OpenFlows FLOOD General FAQ's

Product(s): OpenFlowsTM FLOODTM
Version(s): 10.XX.XX.XX
Area: Help and Documentation

This article provides answers to some of the most commonly asked queries on OpenFlows FLOOD. The content provided here covers some basic aspects of modeling in FLOOD, data import and conversion, product features and capabilities, output interpretation and hydrological simulation.

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Common FAQ's

Does OpenFlows FLOOD offer flexibility in data exchange with other products (viz. HEC-RAS, HEC-HMS, XPSWMM) and data sources (viz. CSVs, CAD and shapefiles)?

You can import data from several sources in its original format as DTM (XYZ ASCII, GeoTIFF raster, ESRI ASCII Grid raster, ESRI ADF binary raster, Bentley TIN, ESRI shapefile, NASA SRTM, etc), land use and soil texture (e.g. ESRI shapefiles, raster), breaklines and buildings (e.g. ESRI shapefiles), etc. but you cannot directly open HEC (E.g. HEC-RAS, HEC-HMS) or XPSWMM formats in OpenFlows FLOOD. If the original data used in those models is present, a new FLOOD model can be built using its customized tools for pre-processing and data input.

Can we use remote machines to run simulations through Bentley Cloud?

FLOOD simulations can be run on different machines from the one/s where the model was built. You can export and reimport on the other machine using FLOOD or use just the project folders and executables required. MOHID Land (FLOOD engine) and the SewerGEMS engine use OpenMP technology that can leverage the optimized usage of the CPU (and more cores in cloud computing machines); 

What is an accepted grid size/resolution and simulation time for a typical model?

Grid resolution, number of grid cells and simulation time depends greatly on the type of flood model you are building and running. The computational grid should cover the area of interest and its resolution should be sufficient so that the model is able to discretize flow in a accurate way. Because OpenFlows can cover all types of flooding (urban, riverine and coastal) and it has different numerical engines (1D open channel natural drainage network, 2D overland flow, 3D groundwater flow all of them dynamically coupled in runtime plus SewerGEMS, and including 2D/3D coastal hydrodynamics), FLOOD can cover computational grids from less than a meter resolution to km, covering areas of less than an ha to thousands of km2. Simulation time varies a lot, and because FLOOD engines have adaptive time step, the time it takes to compute a simulation using the same grid depends greatly on the initial and boundary conditions that were setup for the simulation (e.g. simulating in the same computational grid a scenario representing precipitation events for different return periods will take more time to compute for bigger return periods, as there will be more water flowing over the terrain, infiltrating, etc). 

What are the minimum system requirements in terms of memory, speed and resolution to run OpenFlows FLOOD?

Operating System  Windows Vista, 7, 8.1, 10 or later (64 bit)
Processor 1.8 GHz or faster processor
Memory 2 GB of RAM
Disk Space 900 MB
Display Resolution 1280x1024 or higher resolution
Display Color Depth 32 bits

What are the capabilities and limitations of using OpenFlows FLOOD?

OpenFlows FLOOD uses two different numerical engines to address flood modelling: MOHID Land and MOHID Water. Additionally to be able to simulate urban flooding, MOHID Land is coupled in runtime with OpenFlows SewerGEMS Explicit SWMM solver. MOHID Water is a three-dimensional numerical model to simulate hydrodynamics of surface water bodies (rivers, reservoirs, estuaries, coastal areas, oceans). Its hydrodynamic model solves the primitive continuity and momentum equations for the surface elevation and 3D velocity field for incompressible flows, in orthogonal horizontal coordinates and generic vertical coordinates, assuming hydrostatic equilibrium and Boussinesq approximations. MOHID Land is a physically-based, finite volume spatially distributed, continuous, variable time step integrated model to simulate water flow (and transport of water constituents) in inland waters: watershed surface runoff, rivers and groundwater. MOHID Land uses a physical approach to hydrological processes, is especially suited for detailed, complex short-term processes as fast floods, inundation progression, natural-artificial drainage interaction, as well as represent long-term processes in hydrology, soil properties transformation and vegetation dynamics. It comprises of three coupled modules: 1D river hydraulics St. Venant equations with semi-implicit time discretization for the friction term, 2D overland flow using 2D St. Venant equations with similar time integration techniques as the 1D engine and with direct spatial rainfall and infiltration (SCS Curve Number, Green-Ampt method, Richards equation).

To understand more about OpenFlows FLOOD see the below article;

Bentley OpenFlows FLOOD

Can OpenFlows FLOOD be used for Pollution Control Assessment?

FLOOD allows for simulating the transport and decay of pollution (e.g. eutrophication, coliform, BOD, oil spills, etc.) in receiving bodies viz. rivers, coastal waters, groundwater and can also be used to test mitigation strategies in catchments to achieve good water quality in the receiving bodies.

Can OpenFlows FLOOD be used for groundwater simulation with respect to infiltration?

FLOOD can simulate infiltration using Green-Ampt method, SCS Curve Number or the full 3D expansion of the Richards equation (head driven). It can also simulate evapotranspiration. OpenFlows FLOOD has a vegetation growth model for more complex, long term simulations and groundwater flow in saturated and unsaturated parts (Richards equation). The 3D soilwater/groundwater flow module handles water transport due to the balance between pressure (gravity and suction) and resistance trough the medium, infiltration, evapotranspiration and link with the river (groundwater flow). Soil fluxes are calculated by the Buckingham-Darcy equation (Jury et al,1991) and the connections with surface runoff and river are done using the same formulation where the surface water Head is the water level.

How is information exchange between 1D and 2D models handled in OpenFlows FLOOD?

FLOOD manages data input and model construction for the 2D model. SewerGEMS manages data input and model construction for the 1D model. Model engines from both can run at the same time in FLOOD and results from the two engines can be explored in FLOOD as maps and time series data. In case of any change in 1D model in SewerGEMS it can be reimported in FLOOD. It is also possible to make changes to the 1D model directly in FLOOD through the text editor option inside FLOOD. It is planned to improve this workflow for future versions.

Does OpenFlows FLOOD have a built in DTM library?

Flood can automatically and seamlessly import any DTM from the NASA DTM database (accuracy of 90 m which will be 30 m in next releases). It is also possible to import any DTM from the NOAA database ( For higher accuracy, you should import your own DTM, which can be in the following formats: XYZ ASCII, GeoTIFF raster, ESRI ASCII Grid raster, ESRI ADF binary raster, Bentley TIN, ESRI shapefile

Does OpenFlows FLOOD have a built-in land use type for the area selected similar to other solutions such as PC SWMM 2D or InfoWorks software?

This kind of information needs to be provided by the user in FLOOD in shapefiles, for instance; and can be easily obtained from different online data sources.

How are catchments generated in OpenFlows FLOOD?

Catchments are automatically assigned and defined (including natural drainage) based only on the DTM file (based on the ground elevations).

Can OpenFlows FLOOD simulate coastal events such as tsunamis?

For coastal events, FLOOD can take into consideration astronomical tides, which can be automatically generated internally based on the reconstitution of harmonic components (with amplitude and phase) defined externally, either by linking to a global tide model such as FES 2014; or customized by the end-user. In the new release for February 2020, global integration with FES2014 is seamless – which means that user can generate tidal currents globally with minimum effort.

For tsunami simulation, it is necessary to define the following data: epicenter coordinates, time/hour, fault depth, fault length and width, vertical terrain displacement, fault direction in degrees. Most of this information is usually public and can be extracted from the earthquake report. The method used to compute the tsunami is based on OKADA 1985.

What is the typical workflow for modeling urban flood simulation (UFS) in OpenFlows FLOOD?

The general workflow to be followed for UFS is;

1. Bring in the prepared Digital Terrain Model (DTM) in FLOOD

2. Import additional information / layers (use land type, pervious / impervious areas, roughness coefficient, etc.) in FLOOD

3. Create a computational grid where we define the study area and also the accuracy of the model, by defining the size of each square within the grid where calculations are performed

4. Create grid data files, where the grid is combined with the DTM and the additional information layers

5. Import sewer / storm network (direct import from OpenFlows SewerGEMS / CivilStorm)

6. Import / define inlets of the sewer / storm network

7. Insert rainfall information  

Can OpenFlows FLOOD be used for designing and analyzing structures such as dams?

You can use FLOOD to support in the identification of the design flow or upstream flooded areas, but for the structural design of a DAM you would require either PLAXIS to design and analyze embankment dams or STAAD.Pro for reinforced concrete dams.

What is the recommended grid size in OpenFlows FLOOD?

Computational grid size is always a compromise between the spatial resolution that we expect to fit the scale that we are studying; and the amount of time that we may have to wait for the model results.

If we are looking for spatially high resolution (e.g. to analyze flood risks in a critical infrastructure) such as 1 or 2 meters, we might have to limit the range of the model to an area that is not bigger than 1km2 (1000 x 1000 cells). More than 1 million grid cells might result in higher computational time. If we want to analyze bigger areas, we might either decrease spatial resolution, increase computational power or build additional model domains (which can also be downscaled ) for other areas. If we are analyzing results in a watershed scale, 1 meter discretization is clearly exaggerated, meaning that we might reduce significantly the spatial resolution, and therefore cover (model) a much bigger area.

Does OpenFlows FLOOD allow rain calculation on mesh? Does it allow the use of traditional hydrological models (SCS, Wallingford Method, etc.)?

SewerGEMS is a 1D model for the design and analysis of piped networks (storm and sewer). The storm water enters the network via rainfall-runoff methods applied at the catchment level. SewerGEMS can be used in conjunction with OpenFlows FLOOD to provide an integrated catchment approach, in which the rainfall landing on the 2D mesh can travel overland and enter the network at manholes. The 2D mesh receives the rainfall according to what is imposed: it can be space and time variant values. Those can derivate from timeseries defined by SewerGEMS, but they can also be imposed from complex meteorological models (e.g. WRF).

How often are program updates for OpenFlows FLOOD made available?

It is planned to provide product updates every six (6) months.

What kind of calculation methods do the 2D and 1D methods in OpenFlows FLOOD use?

1D – finite difference solution of the 1D St Venant equations

2D – finite volume solution of the 2D (depth averaged St Venant equations)

Can OpenFlows FLOOD integrate with MapInfo, ArcView and ArcMap? Does it allow to import / export from and to MapInfo, ArcView and ArcMap?

FLOOD would not require import / export from and to MapInfo, ArcView and ArcMap as it is in itself a GIS solution.

Does OpenFlows FLOOD allow import and export with AutoCAD?

In a 1D-2D implementation, SewerGEMS can import / export to DXF format. Then OpenFlows FLOOD can import the drainage network from SewerGEMS, but there is no direct import from AutoCAD in FLOOD.  

Does OpenFlows FLOOD allow to work with GRID models (LIDAR, etc.) and TIN models directly?

Currently FLOOD, by default works with gridded digital terrain models (DTM’s). We can easily create (grid) digital terrain models based in raster / GRID, vector (shapefiles) or XYZ (points), ASCII files that can result from NASA HGT files (from Shuttle Radar Topography Mission) or from an export of TIN files (the export process as to be made outside OpenFlows FLOOD).

Does it allow access to the results at any point in the simulation without having to determine those points before running the simulation?

FLOOD results can be provided in:

• Gridded numerical data files (HDF5 format). These files provide outputs in the whole model domain, and it’s possible to access results in any point, and in any output instant. It also enables the visualization in a map. • Predefined timeseries files. These files are ASCII time series for specific points, defined prior to the model running. 

Does OpenFlows FLOOD allow to share databases among different users, keeping control of versions?

Databases can be shared among different users, but version control is not possible. In addition, in OpenFlows FLOOD has “workspaces” to hold project information and files. It’s possible to import / export workspaces with all the model configurations, map layers, and results (optional).

Does OpenFlows FLOOD allow to do a custom validation of the data?

FLOOD does perform custom validation. The most intuitive way would be to generate one or multiple model timeseries in specific location(s) where measured data exists (e.g. for water column); then compare and analyze timeseries for instance, in a spreadsheet application (like MS Excel). Another possibility is to compare map visualization with satellite imagery. Usually this is mainly done by “eye-ball” method.

Can OpenFlows FLOOD provide 3D coastal flood risk analysis with fluvial and pluvial sources of flooding?

OpenFlows FLOOD includes the MOHID Water module for this. MOHID Water can be used for both 2D and 3D modelling. 3D modelling is more complex to implement (and more computational demanding) but can be justified in some very specific cases (with secondary circulation patterns). But usually, 2D hydrodynamic modelling is enough. MOHID Water can be used to explicitly simulate storm surge, combining effects from astronomic tide, wind, and pressure. Wave modelling (refraction or diffraction) is not explicitly included in OpenFlows FLOOD, but wave effects can be ingested into MOHID Water, so that sea water elevation can be computed not only based in tides, wind and pressure, but also based in wave effects (this effect is called wave setup). The same for atmospheric model.

Note: The MOHID engine can also simulate fetch effect due to wind (explanations later, below)

Does an imported object, like a shapefile for soil types for example, need to be added to the map for it to be included in a simulation?

No. OpenFlows FLOOD and the Urban Flood Simulator doesn’t use directly objects in map. In fact, shapefiles are not used at all (they always need to be exported to a MOHID format). Nevertheless, there is no relation between layers in map and model running & configuration.

Are wind properties used for 2D / 3D coastal flood modeling for oil spill or pollutant tracking models?

Wind properties are also used in coastal circulation modelling (hydrodynamics). As a matter of fact, wind is the important factor responsible for storm surge (due to a long fetch of winds spiraling inward toward the storm), and we can simulate wave height due to fetch explicitly in OpenFlows FLOOD. Therefore, wind is always used for coastal circulation (including spills or drifting objects). Also, wind modules (not direction) are used in the MOHID Land engine (for evapotranspiration process).

Can structures (such as sluice gates, weirs etc.) be added in OpenFlows FLOOD? If so can they be logically controlled?

Yes. Structures such as culverts, sluice gates, weirs etc. can be used and controlled and configured in the SWMM engine of SewerGEMS or CivilStorm. All these would be imported when the stormwater model is linked to the FLOOD model.

Can SewerGEMS / SWMM Models be plugged into OpenFlows FLOOD for off-line modelling? Then does this mean that the model editing would be done in external packages?

This is available in OpenFlows FLOOD. The SWMM engine is available inside OpenFlows FLOOD, so you just need a calibrated SewerGEMS project (prepared with the Explicit SWMM Solver). The main purpose is online dynamic modelling with the 1D underground model linked to 2D surface flow modelling. But it can be used offline as well. Some discharges need to be included in OpenFlows FLOOD to “replace” SWMM exchanges.

If you want to edit the SWMM / SewerGEMS project, you need to use SewerGEMS or an EPA-SWMM interface. Then, in OpenFlows FLOOD, there are 2 different tools called “Connect SWMM model to MOHID Land model”, or “Connect SewerGEMS model to MOHID Land model” which needs 2 inputs:

• Digital terrain model (grid data) used in 2D model;

• .inp project file in the case of a SWMM model or .stsw file in case of a SewerGEMS model.

The outputs generated, that will be used by OpenFlows FLOOD’s Urban Flood Simulator are:

• A modified .inp file (this file is modified for several reasons: to remove sub-catchments, rainfall, and evaporation; to ensure georeferencing compatibility; to adjust all units to CMS; to compute ponded area, based in the 2D cell area, and the number of nodes in the same cell);

• Connection Points (a new grid data mentioning, in each 2D cell, the number 1D nodes to be linked to 2D model. In case of existing channels, the SewerGEMS connection tool also generates an updated DTM and a mapping file with the surface-channel links.

What are the flow exchange mechanisms between the 1D and 2D models connected models?

In OpenFlows FLOOD a weir equation is defined to control the inlet capacity with the water level on the surface. In the case of a manhole, we just need consider the number of manholes in the grid cell, and the ponded area = grid cell area / number of nodes in that grid cell.

If the SWMM / SewerGEMS Model is calibrated and uses hydrological models accounting for the run-off, how does this work in OpenFlows FLOOD? Is the SWMM/SewerGEMS hydrology simply removed and the 2D mesh is used or can the areas already accounted for in the SWMM / SewerGEMS model be excluded from the mesh used in the rainfall run-off calculations?

Hydrology processes must always be directly computed through 2D model in OpenFlows FLOOD. The 2D model will calculate the run-off based on direct rainfall (precipitation) calculations. In SewerGEMS, you should ideally remove all the inflow derived from run-off, to avoid duplication of flow. However, if you want to consider any inflow to a node you can keep it when linking in OpenFlows FLOOD.

Does the calculation grid have curvilinear grid and nested grid options?

Yes, both curvilinear grids and nested grids are available in OpenFlows FLOOD.

Can OpenFlows FLOOD model snow-melt?

Not at present but there are plans to incorporate this in the near future.

Do the equations for porous media and infiltration (viz. 3D solving Richard’s equation, SCS CN etc.) account for recovery in between events?

Yes. They do take into account the recovery between events

Does water quality analysis use ADE equations for hydraulic simulation in OpenFlows FLOOD?

Yes. You can simulate first order decay of fecal coliform, the nitrogen cycle, the phosphorus cycle and dissolved oxygen.

Is it possible for users to write their own equations in OpenFlows FLOOD?

Yes. It is possible since the code is opensource. However, they would have to modify MOHID Land source code for this.

Can OpenFlows FLOOD simulate first flush modelling (i.e. concentration entering system highest at first rainfall)?

Yes. Check this article:

Does the water quality modelling extend through all the hydraulic models – i.e. could a pollutant be added into a 1D river model, which then infiltrates into the ground water as well as travelling out of the river into a 2D reservoir for example? One assumes that this can be used for modelling ground water extraction quality. Do all models cover all water quality parameters?

Yes, except for runoff (which is not so important, because this is so fast). In the previous article water quality was still incomplete, but now it’s fully complete . We can also simulate water quality exchange from a river to an estuary (MOHID Water). For instance: to study the propagation of fecal coliforms in a river into an estuary and vice-versa.

What equations and models are used in OpenFlows FLOOD for sediment transport?

Several different approaches can be used – more dynamic or more static. You can model the sediment transport with a Lagrangian transport model or using a typical Eulerian model.

You can also model the dynamic modification of the seabed / bathymetry (morphodynamics), coupled to a wave model (SWAN). In this case, this is high resolution modelling, and the seabed modification changes the circulation patterns. That’s not presently available in OpenFlows FLOOD; but we have the possibility of providing a plugin to these kind of applications (E.g. Nearshore Simulator Plugin).

Here are some reference links:

How is Dam break analysis done in OpenFlows FLOOD?

In the latest release of OpenFlows FLOOD, you can compute both instantaneous removal or gradual removal – in this case by generating a dam-break hydrograph based on user specified geometry of the dam and reservoir.

Can dam break analysis be used to model lateral embankment failures to replicate a riverbank breach?


Are agricultural practices (using pesticides and fertilizers) considered in water quality modeling in OpenFlows FLOOD?

Yes, agricultural practices such as using pesticides and fertilizers are incorporated by modifying the Manning’s roughness coefficient.

Can breakline be used to update the cell elevations in the grid used?

To do this you have to create a breakline file ( a .lin file created inside OpenFlows FLOOD) with the line, which needs to have the cell elevations. Then, the tool in OpenFlows Flood will use this line to interpolate the grid cell elevation values based on the selected breakline. So, the answer is yes. It can be used to update the cell elevations in the grid used.

When using layers for building the model – I assume that land use can be used to adjust roughness (dependent on time of year etc.? and will therefore be used in conjunction with Manning’s n or is it to look at curve number and will affect permeability), impermeable cover is used for the infiltration into the ground, buildings can be removed from the mesh (no flow areas) to allow only for flow between buildings, and Manning’s n is used to populated the roughness used in the solutions equations?

The buildings should be used to have different manning, as well as to specify a different impermeability fraction. That can be done inside FLOOD. Buildings can indeed be removed from the mesh.

Are LIDS / SuDS with culverts modeled in 1D / 2D linked models in OpenFlows FLOOD?


How does OpenFlows FLOOD handle cross sections?

OpenFlows FLOOD, is able to read cross sections from a text file. Then, it generates an internal file (.dat), which is directly used for interpolation of cross sections to the new drainage network.

What is the 2D computational grid size limit?

The maximum size of grid we have tested so far is having up to 1 million cells i.e. a 1000 x 1000 grid. Usually this is enough for computation even at the highest level.

However, this can impact simulation speeds depending on the number of processors you have and the RAM available. If you are working on a high specifications machine then this would not matter much but for a system working with minimum specifications, the simulation speeds would be severely impacted for such large grids.

In reality there is no limit to the grid size however it is advised to choose an appropriate size depending on your study and machine capabilities.

If water treatment facilities are added to the river, can the scheme be re-simulated in OpenFlows FLOOD?

Yes, in case for example that you would want pump water off from the river for a treatment plant, and then reinject the treated water downstream. For that you'll need to: 

impose a negative discharge (water removal) located in the treatment plant water inlet; the flow should be equal to the treatment plant capacity (MOHID Land will remove the water with the ambient concentrations)

impose a positive discharge located in the treatment plant discharge; the flow should be equal to the negative discharge, but you should impose manually the concentrations regarding the water properties.

If the river flow is below treatment plant capacity, all water is removed from the river (and then reinjected)

If the river flow is above the treatment plant capacity, some water won’t be treated and will mix with the treated water injected downstream.

You can find how to implement point discharges here:

See the section “Creating your own project - Simulating point discharges”

Can OpenFlows FLOOD import a real-world model directly? What format does it accept? Can we extract the terrain model directly from the real model in OpenFlows FLOOD?

Reality models obtained by photogrammetry techniques in ContextCapture can be consumed in FLOOD. ContextCapture is able to generate 3D models (scalable mesh models) that can be opened in LumenRT, and similarly can also generate 2D standard (orthophoto) maps, which are geo-located raster (gridded) files.

OpenFlows FLOOD is able to read the orthophoto maps in RASTER (gridded) files in GDAL format (e.g. .geotiff, ESRI Raster grid). After that, inside OpenFlows FLOOD, we can generate / interpolate a digital terrain model for the desired grid mesh that we want.

So the workflow for this would be;

ContextCapture > Raster (orthophoto map) > OpenFlows FLOOD > Digital Terrain Model

Is there a separate calculation module for dam break simulation and water quality analysis in OpenFlows FLOOD? How to use it?

There is no separate module for dam break analysis. The simulation of a dam break from a modelling point of view, is simply a huge positive point, line or polygon discharge imposed in a river or in any other area (so you need to activate ModuleDischarges), and then running MOHID Land / runoff normally, using default OpenFlows FLOOD MOHID Land engine module options.

For water quality analysis you will need additional modules, depending on what properties you want to simulate, and where: Is it only in rivers? Soil? Are you looking for nutrient cycles and complex ecological / biogeochemical processes? Or are you just looking for a couple of properties transport, diffusion and decayment (e.g. fecal coliforms and microbiology in general)? In the next link you will find some examples on how to implement complex water quality simulation with nutrient transport and transformation, or simply using fecal coliform discharges (we don’t consider this water quality but water contamination).

Check: for more information.

What initial data are needed for water quality analysis in OpenFlows FLOOD?

If you want to model nutrient cycles, you would need several data and parameters, although default values could be assumed.

What are the outputs of infiltration study in OpenFlows FLOOD?

You can model infiltration in saturated and unsaturated soils. Infiltration is often considered with simple approach just as a scheme for water removal from the surface, reducing water column (reducing flood peak). However, in OpenFlows FLOOD, you can indeed model the water infiltration velocity on the soil (porous media), considering that information to quantify and qualify groundwater processes.

If you simulate infiltration, you are able to visualize accumulated water infiltration in each grid cell

See Also

OpenFlows FLOOD TechNotes and FAQ's

Learning Resource Guide for users of OpenFlows FLOOD