The elements of the storm water network are made to safely drain most water during a heavy rainstorm. Flood risk modelling (mathematical modelling) with hydraulic software is a good way to check the network's level of service, especially now that technology has changed. Flood risk modelling is an essential method for checking the network sufficiency and adequacy for different kinds of rain. Modelling flood risk should be done for the whole network to ensure that flows are connected and get accurate results. In this study, one-dimensional flood risk modelling is needed for a sub-network that is part of Qatar's full storm water network. The flood risk modelling is conducted using InfoWorks ICM software. Running the whole model takes a lot of computational data processing. Splitting the entire stormwater network to the concerned sub-network without considering the hydraulic effects of upstream and downstream flows at the boundary conditions of the sub-network will lead to erroneous conclusions. In this research, a new method for hydraulically dividing the network into sub-networks is presented, taking into account the characteristics of the boundaries. This method increases the efficiency and viability of hydraulic modelling for sub-networks in big networks. For purposes of validation, the results of the reduced model in terms of flood depths and volumes are compared with those of the full model. The results of the split model are in good agreement with the entire model.