Art. LI. —On the Surface Fall of Water, as a guide for Under Drainage.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 2, 1869, Page 213

Art. LI. —On the Surface Fall of Water, as a guide for Under Drainage. By James Baber, C. E. [Read before the Auckland Institute, July 5, 1869.] The practical part of drainage is an agricultural subject, but the principles from which rules for practice are deduced, belong to general science. In this colony it is of importance that drainage of land be conducted on proper principles. To examine the properties of one of these is the object of this paper. Water on the surface, descending from a higher to a lower level, follows the general law of bodies in motion, moving in the line of least resistance. At any point in the descent, this line will be found to be at right angles to the level or contour-line of the surface at that point. Water drains off an even surface in straight lines perpendicular to the contours, or in curves having chords in the same direction. So if the courses of water over any land be carefully marked, and lines be drawn at right angles to these courses, the line so drawn will form parts of the contours of the surface. * A diagram to illustrate this has been omitted. —ED. A drain laid in the line of these courses will possess the following properties:— Water will enter it on both sides with an equal pressure, the depths from the surface being equal. It will drain equally an equal distance from each side, for if any two equidistant points be supposed at the depth of the drain, on what may be termed the drainage surface, on opposite sides, and opposite to the line of drain, these points and the drain will be on the same level. The drain will not leak, water will not enter on one side and escape through the joints on the other side, for having sunk through the soil to the level of the drain, it must descend through the pipe, that being in the line of least resistance. The forces which chiefly act on water descending from the surface of land to the drainage level are:—impulse from water in motion, and gravitation. The capillary and molecular attractions, and the absorbent powers of the soil, vary so much, that they need not be calculated for general rules. These two principal forces will operate in the line of descent, at right angles to the contour. The deduction from this principle is, that the nearer a line of drainage approaches the perpendicular to the contour, the more efficient that drainage will be.