Key Features

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 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.

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.

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.

Infiltration and Evaporation Losses

Spatially variable infiltration for the channel or floodplain can be computed with either Green-Ampt or SCS methods.  Surface water evaporation can computed for both floodplain and channel flow.

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. Culvert flow can occur between grid elements that are not contiguous.  Reverse flow is possible.

Levees and Levee 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.  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.

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 on 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 nine 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 program creates shaded contours, line contour 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.   A high resolution, ArcGIS integrated mapping program MAPPER.NET is also available.