At “incomplete” falls, i.e., where the tail water level is above the crest, it is not unusual to construct a low-level arch, which forms a syphon. The object is to allay the surging of the surface water.
The question of skew bridges has been dealt with in [Art. 3]. Another question is that of the heights of bridges. Irrigation channels, especially the smaller ones, are very frequently at a high level, and bridges have ramps which are expensive to make and to maintain, and are inconvenient. The lowering of distributary bridges in such cases, so that they become syphons, or nearly so, has often been advocated and is frequently desirable. The bed should slope down to the floor and up again. The heading up can be reduced by giving ample waterway, but it will not be necessary to do this if there is head to spare. The fall in the water surface can be recognised and shown on the longitudinal section. The structure becomes one of the incomplete falls above described. The crown of the arch can, if desirable, be kept above F.S. level, so that floating rubbish will not accumulate.
The width between the parapets of a regulator can be 10 feet in the case of a driving road. It may be less, according to the width of the bank, in other cases.
The upper layer of the floor of a bridge or regulator is of brick on edge. Below this there is a layer of brick laid flat, and below this, concrete of a thickness ranging from ·5 feet to 3 feet. The thicknesses of piers range from 1·5 to 3 feet.
The bricks used for canal work in Northern India are 10 inches long, 4⁷⁄₈ inches wide, and 2³⁄₄ inches thick. The thicknesses of walls are about ·83, 1·25, 1·7, 2·1, 2·5 feet, and so on.
The slopes of ramps should be about 3 in 100 for district roads, and 5 in 100 for village roads.
Railings should be provided along the tops of high walls and top of pitching near to public roads or canal patrol roads. Bumping posts should be provided for all parapets, and should not be so placed as to seriously obstruct the roadway.
The quarters for the regulating staff should, when convenient, be in the fork between the two principal branches. They may be on the bank—with foundations on pillars carried down to ground level—but not in such a position as to obstruct the road or any road likely to be made. Rests consisting of two parallel timbers bolted to blocks of masonry reaching up a foot from the ground, should be provided for the needles or planks. The bolt head should be countersunk so as not to damage the needles and planks when they are hurriedly laid down.
When two or more works are close together they should be made to conform, and the whole site should be considered with reference to a neat and suitable arrangement of works, ramps and roadways. If an outlet is near to a minor or distributary head the parapets of the two should be in line. If two masonry works of any kind are near together it is often suitable to pitch the intervening space. If there are outlets or distributaries on opposite banks they should be exactly opposite each other. Where a road crosses a bridge or regulator, the bank should be at the same level as the road, the bank being gradually ramped back to its original level. The space in front of any quarters should have a slight slope for drainage, but otherwise be at one level and be connected with the road or bank by proper ramps. The berm or bank should be made at the exact level of the top of any pitching or side wall which adjoins it. Wing walls are frequently made too short, so that the earth at their ends forms a steep slope and is worn away, and the bank or roadway is cut into. The walls should extend to such a point that the earth at their ends cannot assume a slope steeper than the slope of the bank.
It is obvious that for every masonry work there should be a large scale site plan[23] showing all roads, ramps, and adjoining works, both existing and proposed roads being shown for some little distance from the work.