Bed evolution numerical model for rapidly varying flow in natural streams

A numerical model suitable for the reproduction of bed evolution in natural alluvial rivers and in channels of complex geometry is presented. It is based on a conservative formulation of one-dimensional shallow water equations, which includes an original treatment of the momentum equation source term. The proposed model has already been cross referenced with several test cases and experimental data found in the literature for fixed beds. In this study, however, the focus is on mobile bottom beds. The selected test cases are representative of some of the characteristic bed configurations that often occur in natural streams, and, therefore, are suitable for verifying the versatility of the model and its potential usefulness in the treatment of alluvial rivers. The MacCormack explicit finite difference scheme has been adopted for the numerical implementation. The liquid and solid phases are then solved by means of a semi-coupled procedure. A variety of bed evolution mechanisms and different water regime conditions are investigated in order to verify the model response in the cases of erosion, deposition and bed morphology evolution. The results show a good correspondence with the experimental data. Furthermore, the proposed model shows a notable numerical stability even when applied to test cases that are particularly difficult under the numerical computational profile, where the relative model of the standard formulation of the conservative balance equations shows evident numerical instability

ONE-DIMENSIONAL HYDRODYNAMIC MODELS IN RIVER RESOURCE MANAGEMENT AND POLICY DESIGN

This thesis concludes a three-year doctoral program in hydraulic engineering. The research work focused on the development, testing and application of one-dimensional numerical models to river resources management and policy design issues, pointing out the key role numerical modeling can play to support river managers in the decision process where large-scale, eco-morphological river changes are supposed to occur. These latter may result in overwhelming environmental and socio-economic costs for the population and communities living along rivers, especially in developing countries: river morphological evolution cannot be neglected anymore when planning river resources use in large watersheds, and the need for tools capable of informing river managers is nowadays manifest. One-dimensional models, even though introducing simplifications in the description of hydrodynamics and sediment movement processes, do have some advantages. Provided modeling assumptions are correct, they are able to provide indicators that - despite not capturing all details - allow analyzing trends of morphological processes at reach-scale or basin scale. Moreover, they are time-saving and open the way to long-term analyses and implementation in optimization procedures. Models based on the shallow water equations for the liquid phase, together with the Exner equation for river bed evolution, have been used: two different numerical schemes for solving the governing equations have been implemented trying to improve model robustness and versatility. Even the simplest numerical models, built on simple numerical schemes, require a long case-specific model building work. Understanding of the river morphology evolution drivers and basic physical processes, hydrological and topographical data collection and pre-processing, adaptation of the model to the specific case study and specific features (e.g. control rule for the Isola Serafini diversion barrage, time-varying bifurcation for the Red River), choice of suitable closure equations (e.g. sediment transport formula), are all essential steps that influence reliability of results and claim the same carefulness as the development of the numerical model itself. Details are given about the model building process for each case study. We analyzed three different case studies of river system management in which numerical model simulations give a relevant contribution to the decision-making process. - River Po (Italy), in its lower course, has undergone a severe bed lowering process during second half of the 20th century. The Isola Serafini hydropower plant, located on Po river, operating since 1962, is served by a 330 m wide barrage that still affects the hydrological regime and sediment supply downstream. Alternative operating rules for the barrage, over a time horizon of ten years, have been analyzed by the means of a one-dimensional hydro-morphological numerical model. A multi-objective optimization framework was implemented, to assess the effects of the operating rules on hydropower revenue and river bed incision. We adopted a surrogate modeling technique (Global Response Surfaces in the Learning and Planning procedure) to embed the hydro-morphological model in the optimization procedure in a multidisciplinary approach. The obtained results are encouraging and show that with a moderate loss in hydropower revenue, the decrease in river bed degradation can be remarkable. - The Red River (Song Hong) in northern VietNam has experienced severe river bed degradation along its lower course as well, but over a shorter period (last 15 years). The continued decrease of the water levels in the dry season aggravated water scarcity for agriculture. These outcomes can be attributed to strong instream sediment mining, major upstream impoundments, climatic and land use changes. The IMRR (Integrated and sustainable water Management of Red-Thai Binh Rivers System in changing climate) project, run by the Politecnico di Milano university with several Vietnamese agencies and authorities under the supervision of the Italian Ministry of Foreign Affairs, has faced several challenges inherent to water resources use in the Red River basin; the main river morphological evolution aspects of the last decades have been thoroughly analyzed and a finite-volume numerical model of the lower course of Red River has been set up and tested. The model has been used to evaluate the sensitivity of the river stretch to discharge modulation operated by upstream reservoirs, and to estimate instream sand mining rates to draw up some prediction scenarios. We showed that sediment mining rates, as expected, could be much larger than licensed amounts; this accelerates incision in the first reach and reverses the natural restoration trend in the following reaches, aggravating water level lowering. The flow control operated by reservoirs, conversely, appears to affect much less the morphological processes in the studied reach. - In January 2014, a serious flooding event (due to a levee breach) occurred in the Modena Province (Italy) along Secchia River. Concerns have increased about the role of vegetation in Secchia river channel during big floods. A detailed characterization of riparian vegetation features has then been produced by the Po River Interregional Agency (AIPo) to point out critical issues about current vegetation pattern along Secchia river banks and possibly plan a maintenance - remediation strategy. This characterization report constitutes a good basis for a hydrodynamic modelling work; a 1D finite-volume model, accounting for the influence of different vegetation patterns and densities on flow resistance has been applied to a 60 km long stretch of Secchia river. The agreement between simulation results and gauged data on a real flood event is encouraging and opens the way to the use of the model for evaluating the effectiveness of future vegetation management plans