<p>Lateral-intake multiple forebays are widely used in urban pumping stations, but there is still insufficient understanding of their hydraulic characteristics, making it difficult to provide scientifically based guidance for determining optimal start-up combinations, which are critical to pumping station efficiency. This study numerically investigated the hydraulic characteristics of lateral-intake multiple forebays under different start-up combinations and, for the first time, applied entropy production theory to analyze the causes and locations of hydraulic loss. The lateral-intake causes undesired flow patterns, including helical mainstream and flow recirculation in the forebays, and mainstream one-sided concentration and backflow in the sumps. These undesired flow patterns become more significant with reduced standby pumps and increased inflow Froude numbers, directly degrading flow uniformity and increasing total hydraulic loss. Keeping the downstream pumps in operation for one-pump and two-pump standby conditions helps to improve forebay flow uniformity and reduce total hydraulic loss. Entropy production method revealed that turbulence dissipation is the main cause of total hydraulic loss in the forebays, accounting for over 90%, which is induced by helical mainstream and flow recirculation and impingement, occurring near the forebay inlets, sump piers and suction pipe mouths. By comprehensively evaluating flow patterns, uniformity, and hydraulic loss, the optimal start-up combination for different standby pump requirements was concluded as operating the downstream pumps while keeping the upstream pumps on standby. This provides a theoretical basis for investigating hydraulic characteristics and optimizing pump operation in urban pumping station forebays.</p>