Waterways from draining excess water for fields

In order to avoid problems of erosion at the site of emptying and to reduce the speed of watershed runoff following rains; soil conservation structures should be designed for water retention and infiltration whenever possible. If it is judged necessary to drain water from a field, special care should be taken in selecting areas in which to deposit all the diverted drainage water.

Possible drainage areas include pasture areas with a thick ground cover, orchards, or forested areas; where infiltration of the diverted water can probably occur with a minimum of erosion, especially if the water is spread over a large area. Existing waterways may also be used as drainage sites, although one should avoid exaggerating erosion problems by diverting water into areas of active gully formation. In all of these cases, if erosion problems are noted upon diverting runoff water, then a permanent, protected site for receiving runoff should be designed and constructed as soon as possible.

Protected drainageways can be formed by reshaping natural drainageways or digging artificial drainageways of a low, broad shape, protecting them from erosion by lining with rocks, planting grass, and/or placing drop structures or check dams periodically. (Fig. 19).

Fig. 19. Water drainageway protected against erosion by rock lining.

Waterway design specifications are given in more complete soil conservation guides (Hudson, 1981; Suarez Castro, 1980), allowing the construction of waterways of an adequate size and the selection of an adequate lining method based on local metereological conditions, soil properties, the area of land involved land slope, and the type of protecting lining to be used. If sufficient data is available for the work area, such information can be useful in designing a drainageway of sufficient capacity, without overdesigning it and involving excess work. If however, insufficient local data is available, or it is desired to teach farmers the conservation techniques without discouraging them with complicated tables or formulas, then the appropriate size can be estimated. Hudson (1981) mentions an extremely simplified method ignoring local climatic or edaphic factors: construction of a shallow (30 cm) drainageway measuring one meter wide for every hectare of land area in the drainage basin. While this is probably oversimplified, it does allow one to design drainages of probably adequate dimensions without having to deal with complicated methods.

When no appropriate drainage area is available around a field, then retention wells can be dug and water diverted into, and stored in them while it gradually enters the soil or evaporates. (Fig. 20)

Fig. 20. Retention well as site for diverting runoff water.

Special care should be taken to have the waterways and retention wells completely constructed and well vegetated, if grass lined, before diverting drainage water into them. This can be done by preparing them a season or two ahead of time or by preparing a temporary drainage site until the permanent one is ready.