A probabilistic approach to an upper-body region simulation in a bedload-dominated delta: implications for neck area morphology and plant colonization

The uncertain effects of water–sediment input on the delta morphology should be carefully considered given the current high occurrence frequency of extreme climate events. To determine the formation and evolution of the upper-body region in a bedload-dominated shallow-water delta, jet theory was fundamentally utilized and resolved into 2-D plane flow fields based on the similarity solution method, and a newly parameterized bedload transport rate equation, including both the partial bedload within relatively small suspended loads and the flow regime effects, was combined with grain-size transformation between transporting and bed material to establish a bed deformation equation in this study. In a lab-scale case, the coupled deterministic model of the upper-sectional delta topography was extended based on two independent stochastic events, considering the impacts of the transverse position deviation from the longitudinal centerline in the delta and the deviation of the (least-square optimal) flow regime influencing parameter from its classic default value during diffusion and the drift term of the established stochastic differential equations. The stochastic solutions were highly consistent with limited measurements (though without reasonable parameter estimation), especially the median solution in the two-sided parts of sections and the 75th percentile solution in the central parts of the sections. Furthermore, the prediction calculations corresponding to different roughness conditions resulting from a possible improvement in the ecological environment revealed that the neck area of a mature upper delta contains both the riskiest and safest parts when faced with a storm, and that plant colonization may generate a local positive effect along with a greater accumulation of real data and a more certain determination of flow-sediment interactions in the delta, pointing a way to disaster risk reduction such as the 2021 heavy rainfall disaster in Henan Province (China) to some extent.