FLO-2D is a complete flood routing hydrologic and hydraulic model with many urban detail features, river and floodplain interface, sediment transport, storm drain component, mudflow and groundwater modeling.
Overview
FLO-2D is a flood routing model that simulates river, alluvial fan, urban and coastal flooding.
FLO-2D can tackle any diverse flooding problem including:
River overbank flooding
Watershed rainfall and runoff
Urban flooding with street flow, flow obstruction and storage loss
Overland tsunami/hurricane surges
Storm drain modeling
Mud and debris flows
Unconfined alluvial fan flows
Surface and groundwater interaction
Dam and levee breach
Tailings dam failure and volume prediction
Flood insurance studies
FLO-2D is a combined hydrologic and hydraulic model so there is no need to separate rainfall/runoff and flood routing. It is on the list of Corps approved models. For a brief overview of the model components For additional information download the Demo Model or Contact Us.
Solution Algorithm
The model uses the full dynamic wave momentum equation and a central finite difference routing scheme with eight potential flow directions to predict the progression of a floodwave over a system of square grid elements.
Creating a Grid System
FLO-2D requires two sets of data: topography and hydrology. Topography can be represented by a digital terrain model (DTM) points, contour mapping or survey data. The grid element elevations are assigned from an interpolation of the DTM points. A pre-processor program called the Grid Developer System (GDS) generates the grid system and assigns the elevations. A typical grid element size will range from 10 ft (3 m) to 500 ft (150 m). The number of square grid elements is unlimited.
Backgound Images
Aerial images can be imported to the GDS as background to assist graphical editing. The GDS requires a world file to read images.
Volume Conservation, Routing Algorithm Stability and Timesteps
The key to accurate flood routing is volume conservation. FLO-2D tracks and reports on volume conservation. Numerical stability is linked to volume conservation and when the model conserves volume the model runs faster. Computational timesteps are incremented or decremented according to numerical stability criteria for floodplain, channel and street flow.
Inflow Hydrographs or Rainfall
Inflow hydrographs can be assigned to either the channel or floodplain nodes. The number of inflow nodes are unlimited. Any ASCII data format hydrograph can be used as input. FLO-2D can also perform as a rainfall runoff model and rain can occur on the flooded surfaces.
Replicate Historical Rainfall Events with NEXRAD data
Model calibration can be performed with NEXRAD rainfall data. NEXRAD ASCII grid rainfall data can be interpolated to the grid elements using the GDS. A file RAINCELL.DAT is generated so that each grid element distinct rainfall data in the NEXRAD recorded intervals (typically 5 or 15 minutes). A historical rainfall event can then be simulated with spatially and temporally varied rainfall. Adjusted NEXRAD data to rain gages is necessary to compile rainfall data.
Infiltration and Evaporation Losses
Spatially variable infiltration for the channel or floodplain can be computed with either Green-Ampt, SCS curve number or Horton methods. Combined Green-Ampt and SCS methods will enable curve number rainfall losses to be model with transmission losses. Surface water evaporation can computed for both floodplain and channel flow.
Channel Flow and Exchange of Channel and Floodplain Discharge
One-dimensional channel flow is simulated with rectangular, trapezoidal or surveyed cross sections. Unconfined floodplain flow is simulated in eight directions (4 compass directions and 4 diagonal directions). Overbank flow or return flow to the channel is simulated for each timestep. For detailed simulations the channel can be larger than the grid element. Tributary inflow is unlimited. The GDS can convert HECRAS cross sections into a data file formatted for FLO-2D.
Street Flow
Streets are simulated as shallow rectangular channels with a curb. Streets can intersect and exchange flow with the floodplain.
Hydraulic Structures
Hydraulic structures can represent bridges, culverts, weirs or other hydraulic control features. Hydraulic structures are simulated by user specified discharge rating curves or tables assigned to either channel or floodplain elements. Reverse flow is possible. Culvert flow can occur between grid elements that are not contiguous. The generalized culvert equations will account for inlet and outlet control.
Storm Drain System
There is a fully integrated surface water and storm drain system. The storm drain components can include an unlimited pipe system, inlet, outfalls and manhole covers. Inlet control is simulated with five storm drain inlet types. Manhole covers can be popped. Outflow through the inlets or junction boxes with manholes is based on the comparison between pipe pressure and surface water elevation. There are numerous types of outfalls which may include underwater outfalls and flapgates.
Levees and Levee and Dam Breach Failure
Levees, road embankments and dams can be simulated by specifying crest elevations on a grid element boundary. There a several levee failure options including a comprehensive breach erosion model with a choice of nine sediment transport equations. Levee breaches can be initiated with fragility curves.
Buildings and Flow Obstructions
Floodplain storage loss due to buildings or features can be modeled. A portion or the entire element can be removed from potential inundation. Grid element flow exchange can be partially or entirely obstructed in all of the eight flow directions.
Roof Runoff
Rainfall runoff from roofs can be simulated with downspout inlet control and parapet wall storage. Variable depth tolerance values (TOL) can be assigned to accommodate other roof storage. The rainfall runoff from the roof will be added to the ground water surface around the building based on the roof slope. A positive roof head enables the flow to be added to the ground surface water while the flooding will still go around the building.
Low Impact Development
Lot-size flood retention storage for site development can be simulated with a spatially variable tolerance depth (TOL) value. This may include bio-retention, green roofs, rain gardens, permeable pavement, drainage disconnection, swales, and on-site storage. TOL values are assigned to represent composite Low Impact Development (LID) techniques on a given grid element. Different grid elements may represent different LID techniques.
Distributary Channel Flow
Overland flow can be simulated in small rills and gullies instead of sheet flow. The small distributary channels expand as more flow enters the gully. This distributary flow improves the time of concentration for floods progressing over alluvial fans.
Mud and Debris Flows
Mudflow is simulated by the FLO-2D model using a quadratic rheological model that includes viscous stress, yield stress, turbulence and dispersive stress terms as a function of sediment concentration. Viscous mudflows may cease flowing and conversely, mudflows can be diluted by inflow.
Sediment Transport
Sediment transport is computed for both channel and overland flow using one of eleven available equations. Sediment volume is conserved on a grid element basis. Scour and deposition are non-uniformly distributed on channel cross sections. Sediment routing by size fraction and armoring can be simulated.
Groundwater and surface water exchange
The FLO-2D model is linked with the USGS groundwater MODFLOW model at runtime. Groundwater and surface exchange can occur in both directions.
Limiting Froude Numbers
Limiting Froude numbers can be assigned to the channels, streets and floodplain grid elements. When the limiting Froude number is exceeded in a particular grid element, the model will increase the roughness value to suppress numerical surging. It is efficient for the model flood routing to calibrate n-values for reasonable Froude numbers.
Model Output, Results and Mapping
Text output is written to ASCII files. The Post-processor MAPPER programs create shaded contours, line contours or grid element flow depth plots and hazard maps. Flood damages can be assessed and the FLO-2D output can be viewed as a flood animation. MAPPER will also automatically generate shape files that can be imported directly to ArcGIS. A DFRIM tool is available for FEMA FIS studies.
Results can be written now to the TIMDEP output file using a HDF5 or NetCDF binary format. FLO-2D PRO writes the TIMDEP output file in different formats according to the value specified on the ITIMTEP variable:
0: NO TIMDEP.OUT RESULTS
1: ONLY TIMDEP.OUT IS WRITTEN
2: TIMDEP.OUT and HDF5 ARE WRITTEN
3: TIMDEP.OUT and NETCDF4 FILES ARE WRITTEN
4: ALL OUTPUT FILES ARE WRITTEN
For options 0, 1, 2, 3 or 4 the output timestep TIMTEP must be specified. The number of output variables printed in the TIMDEP files was also expanded.
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