A tool called WetSpa-Urban was developed to respond to the need for precise runoff estimations in an increasingly urbanized world. WetSpa-Urban links the catchment model WetSpa-Python to the urban drainage model Storm Water Management Model (SWMM). WetSpa-Python is an open-source, fully distributed, process-based model that accurately represents surface hydrological processes but does not simulate hydraulic structures. SWMM is a well-known open-source hydrodynamic tool that calculates pipe flow processes in an accurate manner while runoff is calculated conceptually. Merging these tools along with certain modifications, such as improving the efficiency of surface runoff calculation and simulating flow at the sub-catchment level, makes WetSpa-Urban suitable for event-based and continuous rainfall–runoff modeling for urban areas. WetSpa-Urban was applied to the Watermaelbeek catchment in Brussels, Belgium, which recently experienced rapid urbanization. The model efficiency was evaluated using different statistical methods, such as Nash–Sutcliffe efficiency and model bias. In addition, a statistical investigation, independent of time, was performed by applying the box-cox transformation to the observed and simulated values of the flow peaks. By speeding up the simulation of the hydrological processes, the performance of the surface runoff calculation increased by almost 130%. The evaluation of the simulated 10 minute flow versus the observed flow at the outlet of the catchment for 2015 reached a Nash–Sutcliffe efficiency of 0.86 and a bias equal to 0.06.

Nowadays there is a need to overcome the effects caused by rapid urbanization with more innovative methods. Recently, source control approaches, known as Low Impact Development (LID), are being used by urban planners to cope with water-related problems due to their cost-effectiveness and reliability. To do this, allocation of LID techniques in most suitable locations is done by introducing a new modeling tool called LID locator to the WetSpa-Urban software package and tested in the Watermaelbeek catchment situated in the Brussels capital region. In the first step, almost half of the catchment is eliminated from the implementation of LIDs by considering only pixels with high runoff. Also, by considering the sewer network behavior, the total area of high potential pixels for implementing LID is reduced to the range of 37% and 10% of the entire area of the catchment for different scenarios. In the next step, the selected area is further eliminated by use of suitable location algorithm based on size and other installation limitation of each type of LID. Then, the results are used as an input for the newly developed cost-optimization tool. In the end, the best scenario (10-year design storm with the current state of sewer network) is selected based on their performance in surface runoff volume reduction and the total cost. As a result, maximum flow and total volume at the outlet is reduced by 21 and 8 % for the most cost-optimized scenario in January and February 2015. In addition to considering sewer network 2 performance, land characteristics and LID installation limits for LID implementation, producing maps indicating the most cost-efficient locations and combinations for LIDs and visualizing them through the GUI make this tool a user-friendly product. This tool was applied to this case study in Brussels however it is applicable for other urban catchments.

Nowadays there is a need to overcome the effects caused by rapid urbanization with more innovative methods. Recently, source control approaches, known as Low Impact Development (LID), are being used by urban planners to cope with water-related problems due to their cost-effectiveness and reliability. To do this, allocation of LID techniques in most suitable locations is done by introducing a new modeling tool called LID locator to the WetSpa-Urban software package and tested in the Watermaelbeek catchment situated in the Brussels capital region. In the first step, almost half of the catchment is eliminated from the implementation of LIDs by considering only pixels with high runoff. Also, by considering the sewer network behavior, the total area of high potential pixels for implementing LID is reduced to the range of 37% and 10% of the entire area of the catchment for different scenarios. In the next step, the selected area is further eliminated by use of suitable location algorithm based on size and other installation limitation of each type of LID. Then, the results are used as an input for the newly developed cost-optimization tool. In the end, the best scenario (10-year design storm with the current state of sewer network) is selected based on their performance in surface runoff volume reduction and the total cost. As a result, maximum flow and total volume at the outlet is reduced by 21 and 8 % for the most cost-optimized scenario in January and February 2015. In addition to considering sewer network 2 performance, land characteristics and LID installation limits for LID implementation, producing maps indicating the most cost-efficient locations and combinations for LIDs and visualizing them through the GUI make this tool a user-friendly product. This tool was applied to this case study in Brussels however it is applicable for other urban catchments.