The reduction in pollutional load realised by storm water settling basins is potentially reduced due to the occurrence of a partially mixed situation in the basin or due to resuspension of settled material. The decrease in efficiency can theoretically be avoided by means of partially bypassing the basin. In order to quantify the potential increase in efficiency a settling basin in Amersfoort is taken as an example. This basin seems to behave like an almost completely mixed system, bypassing would increase the overall efficiency from ca. 34 % to ca 39 % for three overflows.
The dynamic behaviour of settling basins, scouring conditions and the boundary conditions for which settling basins are to be designed are the research needs for further development in the field of storm water settling basins.