Optimal secondary clarifier performance is crucial to meet treatment requirements, especially when treating peak wet weather flows (PWWFs), to prevent high effluent suspended solids (ESS) concentrations and elevated sludge blankets. Field stress test and state-of-the-art computational fluid dynamic (CFD) modeling were used to evaluate the effect of tank side water depth on performance and capacity for two different units treating the same mixed liquor characteristics. The results showed that the deeper clarifiers had more capacity to accept inventory transfers during wet weather events, and thereby accept substantially higher surface overflow rates and solids loading rates. However, it should be noted that other design elements were also influential including the center well size, the sludge removal mechanism and the launder type. On-site stress test and CFD modeling were found to be an effective method to determine: 1) capacity of existing clarifiers, 2) performance limitations of clarifiers, 3) effects of clarifier modifications that can potentially improve performance and increase their capacity, and 4) design considerations for new clarifiers.