Abstract:
A benthic barrier in which a polygonal shaped mat having straight sides is mounted on a similarly polygonal frame, but in which the frame sides bow out slightly between corners, causing the mat to stretch slightly and become taut. The frame comprises a plurality of straight leg members and a number of corner members as required by said polygonal shape. The corner members encompass interior angles which are from about 1 to about 5 degrees greater than the interior angle of the polygon defined by the straight sides of the mat. The frame member legs are rigid, but slightly flexible such that when assembled into the mat as an assembled frame, the straight legs are forced to bow out slightly between the corners, pulling the mat taut.

Description:
FIELD AND BACKGROUND 
       [0001]    The present invention relates to the field of “benthic barriers,” so called because they are barriers to the bottom of an area of water. The benthic zone is the ecological region at the lowest level of a body of water such as a lake, including the sediment surface and some sub-surface layers. Such barriers are typically placed in “photic zones,” which are areas where enough sunlight reaches the benthic area to permit photosynthesis, allowing aquatic plant growth. Benthic Barriers are intended to block sunlight from reaching lake bottoms, thereby preventing photosynthesis and eliminating unwanted, aquatic plant growth in specific areas where benthic barriers are placed. 
         [0002]    Know by common names such as, “lake weed control mats,” “seaweed mats” and simply, “weed mats,” crude versions of benthic barriers have been used in the United States for well over 100 years. Early benthic barriers were first fashioned of materials such as burlap and canvas to block sunlight and were held on the bottom primarily with rocks. Primitive frames were sometimes constructed with iron bars car fencing to, hold the fabrics in place. In some mats, metal bars, like reinforcing rod, have been placed transversely across the mat. Others attach frames on the perimeter of the fabric. Remarkably, many homemade frames were made of wood (which floats). Other frames have been fashioned from steel reinforcing rod, which is a soft, flexible bar, with a very low “bend moment.” 
         [0003]    Some of these barriers today comprise a polymeric fabric mat. In some, the polymeric fabric mat is surrounded by a rectangular frame to which the mat is secured. Some mats include small gas relief openings in the body of the mat, so that gas coming up from beneath the mat can escape through those openings. A mat sold by Lacey Inc. is provided with semi-circular slits in the mat, to define openings with adjacent flaps. This allows the mats to settle to the bottom of the lake more quickly, and also facilitates the escape of gas which tends to bubble up beneath the mat and lift the flaps out of the way. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is a benthic barrier in which a polygonal shaped mat having straight sides is mounted on a similarly polygonal frame, but in which the frame sides bow out slightly between corners, causing the mat to stretch slightly and become taut. The frame comprises a plurality of straight leg members and a number of corner members as required by said polygonal shape. The corner members encompass interior angles which are from about 1 to about 5 degrees greater than the interior angle of the polygon defined by the straight sides of the mat. The frame member legs are rigid, but slightly flexible such that when assembled into the mat as an assembled frame, the straight legs, are forced to bow out slightly between the corners, pulling the mat taut. 
         [0005]    Surprisingly, this facilitates the removal of naturally occurring gases from below the barrier, rather than collecting beneath the mat and making it unstable. These and other objects, advantages and features of the invention will be further understood and appreciated by reference to the drawings and written description set forth below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a plan view of a preferred embodiment benthic barrier with mat and frame assembled together, and with a support grid secured thereto on the underside of the mat; 
           [0007]      FIG. 2  is an enlarged view of a corner portion of the benthic barrier; 
           [0008]      FIG. 3  is a perspective view of the mat, the assembled frame, and the supporting grid exploded away from one another. 
           [0009]      FIG. 4  is a plan view of the mat and frame members separate from each other; 
           [0010]      FIG. 5  is a plan view of a frame corner member; 
           [0011]      FIG. 6  is a perspective view of frame members, which are positioned within receiving pockets of the mat, being assembled together; 
           [0012]      FIG. 7A  is a close up view of the joining ends of frame members spaced slightly apart; and 
           [0013]      FIG. 7B  is a close op view of the joining ends of frame members joined together. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    The benthic barrier  1  of the preferred embodiment comprises mat  10 , frame  20 , and optionally a grid under support  30 , shown assembled in  FIG. 1 , and exploded away from one another in  FIG. 3  ( FIGS. 1 and 3 ). Frame  20  is formed of plural frame members  21  and  22  which comprise legs which are straight when frame  20  is unassembled ( FIG. 4 ), but which are forced to bow out slightly by the configuration of the corners  21   c  when assembled ( FIGS. 1, 2 and 3 ). The barriers  1  can come in varying polygonal shapes and dimensions, such as rectangles 8 ft. by 8 ft., 12 ft. by 14 ft., 12 ft. by 24 ft., etc. The various components of barrier  1  are numbered herein as follows: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Benthic barrier 
                  1 
               
               
                   
                 Mat 
                 10 
               
               
                   
                 Gas relief ports 
                 11 
               
               
                   
                 Corner cutaway 
                 12 
               
               
                   
                 Frame pockets 
                 13 
               
               
                   
                 Frame 
                 20 
               
               
                   
                 Corner frame members 
                 21 
               
               
                   
                 Straight legs 
                 21a and 21b 
               
               
                   
                 Corner 
                  21c 
               
               
                   
                 Straight frame members 
                 22 
               
               
                   
                 Frame socket ends 
                 23 
               
               
                   
                 Frame plug ends 
                 24 
               
               
                   
                 Grid 
                 30 
               
               
                   
                 Grid connectors 
                 31 
               
               
                   
                   
               
             
          
         
       
     
         [0015]    Mat  10  is preferably made of a water and gas permeable material such as a non-woven fabric. Materials Which as not permeable to water and gases are lethal to all small benthic or that help sustain the health of a body of water. Preferably the non-woven fabric is of a polymeric material which is resistant to degradation and decay. Non-woven geotextile fabric is preferable. It is a non-elastic material, but it is sufficiently slightly stretchable, such that it can be made taut when frame members  21  and  22  are located in pockets  13  and assembled together, with the sides of frame  20  bowing out slightly. Pockets  13  are formed by folding mat  10  over on itself and sewing or welding overlapping edge portion to the body of the mat. 
         [0016]    Mat  10  is polygonal in configuration, e.g. rectangular as shown, with straight sides ( FIGS. 3 and 4 ) At its sides, mat  10  is formed into pockets  13 , for receiving frame members  21  and  22  ( FIGS. 1, 3 and 4 ). Mat  10  is cut away  12  its corners to facilitate insertion of frame members  21  and  22  into pockets  13 . The cut away portion is triangular, and preferably leaves a cut away corner sufficiently large that frame members  21  and  22  can be conveniently joined at the cut away corner ( FIG. 6 ). The term “cut away” as used herein refers to the fact that a portion of the coiner is missing. It does not mean literally that the mat was actually “cut,” though that is one way to create the cut away corner. The corner of the mat could alternatively have been folded over, the mat could have been produced without the corner portion, etc. 
         [0017]    Even though they are gas permeable, mats  10  also include gas relief ports  11 , which facilitate release of gas which builds up beneath benthic barrier  1  in use, faster than it can dissipate through the permeable non-woven fabric of mat  10 . These may be a small opening cut in mat  10 , or a flap cut into mat  10 , which covers the opening but allows gas build up to escape. The opening is about 1-3 inches in diameter. As shown in the drawings, the opening and its associated flap are semicircles, or “half-moons” with a 1.5 inch radius. However, other configurations for the “port” could be used, e.g. “v” shaped, with or without flaps, rectangular, etc. 
         [0018]    Frame  20  is formed of corner defining members  21  and straight frame members  22 . Corner members define the corners  21   c  of frame  20 , and include straight legs  21   a  and  21   b  extending away from corner  21   c . Leg  21   a  is longer while leg  21   b  is relatively short. Straight frame members  22  would typically be 4 to 6 feet long. The long legs  21   a  of corner members  21  are preferably about 4 feet long, while short leg  21   b  is about 1 foot long. The frame members are made of a material which is rigid, but the legs of which can be forced. 
         [0019]    The ends of each frame member  21  and  22  serve either as a narrowed plug  24  or a socket  23  for receiving plug  24  ( FIGS. 5, 6, 7A and 7B ). Plug end  24  is swaged to reduce its diameter so that it fits into the socket end of the next adjacent frame member (See especially  FIGS. 7A and 7B ). It both forms a tight joint, but also assists in allowing the long straight legs of frame  20  to flex between corners. 
         [0020]    The interior angle A encompassed by the corner portion  21   c  of corner member  21  is from about 1 to about 5 degrees greater than the interior angle of the polygon defined by mat  10  ( FIG. 5 ). Thus mat  10  is rectangular, such that its straight sides, extended to intersection, would encompass a 90 degree angle. However angle A in  FIG. 5  is actually 95 degrees. 
         [0021]    To assemble frame  20  to mat  10 , mat  10  is laid out flat on the ground with the frame members  21  and  22  surrounding it, as shown in  FIG. 4 . The straight members  22  are inserted into pockets  13  first, one at a time. Pockets  13  are about 3-5 inches wide, which facilitates insertion and manipulation of frame members into and within pockets  13 . The end of the previously inserted frame member  22  is left exposed so the next frame member can be easily joined to it. The joints are preferably taped with duct tape. 
         [0022]    The corner members  21  are joined last. The long leg  21   a  of each corner member is joined to its adjacent, straight leg at each corner. Because of the cut away corners  12  of mat  10 , the joining of the socket end of short leg  21   b  of each corner member to the plug end  24  of the free end of an adjacent straight member  22  is facilitated (See  FIG. 6  and  FIG. 2 ). As noted above, the cut away portion is triangular, and preferably leaves a cut away corner sufficiently large that frame members  21  and  22  can be conveniently joined at the cut away corner ( FIG. 6 ). Thus for a corner frame member with a short leg  21   b  of about a foot in length, the hypotenuse of the cut away is about 25 inches such that the leas of the cut away triangle are about 18 inches. This leaves about 5-6 inches of the adjacent straight frame member  22  for a user to grasp, while joining short leg  21   b  to straight, frame member  22  ( FIG. 6 ). The joint between joined plug  24  and socket  23  can be seen in  FIG. 2 , in the opening left by the cut away corner portion of mat  10 . 
         [0023]    As shown in  FIG. 6 , the assembler has grasped the end of straight frame member  22  with his right hand, and the short leg  21   b  of corner member  21  with his left. He pulls his right hand towards his body while pushing his left hand away, thus creating a separation between plug  24  and socket  23 . Plug  24  is then inserted into socket  23  to complete the joining process, if the next adjacent corner or corners have already been joined, this joining will force the straight leg(s)  22  and/or  21   a  to bow outwardly slightly, and will also force the insertion of plug  24  into socket  23 . Mat  10  is dimensioned such that this bowing of the side members of frame  20  draws mat  10  taught, placing it in tension within frame  10 . 
         [0024]    Example: Consider a mat with the dimensions of 12-feet by 24-feet. By creating corners that are 95 degrees (instead of the 90-degree corners of a standard rectangle) the center of the frame bows out to 13-feet in the center. Stretching non-woven fabric, sewn 12-feet wide, over a frame that is 13-feet wide M the center, pulls the frame inward while pulling the fabric taunt, much as a bow holds a bowstring taunt, by being cambered outward, away from the bowstring. 
         [0025]    This taut condition of mat  10  within frame  20  leads to unexpected results. Preventing photosynthesis through the use of a benthic mat is relatively simple, and easily understood by those of ordinary skill in the art. Releasing gases from, the lake bottom is not. As aquatic plants die, the resulting biomass is broken down and digested by bacteria. During the decomposition process, several gases are released including oxygen (at first), nitrogen, carbon dioxide, and methane. Undigested biomass that sinks beneath the lake bottom soil must be digested by anaerobic bacteria, releasing: ammonia, carbon dioxide and hydrogen sulfide. Surface gases tend to dissipate quickly, within two months, provided there&#39;s enough oxygen and aerobic bacteria to digest the plant matter. Deeper gases, like hydrogen sulfide, may have been created and trapped for hundreds or thousands of years. All of these gases rise and dissipate, when allowed to. 
         [0026]    However even in mats which are gas permeable, and even in those with gas relief openings, these gases can bubble up so fast that they are trapped beneath the benthic mat. As more gases accumulate, the entire benthic barrier often lifts off the bottom, floating and losing its shape until the gases escape. Because assembled frame  10  holds the fabric mat  10  taut, particularly in the center, gases migrate more efficiently to the edges of the mat, and to gas-release ports  11  if they are used. There is less “loose” fabric to form pockets where gases can collect. 
         [0027]    Optionally, a grid  30  such as a geogrid can be joined to the assembled frame  20  and mat  10 , through the use of zip ties or the like. This gives benthic barrier  1  even greater stability on the lake floor, as it helps keep the barrier from shifting. Grid  30  creates a firm surface on which to walk over soft lake bottom soils. It acts as a giant “snowshoe” by distributing weight over a larger area, and hinders “lateral” soil movement. 
         [0028]    This unique frame and fabric system allows for a relatively light, (60 to 70 lbs. dry) yet very study and extremely effective product that is easily installed, cleaned, moved and removed, it is stable in use, allowing gas bubbling up from below to flow readily to the edges and/or gas relief parts. Of course, it is understood that the forgoing is a preferred embodiment of the invention, and that various changes and alterations can be made without departing from the spirit and scope of the invention.