Capacity and Noncapacity Sediment Transport Characteristics in the Overtopping-Induced Dam-Breaching Process

Abstract

Although the applicability of physically based noncapacity sediment transport models has received increasingly wide recognition, debate still exists: it was reported that for overtopping-induced dam-breaching flows, the noncapacity model may either produce unphysical oscillation in bed profiles or induce large deviations from the measured data. In this paper, the applicability of the noncapacity model for overtopping-induced dam-breaching is investigated by theoretical analysis and numerical case studies. It is shown that the governing equations of the capacity model can be derived from those of the noncapacity model by incorporating the instant sediment adaptation assumption. For instant sediment adaptation, a threshold value of the net sediment exchange flux (or the sediment adaptation coefficient α) can be derived as an upper limit. Using this threshold value, the simulated overtopping processes by the noncapacity model resemble those of the capacity model with negligible difference. Moreover, numerical case studies were conducted with a series of reasonable (not exceeding the threshold) α values, which reflect different adaptation features. Specifically, the noncapacity model can give either smooth or waving beds, depending on the sediment adaptation feature. A relatively slow sediment adaptation feature leads to longitudinal increasingly large discrepancy between the transient concentration and the capacity regime, which results in an increasing net erosion flux along the slope, and thus a waving bed. In contrast, a sufficiently fast adaption leads to almost constant net erosion flux along the slope, and thus a smooth bed was computed. These indicate that the noncapacity model with a reasonably specified sediment adaptation coefficient is generally applicable for overtopping dam-breaching processes, whereas the capacity model is applicable for processes with relatively coarse sediments and accordingly sufficiently fast sediment adaptation. This paper resolves a recent debate about the applicability of noncapacity models to overtopping-induced dam-breaching processes. Although recent research indicated that the noncapacity model may either produce unphysical oscillations in bed profiles or induce large deviations from the measured data of overtopping-induced dam-breaching processes, this study demonstrates the opposite. First, the capacity model is a reduced case of the noncapacity model. The former can be obtained by incorporating the instant sediment adaptation assumption into the latter. A threshold expression of the net sediment exchange flux, which fulfills instant sediment adaptation, is also derived. Consequently, it is demonstrated that with a reasonably specified sediment adaptation coefficient, the noncapacity model can also well reproduce the concerned overtopping-induced dam-breaching process. Second, the sediment adaptation feature affects the computed bed profiles considerably. A relatively slow sediment adaptation feature favors the formation of a waving bed, whereas a sufficiently fast adaption may produce a smooth bed.