Modelling tools - MIKE11 Part1 - Introduction

IHE 2002-2003

Modelling tools - MIKE11Part1-Introduction

I. Popescu 2 Fluid dynamics-exercise-modelling packages@IHE/2002

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)


Hydraulic variables


Equations

• Mass conservation

qt

A

x

Q

• Momentum conservation

0

2

2

ARC

QgQ

x

hgA

x

AQ

t

Q


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


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 i Cross section i+1 Cross section i-1

Space

Time

Reach


Flow description

– Neglect first two terms

0

2

2

ARC

QgQ

x

hgA

x

AQ

t

Q

• Diffusive wave ( backwater analysis)

– Neglect three terms

0

2

2

ARC

QgQ

x

hgA

x

AQ

t

Q

• Kinematic wave (relatively steep rivers without backwater effects)


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

i i+1 i-1

Space

h1h3

h5

h7

2

4

6

Q

Q

Q

Center point


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 - branches


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

h

R


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

x

ghvtCr

2 to1x

tv


Structures

• Broadcrested weirs• Special weirs• User-defined culverts• Q-h calculated culverts• Dam break structure


Mike 11 main menu


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 !!

Modelling tools - MIKE11 Part1 - Introduction

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