As a part of the GrönNano project at Luleå University of Technology, a sustainable drainage system, composed of infiltration surfaces and a wale,has been subject to hydrological measurements and modeling. The aim of this study was to identify the conditions under which it is possible to model the hydrological processes of that system adequately, using the two-dimensional hydrodynamic modeling software MIKE 21 equipped with an infiltration and leakage module. To achieve this, a model was set up with site-specific data followed by sensitivity analyses, calibration, validation and finally evaluation against a corresponding model, set up in the integrated hydrological modeling system MIKE SHE. From a sensitivity analysis it was indicated that too large time steps may introduce undesired volume losses when using the infiltration and leakage module. As a consequence, along with limitations of the simple infiltration model, this module and its principles were questioned and recommended to be treated with great care in inland applications. A second sensitivity analysis showed that hydraulic roughness associated with the swale is of great importance. Based on this, a proper two-dimensional representation of the swale was considered to be a critical point for predicting discharge correctly. Therefore it is of interest to further examine the influence on this, of for example the model grid resolution. The MIKE 21 model was successfully calibrated for a selected rainfall event but could not be fully validated for a following longer period. Nor could the model be satisfactorily evaluated against the preliminary version of the MIKE SHE model. However, from this it could be concluded that there were great uncertainties in the accuracy of rainfall and calibration data.Infiltration modeling – A complement to a proven rainfall-runoff model for urban stormwater management. The global situation of extensive urbanization and climate changes demands innovative approaches for implementing sustainable drainage systems (SUDS) in urban areas. By using rainfall-runoff computer models, the design and integration of SUDS may be optimized to meet both environmental and economical preconditions. The broad spectrum of available rainfall-runoff modeling tools offers the possibility of efficient large-scale applications as well as representation of the full hydrological cycle. However, with the computer power of today, it is hard to fulfill both these criteria simultaneously, a fact encouraging the “middle way”. Luleå University of Technology recently initiated a project called GrönNano, with the aim to develop new ideas and modern technology within the field of sustainable stormwater drainage in urban areas. As a part of this project, DHI Sweden were involved to set up a computer model, MIKE SHE, describing all the important hydrological processes in an existing sustainable drainage system. This was done in order to acquire knowledge about these processes and to evaluate the functions of this system. A fully integrated MIKE SHE model requires significant amount of input data and running time for describing the hydrological processes in detail. In the master thesis Two-dimensional hydrodynamic modeling of overland flow and infiltration in a sustainable drainage system by Karl Gunnarsson, a study has been performed to examine under which conditions the same system can be adequately modeled using the two-dimensional (2D) hydrodynamic modeling tool MIKE 21. This type of surface-runoff simulation tool, originally developed for marine applications, is in general more efficiently applied on a larger scale compared to more complex tools like MIKE SHE. Although 2D hydrodynamic models are extensively used for flood risk analysis in urban areas, they seldom include a physical representation of the soil infiltration process. However, just recently, MIKE 21 was taken one step in this direction by implementation of a simple infiltration and leakage feature. The use of this new function for inland flooding applications is questioned in the study of the master thesis; mainly due to the reason that the infiltration-leakage concept and parameters of the model are not considered to be sufficiently analogous to the corresponding processes and properties in nature. Moreover, the infiltration and leakage feature was revealed to cause errors in the water volume balance, if the basic model was not set up properly. The performance of the MIKE 21 model was evaluated by comparing the behavior of modeled outflow from the drainage system with observed flow data from measurements in a swale of the drainage system. Although the model performed well for a selected calibration period, it performed rather poor for a set of independent validation periods. The main issue related to this, is considered to be the difficulties of incorporating the swale as a 2D feature in the 2D model grid. In that case, a proper representation of the swale flow would probably require a rather fine grid resolution of the model. In contrast, for efficient application of the model on a large scale, a coarse grid resolution would be required. Thus, for future similar applications of this model concept, it is necessary to either find an optimal way of representing swales in 2D, or to use a one-dimensional representation of the swale instead, by establishing a link to another modeling system. It is also crucial to be aware of the limitations of the MIKE 21 infiltration and leakage model, keeping in mind the great deviations from the processes in reality and that the basic principles may not be optimally or even intentionally designed for inland applications
