The investigation of the run-off conditions in the municipal catchment areas is a very old hydrological discipline. Given the fundamental differences between the natural and urbanised catchment areas, it is often not possible to apply classic methods used in hydrology in various calculations. Therefore, special working methods and calculation methods have been developed for the municipal hydrology and they are applied to the modelling of the run-off from the urban areas.
A major difference compared to hydrology is the completely different timing and size. The catchment size with respect to the urban area is tens of hectares (max. hundreds of hectares), in hydrology we are talking square kilometres, the time parameter of the hydrological processes is months, in exceptional cases days and hours. The time parameter of the rainfall/run-off phenomenon in the municipal catchment area is minutes, only in some types of assignments longer time periods are interesting (e.g. a year).
The receiving bodies of water in municipal areas are usually affected by the agricultural production in the surroundings of the town, potentially together with other municipal catchment areas located on the same receiving body of water. Therefore, in order to reduce the pollution discharged from the municipal catchment area to the receiving body of water it is necessary to take these facts into consideration. The picture shows a typical situation. We have two localities situated on a single water course, the surroundings of which is intensively used for agricultural purposes. The receiving body of water and the tank in the urbanised area are strongly polluted by the combined system overflows and WWTP effluents. The receiving body of water and the tank are also affected by the agricultural operations. The task related to the ecological solution to this area is to keep the water in the tank under certain limit values of pollution at an acceptable price. This means that the agricultural production must be governed by certain rules, the water effluent from the tank and the receiving body of water must be controlled and the final design and operation of the municipal drainage system must also respect certain rules of ecological management of the whole area. This indicates that the task of the municipal drainage is to propose such a solution that guarantees that the pollution of the receiving body of water caused by the outlets of the separate rain sewerage, combined system overflows and WWTP effluents is minimum and in compliance with the comprehensive solution covering the whole area.
We are building the municipal drainage systems in order to increase the comfort of living and convey wastewater from the urban catchment; however, at the same time the drainage systems are constructed to protect the receiving bodies of water against pollution. When designing municipal drainage systems we principally address an unsolvable contradiction: „protection of people against the nature and, on the contrary, protection of the nature against people“.