MISTAKE NO. 4—TOO SMALL TILE.
No mistake has become apparent sooner than this. The following observations will account for this, and also aid in correcting it. The whole area of land which naturally discharges toward the drain is not always taken into account. It is generally thought that land lying at some distance from the drain, though sloping toward it, does not affect the capacity required for the drain, whereas in times of heavy rains, when drains are taxed to their utmost, water flows from those more distant parts over the surface to the ground acted upon by the tile drain. We must then provide for the drainage not only of land contiguous to the drains but for an additional amount of water coming from adjoining slopes.
Another popular error is that the diameter of the tile is the measure of its capacity, whereas the grade upon which it is laid is as important as the size of the tile. The extreme porosity of many of our soils, and the lack of thorough lateral drainage is another thing by reason of which main drains become over-taxed, simply because drainage water is not held in check by close soils, or distributed by lateral drains, but is brought in large quantities over the surface to the drain line, and must be taken away in a short time or injury is done to the land. In making mains or sub-mains it is better to err in making them too large than too small.