MIKE+ | Examining simulation summaries

Description
This is a guide on how to examine simulation summaries from MIKE+ river, collection system, and 2D overland models.  

Simulation summaries are quick sources of information for initial assessment of the soundness of model setups and their results.

By displaying information on model boundary inputs, system outflows, and water balances simulation summaries help identify and correct potential setup errors. Early inspection of simulation summaries may prevent time-consuming examination of model results, as major setup errors may become evident.

Below are examples of simulation summaries with tips on how to decipher them during the model checking process.

#01 - Rainfall Runoff (RR) Simulation
Catchment model simulation summaries (xx_Summary.html) are automatically generated in project results folders. The file name is prefixed by the simulation ID.

Various sections are presented in the summary file. Check the ‘Rainfall Runoff Summary’ section for details on the inputs and outputs for the catchment computation (Fig. 1). Examine the Continuity Balance table:

• Net rainfall: Confirm rainfall inflow volumes correspond to expected values based on applied boundary conditions for the simulation. Net Rainfall in the summary file is the applied rainfall depth multiplied by the total (user-defined or geometric) catchment area.
• Runoff discharge: Estimate expected runoff volume based on the total area contributing to runoff and the applied rainfall depth. Note the runoff amount and compare it to subsequent network simulation inflows.

Note: ‘Catchment Discharge’ is only relevant if catchment discharge boundaries are active for the simulation. These pertain to catchment-based flows, for example, wastewater inflows based on person equivalents (PEs) in a catchment, discharge based on catchment area, etc.

Fig. 1 - Rainfall Runoff Summary section

#02 - Network (HD) Simulation
Specify the generation of network simulation summaries (xx_Summary.html) via the Simulation Setup editor (Fig. 2). A properly filled HD Summary Continuity Balance table is obtained only if a Summary file is requested for a simulation. For non-integrated model simulations, the file is generated in the project results folder. The file name is prefixed by the simulation ID.

 
Fig. 2 – Request for a Network Simulation Summary on the Simulation Setup editor

The HD Summary and Rainfall Runoff Summary sections are presented in the same *.html file when running simultaneous Catchment and Network simulations (Fig. 3).

Fig. 3 - HD Summary and Rainfall Runoff Summary continuity balance tables from simultaneous catchment and network simulations

Some notable HD summary items include

• Internal nodes inflow: Total inflows into network nodes, such as runoff entering collection system nodes to which catchments are connected.
• Reaches inflow: Total inflows into network reaches or links, such as catchment runoff inflows into connected river reaches or pipes.
• Water generated in empty parts of the system: Total amount of numerical water added to the system to maintain minimum water depths at grid points needed to perform calculations. Minimise numerical water through proper initialisation of the system (i.e., hot starting), or lowering ‘Minimum water depth’ thresholds via the MIKE 1D Engine Configuration dialog.

Check that inflows into the network correspond to amounts defined as boundary inputs. For example, in the summary shown in Fig. 3, catchment and network flow computations were run simultaneously for a system comprised of a river and stormwater collection system network. Rainfall was applied to all catchments, which were connected to either collection system manholes or river reaches.

Therefore, in this case, one should expect that computed runoff volumes (Rainfall Runoff Summary ‘Runoff discharge’ item) correspond to total network inflows (i.e., HD Summary ‘Internal nodes’ and ‘Reaches’ inflows). Otherwise, the setup may need to be checked for sources of unanticipated inflows or volume loss.

#03 - 2D Overland Simulation
2D model simulation summaries (xx_m21fmVolumeBalance.html) are generated in the project directory. The file name is prefixed by the project setup ID.

Fig. 4 - Example 2D overland simulation volume balance summary table

The 2D model simulation summary presents volume balance items such as:

• Initial volume in model area: Results from initial conditions defined in the 2D domain may be used to verify if the 2D model had been correctly initialised for the simulation. 
• Final volume in model area: Total water volume in the 2D model domain at the end of the simulation period. Comprised of water in areas where depths are greater than the dry depth (Final volume in wet area), and water in areas where depths are less than or equal to the dry      depth (Final volume in dry area).
• Total volume from source: Net inflows into the 2D model obtained from subtraction of outflows (e.g., sinks) from inflows (e.g., sources). Flow exchanges with a coupled network model are also logged under this item for integrated flood models (see succeeding section).
• Total volume from precipitation/evaporation: Total losses from evaporation and infiltration subtracted from precipitation input defined in the 2D model.
• Total volume from boundaries: Net inflows into the 2D system resulting from defined open 2D boundary conditions (e.g., inflow, water level, etc.).

Examining the above items in the summary file provides a quick way of catching potential 2D model setup mistakes such as when water volume contributions are unexpectedly reported for an item meant to be disabled.

#04 - Integrated Flood Model Simulation
Simulation summaries from integrated flood models comprised of coupled 1D network and 2D overland systems are presented in separate files for each component. The main summary files are:

• Summary.html: Summary file associated with the 1D network and catchment models. Presents the HD Summary section and Rainfall Runoff section (when relevant or active). Located in the project directory (not the results folder). File name prefixed by the simulation ID. Ensure         generation of correct network simulation summaries by defining summary file generation in the Simulation Setup editor (also see Fig. 2).
• m21fmVolumeBalance.html: Associated with the 2D overland model component of the coupled model. Located in the project directory (not the results folder). File name prefixed by the project setup ID.

Examine both files when checking the overall water balance for the integrated flood model.

m21fmVolumeBalance.html
The 2D model summary presents a similar table as described previously for (uncoupled) 2D models. But for integrated models, the ‘Total volume from source’ section is expanded to include items tracking flow exchange volumes with the coupled network model.

Flow exchanges with the network model, which could comprise a river model, collection system model, or both, are labelled prefixed with ‘MIKE 11’ or ‘MIKE URBAN’. These labels simply denote that they are from a river network or a collection system network model, respectively. They do not necessarily denote the software with which the network models are built, as the labels may misleadingly indicate. Also, note the items listed in the 2D model summary are presented from the 2D model point of view. i.e., ‘MIKE 11 outflow target’ pertains to expected flows from the 2D model to the river model via coupling points.

Under the 2D model summary for an integrated model, note the following items as they can indicate potential water balance errors in the overall flood model system:

• River and/or collection system inflow/outflow targets: These are the ‘target’ inflow or outflow exchange volumes as estimated from head level differences detected in calculation time steps.
• River and/or collection system inflow/outflow corrections (see outlined items in Fig. 5): These are volume corrections to the estimated ‘targets’ based on actual head level difference detected during a coupling time step. Note that actual 2D computation time steps may        differ from the coupling and 1D model time steps. Flow exchange estimates may differ from available volumes by the time flow exchanges occur, hence the corrections.

The volume error occurs as the coupled 1D network model assumes exchanges with the 2D model based on ‘target’ volume values ignorant of the ‘corrections’.


Fig. 5 – Example 2D model simulation summary from a coupled flood model

Summary.html
The 1D network model summary presents a similar table as described previously for (uncoupled) network models. However, for integrated models, the ‘Total inflow’ and ‘Total outflow’ sections are expanded with items tracking flow exchanges with the coupled 2D model. The item ‘Total inflow from surface coupling’ indicates flow exchange volumes going from the 2D to the network model, while ‘Total outflow from surface coupling’ indicates volumes from the network model spilling onto the 2D system.

Fig. 6 - Example 1D network model simulation summary from a coupled flood model

Reconciling items between summaries
Simulation summaries from integrated flood models will comprise of both 1D network and 2D model summaries. Examine both files by reconciling the respective items from both as initial sanity checking for the integrated model.

• Inputs to the overall system comprise of ‘Total volume from precipitation/evaporation’ (2D), Total volume from boundaries’ (2D), ‘Water level boundaries inflow’ (1D), ‘Internal nodes inflow’ (1D), and ‘Reaches inflow’ (1D).
• Compare runoff discharge to the sum of ‘Internal nodes inflow’ (1D), ‘Reaches inflow’ (1D), and ‘Runoff diverted to surface’ (1D).
• Check that ‘MIKE URBAN/MIKE 11 outflow targets’ correspond to ‘Total Inflow from Surface Coupling’, and that ‘MIKE URBAN/MIKE 11 inflow targets’ match ‘Total Outflow from Surface Coupling’ and ‘Runoff diverted to surface’ items (1D). ‘Runoff diverted to surface’ directly flow onto the coupled 2D model.
• Water balance errors comprise of ‘Water generated in empty parts of the system’ (1D), ‘MIKE 11 outflow correction’ (2D), ‘MIKE URBAN outflow correction’ (2D), and any Continuity Error items reported at the end of the volume balance tables for both the network and 2D models.

Conclusion
Take advantage of readily available information from simulation summary files as they provide quick indications of:

• Whether model inputs have been correctly defined.
• If input between systems occur as expected, such as runoff input to networks, or flow exchanges between network and overland systems.
• Whether adjustments to computation parameters or the model setup are needed to address detected unacceptable volume balance errors.

Lastly, use summary files to perform overall model validity checks before attempting detailed scrutiny of model results.

FURTHER INFORMATION & USEFUL LINKS

Manuals and User Guides
MIKE+ Documentation Index
MIKE+. River Network Modelling User Guide
MIKE 21 Flow Model FM Hydrodynamic Module User Guide

Training options
MIKE+ – Getting started with river modelling
MIKE+ 2D Overland Flow – Getting started with urban flood modelling
MIKE+ CS - Getting started with urban drainage modelling