When the water-level is raised by a weir or by narrowing the channel--though in the latter case the raising may not be permanent--it is generally best to commence the work from the upstream end. The raising of the water-level will then not interfere with the execution of the rest of the work. But in a case of widening, where the water-level upstream of the work is lowered, the work can conveniently be begun at the downstream end, and the remark applies also to a case of straightening, provided that the new channel is not so small that it at first causes no lowering. In any case in which there is a doubt whether the whole of the scheme will be carried out, the reach to be dealt with first can be decided on according to circumstances. There is no general reason for selecting an upstream or downstream reach, except that any raising or lowering of the water-level will extend upstream of the reach and not downstream of it (CHAP. I., _Art. 4_). Training walls and groynes, if made with stakes or fascines or any materials except stone, require careful watching and maintenance. CHAPTER IX CANALS AND CONDUITS 1. =Banks.=--All banks which have to hold up water should be carefully made. The earth should be deposited in layers and all clods broken up. In high banks the layers should be moistened and rammed. The dotted lines in fig. 29 show two possible courses of percolation water. The vertical height--from the water-level to the ground outside the bank,--divided by the length of the line of percolation is the hydraulic gradient, as in the case of a pipe, and this gradient is more or less a measure of the tendency to leakage. A bank which has water constantly against it nearly always becomes almost water-tight in time. The time is less or greater according as the soil is better, and according to the amount of care with which the bank is made. [Illustration: FIG. 29.] The side slopes of banks vary with the soil. Generally they are 1½ to 1, but they are sometimes 2 to 1 or even 3 to 1 if the soil is bad or sandy, or if great precautions against breaches are to be taken. Leakage can sometimes be stopped by throwing chaff or other finely divided substances into the water at the site of the leak. In other cases it is necessary to dig up part of the bank, find the channel by which the water is escaping, and fill it up by adding earth and ramming. On some navigable canals in France it was at one time the custom to lay the reach dry, when a bad leak occurred, and to dig away the bank and lay slabs of concrete or puddle over the place. This plan was abandoned, and instead of it sheet piles are driven in. They are then withdrawn one at a time and, if any leakage occurs, the space is filled with concrete.