International Journal of New Technology and Research (IJNTR) ISSN:2454-4116, Volume-4, Issue-2, February 2018 Pages 34-41 34 www.ijntr.org Abstract—The current research work presents one-dimensional numerical simulation results of water surface elevation along a natural river, in order to investigate an accurate operation of three existing dams, with flat and arched gates, for flood protection. The study reports an extensive numerical simulation procedure of four different operations of the dam gates, including all gates opened, all gates closed, only flat gates opened and only arched gates opened, for flood risk estimation through the studied river reach. The one-dimensional hydrodynamic numerical simulation procedure of water surface profile variation, under steady flow conditions, through the natural river, was performed for available maximum inflow discharges of 5, 25 and 100 year flood period. All numerical results are graphically presented and comparisons between different gate operations provide an accurate investigation of a safe dam handling in order to prevent the surrounding river area against flood events. The applied hydrodynamic numerical model, “HEC-RAS” model, is a useful and accurate methodology for flood risk assessment in a natural river area, with complicate geometries and different hydraulic constructions. Index Terms—Hydrodynamic Numerical Simulation, Flood Risk Estimation, Gate Operation I. INTRODUCTION In river control engineering works an accurate and reliable quantitative estimation of water surface variation is of paramount importance for a safe river design and flood protection procedure. The foundation of hydraulic constructions along alluvial channels produce a non-uniform flow pattern and underestimation of flow depth may led to flood risk while overestimation provides unnecessary construction cost. Significant advances have been made in numerical simulation models, applied to free-surface flows in natural rivers, as the flow pattern of the aforementioned regions is highly complex. Several numerical simulation methodologies have been developed with the aim of simulation detailed information about the flood extent, flood water depth, how the flow affects several structures and other flood characteristics which are necessary in flood risk management. Free-surface variation and flood characteristics simulation with flood risk evaluation are often determined using several one-dimensional and two-dimensional hydrodynamic models as Miller and Chaudhry (1989), Bousmar, Scherer and Zech Evangelia D. Farsirotou, Department of Civil Engineering T.E., Technological Educational Institute of Thessaly, Larissa, Greece Athanasios F. Blantas, Department of Civil Engineering T.E., Technological Educational Institute of Thessaly, Larissa, Greece (1998), Horritt and Bates (2002), Farsirotou, Soulis and Dermissis (2002) and Bradford and Sanders (2002). The application of a one-dimensional (1D) or a two-dimensional (2D) hydraulic flood propagation model for flood hazard and risk assessment makes a focus on how well can predict the spatial-dynamic characteristics of floods and how the model results can be transformed into a flood risk assessment. Furthermore, Ahmad and Simonovic (1999) compared free surface variation results on a natural river area using 1D and 2D hydrodynamic modeling approaches. Capart, Eldho, Huang, Young and Zech (2003) developed a one-dimensional finite volume algorithm for the treatment of irregular bathymetry of open channel flow and performed a simulation of a severe flood through a complex river system. Comparison between 1D/1D and 1D/2D Coupled (Sewer/Surface) hydraulic models for urban flood simulation from Leandro, Chen, Djordjevij and Savij (2009) shows that flow over the terrain is better modeled by 2D models, whereas in confined channels 1D models provide a good approximation with less computational effort. Farsirotou, Klonidis and Soulis (2013) performed a three-dimensional numerical simulation of supercritical flow in an expansion channel and Farsirotou and Kotsopoulos (2015) focus on the effect of abrupt changes in river topography on free surface flow variation performing experimental and one-dimensional numerical simulation analysis and the results were satisfactorily compared. The main purpose of the current research work is to numerical simulate water surface variation along a natural river, with different hydraulic constructions, and to investigate an accurate and safe operation of three dams in the river reach in order to prevent flood risk in the studied area and control the potential risk of flood. Four different scenarios of dam gate openings are numerically simulated and comparisons of free surface flow elevation from each estimation are graphically presented. A. One-dimensional water surface numerical simulation The Hydrologic Engineering Center’s River Analysis System, “HEC-RAS” (2010) numerical model was used in order to perform one-dimensional water surface profile calculations for steady flow through a natural river, Enipeas river reach. Water surface profiles are computed from one cross section to the next by solving the energy equation to a body of water enclosed by two cross sections at locations 1 and 2, presented in Fig 1., as: e 3 1 1 1 1 3 2 2 2 2 h g 2 V a Z Y g 2 V a Z Y       (1) Hydrodynamic Numerical Simulation and Flood Risk Assessment in a Natural River Evangelia Farsirotou, Athanasios Blantas