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What are the differences in OpenFlows FLOOD and HEC-RAS®?
HEC-RAS or “Hydrologic Engineering Center's River Analysis System”; is a software developed by US Army Corps of Engineers. This software helps the user to perform one-dimensional (1D) steady flow analysis, one and two-dimensional unsteady flow calculations analysis, sediment transport/mobile bed computations and water temperature/water quality modeling.
OpenFlows FLOOD on the other hand, is a single tool to implement an integrated modeling solution capable of imposing spatially and temporally varying rainfall (using direct rain-on-grid). With this you can impose rainfall from a constant value, or from one or more time-series (hydrographs) or from a meteorological model. Once the rainfall hits the surface, rainfall water is converted in 2D overland flow (based in topography), and FLOOD can estimate losses by infiltration and evapotranspiration (both variable in space and time). The infiltration / flow through porous media (saturated and unsaturated) to the ground can also be modeled (in 3D) with variable layer thickness. There is also interaction with the (1D) drainage network through rivers and canals.
Water moves through all the mediums (atmosphere, porous media, soil surface, and river network) based on mass and momentum conservation equations. The interactions between the different processes (like water exchange between aquifer and river) are calculated dynamically by the model, using the hydraulic gradients. MOHID Land can model the whole system described above, however the 1D surface drainage network can also be modeled using SewerGEMS (SWMM engine), with the advantage of this solver including a wide panoply of hydraulic structures (including culverts, etc.), among other features and capabilities.
If stormwater drainage network exists, OpenFlows FLOOD can also integrate this subsurface network, dynamically interacting with both 2D surface runoff and (1D) surface drainage network system. Bidirectional interaction between surface runoff and stormwater drainage network is done through manholes or inlets / storm drains. For more information on this, see here.
FLOOD can also be used for modeling coastal flooding, including storm surges and tsunamis and for modeling water quality (nutrient cycles) and water contamination (microbiology or other contaminants) and sediments transport.
Differences in OpenFlows FLOOD and HEC-RAS
Some of the major main differences in terms of features between OpenFlows FLOOD and HEC-RAS are as follows;
• All the equations and numerical methods, source code, etc. from the 2D flood model (MOHID) is available online, because this is a collaborative, free, opensource “model”. HEC-RAS is free, but not open-source – which means that an advanced user is not able to check source code, or to modify it.
• OpenFlows FLOOD numerical solver (MOHID) was analyzed for comparison with a benchmark study from UK Environment Agency. This methodology has been assumed as the major world reference in terms of benchmarking 2D flood modelling. OpenFlows FLOOD results are compatible with most solutions.
• OpenFlows FLOOD can be used as a single tool for integrated 2D flood model, while HEC-RAS doesn’t have direct rain-on-grid, infiltration or evapotranspiration (you would need an additional tool called HEC-HMS for other hydrological parameters and processes).
• OpenFlows FLOOD is a full GIS-enabled GUI. You can handle GIS data inside (e.g. Bing Maps; shapefiles, RASTER files, etc.), including results visualization. HEC-RAS visualization needs to be handled in separate tools, including RAS Mapper (although is connected to HEC-RAS), which is not fully GIS-enabled. HEC-GeoRAS is also an ArcGIS extension for HEC-RAS and requires 3rd party software such as ArcGIS.
• You can directly view 4D visualization of OpenFlows FLOOD output results in Bentley’s LumenRT, integrating life-like digital nature into simulated infrastructure designs, and create high-impact visuals for stakeholders.
• OpenFlows FLOOD can integrate with Bentley’s OpenCities Planner, a cloud-based service that enables visualization of 2D, 3D, and GIS data in a city-scale digital twin where you can design, visualize, and communicate projects from large-scale city developments to detailed architectural design.
• OpenFlows FLOOD is a complete solution for flood modelling (urban, river, coastal) .HEC-RAS doesn’t reproduce coastal flooding and is not capable of simulating urban flooding because it doesn’t have stormwater drainage modeling capability.
• OpenFlows FLOOD model applications can be easily ingested by Action Server for implementing a flood forecasting / early warning system. HEC-RAS also has HEC-RTS (Real Time Simulation). In OpenFlows FLOOD you don’t need to specify the area or time of concentration for a catchment since Digital Surface model together with 2D calculation (Saint-Venant and Manning-Strickler equations) will do this. Additionally, you don’t need to enter depression data as the 2D surface contains topographical data which is considered in runoff estimation. FLOOD also has tools to remove depressions from the ground surface if required.
• OpenFlows FLOOD provides more realistic runoff hydrographs and estimations whereas CivilStorm / SewerGEMS /StormCAD rely on standard runoff estimation methods such as SCS CN, IDF Curves etc. for runoff estimation which can be useful for a more conservative design.
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