The banks of a stream or watercourse should be inspected occasionally, especially on the up-stream side of a culvert, in order to note whether they are stable or crumbling away, as then the course of the stream may be widened or diverted, and so erode the toe of an embankment and cause a slip of earth. Splayed wing-walls are a protection against such a danger besides aiding the flow of water through them, and they also lessen the chance of a damming back of the water and undermining, as they increase the discharge; for instance, a splay of 53° has been found to increase the flow about 25 per centum. The surface of the material of which a culvert is constructed should be as smooth as practicable, so as to reduce the friction of water flowing past it, which, in the case of unplaned timber, cast and wrought ironwork, ashlar masonry, brickwork, and concrete, is about the same, but is some 30 per cent. more when the surface consists of rubble masonry. When an embankment crosses a narrow valley, in which no watercourse exists, instead of a culvert, a bed of loose stones has been placed upon the ground under the whole area of the seat of an embankment at its greatest depth, a proper fall and bed being given to it. The toe of the slope on the up-stream side should be protected from any wash, and the stone layer be carried a few feet beyond the foot of the slope on the lower side. The following general principles it is well to remember in designing culverts. When a culvert is of uniform section, which is almost invariably the case, it should have the same inclination throughout. Avoid, or ease, all bends as much as possible. Have splayed wing-walls. Make provision against undermining by scour or percolation of water. Have smooth and even surfaces so as to reduce the friction and increase the discharge. Have an arched invert to a culvert, and a flat bed stone to all small surface drains, with complete connection to the side walls. In some countries having a severe climate, or in high mountainous districts where the soil is rock and a heavy discharge of flood-waters occurs, instead of placing a culvert and gathering the waters at or about the level of the toe of an embankment, in deep hillsides or ravines an unlined tunnel is made under the embankment in the rock, thus avoiding a masonry or brickwork structure, which could only be set in the summer months, and preventing the waters touching the seat of an embankment and promoting a slip.