawn can be maintained at a low level even if the amounts of the light-shielding structures are drastically reduced compared to the prior art. [0012] In the method for preventing water quality contamination of the invention, the water area-dividing fence is preferably one 15 comprising a water flow-blocking fence which blocks the flow of water in a region from the surface of the reservoir to a prescribed depth. [0013] Because algae require light for growth, they mainly inhabit regions from the water surface to a prescribed depth (generally 5-10 m from the surface). Thus, by blocking the flow of water in the 20 region from the surface of the reservoir to a prescribed depth, it is possible to efficiently inhibit algae inhabiting the region near the surface from migrating into the water quality contamination-prevented water area. [0014] In addition, preferably a section of the water flow 25 blocking fence used for the method for preventing water quality contamination according to the invention is a fixed-type water flow 4 FPO4-0044-00 blocking fence which is set so as to be attached at the bottom in a region shallower than a prescribed water depth (preferably within a water depth of 5-10 m) in the reservoir, while the remaining section of the water flow-blocking fence is an anchored-type water flow-blocking 5 fence which is set so as to be free of the bottom in a region deeper than the prescribed water depth in the reservoir. [0015] This construction can reliably prevent migration of algae between water areas by the fixed-type water flow-blocking fence in the region shallower than the prescribed water depth. Migration of algae 10 between water areas is also efficiently inhibited between the reservoir surface and the prescribed depth by the anchored-type water flow blocking fence which is free of the bottom in the region deeper than the prescribed water depth. The anchored-type fence allows automatic adjustment to changes in the water level due to volume swelling caused 15 by flooding or the like, in order to consistently provide efficient inhibition of algae migration. Moreover, since it is free of the bottom, water exchange can freely occur in regions deeper than the water flow blocking fence, thereby allowing free movement of aquatic life such as fish and crustaceans in the water under the anchored-type water flow 20 blocking fence. [0016] The fixed-type water flow-blocking fence portion of the water area-dividing fence used for the method for preventing water quality contamination of the invention is preferably a concrete wall, a steel sheet or a combination thereof. 25 [0017] In addition, the anchored-type water flow-blocking fence used for the method for preventing water quality contamination of the 5 FPO4-0044-00 invention is preferably constructed with a float situated on the water surface and a curtain having one side connected to the float and being suspended from the float. [0018] With this type of construction, the anchored-type water 5 flow-blocking fence will be capable of reliably blocking the water flow to the prescribed depth from the water surface even when the water level changes due to volume swelling, for example. In addition, even if the water flow changes drastically such as in cases where water is discharged from the reservoir, the curtain can adapt to changes in the 10 water flow since it is suspended from the float, thereby rendering the curtain more resistant to damage than if it were attached to the bottom. [0019] Preferably, a plurality of curtains are provided, each having a roughly quadrilateral shape and with the curtains situated so that the side edges of adjacent curtains overlap each other. 15 [0020] This construction will produce gaps between adjacent curtains allowing water to easily move past, thereby helping to reduce damage to the curtains and damage to the water flow-blocking fence when the volume of moving water increases due to flooding or the like. In addition, since a plurality of curtains are situated with the side edges 20 of adjacent curtains overlapping each other, it is possible to effectively block the flow of water between adjacent curtains during normal periods, thus helping to more effectively inhibit migration of algae. [0021] In the method for preventing water quality contamination of the invention, preferably 1/3 to 2/3 of the volume of the water 25 quality contamination-prevented area is shielded from light by the light-shielding structures. 6 FPO4-0044-00 [0022] Since the water quality contamination-prevented area is partitioned from the other water areas by the water area-dividing fence as described above, and inflow of algae is thus efficiently inhibited, it is possible to prevent generation and/or proliferation of algae in the 5 water quality contamination-prevented area by shielding light from 1/3 to 2/3 of the volume of the water quality contamination-prevented area. The water in the water quality contamination-prevented area is withdrawn for use. Thus, the volume of algae in the withdrawn water can be efficiently reduced without shielding 1/2 to 2/3 of the volume of 10 the entire reservoir. [0023] The water flow-blocking fence of the invention blocks the flow of water in a region from the reservoir water surface up to a prescribed depth, the fence being characterized in that it is constructed with a float situated on the reservoir water surface and a plurality of 15 roughly quadrilateral curtains each having one side connected to the float and suspended from the float, and in that the plurality of curtains are situated in such a manner that the side edges of adjacent curtains overlap each other. [0024] With this construction, the float will adapt to changes in 20 the water level occurring with water volume swelling and the like, thereby allowing the water flow to be reliably blocked from the water surface to the prescribed depth. In addition, gaps are produced between adjacent curtains to facilitate passage of water, thereby reducing damage to the curtains and damage to the water flow-blocking fence 25 when the volume of moving water increases due to flooding or the like. Furthermore, since the curtains are situated with the side edges of 7 FPO4-0044-00 adjacent curtains overlapping each other, it is possible to effectively block the flow of water between adjacent curtains during normal periods, thus helping to more reliably inhibit migration of algae. Brief Description of the Drawings 5 [00251 Fig. 1 is a schematic diagram showing a reservoir employing an embodiment of the method for preventing water quality contamination according to the invention. [0026] Fig. 2 is a cross-sectional view of a reservoir illustrating a mode for installation of a water area-dividing fence. 10 [0027] Fig. 3 is a front view of a fence unit according to an embodiment of a water flow-blocking fence of the invention. [0028] Fig. 4 is a cross-sectional view of a fence unit. Best Mode for Carrying Out the Invention [0029] Preferred embodiments of the invention will now be 15 explained with reference to the accompanying drawings. Throughout the explanation which follows, corresponding elements will be indicated by like reference numerals and will be explained only once. The dimensional proportions in the drawings do not necessarily match those in the explanation. 20 [0030] Fig. 1 is a schematic diagram showing an embodiment of employing a method for preventing water quality contamination according to the invention in a reservoir. The reservoir 1 is provided with an inlet channel 2 through which water flows into the reservoir 1, a discharge channel 3 through which water is discharged for adjustment 25 of the water volume of the reservoir 1, and a withdrawal channel 4 through which water is withdrawn from the reservoir I as water for 8 FP04-0044-00 public use. [0031] In the method for preventing water quality contamination according to this embodiment, the reservoir 1 is divided into two water areas by a water area-dividing fence 5, as shown in Fig. 1. The 5 construction of the water area-dividing fence 5 will now be explained with reference to Figs. 2 to 4. [0032] The water area-dividing fence 5 inhibits migration of algae between the water areas of the reservoir 1. Here, "algae" refers to epiphytic algae or swimming algae such as water-bloom. Also, 10 "inhibits migration of algae" does not necessarily mean total inhibition of algae migration, but may be a reduction to less than 1/10 and preferably less than 1/100 of the volume of migration which would occur without the water area-dividing fence 5. [0033] The water area-dividing fence 5 is not restricted so long as 15 it inhibits migration of algae, and it may even be an "oil fence". However, it is preferably one with a water flow-blocking fence which blocks the flow of water in the region from the water surface to the prescribed depth (preferably 5-10 m from the surface). Here, "blocks the flow of water" does not mean that the movement of water is 20 completely blocked, as movement of a small amount of water is permitted through the linked sections of the water flow-blocking fence. [0034] Also, the reservoir 1 will usually have a shallower water depth at sections near the banks and a deeper area near the center, as shown in Fig. 2. In a water area-dividing fence 5 which divides this 25 kind of reservoir 1, the sections that partition regions which are shallower than the prescribed water depth, such as near the banks, are 9 FPO4-0044-00 preferably fixed types set on the bottom, being attached to the bottom or bank of the reservoir, while the sections partitioning regions which are deeper than the prescribed water depth, such as near the center of the reservoir 1, are preferably anchored types which are free from the 5 bottom and anchored to floats or the like. The prescribed water depth may be a depth inhabited by swimming algae such as water-bloom, for example, and is preferably a water depth of 5-10 m. [0035] A part of the water area-dividing fence 5 of this embodiment consists of fixed-type water flow-blocking fences 5a,5b 10 each of which is attached to the bottom, while the other part consists of an anchored-type water flow-blocking fence 5c which is not attached to the bottom, as shown in Fig. 2. [0036] The fixed-type water flow-blocking fences 5a,5b are situated in regions which are shallower than the prescribed depth, and 15 are fixed to the bottom or bank of the reservoir. The fixed-type water flow-blocking fences 5a,5b are concrete walls, steel sheets or combinations thereof. [0037] Also, the fixed-type water flow-blocking fences 5a,5b preferably have heights reaching the maximum expected water level. 20 This will prevent swelling water from rushing over the fixed-type water flow-blocking fences 5a,5b. [0038] The anchored-type water flow-blocking fence 5c is situated between the fixed-type water flow-blocking fences 5a,5b, in a region deeper than the prescribed water depth of the reservoir 1. 25 [0039] The anchored-type water flow-blocking fence 5c used for this embodiment has fence units U 1

-U

9 linked in series, as shown in Fig. 10 FPO4-0044-00 2. Each of the fence units have the same construction, and therefore the following explanation will refer only to fence unit U 1 . [0040] Figs 3 and 4 are front and cross-sectional views, respectively, showing the construction of the fence unit U 1 . The fence 5 unit Ui is constructed with a float 50 and a quadrilateral curtain 51 having one side attached to the float 50. [0041] The float 50 is composed of a rod-like buoyant member 50a having a prescribed length and a covering layer 50b attached in a manner wrapping around the circumference of the buoyant member 50a. 10 According to this embodiment, the buoyant member 50a is composed, for example, of a hollow structure made of a hard synthetic resin such as polyethylene terephthalate, or a solid or hollow structure made of a hard synthetic resin foam body, and it is constructed so as to provide sufficient buoyancy for the entire curtain 51. 15 [0042] The covering layer 50b used is preferably composed of a flexible sheet-like member. More specifically, there may be used a sheet material made of rubber such as synthetic rubber or natural rubber, or a synthetic resin such as soft polyvinyl chloride resin, polyethylene resin or polypropylene resin, and canvases made of 20 various synthetic resin fibers. The length of the covering layer 50b in the axial direction of the buoyant member 50a is roughly the same as the length of the curtain 51 in the axial direction of the buoyant member 50a. [0043] The curtain 51 is connected to the buoyant member 50a by 25 being sandwiched and fixed by both edges of the sheet-like member serving as the covering layer 50b attached in a manner wrapping 11 FPO4-0044-00 around the circumference of the buoyant member 50a, as shown in Fig. 4. The curtain 51 is preferably, for example, a flexible curtain such as a rubber sheet, soft plastic sheet, canvas or the like. The means for attachment of the covering layer 50b and curtain 51 is not particularly 5 restricted but is preferably, for example, solvent welding, hot-air welding, bonding treatment with an adhesive, or fastening treatment with a physical fastening tool such as a bolt fastener. Combinations of such means may also be employed. [0044] The lower part of the curtain 51 is wrapped to form a 10 cylindrical plumb body insertion part 51 a, and a plumb body 53 made of metal or the like is inserted and fixed in the plumb body insertion part 51a. [0045] The length from the buoyant member 50a end to the plumb body insertion part 51 a end of the curtain 51 is determined so as 15 to correspond to the prescribed depth from the water surface at which the water flow is to be blocked, but normally it will be in the range of 5-10 m. This is because a length of 5-10 m for the curtain 51 in the depthwise direction will allow effective inhibition of algae migration since algae are mainly present in a region 5-10 m from the water 20 surface. The length in the direction perpendicular thereto is preferably about 10-15 m for convenience of transport by trucks or other vehicles. [0046] Doubled sections 51d,51e are formed at the edges 51b,51c of the curtain 51. The buoyant member 50a does not extend to the sections of the float 50 corresponding to the doubled sections 51d,51e. 25 That is, the prescribed length of the buoyant member 50a is approximately equal to the length of the curtain 51 in the axial 12 FPO4-0044-00 direction of the buoyant member 50a minus the length of the doubled sections 51d,51e. The sections of the float 50 in which the buoyant member 50a is absent, i.e. the sections consisting only of the covering layer 50b, will hereinafter be referred to as the linked sections 5 50b',50b". [0047] The total area of the doubled sections 51d,51e of the curtain 51 is preferably 5-20%, and more preferably 7-15%, of the total area of the curtain 51. If the area of the doubled sections 51d,51e is too small, the degree of overlap of each curtain 51 will be insufficient and 10 may result in inadequate blockage of water flow at the doubled sections 51d,51e. If the doubled sections 51d,51e are too large the curtain 51 will not be effectively utilized and the cost performance will be reduced. Also, since excessively large doubled sections 51d,51e essentially function the same as a single sheet, the load from the water 15 flow will increase, tending to damage the curtain 51. [0048] The anchored-type water flow-blocking fence 5c is formed by linking fence units U 1

-U

9 in series as shown in Fig. 2, along the line of division of the water areas on the water surface, so that the doubled sections 51 d,5 1 e of the curtain 51 overlap each other. 20 [0049] Linking of the fence units U 1

-U

9 is accomplished by linking adjacent fence units such as, for example, linking the fence unit

U

1 and fence unit U 2 by overlapping the doubled section 51c of the fence unit U 1 and the doubled section 5 1b of the fence unit U 2 while overlapping the linking sections 50b',50b" of the covering layer 50b of 25 each fence unit U 1

,U

2 , and joining the overlapping sections of the linking sections 50b',50b" with connecting gear such as bolts (not 13 FPO4-0044-00 shown). [0050] In other words, each fence unit of the anchored-type water flow-blocking fence 5c is linked so that the linked floats 50 of the fence units constitute a single float, with the plurality of curtains 51 5 being situated on the float in such a manner that the side edges 51 b,51 c of the adjacent curtains 51 overlap each other. [0051] Also, the water area-dividing fence 5 is constructed with the fence units U 1

,U

9 at both edges of the anchored-type water flow blocking fence 5c anchored with individual ropes to the fixed-type 10 water flow-blocking fences 5a,5b for attachment to the fixed-type water flow-blocking fences 5a,5b. The anchoring ropes preferably are given a certain degree of slack. The slack will allow the horizontal position of the anchored-type water flow-blocking fence 5c to shift within a relatively small width that will not cause any significant 15 problem. [0052] A method for preventing water quality contamination according to this embodiment will now be explained based on the type of water area-dividing fence 5 described above. [0053] As mentioned above, the water area-dividing fence 5 is 20 used to divide the reservoir 1 into two water areas, as shown in Fig. 1. Here, the water area comprising the inlet channel 2 and withdrawal channel 4 in the divided reservoir 1 shown in Fig. 1 is designated as the water quality contamination-prevented water area A. [0054] A portion of the water surface of the water quality 25 contamination-prevented water area A is then shielded from light to inhibit generation and/or proliferation of algae. Light-shielding of the 14 FPO4-0044-00 portion of the water surface of the water quality contamination prevented water area A is accomplished by anchoring or floating light shielding structures 6 at sections of the water surface of the water quality contamination-prevented water area A, as shown in Fig. 1. The 5 light-shielding structures 6 are not particularly restricted so long as they shield the water surface from incident sunlight rays, and they may be, for example, flat or moderately high floats having circular, quadrilateral or hexagonal shapes and composed of opaque foam plastic, hollow plastic molded bodies or the like, or artificial floating 10 islands constructed by loading dirt, and if necessary planting grass or trees, on the floats. [0055] These light-shielding structures 6 may be in the water quality contamination-prevented water area A in a freely movable floating (freely floating) state, or they may be anchored with ropes or 15 the like to bottom anchors or to fixed members on the water bank, in a manner restricting their movement (anchored floating). [0056] The number of light-shielding structures 6 situated in the water quality contamination-prevented water area A is not particularly restricted, and the manner of installation (freely floating/anchored 20 floating) and their shapes and sizes may be of a single type or a combination of types. However, 1/3 to 2/3 of the water volume of the water quality contamination-prevented water area A is preferably shielded from light by the light-shielding structures 6. This will more efficiently prevent generation and/or proliferation of algae in the water 25 quality contamination-prevented water area A. [0057] Water is withdrawn from the withdrawal channel 4 of the 15 FP04-0044-00 water quality contamination-prevented water area A which is partially shielded from light by the light-shielding structures 6 in the manner described above. The average retention time of public use water withdrawn from the water quality contamination-prevented water area 5 A is preferably two weeks or longer, and more preferably 4 weeks or longer. Retention of public use water in the reservoir 1 for such a period of time will allow purification of inflowing harmful substances such as E. coli. The average retention time in the water quality contamination-prevented water area A is determined by dividing the 10 average water volume of the water quality contamination-prevented water area A by the average withdrawal volume. [0058] The function and effect exhibited by the method for preventing water quality contamination of this embodiment will now be explained. 15 [0059] According to the method for preventing water quality contamination described above, the water quality contamination prevented water area A is the water area comprising the inlet channel 2 and withdrawal channel 4, of the water areas of the reservoir 1 divided by the water area-dividing fence 5, and 1/3 to 2/3 of the water volume 20 of the water quality contamination-prevented water area A is shielded from light by the light-shielding structures 6 to inhibit generation and/or proliferation of algae. Water from the water area having inhibited generation and/or proliferation of algae is withdrawn as water for public use. 25 [0060] Since the water area-dividing fence 5 blocks the flow of water from the water surface to the prescribed depth, it is possible to 16 FPO4-0044-00 efficiently block inflow of algae from other water areas to the water quality contamination-prevented water area A. [0061] Moreover, since the inlet channel 2 and withdrawal channel 4 are in the water quality contamination-prevented water area 5 A, water flow occurs easily from the inlet channel 2 to the withdrawal channel 4. Thus, migration of algae into the water quality contamination-prevented water area A is reduced compared to, for example, cases where inlet channels are present elsewhere than in the water quality contamination-prevented water area A. 10 [0062] In addition, since light is shielded by the light-shielding structures 6 which are anchored or floating in the water quality contamination-prevented water area A, into which inflow of algae from the other water areas is inhibited as explained above, the light-shielding structures 6 only have to inhibit generation and/or proliferation of algae 15 in the water quality contamination-prevented water area A. Consequently, generation and/or proliferation of algae can be effectively inhibited by the light-shielding structures 6. [0063] Furthermore, while a larger reservoir 1 requires a correspondingly larger light-shielding area if the object is to inhibit 20 algae throughout the entire reservoir 1, in the case of this embodiment, even a large reservoir is divided by the water area-dividing fence 5 to form a smaller water quality contamination-prevented water area A as a water area comprising an inlet channel 2 and withdrawal channel 4, and therefore the light-shielding region is not dependent on the size of the 25 reservoir. Also, since the water quality contamination-prevented water area A which is the area to be shielded from light is a limited area, it is 17 FPO4-0044-00 possible to efficiently inhibit generation and/or proliferation of algae with fewer light-shielding structures 6 than by the prior art. [0064] Moreover, since the inlet channel 2 and withdrawal channel 4 are at the bank sides, the water quality contamination 5 prevented water area A therefore is a water area near the banks of the reservoir. Since algae inhabit a zone up to a prescribed depth from the water surface as explained above, algae tend to grow more near the banks than at the center. On the other hand, when 1/3 to 2/3 of the total volume of water of the entire reservoir 1 is shielded from light, for 10 example, the deep sections at the center of the reservoir 1 are also shielded from light. In contrast, when only 1/3 to 2/3 of the water volume of the water quality contamination-prevented water area A is shielded from light by the light-shielding structures 6, it is possible to effectively shield light from the areas near the banks, i.e. the areas 15 where algae are generated and/or proliferate more easily. It is thus possible to more efficiently and reliably inhibit generation and/or proliferation of algae than when the entire reservoir 1 is shielded from light. Consequently, the algae content of public use water withdrawn through the withdrawal channel 4 in the water quality contamination 20 prevented water area A can be kept to a very low level. [0065] Also, the water area-dividing fence 5 used in this method for preventing water quality contamination is composed of fixed-type water flow-blocking fences 5a,5b and an anchored-type water flow blocking fence 5c, as explained above. The anchored-type water flow 25 blocking fence 5c is situated in a region shallower than the prescribed water depth, and the length of the curtain 51 is a length corresponding 18 FPO4-0044-00 to the prescribed depth from the water surface (preferably 5-10 in). In other words, the flow of water deeper than the prescribed depth from the water surface in the region formed by the anchored-type water flow-blocking fence 5c is not blocked. This allows free water 5 exchange to occur in the water area which is deeper than the prescribed depth from the water surface. It is thus possible to effectively inhibit migration of algae, which mainly inhabits a region to a prescribed depth from the water surface, into the water quality contamination prevented water area A, while still allowing aquatic life such as fish 10 and crustaceans to freely move under the anchored-type water flow blocking fence 5c, thus avoiding adverse effects on the ecosystem and adequately protecting the natural environment. [0066] In addition, the anchored-type water flow-blocking fence 5c of the embodiment described above has fence units U 1

-U

9 linked in 15 series, in such a manner that the side edges 51b,51c of the adjacent curtains 51 overlap each other. Thus, when the volume of moving water increases due to flooding or the like, gaps are produced between the curtains 51 allowing water to easily move past, thereby helping to reduce damage to the curtains 51 and damage to the water area 20 dividing fence 5. Also, by attaching a plumb body 53 at the lower end of each curtain 51, it is possible to improve the stability of the curtain 51 in the water, and thus prevent swinging of the curtain 51 by water flow and formation of gaps between adjacent curtains 51 during normal periods. Thus, attachment of the plumb body 53 at the lower end of the 25 curtain 51 can improve the effect of water flow blockage. [0067] Also, by adjusting the number of connected fence units 19 FPO4-0044-00

U-U

9 , it is possible to set a desired length of the anchored-type water flow-blocking fence 5c along the water surface. [0068] The present invention is not limited to the preferred embodiment described above, and various modifications thereof may 5 be implemented. For example, while a water flow-blocking fence is used as the water area-dividing fence 5 in the embodiment described above, there is no particular restriction to this mode so long as it inhibits migration of algae, and an "oil fence" may even be used. [0069] Also, in the embodiment described above, the anchored 10 type water-blocking fence 5c is a water flow-blocking fence wherein each fence unit is linked and the connected floats 50 of each of the fence units are considered to be one float, with the sides of the curtains on the float connected and situated on the float in such a manner that the side edges of adjacent curtains overlap, but there is no particular 15 restriction to this mode, as it is sufficient if the float is situated along the water surface and a curtain has one side connected to the float and is set in a manner suspended from the float. Therefore, for example, the anchored-type water flow-blocking fence may also incorporate a "polluted water anti-dispersion fence" (silt fence) as either all or a 20 portion thereof. [0070] In the embodiment described above, the anchored-type water flow-blocking fence 5c has each fence unit linked together and attached to the fixed-type water flow-blocking fences 5a,5b, but alternatively, for example, the floats may be anchored with a rope to an 25 anchoring structure such as a concrete block immobilized on the water bottom or bank, for positioning in the horizontal direction. In this case 20 FPO4-0044-00 as well, the anchoring ropes are preferably given a certain degree of slack to allow for variations in the water level. [0071] In addition, although nine fence units U1-U 9 are linked as the anchored-type water flow-blocking fence 5c, there is no need for a 5 limitation to nine units, and any number of necessary fence units may be linked depending on the size of the reservoir. [0072] Also, the fixed-type water flow-blocking fences 5a,5b in this case are concrete walls, steel sheets or combinations thereof, but alternatively hard panel structures such as plastic boards, for example, 10 may be used. Industrial Applicability [0073] As explained above, the method for preventing water quality contamination of the invention allows effective prevention of water quality contamination by algae in water areas which flow into 15 withdrawal channels. Also, by applying the water flow-blocking fence of the invention as a method for preventing water quality contamination, it is possible to effectively, reliably and efficiently prevent water quality contamination by algae in water areas which flow into withdrawal channels. 21

Claims (8)

1. A method for preventing water quality contamination by using light-shielding structures to shield a portion of the water surface of a reservoir having at least one inlet channel and at least one withdrawal 5 channel, in order to inhibit generation and/or proliferation of algae, the method being characterized by installing in said reservoir a water area-dividing fence which inhibits migration of algae, so as to partition off a water quality contamination-prevented water area having at least one inlet channel and at least one withdrawal channel from the 10 other water areas, and by anchoring or floating said light-shielding structures on at least a section of the water surface of said water quality contamination-prevented water area for shielding of light thereto.

2. A method for preventing water quality contamination according to claim 1, characterized in that said water area-dividing 15 fence is one comprising a water flow-blocking fence which blocks the flow of water in a region from the surface of said reservoir to a prescribed depth.

3. A method for preventing water quality contamination according to claim 2, characterized in that a section of said water flow 20 blocking fence is a fixed-type water flow-blocking fence which is set so as to be attached at the bottom in a region shallower than a prescribed water depth in said reservoir, while the remaining section of said water flow-blocking fence is an anchored-type water flow blocking fence which is set so as to be free of the bottom in a region 25 deeper than said prescribed water depth in said reservoir.

4. A method for preventing water quality contamination 22 FPO4-0044-00 according to claim 3, characterized in that said fixed-type water flow blocking fence is a concrete wall, a steel sheet or a combination thereof.

5. A method for preventing water quality contamination according to claim 3, characterized in that said anchored-type water 5 flow-blocking fence is constructed with a float situated on the water surface and a curtain having one side connected to said float and being suspended from said float.

6. A method for preventing water quality contamination according to claim 5, characterized in that a plurality of curtains are 10 provided, each having a roughly quadrilateral shape and with said curtains situated so that the side edges of adjacent curtains overlap each other.

7. A method for preventing water quality contamination according to any one of claims 1 to 6, characterized in that 1/3 to 2/3 of 15 the volume of said water quality contamination-prevented area is shielded from light by said light-shielding structure.

8. A water flow-blocking fence which blocks the flow of water in a region from a reservoir water surface up to a prescribed depth, the water flow-blocking fence being characterized in that it is 20 constructed with a float situated on said reservoir water surface and a plurality of roughly quadrilateral curtains each having one side connected to said float and suspended from said float, and in that said plurality of curtains are situated in such a manner that the side edges of adjacent curtains overlap each other. 25 23