GRADIENT OF DRAINS

Opunake Times, 26 August 1947, Page 4

GRADIENT OF DRAINS

ADVICE TO FARMERS GOOD FALL ESSENTIAL Tile drains, and in fact any farm drains, should cross the direction of the land's greatest local fall, so long as this crossing allows sufficient fall in the drains themselves. Too great a fall may result in under-scour as a result of water passing through the underside of tile joints and running along_ the bed of the tiles. What constitutes a s maximum safe fall depends on the nature of the sub-soil in which the tile is laid. It is suggested: that about 1 in 50 should be a maximum for light-textured soils, and up to 1 in 15 for stiff clays. Such steep falls are necessary only under exceptional circumstances and usually only in the vicinity of outlets. They should be avoided.if possible, or glazed jointed pipes should be used. Fall must be uniform throughout the length of the drain, or it may increase towards the outlet. It should never be reduced appreciably unless a settling pit (a large pipe or concrete tank, the bottom of which should be at least one foot below the bottom of the tiles) is provided at the point of change of gradient. Doubling the fall in a drain will increase its water-carrying capacity by about one-third, and halving the fall will reduce the capacity by one-quarter.