Sometimes when a main embankment is set far back, a subsidiary embankment of smaller section is constructed closer to the stream. This is often objectionable. The smaller embankment is liable to breach, and the water then rises suddenly instead of gradually against the main embankment, which is thus endangered to some extent, especially as it is dry instead of being soaked. It is often said that one effect of embanking a reach of a river is to increase the severity of floods further downstream. The importance of this is generally exaggerated. The narrowing of the flood stream in the embanked portion causes the flood to travel more quickly and rise higher in that particular reach. At a place further downstream the same effect is produced, but in a less degree and only because of the increased velocity and consequent reduction in the flattening out of the flood wave, especially when the rise is soon succeeded by a fall. When there is a gradual rise lasting for a considerable time--and this is most likely to cause a high flood--there is no rise of the flood-level downstream of the embanked reach, except such as is due to the increase in the discharge of the stream consequent on the absorption and evaporation being less than before, owing to the reduced area of flooding in the embanked reach. In the case of a long-continued rise, such as that just mentioned, it is the reach immediately upstream of the embanked reach which will, to some extent, share in the increased height of the floods. An embankment may suitably have side slopes of 4 to 1 on the river side and 3 to 1 on the land side, with a top 10 feet wide and 3 feet above high flood-level. On the Irrawaddy the top width is generally 8 feet. For very high and very low embankments it is 10 feet and 3 feet respectively. In Holland 1 foot above high flood-level was at one time supposed to be the rule, but in practice it was usually 4 feet. With sandy soil the riverward slope prescribed was 6 to 1. Such flat slopes are not necessary if fascining or stiff soil is used as a protection. On the Rhine the top width of embankments consisting of gravel and sand has been made about 15 feet, but the side slopes were 1½ to 1 and 1 to 1. The embankments had spurs to keep off the current. Sand, protected as above, makes a good embankment, and rats do not burrow into it. Of course, if a breach occurs in an embankment consisting mainly of sand, it will enlarge very quickly. In some cases an embankment has a core wall of sand or of clay puddle. In Holland, on sandy soil, a trench 8 feet wide is made and taken down to the clay.