Combined urban and rural models for integrated river basin management

Traditionally, rainfall runoff processes are considered from a most different point of view in rural and urban hydrology. Usually they are modeled independently, if at all they are linked by data transfer interfaces. Only a few meso scale models (including SWMM) are known that are capable of modelling both urban and rural catchments simultaneously at a high spatial resolution; however, detailed interactions between the different hydrologic sub-systems are hardly taken into account. In the field of urban hydrology successful attempts have been made to model sewer systems including wastewater treatment plant (WWTP) and receiving water body. However, pollutants from rural areas are mostly neglected when modeling water qual-ity of receiving water bodies. Besides other reasons, the complexity of an integrated model, taking into account rural and urban systems as well as their interactions is a major issue for the lack of such a model. At the moment, however, for the realisation of the EU-Water Framework Directive such models are urgently needed.

In this paper, an approach is described, which takes into account the interaction of urban and rural hydrology by a 2-layer modeling system. For this approach, two existing models have been modified and combined with pre-defined points of interaction. The main goal of the de-velopment was the improved flow peak simulation in streams loaded with multiple combined sewer overflows from several sewer networks. Criteria for water quality in the receiving water bodies are neglected in this discussion, but were kept in mind by the authors for further de-velopment of the integrated model system.

The models chosen are a highly distributed deterministic GIS-based hydrological grid-model for rural areas as well as a lumped deterministic hydrological pollution load model, that is based on the concept of hydrological similar units for urban areas, respectively. At this stage, the urban hydrologic model also contains a module for pollution transport and a simple com-ponent for the hydraulic representation of a WWTP. In the 2-layer approach, one layer each represents the rural and urban system. Within each time step, both layers are simulated simultaneously allowing for feedback processes.

Concerning the modification of surface and subsurface flow patterns in urban areas further assumptions had to be made. E.g. the interflow process underneath urban areas is “deacti-vated” in the according grids of the rural model; hence only vertical underground flow proc-esses in the soil zone are considered. Overland flow from rural areas into an urban settlement will be processed according to the flow patterns given in the urban system .

The model is designed to handle any degree of urbanisation in a mesoscale catchment up to 1000 km². It can account for urban areas located outside the rural catchment but draining into the rural catchment boundaries; and for urban areas located inside the rural catchment, but draining beyond the boundaries of the rural catchment. The model’s functioning will be demonstrated by means of a case study .