Modelling tools - MIKE11R. 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)
Fluid dynamics-exercise-modelling packages@IHE/2002
The modelling process
Understand the problem
reason to model a system ( e.g. what if a dam is built?)
collect and analyse data
Assumptions about the system
Computer simulation
computer program
improve the model
water surface profiles (HEC2)
habitat modelling (PHABSIM)
1D models (MIKE11, SOBEK)
finite differences
finite elements
boundary elements
Fluid dynamics-exercise-modelling packages@IHE/2002
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
I. Popescu
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
continuity equation (mass conservation)
Assumptions
bottom slope is small
flow everywhere is paralel to the bottom ( i.e. wave lengths are large compared with water depths)
I. Popescu
water level h
All other variables are function of the independent or dependent variables
I. Popescu
Depending on how many terms are used in momentum equations
full Saint Venant equations (dynamic wave)
explicit methods
implicit methods
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.
I. Popescu
I. Popescu
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
internal “boundary conditions” - hydraulic structures ( here Saint Venant equation are not applicable)
Initial condition
a discharge hydrograph of a specific event
Typical downstream boundary conditions
time series of water level ( tidal cycle)
a reliable rating curve ( only to be used with downstream boundaries)
I. Popescu
Required at representative locations throughout the branches of the river
Must accurately represent the flow changes, bed slope, shape, flow resistance characteristics
I. Popescu
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
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
I. Popescu
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 !!