Computational Control Parameters
Computational Control Parameters includes options for specifying parameter values for variables used to control the behaviour of the computational engine within MIKE HYDRO.
The River module parameters includes entries for refining the numerical solution scheme in model simulations as well as specific settings for numerical methods applied engine specific control parameters.
Grid spacing (or Maximum dx) is the maximum grid-spacing allowed between two adjacent h-points (water level calculation points) in a the numerical scheme of a River simulation.
Generally, the methodology for creating calculation points in the River model is as follows:
· At locations where Cross sections are present in the cross section file, h-points are always created.
· At locations where rivers are connected, a ‘connection node’ is created internally in the simulation engine and h-points will be automatically created at these locations as well (in case connections are defined where no Cross sections are defined)
· If the defined value of Maximum dx is smaller than the distance between cross sections, a number of additional h-points are automatically inserted at engine defined locations to ensure that the maximum distance between h-points is less than or equal to the defined Maximum dx. The additional h-points generated from the Maximum dx criteria in the model utilise linearly interpolated values for hydraulic parameters (Area, Radius, Width etc.) from the nearest physically defined up- and down-stream cross sections.
Maximum dx may therefore be used to increase the spatial resolution of the river model by forcing an additional number of calculation points in between physically defined cross sections. (The lower Maximum dx, the higher the resolution).
Maximum dx always utilise a global value, which per default is rather large (10 km). It is possible to define branch specific local variation of Maximum dx in the Local values table.
Global value
The Global value of Maximum dx will be applied throughout the entire river network except for branches defined with a local values.
Local values
It is possible to define a Maximum dx for each individual river branch in the model setup. To define a local value, click the Append button
above the Local values table, select a Branch from the drop-down menu and enter a value for Maximum dx. For areas that are not assigned a local value, the global value of Maximum dx will automatically be applied.
Wave approximation refers to the numerical solution and number of physical terms included in the Momentum equation applied in the Hydrodynamic simulation. For further details, please see the MIKE 1D reference manual.
Four different wave approximation (or flow description) types are available.
· Dynamic: The ‘Dynamic’ wave approximation should be used where the inertia of the water body over time and space is important. This is the case for all tidal flow situations and in river systems where the water surface slope, the bed slope and the bed resistance forces are small.
· Dynamic, high order friction: Similar to ‘Dynamic’ (see above), but in addition, the ‘Dynamic, high order friction’ wave approximation contains specific high order and upstream centred friction terms in the momentum equation. This modification typically allows simulations to be performed at longer time steps than the ‘Dynamic’ wave approximation.
· Diffusive: The diffusive wave approximation is a simplification of the ‘Dynamic’ wave approximation. It assumes that there are no inertial forces (i.e. the inertial terms are dropped from the momentum equation). It is suitable for backwater analysis, slow propagating flood waves and for cases where the bed resistance forces dominates. It is not suitable for tidal flows.
· Kinematic: The kinematic wave approximation assumes a balance between the friction and gravity forces on the flow. It is suitable for relatively steep rivers without backwater effects.
In general it is recommended to use ‘Dynamic’ or the ‘Dynamic, high order friction’ wave approximation. ‘Diffusive’ or ‘Kinematic’ wave approximation are simplifications of the ‘Dynamic’ equations and should only be used when it can be clearly shown that they are adequate. It is important that the assumptions described above are respected. In that case, selecting ‘Diffusive’ or ‘Kinematic’ wave approximation can improve the computational efficiency of the model.
The wave approximation can be selected globally for the system and/or locally for individual Branches.
The Computational parameters includes a range of variables targeting the numerical solution of the hydrodynamic simulation. All variables are defined with default values.
Shallow water equation parameters
· Time centering coefficient for gravity term (Delta):
A value of 0.5 will produce the most accurate calculations provided there are no numerical instabilities, but instabilities are less likely with higher Delta values. Large value of Delta (towards 1.0) has a dissipative effect which can significantly influence model dynamics. High values of Delta should therefore be avoided especially for applications with strong and systematic dynamics such as in e.g. a tidal range application.
· Velocity distribution coefficient (Alpha):
Used in the convective acceleration term of the momentum equation.
· Weighting factor, momentum equation (Theta):
Used in the quadratic part of the convective acceleration term of the momentum equation.
· Threshold water level slope for diffusive wave approx. (Eps):
If the water surface slope becomes greater than Eps, the computational scheme will become fully forwarded upstream. The parameter can be used to control the stability of the computation.
· Enhanced formulation of convective suppression (EnConvSup):
For situations with high Froude numbers combined with small grid spacing the enhanced formulation can be applied (see the MIKE 1D Reference Manual).
Two parameters must be specified:
- – Froude Max:
‘Froude Max’ is the parameter ‘a’ in the enhanced formulation of the suppression term applied to the convective acceleration term in the momentum equation.
- – Froude Exp:
‘Froude Exp' is the parameter `b' in the enhanced formulation.
· Alternative utilization of the enhanced convective suppression (AlConvSup):
If ‘Dynamic’ or ‘Dynamic, high order friction’ has been selected as wave approximation (see Wave Approximation), the ‘Alternative utilisation of the enhanced convective suppression’ may be used to increase the numerical stability of the algorithm.
Structure parameters
· Threshold water level difference below which flow is linearized (Delhs):
The Delhs value defines the threshold for water level difference across a weir for which the solution of flow across the structure changes between a normal structure equation and a linearised solution. If the water level difference is less than the threshold value, a linearised solution will be applied to obtain improved stability of the structure solution.
· Minimum head loss coefficient (ZetaMin):
The minimum head loss coefficient allowed in the computation of flow over structures.
· Max. number of iterations at structures (MaxIterStruc):
The maximum number of iterations permitted at each time step to obtain a solution at a structure.
· Use pre-processed h-Q-h files for bridges generated from previous simulations:
The generation of h-Q-h relations files for bridge-structures, during the initialisation of the simulation, may be time consuming. When this option is checked, the files generated during a previous simulation are re-used, when they exist, but are not updated. If they don't exist, they are generated again during the initialisation of the following simulation.
Miscellaneous
· Threshold depth for slot creation (Delh):
The Delh factor controls the dimensions of an artificial `slot', which is introduced to a cross section to prevent `drying out' of the section. The artificial slot is a small void introduced at the base of the section and allows a small volume of water to remain in the section preventing computational instabilities at low flows. The slot is inserted at height Delh above the river bottom and extends to a depth of 5*Delh below this level.
· Number of iterations at each time step (NoIter):
Each time step in a simulation includes as a minimum one sequence of solving the hydrodynamic equations. The Number of iterations at each time step variable is therefore used to define the number of additional iterations performed in each time step.
The default value is 1, which means that one additional iteration is made, which results in two calculations in each time step. One additional iteration generally provides the accurate and adequate results. Zero additional iterations is generally not recommended (less accurate) whereas the benefit of adding multiple additional iterations also generally is very small when it comes to enhanced accuracy of model results.
· Max. exceedence factor for depths above bank level (MaxBankDepth):
The maximum allowed exceedence factor for simulated water depths compared to the max depth in cross sections can be controlled by adjusting the variable. The factor is calculated as a multiplum of the max allowed depth in a cross section (calculated from bottom level to the highest of the left or right embankment levels).
