1. Modelling tools - MIKE11 Part1-Introduction

2. ‘A model is a caricature of reality.´ R. May
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

3. Understand the problem
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

4. 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

5. 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

6. Mike 11 includes the following modules
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

7. Open channel flow- Saint Venant equations (1D)
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)

8. Hydraulic variables

9. Momentum conservation
Equations
Mass conservation
Momentum conservation

10. Independent variables
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

11. Depending on how many terms are used in momentum equations
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
Full Saint Venant equations are used when there is a rapid change in the water depth over time, and water discharge is significantly higher than the available calibration data
When differences in space are to be computed, the question is if the values in time step j or time step j+1 should be used. If the time step j is used an explicit solution is given. If the values at time j+1 are used, an implicit solution is given. An implicit solution is more stable than an explicit one and longer time step can be used. An explicit solution is simpler to program.

12. Neglect first two terms
Flow description
Neglect first two terms
Diffusive wave ( backwater analysis)
Neglect three terms
Kinematic wave (relatively steep rivers without backwater effects)

13. 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 h1 h3 h5 h7 2 4 6 Q
Center point

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

15. 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)

16. Discretization - branches

17. Discretization - branches

18. 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

19. 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

20. 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

21. User-defined culverts Q-h calculated culverts Dam break structure
Structures
Broadcrested weirs
Special weirs
User-defined culverts
Q-h calculated culverts
Dam break structure

22. Mike 11 main menu

23. View results - Mike view Print results - Mike print
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

24. 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 !!