Large-scale combined sewer systems necessitate accurate hydraulic models with a low computational requirements to depict combined sewer overflows (CSOs) in real-time. A hydraulic model is proposed that incorporates the mass and momentum equations into a series of look-up tables. Flow that enters the interceptors is routed downstream based on a hydraulic performance graph (HPG) which conserves momentum, and a volumetric performance graph (VPG) which conserves mass, established for each conduit. Weirs and sluice gates that control water distribution throughout the combined sewer system are also represented by look-up tables created off-line. During real-time computations, referencing the look-up tables is faster than computing the full equations. The model includes accurate sewer and deep tunnel components imported from Arc-GIS, and is shown to emulate EPA SWMM 5.0 dynamic wave results on a faster time scale. Calibration and timing results show that the model may be successfully applied to evaluate potential operating scenarios more quickly than SWMM.