The flow dynamics adjacent to spur dikes exhibit turbulence and complexity, often resulting in the formation of scouring pits in the riverbed nearby. In regions downstream characterized by robust riverbed mobility, the vulnerability of riprap spur dikes stems from the instability of the upper riprap induced by these scour pits. Current research on scour pits primarily focuses on singular runoff conditions, with a limited exploration into the formation and traits of these scour pits under the combined influence of runoff and tidal currents. This study delves into the formation process and features of scour pits adjacent to submerged riprap spur dikes shielded by flexible mattresses, considering the impact of both runoff and tidal forces, using flume model tests in the tidal zone of the lower Yangtze River as a reference. Our findings reveal that the scour pits at the forefront and rear of riprap spur dikes undergo cyclic scouring and silting influenced by the runoff and tide current’s duration and intensity. The maximum scour depth observed ranges from 60% to 90% of that during runoff alone, contingent upon the ratio of maximum flow velocity at flood tide and ebb tide (denoted as e). This law can be quantitatively elucidated through the concept of the average effect of flow on the riverbed scouring and silting in a unit time (denoted as E). A formula to calculate the maximum scour depth of riprap spur dikes under both runoff and tidal current scenarios, along with a slope formula describing the maximum scour depth relative to the spur dike toe are proposed in this study. These formulations offer versatility across varying flow conditions. Subsequently, we establish an evaluation index pertinent to the safety operation of spur dikes based on the latter formula. This research contributes to a more comprehensive understanding of scour pit dynamics adjacent to spur dikes, especially under combined runoff and tidal influences. The proposed formulae and evaluation index hold promise in enhancing the assessment and maintenance practices for these critical riverbank structures.