Contour ditches (drainage and infiltration ditches)

Contour ditches serve many of the same purposes as contour barriers, in addition they completely stop dawnslope water movement as the water falls into the ditch. These structures are some of the mast useful for small-scale hillside farming singe they require less work than terraces, are simple to build, and can be used to either divert or to retain water. If constructed at a 1% slope* they divert excess water to protected drainageways, reducing soil erosion and leaching of nutrients. (Fig. 11). The uppermost ditch, called stormwater drain by Hudson 1981, is very important if a great deal of water enters from above the field.

Fig. 11. Contour drainage ditch

*Hudson (1981) recommends a 0.25% slope for contour ditches (channel terrapin). Due to the relative inaccuracy of the rustic levels often used (See Apendix 4), a 1% slope is recommended here to insure water flow in the proper direction.

If it is desirable to retain as much water as possible, earthen dikes can be left in the ditches or the ditches can be constructed at a 0% slope so that water infiltrates into the soil and is not diverted outside the field. (Fig. 12)

Fig. 12. Contour infiltration ditch

The ditches are constructed using pickaxes and shovels. If possible, plowing the contour line makes the construction much easier. The first stage in the construction is to excavate a 12 inch wide by 12 inch deep ditch (ditches can be constructed of any size, if desired). Then the banks are formed by cutting a slanted wall at each side. The removed earth is placed in a mound 6 - 9 inches below the lower lip of the ditch. A live barrier is necessary above the upper edge to prevent filling with soil. Especially on steeper slopes, it is often advantageous to plant the live barrier first, several months or one season in advance, so that the ditch will be adequately protected once built.

Table 3 should be consulted for the appropriate distance between ditches on hillsides of a given slope. If the ditches are to be dug with a 1% slope to drain excess water, the 1 or 2 meters before emptying into a protected drainageway should have a slightly steeper slope, (1-2%) to facilitate drainage. Care should also be taken not to drain excess water into neighboring fields, houses, or other areas which may result in problems. (See "Waterways for draining excess water from fields").

TABLE 3 Spacing of contour hillside ditches