Making Benthic Mats

Construction:

• Unfold the mat completely.
• Attach rebar at the two ends using 6” lengths of cord on every other grommet.
• Add additional rebar about every three feet, which is every 2nd or 3rd grommet. It is better to have more rods than not enough. Our current 35’ mats typically have 19 grommets along the length, and placing rebar every other grommet gives 10 rebar/mat; a good number.
• Weave the rebar through the mats by slicing a 2” lengthwise slit in the tarp using a box cutter, about every 2 feet. Keep the holes as small as possible; if they are too big the milfoil grows through the holes!

The best technique for doing this is to lay the rebar in position on the mat, and then drag it off, which will leave a rust line on the mat. Then drag the rebar back onto the mat, cutting holes and weaving as you go. This makes it easier to pass the rebar back from the bottom to the top, and can be done by a single person.

• Tie the ends of the rods to the grommets at the edges of the mat using 6” lengths of cord. (Note that the grommets occasionally do not line up, so if necessary, cut a hole near the edge of the mat to tie the rebar.)
• Tie a foam float to the two corners of one end of the mat. (This makes the ends of the mat roll visible after being dropped on the lake bottom. We used to do both ends, but it got entangled in the protruding rebar ends.)
• When finished, roll up the mat beginning on the end that does not have the floats.
• Tie the tarp roll together with 3 lengths of cord. (There is no need to use slip knots; the divers will use their knives to cut the cords.)

Materials:

• Tarp: Premium, Green/silver Tarp, 20’ X 35’. Kotap America Ltd., Lawrence, New York. Tom Wynne has been supplying these tarps, at cost, for about $50 each.
• Rebar: ½” x 20’ rods. Supplier has been TC Murphy Lumber, at cost, for about $8 per rod. This works out to $80 of rebar per mat

• Avery cord: This is a ribbon-like synthetic cord. Also obtained through Tom Wynne, at cost. A possible labor-saving alternative is to use plastic slip-ties, but we have not tried that.

• Small Styrofoam floats (Can be made from bright colored “noodles”, cut into 3-4” lengths.)

Note: Prices shown for materials are circa 2008.