A semi-empirical method \([\Delta Z = \int_{{t_1}}^{{t_2}} {\alpha \omega \left( {{k_1} - 1} \right){C_{*a}}/\left( {1 - p} \right){\rm{ }}} dt]\) for calculating seawall toe scour depth, derived from the bed deformation equation, treats the ratio of sediment concentration to sediment transport capacity, \({k_1}\), as a fixed constant (\({k_1}=0.34\), calibrated from a limited number of field sample points during the 2011 and 2017 flood events). While this treatment can capture the integral-equivalent effect of specific hydrodynamic processes, its applicability under varying flood and sediment transport processes has not been systematically verified. In this study, a two-dimensional flow–sediment–morphological coupled model was used to investigate the spatiotemporal variation of \({k_1}\) at multiple seawall cross-sections in the Qiantang River Estuary, using typical historical floods and idealized design floods as test scenarios, and to evaluate the applicability of the conventional calculation method. The results indicate that \({k_1}\) is not a fixed constant but varies with flood stage and bed topography. During the 2011 flood peak, \({k_1}\) at the maximum scour point of the Xijiang seawall cross-section decreased to approximately 0.314 and then recovered during the flood recession. At the location of steepest bed gradient, the instantaneous variation was more pronounced, with \({k_1}\) increasing from 0.53 to 102.6 after the flood peak. During the 2017 extreme flood, when the simulated scour depths were in close agreement with measured bathymetric data, the process-integrated equivalent value of \({k_1}\) was 0.59, exceeding the conventional value of 0.34. Results from the idealized flood scenarios further demonstrate that sharp-peaked floods cause a pronounced drop in \({k_1}\) during the flood peak, broad-peaked floods prolong the low-\({k_1}\) period, and double-peaked or compound floods intensify the continuous scour effect. These findings indicate that the applicability of the conventional method depends on the stability of \({k_1}\) and its process-integrated equivalent value. This work provides a reference for seawall toe scour risk assessment, calculation method optimization, and the seawall design and maintenance under the “Thousand-Kilometer Seawall Safety Enhancement Project” in the context of rising sea levels and intensifying compound flood risks.