Interpretation of delayed suspended sediment transport during flood events from the perspective of movement characteristics

Different velocities of flood and suspended sediment peaks during flood events result in the delayed transport of suspended sediment, which is exacerbated by reservoir impoundment. The diversity of flood events, the asynchronous pattern of suspended sediment peaks during flood formation and human activities lead to notable differences in the propagation of flood peaks and suspended sediment peaks and subsequently affect the suspended sediment lag, which poses certain problems and challenges for watershed governance, especially reservoir management. In this paper, the Three Gorges Reservoir (TGR) is selected as the research object, and we adopts the analysis of measured data, K-means cluster analysis, 1-D mathematical model and random forest regression. First, the flood process of the TGR from 2003 to 2020 is selected, and then flood event types with nine flood indices are identified. The influences of different flood types on flood and suspended sediment peak propagation are subsequently clarified. We then systematically analyse the factors influencing flood and suspended sediment peak propagation, explore the contributions of different influencing factors to the suspended sediment transport lag, and assess the reasons for the recent changes in the suspended sediment transport lag. The results show that (1) flood events entering the TGR can be approximately classified into six different types, the long-duration and dwarf-fat type (Type 1, 6.59 %), the sharp-thin and early-peak type (Type 2, 20.88 %), the medium-duration and conventional type (Type 3, 27.47 %), the short-duration and sharp-thin type (Type 4, 25.27 %), the long-duration and dramatic-change type (Type 5, 5.49 %), and the medium-duration and multiple- peaks type (Type 6, 14.29 %). (2) Different flood event types result in different effects on the propagation of both the flood and suspended sediment peak. Under the condition of a reservoir flood limit level of 145 m, the flood types for which Q p propagates are as follows from slowest to fastest: Type 1 > Type 5 > Type 2 > Type 3 > Type 6 > Type 4. Moreover, the flood types for which SSCp propagates are as follows from slowest to fastest: Type 4 > Type 2 > Type 3 > Type 6 > Type 1 > Type 5. (3) The initial SSCp asynchronous relationship is the most dominant factor influencing the lagged transport of suspended sediment in the reservoir area, accounting for 32.80 % to 89.40 % of the variation in suspended sediment transport lag, followed by the impacts of differences in flood types (8.2 % to 52.20 %) and reservoir scheduling (2.4 % to 15 %). (4) Human activities, particularly the damming of the cascade reservoirs in the upper reaches of the TGR area and the adjustment of the water level from 145 to 155 m in the TGR due to reservoir scheduling protocols, lead to an increase in the lagged transport time of suspended sediment peak. These findings reveal the asynchronous and differential propagation characteristics and motion mechanisms of flood and suspended sediment peaks in the reservoir area, thereby providing a solid theoretical basis for refining sediment peak discharge scheduling in the TGR.