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## Modelling tools - MIKE11 Part1 - Introduction

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**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**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**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**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**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**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)**Equations**• Mass conservation • Momentum conservation**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**Cross section i-1**Cross section i Cross section i+1 Time Time step j+1 Reach Time step j Time step j-1 Space Flow description • Depending on how many terms are used in momentum equations • full Saint Venant equations (dynamic wave) • explicit methods • implicit methods**Flow description**• Neglect first two terms • Diffusive wave ( backwater analysis) • Neglect three terms • Kinematic wave (relatively steep rivers without backwater effects)**i-1**i i+1 Time Time step n+1 Q h7 6 Time step n+1/2 Q h5 Q 4 h3 h1 2 Time step n Center point Space 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**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**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)**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**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 R h**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**Structures**• Broadcrested weirs • Special weirs • User-defined culverts • Q-h calculated culverts • Dam break structure**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**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 !!