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Creating a Flow Surface Database

FLO-2D requires two sets of data for any flood simulation, a digital terrain model (DTM points) and either an inflow hydrograph or a discretized rainfall event. The potential flow surface topography is represented by a square grid system. The grid elements are assigned elevations from an interpolation of the DTM points. A processor program GDS (grid developer system) generates the grid system and assigns the elevations. The GDS superimposes a grid system over the DTM points and interpolates grid element elevations using DTM point filters. It automatically generates the FLO-2D floodplain and other data files to start a simple overland flood simulation. Images can be imported to the GDS to assist graphical editing. Any size grid element can be used in the model, but the timestep is governed by wave celerity and small grid elements will require small timesteps and long model run times. A typical square grid element size will range from 10 ft (3 m) to 500 ft (150 m). The number of grid elements is unlimited.

Inflow Hydrographs or Rainfall

Inflow hydrographs can be designated for either channel or floodplain nodes. The number of inflow hydrograph nodes are unlimited. ASCII data format hydrographs generated from HMS or other watershed rainfall/runoff models can be input to a river or overland FLO-2D flood model. FLO-2D can also perform as hydrologic model and spatially variable rainfall data (such as NEXRAD data) can be assigned for depth area reduction or to simulate a moving storm. The rainfall is routed as overland sheet flow or as rill and gully flow until it is intercepted by a main channel. The flood routing can continue in the river channel in the same model creating a combined hydrologic and hydraulic model.

Infiltration and Evaporation Losses

FLO-2D can be applied as a watershed rainfall-runoff model in the upper basin or rainfall can be simulated on the potential river floodplain surface before or during the flood simulation. Runoff losses can be computed as either infiltration seepage or evaporation. Channel or floodplain infiltration is simulated with the Green-Ampt infiltration model and can be spatially variable. Open water surface evaporation is computed for both floodplain and channel flow and can be based on diurnal variation.

Routing Algorithm Stability and Volume Conservation

Computational timesteps typically range from 1 to 30 seconds. The timestep is incremented or decremented according to strict flood routing numerical stability criteria. Numerical stability is linked to volume conservation. The key to any successful flood routing model is volume conservation. When the model accurately conserves volume the model runs faster. Volume conservation is tracked and is reported both during the simulation and in summary output files.

Overland and Channel Flow - Exchange of Channel and Floodplain Discharge

Unconfined overland flow is simulated in eight directions (4 compass directions and 4 diagonal directions). One-dimensional, channel flow is simulated with rectangular, trapezoidal or natural shaped cross sections and actual channel cross section survey data can be used in the model. The channel width can be larger than the grid element allowing for more detailed floodplain simulations. When the discharge exceeds the channel capacity, an interactive routine in FLO-2D will compute the overbank discharge onto the floodplain or the return flow to the channel on a grid element basis. For urban flooding, the model can simulate channel overbank flow through residential areas with street conveyance and flow around obstructions and then have the flow return to the channel downstream. Tributary inflow is unlimited. The GDS can convert HECRAS cross sections into a data file formatted for FLO-2D.

Street Flow

Streets are important to flow distribution on an urban floodplain and can significantly affect the area of inundation. Streets are simulated as shallow rectangular channels with a width and curb height. Streets can intersect and exchange flow with the floodplain similar to river channels. The small roughness coefficients assigned to streets enable the street flow to extend the area of inundation. Streets can be simulated with any of the other model components including rivers and rainfall.

Hydraulic Structures

Hydraulic structures can represent bridges, culverts, weirs or other control structures. Hydraulic structures are simulated by user specified discharge rating curves or rating tables assigned to either channel or floodplain elements. Culvert flow can occur between grid elements that are not contiguous and flow routing in long culverts is estimated. Reference elevations for headwater depth and tailwater effects can be considered. Flow can reverse directions in bridges and culverts.

Levees

Levees or berms can be simulated by specifying crest elevations for any combination of the eight grid element flow directions. Berms, road embankments or small dams can be simulated with the levee component. Levee failure can occur by overtopping or by inundation at a levee elevation for a specified duration. Levee failure is simulated on a grid element basis. Levee breaches can enlarge vertically and horizontally by prescribed rates. A levee freeboard deficiency output file lists the maximum loss of freeboard or those levee elements that are overtopped. This output can be reviewed graphically in MAXPLOT and Mapper

Buildings and Flow Obstructions

Floodplain storage loss due to buildings, topography or even large trees on a grid element basis can be incorporated into a flood model using area reduction factors. A portion of a grid element or the entire element can be removed from potential inundation during the flood simulation. Reduced flood storage forces more flow downstream. The flow exchange between grid elements can be partially or entirely obstructed with a flow width reduction factor for any or all of the eight flow directions.

Rill and Gully Flow

Overland flow can be simulated in small rills and gullies instead of sheet flow. When two grid elements are assigned small multiple channels, discharge between those elements occurs as concentrated flow in small, rectangular channels and not as overland sheet flow. Rainfall on the overland portion of each grid element is routed into the element rill or gully. More than one gully can be specified for a grid element. When the rill or gully conveyance capacity is exceeded, the channel is expanded by an assigned incremental width to contain the gully flow. On the falling limb of the hydrograph, when the flow depth decreases to less than one foot, the channel width is incrementally decreased. Variable rill and gully channel characteristics can be spatially delineated on the grid system.

Mud and Debris Flows

Hyperconcentrated sediment flow 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 and yield stress relationships for various mudflow samples are provided in the manual. Viscous mudflows may cease flowing on very rough surfaces or mild slopes. Conversely, mudflows can be diluted by rainfall inflow. FLO-2D tracks and reports on the sediment volume separately from the water flood volume so that you can assess the relationship between the sediment and water yield from the watershed.

Sediment Transport

Sediment transport is computed for channel and overland flow using one of seven equations: Zeller-Fullerton, Yang's, Englund and Hansen, Ackers and White, Laursen and Toffaleti, or MPM-Woo (for high concentrations of fine sediment). Sediment volume is conserved on a grid element basis. Scour and deposition are nonuniformly distributed on channel cross sections or uniformly distributed on floodplain elements. Sediment routing by size fraction and armoring can be simulated.

Limiting Froude Numbers

Maximum channel, street and overland flow Froude numbers can be designated for a flood simulation. When the limiting Froude number is exceeded in a particular grid element, the model will increase the appropriate floodplain, channel or street roughness value by 0.001. This routine is effective for avoiding supercritical flow on alluvial sand bed systems or for limiting discharge surging. It is also an expedient method to calibrate n-values for channel bankfull discharge. On the falling limb of the hydrograph, the roughness values will decrease until the original roughness value is reached. The maximum compute n-values are written to file so that the increased n-values can be automatically used in the next simulation.

Model Output, Results and Mapping

The floodwave progression over the flow surface can be viewed along with a plot of the inflow hydrograph while the model is running. Spatially and temporally varied output including flow depth, velocity, discharge hydrographs and water surface elevation are written to file. Discharge hydrographs are computed for every channel element in the system. Floodplain grid elements can be grouped together as cross sections to generate flow hydrographs. Maximum depth and velocity files are automatically created and this output along with the area of inundation can be reviewed graphically in Mapper and MAXPLOT. Mapper has a number of output display options including color flow depth and velocity contours, animation and DTM flow depth interpolations. If a DTM point file is available, the DTM points can be subtracted from the grid element water surface to generate a DTM point flow depth and these plotted as color contours resulting in very detailed Mapper colored maps of the inundated area. Mapper automatically generates and saves shape files of flood plots for viewing in ArcGIS(r).

FLO-2D Version 2006.01

FLO-2D Version 2006.01 represents a significant advancement over the previous version 2004.10. The new version includes enhancements in modeling speed and numerical stability, a number of new components, new features in the GDS and Mapper programs and expanded graphical displays of output and results. This offer includes free updates and maintenance for a year.

EXTRA FLO-2D MANUAL

The FLO-2D manual can be purchased alone without the model package.
Shipment of the manual is by regular mail unless a UPS expedited delivery is specified. There is an additional charge for UPS shipment and handling.