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IHE Modelling tools - MIKE11 Part1 - Introduction

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I. Popescu 2 Fluid dynamics-exercise-modelling What is a model ? ‘A model is a caricature of reality.´ R. May A model is a simplification of reality that retains enough aspects of the original system to make it useful to the modeler Models may take many forms –phisical models ( hydrologic models of watersheds; scales models of ships) –conceptual (differential equations, optimization) –simulation models

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I. Popescu 3 Fluid dynamics-exercise-modelling The modelling process Understand the problem –reason to model a system ( e.g. what if a dam is built?) –collect and analyse data Choosing variables Set up mathematical model –describe situation –write mathematical explanation using variables Assumptions about the system Construction of the mathematical model Computer simulation –computer program –input data and runs –validation Simulation experiments –interpret the solution, test outcomes –improve the model

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I. Popescu 4 Fluid dynamics-exercise-modelling Classification of modelling packages According to what is computed –water surface profiles (HEC2) –flood waves (DAMBRK) –water quality in rivers (QUAL2E) –habitat modelling (PHABSIM) How many dimensions are used –1D models (MIKE11, SOBEK) –2D models –3D models (DELFT3D) Particulars of the numerical methods –finite differences –finite elements –boundary elements –etc

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I. Popescu 5 Fluid dynamics-exercise-modelling MIKE11 - General description Software package developed by Danish Hydraulic Institute (DHI) for simulation of flow, sediment transport and water quality in estuaries, river, irrigation system and similar water bodies User - friendly tool for design, management and operation of river basins and channel networks

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I. Popescu 6 Fluid dynamics-exercise-modelling Implementation Mike 11 includes the following modules –HD - hydrodynamic - simulation of unsteady flow in a network of open channels. Result is time series of discharges and water levels; –AD - advection dispersion –WQ - water quality

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I. Popescu 7 Fluid dynamics-exercise-modelling Theory Open channel flow- Saint Venant equations (1D) –continuity equation (mass conservation) –momentum equation (fluid momentum conservation) Assumptions –water is incompresible and homogeneous –bottom slope is small –flow everywhere is paralel to the bottom ( i.e. wave lengths are large compared with water depths)

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I. Popescu 8 Fluid dynamics-exercise-modelling Hydraulic variables

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I. Popescu 9 Fluid dynamics-exercise-modelling Equations Mass conservation Momentum conservation

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I. Popescu 10 Fluid dynamics-exercise-modelling Equations variables Independent variables space x time t Dependent variables discharge Q water level h All other variables are function of the independent or dependent variables

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I. Popescu 11 Fluid dynamics-exercise-modelling Flow description Depending on how many terms are used in momentum equations –full Saint Venant equations (dynamic wave) explicit methods implicit methods Time step j+1 Time step j Time step j-1 Cross section iCross section i+1Cross section i-1 Space Time Reach

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I. Popescu 12 Fluid dynamics-exercise-modelling Flow description –Neglect first two terms Diffusive wave ( backwater analysis) –Neglect three terms Kinematic wave (relatively steep rivers without backwater effects)

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I. Popescu 13 Fluid dynamics-exercise-modelling Solution scheme Equations are transformed to a set of implicit finite difference equations over a computational grid –alternating Q - and H points, where Q and H are computed at each time step –numerical scheme - 6 point Abbott-Ionescu scheme Time step n+1/2 Time step n Time Time step n+1 ii+1 i-1 Space h1h1 h3h3 h5h5 h7h Q Q Q Center point

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I. Popescu 14 Fluid dynamics-exercise-modelling Solution scheme Boundary conditions –external boundary conditions - upstream and downstream; –internal “boundary conditions” - hydraulic structures ( here Saint Venant equation are not applicable) Initial condition –time t=0

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I. Popescu 15 Fluid dynamics-exercise-modelling Choice of boundary conditions Typical upstream boundary conditions –constant discharge from a reservoir –a discharge hydrograph of a specific event Typical downstream boundary conditions –constant water level –time series of water level ( tidal cycle) – a reliable rating curve ( only to be used with downstream boundaries)

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I. Popescu 16 Fluid dynamics-exercise-modelling Discretization - branches

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I. Popescu 17 Fluid dynamics-exercise-modelling Discretization - branches

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I. Popescu 18 Fluid dynamics-exercise-modelling Discretization - cross sections Required at representative locations throughout the branches of the river Must accurately represent the flow changes, bed slope, shape, flow resistance characteristics

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I. Popescu 19 Fluid dynamics-exercise-modelling Discretization - cross section Friction formulas –Chezy –Manning For each section a curve is made with wetted area, conveyance factor, hydraulic radius as a function of water level h R

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I. Popescu 20 Fluid dynamics-exercise-modelling Avoiding Errors Hydraulic jump can not be modelled, but upstream and downstream condition can Stability conditions –topographic resolution must be sufficiently fine ( x) – time step should be fine enough to provide accurate representation of a wave if structure are used smaller time step is required use Courant condition to determine time step or velocity condition

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I. Popescu 21 Fluid dynamics-exercise-modelling Structures Broadcrested weirs Special weirs User-defined culverts Q-h calculated culverts Dam break structure

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I. Popescu 22 Fluid dynamics-exercise-modelling Mike 11 main menu

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I. Popescu 23 Fluid dynamics-exercise-modelling Used files For Simulation –network file *.mwk11 –cross section *.xns11 –boundary *.bnd11 –time series file *.dfs0 –hd parameters *.hd11 –simulation *.sim11 –result file *.res View results - Mike view Print results - Mike print Demos - Cali and Vida rivers

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I. Popescu 24 Fluid dynamics-exercise-modelling Assignement Solve task 1, 2 and 3 using MIKE11. Task 1 you have to built the simulation files and to run the simulation. For task 2 and 3 you must examine the prepared data files, perform a calculation and check the results. If results are not good, identify the cause and correct the situation. Exercises from task 2 and 3 should be submitted before week 49 or on Monday, December 3-rd Good luck !!

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