Some qualitative and quantitative aspects of the surface water in an urban area with separated storm water and waste water systems

A part of the flat urban area of the new town Lelystad was subject of a study about the flow of the surface and subsurface water into and through the open drains. In three catchment areas, the precipitation, the discharge of storm drains, the subsurface drainage and the groundwater level have been measured. From these data the runoff coefficients and the transformation of the rainfall into the inflow and the outflow from the storm water drains are calculated. These outputs are used as the input for modelling the non-steady flow through the open drains. This model is applied for the design of the urban system of open drains, culverts and weirs. Also the waterquality of the system has been investigated. From the measurements it can be concluded that the system of separated storm water and waste water sewers provide a lower output of water pollution into the environment than a combined system with zero overflows.