Combined Urban and Rural Models for Integrated River Basin Management
Traditionally, urban and rural hydrology consider rainfall runoff processes very differently. Usually they are modeled independently, and linked, if at all, by data transfer interfaces. Only a few meso-scale models are known that are capable of modeling both urban and rural catchments simultaneously at a high spatial resolution; and, even in these, 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 plants (WWTP) and receiving water bodies. However, pollutants from rural areas are mostly neglected when modeling the water quality of receiving water bodies. The complexity of an integrated model, taking into account rural and urban systems, as well as their interactions, is one of the major reasons for the lack of such a model. However, such models are urgently needed for the realisation of the EU-Water Framework Directives (EU-WFD, 2000) which aims at achieving ecological improvements.
In this chapter, 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 development was 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 development 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 semi-distributed deterministic hydrological pollution load model that is based on hydrologically similar units for urban areas, respectively. At this stage, the urban hydrologic model also contains a module for pollution transport and a simple component for the hydraulic representation of a WWTP.
In the 2-layer approach, the two layers represent the rural and the urban systems respectively. Within each time step, both layers are simulated simultaneously allowing for the consideration of feedback processes. Concerning the modification of surface and subsurface flow patterns in urban areas, further assumptions had to be made. For example, the interflow process underneath urban areas is “deactivated” in the corresponding grids of the rural model; hence only vertical underground flow processes 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 meso-scale 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 functionality will be demonstrated by means of a case study.
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