dyke
A flood wall often made of rock and earth. It is chiefly British, spelled Dike. It may also be a ditch or channel. In Geology, it is a long mass of igneous rock that cuts across the structure of adjacent rock. The use of a dyke may cause severe flooding on the other side of the river and up-stream of the dyke.

This is also the original meaning of the word, in comparison to the new, generally accepted meaning referring to lesbians.
Because of the dyke across town, the flood was significantly less devistating.

Though it's a risk, there is a small hole in the dyke to let water seep out through more quickly.
