Stochastic Nature of Overbank Flow Turbulence in Straight Compound Channels with Vegetated Floodplains

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Applications of Coupled Explicit–Implicit Solution of SWEs for Unsteady Flow in Yangtze River

In engineering practice, the unsteady flows generated from the operation of hydropower station in the upstream region could significantly change the navigation system of waterways located in the middle-lower reaches of the river. In order to study the complex propagation, convergence and superposition characteristics of unsteady flows in a long channel with flow confluence, a numerical model based on the coupling of implicit and explicit solution algorithms of SWEs has been applied to two large rivers in the reach of Yangtze River, China, which covers the distance from Yibin to Chongqing located upstream side of the Three Gorges Dam. The accuracy of numerical model has been validated by both the steady and unsteady flows using the prototype hydrological data. It is found that the unsteady flows show much more complex water level and discharge behaviors than the steady ones. The studied unsteady flows arising from the water regulation of two upstream hydropower stations could influence the region as far as Zhutuo hydrologic station, which is close to the city of Chongqing. Meanwhile, the computed stage–discharge rating curves at all observation stations demonstrate multi-value loop patterns because of the presence of additional water surface gradient. The present numerical model proves to be robust for simulating complex flows in very long engineering rivers up to 400 km

Abrupt drainage basin reorganization following a Pleistocene river capture

River capture is a dramatic natural process of internal competition through which mountainous landscapes evolve and respond to perturbations in tectonics and climate. River capture may occur when one river network grows at the expense of another, resulting in a victor that steals the neighboring headwaters. While river capture occurs regularly in numerical models, field observations are rare. Here we document a late Pleistocene river capture in the Yimeng Mountains, China that abruptly shifted 25 km^2 of drainage area from one catchment to another. River terraces and imbricated cobbles indicate that the main channel incised 27 m into granitic bedrock within 80 kyr, following the capture event, and upstream propagating knickpoints and waterfalls reversed the flow direction of a major river. Topographic analysis shows that the capture shifted the river basins far from topographic equilibrium, and active divide migration is propagating the effects of the capture throughout the landscape

Applications of Shallow Water SPH Model in Mountainous Rivers

In this paper, the Shallow Water Equations (SWEs) are solved by the Smoothed Particle Hydrodynamics (SPH) approach. The proposed SWE-SPH model employs a novel prediction/correction two-step solution algorithm to satisfy the equation of continuity. The concept of buffer layer is used to generate the fluid particles at the inflow boundary. The model is first applied to several benchmark water flow applications involving relatively large bed slope that is typical of the mountainous regions. The numerical SWE-SPH computations realistically disclosed the fundamental flow patterns. Coupled with a sediment morph-dynamic model, the SWE-SPH is then further applied to the movement of sediment bed load in an L-shape channel and a river confluence, which demonstrated its robust capacity to simulate the natural rivers

Nappe Flow Surges down a Rough-Stepped Sloping Channel

Nappe flow applies to small discharges, and investigation of nappe flow surges helps to understand the mechanism of water flow in stepped open channels. The objective of this paper is to investigate the nappe flow surge propagation down stepped open channels. First, an analytical solution of dam-break shock waves or flood waves is derived. Then, the theoretical solution is used to calculate the wavefront and celerity and is verified using experiments on surging waters down a stepped slope in a 20-m-long and 0.5-m-wide open channel under three conditions: one for smooth bed surface and the others for rough bed surface glued with uniform sediment (of diameter ds = 1.715 mm and ds = 3.5 mm). For calculating the wavefront propagation in a stepped open channel, a formula for resistance, composed of grain resistance and step geometry resistance, is developed. Unlike skimming flow, the roughness of step surface in the nappe flow regime influences the friction factor fs. Results show that the friction factor increases with the relative roughness height (ds/d0). The theoretical solution for the wavefront location and celerity is then revised by combining the friction formula, and the revised solution is tested for a wide range of experimental data. Test results show that the revised solution fits the measured data quite well. Comparing with the solution previously derived, the revised solution derived in this study more accurately predicts the wavefront location and celerity for the nappe flow condition

Abrupt drainage basin reorganization following a Pleistocene river capture

River capture is a dramatic natural process of internal competition through which mountainous landscapes evolve and respond to perturbations in tectonics and climate. River capture may occur when one river network grows at the expense of another, resulting in a victor that steals the neighboring headwaters. While river capture occurs regularly in numerical models, field observations are rare. Here we document a late Pleistocene river capture in the Yimeng Mountains, China that abruptly shifted 25 km^2 of drainage area from one catchment to another. River terraces and imbricated cobbles indicate that the main channel incised 27 m into granitic bedrock within 80 kyr, following the capture event, and upstream propagating knickpoints and waterfalls reversed the flow direction of a major river. Topographic analysis shows that the capture shifted the river basins far from topographic equilibrium, and active divide migration is propagating the effects of the capture throughout the landscape

SPH study of the evolution of water–water interfaces in dam break flows

The mixing process of upstream and downstream waters in the dam break flow could generate significant ecological impact on the downstream reaches and influence the environmental damages caused by the dam break flood. This is not easily investigated with the analytical and numerical models based on the grid method due to the large deformation of free surface and the water-water interface. In this paper, a weakly compressible Smoothed Particle Hydrodynamics (WCSPH) solver is used to study the advection and mixing process of the water bodies in two-dimensional dam-break flows over a wet bed. The numerical results of the mixing dynamics immediately after the release of the dam water are found to agree satisfactorily with the published experimental and numerical results. Then further investigations are carried out to study the interface development at the later stage of dambreak flows in a long channel. The analyses concentrate on the evolution of the interface at different ratios between the upstream and downstream water depths. The potential capabilities of the mesh-free SPH modelling approach for predicting the detailed development of the water-water interfaces are fully demonstrated.The first author acknowledges the Jafar Studentship during her PhD study at the University of Cambridge. The other authors acknowledge the support of the Major State Basic Research Development Program (973) of China (No. 2013CB036402), Open Fund of the State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University (SKHL1404; SKHL1409), Start-up Grant for the Young Teachers of Sichuan University (2014SCU11056) and National Science and Technology Support Plan (2012BAB0513B0).This is the accepted manuscript. The final version is available at http://link.springer.com/article/10.1007%2Fs11069-015-1726-6