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(e) Use of a filter between the bank revetment and <br />• soil may be necessary to prevent the soil from moving through the <br />revetment, Co prevent the revetment from sinking into the soil, <br />and/or to permit natural seepage from the streambank, thus <br />preventing build-up of excessive groundwater pressure. A filter <br />may be composed of fabric, sand, gravel, or graded rock. If a <br />filter is used, you should seek technical assistance to ensure <br />that the filter will be properly matched with the riprap blanket <br />and ttre soil ; <br />(f) The upst.rexm and downstream ends of the riprap <br />blanket should keyed or tied into bank to prevent stream currents <br />from unravelling the riprap. <br />(g) Trees used in combination with riprap should not <br />be obtained from a ripariar, or riverine source. <br />2. GABIONS. Gabions are wire mesh baskets or cages filled <br />with rock which can he placed along banks to armor them. Rock <br />used as filler must be clean, durable and free from appreciable <br />amounts of sand or gravel. River cobble is an acceptable <br />material for filler provided the river cobble is large enough to <br />stay within the mesh and is not obtained from "live" or flowing <br />waters. Gabions may be less effective in high velocity streams, <br />streams with large bedloads, or streams where the water chemistry <br />is deleterious to the gabion mesh. <br />• The use of gabion baskets is authorized provided: <br />(a) Gabions 'shall be placed on well compacted and <br />stable embankments; <br />(b) Gabion walls shall be angled back into the <br />existing bank at the upstream and downstream ends to prevent <br />undercutting; <br />(c) Gabions shall be at least 3-feet in width; <br />(d) The gabion basket shall be made of galvanized <br />steel, woven wire no smaller than l2 gauge. Rubber-coated wire <br />baskets are also acceptable; <br />(e) The limitations on the volume of material per <br />linear foot and on the length of bank line specified for pivrav <br />(r•efer• t.o 1 (a) and (b) above] are app)icable to this method of <br />bank protection; and , ~ <br />L z c)r pc.r ~t• •i- )CCG <br />(f) The gabion baskets should be transitioned or keyed <br />into the bank and securely tied to each other. Proper anchoring <br />is kPy to the success of gabion structures. <br />• <br /> <br />4 <br />