Geomechanical Hazards related to River Hydraulics and Remedial Measures: Selected Case Studies in India

River science and engineering has been one of the important study areas for geologists, hydrologists and engineers. The open channel flow and associated hydraulics often initiate several geomechanical hazards including silting and scouring, meandering and migration, floods, etc. Such hazards may lead to disastrous consequences if adequate remedial measures are not undertaken by proper river training works. This paper presents selected case studies in northern and north-eastern parts of India where such hazards occurred due to migration of river channel and flooding of adjacent lands. The two study areas have been the Kosi and the Brahmaputra river basins. In the former study area, hazards took place due to eastward migration, whereas in the latter case, significant damages occurred due to scouring and erosion. The descriptions of the hazards occurred, and the mitigation techniques adopted have been briefly summarized in this paper. A critical analysis with prediction techniques for flood occurrence probability and erosion potential has been conducted as well. The relevant conclusions are drawn therefrom.

Modelling Weirs in Two-Dimensional Shallow Water Models

2D models based on the shallow water equations are widely used in river hydraulics. However, these models can present deficiencies in those cases in which their intrinsic hypotheses are not fulfilled. One of these cases is in the presence of weirs. In this work we present an experimental dataset including 194 experiments in nine different weirs. The experimental data are compared to the numerical results obtained with a 2D shallow water model in order to quantify the discrepancies that exist due to the non-fulfillment of the hydrostatic pressure hypotheses. The experimental dataset presented can be used for the validation of other modelling approaches.

Turbulence and Flow–Sediment Interactions in Open-Channel Flows

The main focus of this Special Issue of Water is the state-of-the-art and recent research on turbulence and flow–sediment interactions in open-channel flows. Our knowledge of river hydraulics is becoming deeper and deeper, thanks to both laboratory/field experiments related to the characteristics of turbulence and their link to the erosion, transport, deposition, and local scouring phenomena. Collaboration among engineers, physicists, and other experts is increasing and furnishing new inter/multidisciplinary perspectives to the research in river hydraulics and fluid mechanics. At the same time, the development of both sophisticated laboratory instrumentation and computing skills is giving rise to excellent experimental–numerical comparative studies. Thus, this Special Issue, with ten papers by researchers from many institutions around the world, aims at offering a modern panoramic view on all the above aspects to the vast audience of river researchers.

Consumption of Solid Runoff during Erosion of Bottom Slope of Soil Dam

Erosion is divided into two stages in accordance with the accepted design scheme for erosion of a soil dam during overflow. The paper deals with the first stage, when the downstream thrust prism is washed out. The key factor in calculating erosion deformations is the choice of the solid flow rate formula. Studies show that the mechanism of formation and transportation of solid runoff during erosion of dam models from sandy oils is very similar to that previously described by many authors for the condition of river channel erosion. The peculiarity of the process is that the erosion occurs at high speeds. Therefore, solid runoff almost immediately goes into a suspended state. To select the required formula, experiments have been carried out on models of dams made of sandy soils having various granulometric composition. It has been established that at high velocities under the considered conditions, the value of the solid waste flow rate depends solely on hydraulic characteristics of the flow. The influence of physical and mechanical properties of the eroded soil on the value of the flow rate of solid runoff is insignificant, and they may not be taken into account. Calculations have been carried out using formulas known from river hydraulics, which show that none of them gives sufficient convergence with experimental data. Based on the analysis of a large number of experimental data, a formula for the discharge of solid runoff for erosion conditions of dam models during overflow has been obtained in the paper. This has taken into account the fact that the dam erosion by the overflow has a high degree of stochasticity and is difficult to describe theoretically. This is especially evident in conditions of spatial erosion, when, simultaneously with the classical erosion of the bottom, the sides of the eroded hole periodically collapse, which is difficult to take into account in the calculations.

Generalised Serre–Green–Naghdi equations for open channel and for natural river hydraulics

In this paper, we present a new non-linear dispersive model for open channel and river flows. These equations are the second-order shallow water approximation of the section-averaged (three-dimensional) incompressible and irrotational Euler system. This new asymptotic model generalises the well-known one-dimensional Serre–Green–Naghdi (SGN) equations for rectangular section on uneven bottom to arbitrary channel/river section.

AUTOMATIC INCORPORATION OF CIRCULAR RIVERBANK FAILURES IN TWO-DIMENSIONAL FLOOD MODELING

Riverbanks undergo changes caused by river hydraulics and by the possible landslides that change the channel bank profiles. Those failures are an important form of alluvial channel adjustments but are usually difficult to include during morphodynamic modeling. This paper proposes a novel approach combining a 2D depth-averaged hydrodynamic, sediment transport and mobile-bed model, a limit equilibrium slope-stability model, and a bank failure sediment redistribution submodule, into a fully automatic and continuous dynamic simulation to predict morphological changes for a river reach undergoing exceptional flooding. All mesh nodes located within the mass wasting zone will be automatically updated, allowing a new bank face form. The failed materials is redistributed in the transect according to the geometry of the landslides observed at the study site. The Outaouais River at Notre-Dame-Du Nord, Quebec, is used to test the coupling procedure. Typical results showing the effectiveness of the developed framework are presented and discussed.

Numerical Modeling of the Hydro-Morphodynamics of a Distributary Channel of the Po River Delta (Italy) during the Spring 2009 Flood Event

One-dimensional (1D) numerical models generally provide reliable results when applied to simulate river hydraulics and morphodynamics upstream of the tidal influence, given the predominantly unidirectional flow conditions. Such models, however, can also be used to reproduce river hydraulics across the fluvial to marine transition zone when specific conditions occur, as during high discharge events, and the results obtained via these simple modeling tools can provide indicative trends that may guide more structured and detailed modeling of a particularly critical area. In this study, the application of a 1D model setup with hydrologic engineering centers river analysis system (HEC-RAS) for simulating the hydro-morphodynamic conditions of a distributary channel of the Po River Delta (Italy) during a flooding event that occurred in Spring 2009 is presented. The channel bathymetry and the grainsize composition was taken from field measurements, while the dimension of the plume offshore the delta was derived from a MODIS image acquired at the peak of the flood. The comparison between the numerical outcomes and the field evidence shows the reliability of the proposed 1D modeling approach in representing the delta dynamics at a large scale, as well as in showing locations where more spatially detailed studies are needed. The code was also able to adequately reproduce the channel hydro-morphodynamics and the sediment data as derived from a core sample taken a few km offshore during the flooding event of April–May 2009. Through a sensitivity analysis, it is also proven that the dimension of the river plume can influence the evolution of the prodelta, while having a rather negligible effect inland, because of the major stresses induced by the high river discharge during the flood event.

ON THE PROBLEM OF SEDIMENT TRANSPORT ASSESSMENT

The paper discusses the problem of high errors in the design ratios proposed for assessing sediment transport in natural watercourses. The question is why numerous empirical design ratios obtained and successfully used for some watercourses can give an error of 1000% when applied to other rivers. Calculation formulas worked out on hydraulic trays and channels are found to be inappropriate for specific rivers and natural channels. The problem is caused by the complexity of the natural watercourses geometry and the heterogeneity of the bottom sediment composition. Two approaches that are currently used in assessing the transporting capacity of river flows are studied in the paper. They were laid down in river hydraulics as early as the second half of the 18th century in the works by A. Chesi and P. Dubois. The conditions and possibilities of using both methods are considered and theoretically proven in the paper. The approach developed by P. Dubois is distinguished by a detailed study of the influence of individual factors based on numerous experimental models, the construction of fairly rigorous physical models on this basis, the complication of design ratios, the inclusion of new additional parameters. A. Chesi offered the construction of a maximally simplified initial physical model, with the calibration parameters established for specific conditions on the basis of field observations or qualitative estimates, and therefore being less accurate but more stable. It is shown that despite the scientific attractiveness of the approach to constructing and using more complex calculation models containing new additional parameters, the inclusion of additional parameters entails the inclusion of additional errors associated with the estimation of these parameters. The effectiveness of both approaches is proved; the application of one or the other approach is determined by the conditions and nature of the tasks to be solved, as well as the volume and accuracy of the initial data.