مهندسی آبخیزداری میم

Woven-wire check dams are small barriers which are usually constructed to hold fine material in the gully (Fig. 13). They are used in gullies with moderate slopes (not more than 10 percent) and small drainage areas that do not have flood flows which carry rocks and boulders. The dam is either constructed straight across the gully or in a crescent shape with its open end upstream. The crescent shape check dam is commonly used to allow a longer spillway than is possible on a straight one. At the same time it anchors and protects the ends of the dam. An offset equal to about one-sixth of the gully's width at the dam site will generally provide sufficient curvature.

For example, if the gully is 7.5 m wide, the spillway of the dam would be about 1.25 m downstream from the abutments. A row of posts is set 0.6 to 0.9 m deep along the curve of the dam at about one meter intervals. Posts should be placed to form an interval near the centre of the gully for the central portion of the spillway.A trench about 0.2 m deep and about 0.15 m wide is dug along the upstream side of the row of posts.Heavy guage (four mm or more in diameter) woven wire is placed in the ditch against the posts so that 0.25 to 0.3 m of the wire projects above the ground surface along the spillway interval. The coarse mesh should be placed at the bottom of the ditch.

The wire should be stapled securely to the posts. Keep the top of the wire as level as possible along the central portion (the crest of the spillway) to obtain a much better spread of water over the structure. A layer of fine mulch is placed underneath the apron so that a closer bond with the earth is secured. Rocks, brush or sod may be used for the apron. Anchor the brush by pulling the butt ends through the wire mesh, where both the fill and projecting branches will help hold it. Lay enough brush to make an apron at least 1.2 m long and which extends at least 0.6 m on each side of those posts that form the level portion of the spillway. A tie pole is placed across the centre of the apron and anchored to stakes in order to compress the brush. The apron is countersunk by shorter brush used near the upper end, which produces a shingle effect. To promote rapid filling and to seal the structure, straw, fine brush or similar material should be packed against the wire on the upstream side to the spillway crest. This should be covered with a well-packed earth fill with a minimal 1:2 or 50 percent slope. Sodding or placing of rocks along the spillway crest prevents erosion