Dechao Hu

Forward and inverse rehearsals, conducted in flood-defense applications of digital-twin watersheds, respectively require high-fidelity and rapid simulations of river corridor flood dynamics based on hydrodynamic model (HDM). In this study, two key techniques are proposed to improve these rehearsals. First, Technique-1 is proposed for 2D HDM to couple the terrain time series of newly-defined ribbon-like objects, to automatically regulate regional connectivity in flood simulation. It yields an enhanced 2D model capable of performing high-fidelity simulation of river corridor flood dynamics under complex spatiotemporal variations in regional connectivity, for the forward rehearsal. Second, Technique-2 is proposed to combine 1D HDM and fine 2D grid, involving a seed-spread method to interpolate cross-sectional hydrological data onto 2D grid, and a guided-search algorithm for creating newly-defined subdomain-based inundation units. It turns a 1D HDM into a composite model capable of synchronously simulating flood-routing and inundation distributions in a large river system and its adjacent land, for the inverse rehearsal. These new models are tested using a real levee-breach flood, the annual flood in a large river-lake system, as well as the Li River corridor (including urban area with complex building layouts). The enhanced model is further demonstrated by a comparison study using Mike21, while the composite model is shown to run in second-level real-time in the tests of watershed-scale simulations and be two orders of magnitude faster than a 2D HDM. The new models improve systematically the multi-level integrated simulations of river corridor flood dynamics for digital-twin watersheds, and will provide fundamental tools and new perspectives for agent-invoked automatic flood simulations in the AI era.

Weird horizontal shapes of branches, in large branching estuaries, often cause significant flood risks and environment-related problems. People usually resort to engineering methods to improve the horizontal shape of the weird branches and solve related issues. The responses of the riverbed evolution of a branching estuary to anthropogenic activity are complicated because of complex estuarine hydrodynamics and sediment transports, especially when the project locates specially (e.g., at estuary outlets). The North Branch of the Yangtze Estuary has a narrow upper reach which is almost orthogonal to the South Branch and has a trumpet-shaped lower reach with a wide outlet. The weird horizontal shape of the North Branch brings significant flood risks to cities along this branch, the shrinkage of its entrance, and other problems. In this study, a regulation of the North Branch, which is launched at Guyuan Sand (GYS) just outside the exit of the North Branch, is taken as an example. The GYS regulation aims to improve the weird horizontal shape of the North Branch by building new layouts of outlets, by which people decrease the flood risk of the surrounding cities. The GYS regulation is studied using a 2D numerical model. The riverbed evolution of the Yangtze Estuary in a typical hydrological year is simulated, while the water/sediment fluxes at cross-sections of branches in the estuary during a spring/neap tide are quantitatively calculated. It is found that the regulation changes the rotational flows near the shore, and further reshapes the estuarine circulations of mass inside the outlets, especially exchanges of water/sediment between different branches. The regulation directly changes the riverbed evolution at the outlet of the North Branch, and meanwhile has significant indirect influences on the riverbed evolution of the entrance of the North Branch. The varying riverbed evolution at the entrance of the North Branch and the varying water/sediment fluxes, under different designs of regulations, are related and analyzed. An essential improvement for the weird horizontal shape of the North Branch by an engineering method is shown to be possible, while the regulation mechanism of the engineering method and the response of estuarine riverbed evolution to the regulation are clarified. This study provides a new insight for improving estuarine branches with weird horizontal shapes, by reshaping the tidal processes and the accompanying sediment transports in a branching estuary.

Treatment of the free-surface pressure boundary (FSPB) remains a challenge for the three-dimensional (3D) nonhydrostatic hydrodynamic models using a vertical staggered-grid variable arrangement. Through the tests of the small-amplitude wave, it was found that the pressure of the top layer of the vertical grid is semihydrostatic for vertical staggered-grid nonhydrostatic models. Based on this finding, a modified ghost-cell FSPB treatment is proposed and a fully nonhydrostatic staggered-grid model developed. The new method is simple and easy to implement, and has the self-calibration property of its parameters. The model was further tested using a laboratory experiment of nonlinear waves propagating over a bar toward a beach. Sensitivity studies of model parameters and grid scales were performed. The vertical staggered-grid model using the new FSPB treatment is shown to require only coarse vertical grids to achieve accurate simulations of nonhydrostatic free-surface flows.

It is difficult for a one-dimensional river network hydrodynamic model to manage bifurcations. Traditional methods use simplified junction methods to avoid solving physical equations at bifurcations, which can cause physical distortions and errors. In this article, we propose an algorithm that allows a Eulerian–Lagrangian method (ELM) to track through bifurcations then solve advective terms, in combination with velocity–pressure couplings, to solve physical equations at bifurcations. The new method discards the simplifications and assumptions used by traditional models and is more complete in theory. We tested the new method with two ideal examples, and the results showed that the new method is time-step independent and grid independent. A simple bifurcation was used to compare this method with MIKE11.

A prediction–correction solver is presented here for rapid simulation of free-surface flows in dendritic and looped river networks. Rather than solving a large global algebraic system over the entire domain of river networks, the model solves subsystems for subdomains of branches in two steps: prediction and correction. With the help of the prediction–correction method (PCM), the partial linearization technique, the semi-implicit method, and the Eulerian–Lagrangian method (ELM), the model only needs to solve tridiagonal linear systems for branches and is free of any iteration. The new model was tested using a hypothetical looped river network with regular cross sections, the Three Gorges Reservoir (TGR) dendritic river network, and the Jing-south looped river system (with seasonally flooding branches). In the first test, a time-step sensitivity study was conducted and the model was revealed to produce accurate simulations at large time steps when the condition for application of the PCM to river networks was satisfied. In the TGR test, the PCM model provided almost the same histories of water levels and discharges as those simulated by the HEC-RAS model. In the Jing-south test, the mean absolute error in simulated water levels was 0.07–0.24 m, and the relative error in simulated cross-section water flux was 0.5–4.9% compared with field data; the conservation error was generally 2 × 10⁻⁴ to 3 × 10⁻⁴. The PCM model was revealed to be 2–4 times as fast as a reported model, which solves local nonlinear subsystems using two-layer iterations, and 1.2–1.4 times as fast as the HEC-RAS. Using a time step of 1200 s, it took the sequential code 26.8 and 23.1 s to complete a simulation of a one-year unsteady flow process, respectively, in the TGR river networks (with 588 cells) and the Jing-south river system (with 662 cells).

Because of special morphologies and complex runoff–tide interactions, the landward floodtide flows in Yangtze Estuary are observed to spill over from the North to the South Branches, carrying a lot of sediment. To quantitatively clarify the spillover problem, a two-dimensional numerical model using a high-resolution channel-refined unstructured grid is developed for the entire Yangtze Estuary from Datong to river mouths (620 km) and part of the East Sea. The developed model ensures a good description of the river-coast-ocean coupling, the irregular boundaries, and local river regimes in the Yangtze Estuary. In tests, the simulated histories of the tidal level, depth-averaged velocity, and sediment concentration agree well with field data. The spillover of sediment in the Yangtze Estuary is studied using the condition of a spring and a neap tide in dry seasons. For a representative cross-section in the upper reach of the North Branch (QLG), the difference of the cross-sectional sediment flux (CSSF) between floodtide and ebbtide durations is 43.85–11.26 × 10⁴ t/day, accounting for 37.5–34.9% of the landward floodtide CSSF. The mechanics of sediment spillover in Yangtze Estuary are clarified in terms of a successive process comprising the source, transport, and drainage of the spillover sediment.