Overflow spillways

Retention basins and overflow spillways feeding receiving water bodies can help deal with capacity issues in the drainage system.
Spillways and basins in the sewer system protect against urban flooding, against flooding and pollution of watercourses and lakes, and against erosion due to discharges of large amounts of water over a short period.
Overflow spillways can hold back supernatant and organic material, and basins can reduce the flow to watercourses and lakes as well as hold back organic and anthropogenic substances.