FLO-2D Pro

FLO-2D Pro


FLO-2D Pro #1 dynamic flood routing model

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.

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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:


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.

Model License

This end user License agreement (“Agreement”) is a legal contract between you (either an individual or a single business entity) (“Licensee”) and FLO-2D Software, Inc. By clicking the “I Agree” button during installation or by installing or otherwise using the software application, you agree to be bound by the terms and conditions of this Agreement. If you do not agree to the terms and conditions of the agreement, do not install or use the FLO-2D Software. The following FLO-2D Software License terms are binding upon any Licensee who uses this software.

Grant of Rights

With the purchase of the annual subscription service, FLO-2D Software Inc. grants to Licensee, and Licensee hereby accepts, a non-exclusive, non-transferable, royalty-free License, for use by Licensee only, of the FLO-2D Software package that includes the FLO-2D Pro computer model and processor programs commonly known and referred to as FLO-2D Pro. The License includes the right to copy the FLO-2D Licensed Software only for use within the Licensed office. The license permits the use of the software by Licensee or its regular employees on any and all computers owned by Licensee within or located at the office as indicated by the Licensee’s address appears in documents submitted to FLO-2D. The License granted above does not include the right to copy or distribute the FLO-2D Licensed Software to any person or entity outside the Licensed office. The License does not permit the use of the FLO-2D Licensed Software on a laptop or portable computer outside of Licensed office.

The parties agree that all rights, including, but not limited to, rights under the federal copyright laws in and to modifications, if any, to the FLO-2D Licensed Software shall remain the sole and exclusive property of FLO-2D. The parties further agree that all rights, including but not limited to rights under the federal copyright laws in and to the Licensed Software shall remain the sole and exclusive property of FLO-2D Software, Inc. No rights or licenses to the Licensed Software, other than those granted herein are granted, whether expressly, by implication, estoppels or otherwise. Any attempted assignment of this License or any rights or obligations hereunder, without the prior written consent of FLO-2D, shall be null and void and of no effect and a material breach and default of this License agreement. No modification of this License shall be binding on the parties hereto unless such modification is in writing and duly signed by both parties.

Terms of License

This License Agreement shall continue in effect until terminated. The term of the FLO-2D Pro License shall be the term of the Pro Annual Subscription Service, starting on the date of the payment of the Subscription Service and ending on the same day one year later. Multiple years of Subscription Service may be purchased which extends the term of the License for the duration of the Subscription Service. Without prejudice to any other rights, this Agreement will terminate automatically if you fail to comply with any of the limitations, commit default or fail to meet other requirements described herein.


The FLO-2D model and its accompanying software and processor programs may not be sold, resold, leased, lent, rented or distributed to any other individual or organization outside the Licensed office. The FLO-2D model and accompanying software and processor programs cannot be copied outside the Licensed office or to any other person, decompiled, disassembled, reverse engineered, recreated as a derivative program or otherwise used except as stated in this agreement.

Disclaimer of Warranties and Limitations of Liability

FLO-2D Software, Inc. does not make any warranty, either express or implied with respect to the licensed software, its quality, merchantability, or fitness for a particular purpose. All the FLO-2D Licensed Software provided hereunder is licensed “AS IS” and does not warrant that the licensed software is free from claims of infringement or patents, copyrights, trade secrets, or other proprietary rights of others. There are no warranties, either express or implied, and any and all such warranties are hereby disclaimed and negated. FLO-2D Software and its employees do not warrant the performance or results that you may obtain by using the FLO-2D Software or any results generated by the software. The user assumes the entire risk of using the FLO-2D Software. No oral or written information or advice given by FLO-2D Software Inc. or its employees shall create a warranty or make any modification, extension or addition to this warranty. In no event whatsoever, shall FLO-2D Software, Inc. or its employees be liable to the Licensee or to any third parties for any damages caused, in whole or in part, by the use of the licensed software or for any lost revenues, damages to computers or other computer software, lost profits, lost savings or other direct or indirect, incidental, special, or consequential damages incurred by any person, even if advised of the possibility of such damages or claims, arising out the use or application of the FLO-2D Software or the inability to use the software. The liability of FLO-2D Software, Inc. for a defective copy of the FLO-2D Software will be limited exclusively to the replacement of the originally purchased copy of the FLO-2D Software with another copy of the software or the refund of the subscription service fee if the service agreement is returned within 30 days of the date of purchase.


You will, at your own expense, indemnify and hold FLO-2D Software, Inc., and its subsidiaries and affiliates, and all officers, directors, and employees thereof, harmless from and against any and all claims, actions, liabilities, losses, damages, judgments, grants, costs, and expenses, including reasonable attorneys’ fees (collectively, “Claims”), arising out of any use of the Software by you, any party related to you, or any party acting upon your authorization in a manner that is not expressly authorized by this Agreement.


Without prejudice to any other rights, FLO-2D Software, Inc shall have the right to terminate this Agreement and the License granted herein if the Licensee fails to comply with or commits a material breach of the terms and conditions of this Agreement or commits an act of or is subject to a Default. A “Default” means any one or more of the following events: The distribution, exchange, or offer or promise to distribute or exchange one or more copies of the FLO-2D Software by the Licensee, whether by sale, license, lease or otherwise, and whether or not any consideration is received for any such transfer or offer or promise. A Default will also occur upon failure to pay the full invoice purchase price of the model within the prescribed payment period on the invoice included with the model or upon failure to pay any outstanding debts or invoices to FLO-2D Software, Inc. or Riada Engineering, Inc. within 90 days of the invoice date. Upon the occurrence of a Default, FLO-2D shall provide written notice to the Licensee and the Licensee shall have fifteen (15) days from the Licensee’s receipt of said notice of Default to cure the same. If the Licensee does not affect such a cure within the prescribed time, then this Agreement and the FLO-2D License shall be terminated. Within fifteen (15) calendar days after the Licensee’s receipt of notice as provided for above, Licensee shall deliver to FLO-2D Software, Inc. all hard copies, including but not limited to, all archival and backup copies for the FLO-2D Software and all documentation related thereto and shall delete (remove) all electronic copies of the FLO-2D Software and documentation from any and all computers in the Licensed office.

Governing Law

This License shall be deemed made and accepted in and governed by the laws of the State of Arizona in the United States of America without regard to the conflicts of law rules thereof. The state and federal courts located in Arizona shall have non-exclusive jurisdiction and venue to hear all disputes, national or international, arising out of or related to this License. Any claim or dispute arising in connection with this Agreement shall be resolved in the federal or state courts situated within the State of Arizona. To the maximum extent permitted by law, you hereby consent to the jurisdiction and venue of such courts and waive any objections to the jurisdiction or venue of such courts.


FLO-2D Software, Inc. reserves the right, in its sole discretion, to amend this License Agreement from time to time. If there is a conflict between this Agreement and the most current version of this Agreement, posted at www.flo-2d.com, the most current version will prevail. If you do not accept amendments made to this agreement, then this license will be immediately terminated.


© Copyright 1989, 1993, 2004. FLO-2D is copyrighted by J. S. O’Brien. All rights reserved. The FLO-2D software and manual are protected by U.S. Copyright Law (Title 17 US Code). Unauthorized reproduction and/or sales may result in imprisonment and/or fines (17 USC 506). Copyright infringers may also be subject to civil liability. The Software and Documentation are provided with Restricted Rights. Use, duplication, or disclosure by the government is subject to restrictions as set forth in subparagraph (c)(f)(ii) of the Rights in Technical Data and Computer Software clause at DFARS 252.227-7013 or subparagraphs (c) (1) and (2) of the Commercial Computer Software-Restricted Rights at 48 C.F.R. S:52.227-19, as applicable.


Use these project examples to learn about the various model components. The data files are installed in the FLO-2D Folder along with the software.

Barnard Creek Mudflow Alluvial Fan, Centerville, Utah

Barnard Creek is a completely urbanized alluvial fan with a small debris basin at the fan apex. FLO-2D was applied to delineate the potential water flood and mudflow hazard on the fan. Streets, buildings and the debris flow overflow were simulated. The various flood scenarios that can be simulated include water flooding, rainfall, mudflow, and rainfall and mudflow.

Diamond Alluvial Fan, Las Vegas

A consulting firm conducted an alluvial fan rainfall/runoff study above a proposed development in Las Vegas, Nevada. Rainfall was simulated in the upper basin and runoff was routed to the development site. This project represents a good example of simulating the hydrology of the combined basin and fan complex.

California Aqueduct, Central Valley, California

Flows in the California Aqueduct were simulated using a trapezoidal channel and a uniform slope. The channel was represented by 97 500 ft grid elements. Knowing the design discharge and the channel geometry, slope and roughness, the flow hydraulics can be compared with the analog solution to Manning’s equation for steady, uniform flow. FLO-2D correctly predicts the flow depth and velocity.

Monroe Creek Alluvial Fan, Richfield, Utah

Monroe Creek bisects a large alluvial fan in central Utah. There is a significant supply of boulders from the upstream watershed. The Corps of Engineers used FLO-2D to conduct an unconfined flood simulation of overbank flows. Rectangular, trapezoidal and natural shaped cross sections were used to represent the channel geometry. Overbank flooding and return flows to the channel were simulated to delineate the flood hazard.

Rio Grande, New Mexico

The Middle Rio Grande from Cochiti Reservoir to Elephant Butte Reservoir (173 miles) is being modeled through a joint project of the Corps of Engineers, Bureau of Reclamation and Fish and Wildlife Service. A portion of the entire reach is provided to demonstrate the cross section routine in the FLO-2D model. The reach from San Acacia Dam to San Marcial USGS gage (about 40 miles) is presented. Levees are simulated.

Rogue River, Oregon

The Rogue River FLO-2D project in Oregon was developed by a consulting firm. The 15 mile reach of river includes numerous split channels and old meander bends. The floodplain was mined for gravel and a number of deep gravel pits are part of channel-floodplain interaction. A weir is defined as the channel outflow and provides water surface control for the lower end of the system.

Whiskey Petes Alluvial Fan, Stateline, Nevada

An alluvial fan flood hazard delineation study was conducted above a casino resort. Flows over the alluvial fan were collected at railroad berms and directed into culverts that could overtop the railroad embankment. The culvert outflows were directed at the casino. A concrete channel was designed to collect the flows upfan of the casino and convey them laterally across the fan. Sediment transport was analyzed to determine the potential loss of channel conveyance due to sediment deposition. Flow runup in the channel was a design consideration.

Modeling Tsunami Waves and Ocean Storm Surges with FLO-2D®
O’Brien, J. S.
Presented at: 2005 American Water Resources Association, 2005 Summer Specialty Conference, Institutions for Sustainable Watershed Management, Honolulu, Hawaii.
Abstract: Overland floodwave progression of ocean storm surges from hurricanes and fast rising tsunami waves can be simulated with the FLO-2D® two-dimensional flood routing model. Ocean surge flooding can be simulated by assigning water surface stage and duration to the coastline grid elements. FLO-2D® is a volume conservation model that is effective for analyzing riverine or unconfined alluvial fan flooding, but it can also simulate storm surges through coastal urban areas with detailed resolution. Various ocean storm surges were simulated for the City of Waikiki, Oahu, Hawaii using an existing FLO-2D® watershed model. The results illustrate that the area of inundation is a function of both wave height and duration as they progress through the downtown Waikiki area and into the Ala Wai Canal that bisects the city.
Hazard Zone Delineation for Urbanized Alluvial Fans
R. García, J.J. Rodríguez and J.S. O’Brien
Presented at: 2004 ASCE World Water & Environmental Resources Congress – Arid Lands Symposium, Salt Lake City, Utah.
Abstract: A method is proposed to delineate hazard maps for flooding and mud and debris flow events, based on the application of a two-dimensional flood routing model FLO-2D. The method defines hazard levels based on flood event frequency and intensity. The FLO-2D model has been enhanced with pre- and post-processor programs to automate data input and to generate hazard maps in ArcView GIS format. The proposed methodology was tested in twenty three sites in the Caracas and Vargas State region in Venezuela. This paper describes one application of the proposed method to the community of Tanaguarena on the Cerro Grande alluvial fan. The model results compare very well to the maximum flow depths and area of inundation observed during the December 1999 Vargas debris flow disaster. The hazard maps for the region are being used by local agencies to develop emergency plans and new land use policies. The methodology is being expanded to other flood hazard regions in Latin America.
Real Time Rainfall-Runoff Modeling on Alluvial Fans , Floodplains and Watersheds
J. S. O’Brien and Bing Zhao
Presented at: 2004 ASCE World Water & Environmental Resources Congress – Arid Lands Symposium, Salt Lake City, Utah.
Abstract: Real time rainfall runoff modeling is rapidly advancing and soon will be the framework of a predictive early flood warning system. The Flood Control District of Maricopa County has supported the development of spatially and temporally variable rainfall simulation in the FLO-2D model. FLO-2D is a two-dimensional flood routing model that can simulate rainfall- runoff. Initially, FLO-2D was designed to simulate uniform rainfall on a finite difference grid system of a watershed or floodplain. The model system has been expanded to interpolate ASCII grid file rain data (such as NEXRAD rain data or the Maricopa County rain gage data) to incorporate spatially and temporally variable rainfall data. FLO-2D can also simulate a moving storm system. The variable rainfall-runoff can be simulated with multiple inflow flood hydrographs routed over urbanized alluvial fans and floodplains. Spatially variable rainfall losses are computed with the Green-Ampt model. The assignment of the Green-Ampt parameters are automatically generated by a processor program. Simulating spatially and temporally variable rainfall enables monitored rain storms to be replicated, design storms to be predicted, or real-time network rain gages data to be simulated as a projected flood event. The new rainfall components in the FLO-2D flood routing model set the stage for integration for a predictive early flood warning system.
Reasonable Assumptions in Routing a Dam Break Mudflow
J. S. O’Brien
Presented at: 2003 Third International Conference on Mud and Debris Flows, Proceedings of Debris Flow Hazards Mitigation: Mechanics, Prediction, and Assessment, Davos, Switzerland, Rickenmann & Chen, eds., Millpress, Rotterdam, V. 1.
Abstract: An active landslide threatens to dam the North Fork Cache Creek in Northern California. Releases from an upstream reservoir would result in overtopping and breaching the landslide dam resulting in a mudflow or mudflow. The FLO-2D model is applied to route the landslide dam breach mudflow and map the hazard area of inundation. Two dam landslide scenarios are analyzed. One scenario has a peak discharge in excess of 1 million cfs. By making reasonable assumptions regarding dam breach parameters, sediment concentration and mudflow fluid properties, the potential mudflow hazard can be mapped.
Simulation of Flooding and Debris Flows in the Cerro Grande River
Maria E. Bello, J.S. O´Brien, J.L. López , and R. Garcia-Martínez
Presented at: 2003 Third International Conference on Mud and Debris Flows, Proceedings of Debris Flow Hazards Mitigation: Mechanics, Prediction, and Assessment, Davos, Switzerland, Rickenmann & Chen, eds., Millpress, Rotterdam, V. 1.
Abstract: The December 1999 3-day storm along the north-central coast of Venezuela initiated widespread landslides that evolved into mud and debris flows in watersheds throughout the coastal State of Vargas. One of the many urbanized alluvial fans impacted by flooding and debris flows was the community of Tanaguarena at the mouth of the Cerro Grande River canyon. The Institute of Fluid Mechanics, University of Central Venezuela, is currently designing flood and debris flow mitigation on this alluvial fan. A three-phase analysis of the replicating the 1999 flood was implemented. First, the December 1999 rainfall distribution, intensity and runoff were investigated. Then the 3-day flood hydrograph was estimated with the HEC-1 hydrologic. Finally, a two-dimensional flood routing model FLO-2D with the capabilities of simulating hyperconcentrated sediment flows was applied to route the flows over the alluvial fan. With this calibrated model, flood mitigation can be designed for a selected frequency design flood event. Results indicate that the FLO-2D model can accurately replicate the 1999 flooding on the Cerro Grande fan.
A Case Study of 2-Dimensional Modeling in an Urban Environment
W.S. Ogden and J.S. O’Brien
Presented at: 2001 3rd International Symposium on Environmental Hydraulics, IAHR, Arizona State Univ., Tempe, Arizona.
Abstract: Floodplain development presents many challenges to the floodplain manager and hydraulic engineer. Historic floodplain maps that were created when little or no development existed are no longer valid representations of the existing floodplain condition. Conveyance of floodwaters in wash corridors and arroyo floodplains that were once predominantly overland sheet flow or flow in rills and gullies, are now redirected by houses, fences, small buildings, streets, and other obstructions to flow. In many of these cases, flood containment within the historic mapped flood plain is questionable due to loss of storage and flow path obstruction, and there is a need to quantify the discharge that has been diverted from the floodplain. Traditional one-dimensional backwater models are inadequate to predict the unconfined flow behavior in such urban environments and if used, require too many assumptions regarding flow diversions and potential confinement. Advances in the computational speed of computers has facilitated the reality of using two-dimensional flood routing programs to efficiently simulate these complex urban flood environments with accuracy and detail.
Q01. What type of computer system do I need to run FLO-2D?
Bigger and faster is better. The newest, fastest computer you can afford is recommended. The operating system should be Windows 7 and Up with a 64-Bit OS. Multiple processors are a good idea if you are running models with more than 500,000 cells. Processor speed is also important and we try to use 3.2 GHz or higher. For Laptop’s i7 and i9 processors are good and processor speed is important. Use Windows 10 Pro because Windows Home will throttle the processors.
Q02. Is there any limit to the number of grid elements?
In theory, there is no limit to the number of grid elements in a FLO-2D grid system. Testing has shown that computer memory limitations can limit the number of grid elements the GDS can build. Alternate GIS programs can be used to develop data files when the GDS memory limitation is a factor.

The Basic version of the model has a limit of 50,000 cells.

Q03. Is there a minimum grid element size?
No, but we recommend that you balance the grid element size with the inflow discharge flux. For practical purposes, grid elements less than 10 ft are not recommended. Most flood simulations will have sufficient resolution with 20 ft or 100 ft elements. For large flood events inundating large areas, 250 ft elements or larger are suggested. See the Hints and Guidelines.doc available at the website for detail information regarding the grid element size.

The Basic version of the model has a limit of 100ft or 30m grid element size.

Q04. What happened to the diffusive wave momentum equation?
The diffusive wave approximation to the momentum equation was eliminated in v2006.01. Only the full dynamic wave (FDW) momentum equation is now available for the FLO-2D model. The FDW requires more computer resources but this is now a minor issue with the faster processors and the disadvantages are outweighed by the increased numerical stability over the diffusive wave equation.
Q05. For a simple flood simulation what data files do I need?
The simplest flood simulation is overland flow (alluvial fan) without channels, streets or other components. For this simulation you will need TOPO.DAT, MANNING_N.DAT, CONT.DAT, TOLER.DAT, INFLOW.DAT and OUTFLOW.DAT files. The GDS processor program now creates all the necessary files for a basic flood simulation.
Q06. I have an older version of FLO-2D. Can I download and use the FLO-2D Pro model from the web site?
No, each model version requires the purchase of the update or subscription installation file. The various programs and dll’s need to be registered on the computer. Once the program has been loaded on your computer, you can then download the various updates with bug fixes and enhancements that are posted at the website throughout the year. These programs can be replaced in your FLO-2D subdirectory.
Q07. Can I use culverts in my channel with the HYSTRUC.DAT file?
Yes, both bridges and culverts can be modeled in the channel component. Culverts can now be long spanning several grid elements. Culverts and bridges (with inflow and outflow nodes) no longer have to be assigned to contiguous channel or floodplain elements. Flow can occur in either direction in the model.
Q08. What is going on when the model just stops in the graphics mode?
If a model simulation terminates in the graphics mode, you will be able to see an error message because the model now has error trace back. In addition, you can also review the ERROR.CHK file. If the model encounters an array allocation error, the error message on the screen may be generic and may not indicate the file number or model component where the error is encountered. In this case, please contact us by email and zip the *.DAT files and attach them.
Q09. How can I access support manuals and documentation for FLO-2D?
All documentation files are installed onto the user’s computer when FLO-2D is installed. This includes user manuals, PowerPoint files, example projects and much more. They are located in the Public Documents area. C:\users\public\public documents\FLO-2D Pro Documentation.
Q10. Why doesn’t the model run at all with the channel option?
There are several data dependencies between data files that must be observed. If the channel option is turned on (ICHANNEL = 1 in the CONT.DAT file) and there is channel inflow and you want to view a channel inflow hydrographs (INPLOT =1 in the CONT.DAT), the channel inflow graphics must correctly set (IDEPLT in the INFLOW.DAT file and there must be a channel inflow node with a C character in column 1 of the INFLOW.DAT. The simplest approach to debugging the channel data files is to review the *.BAC files (set IBACKUP =1 in the CONT.DAT file). (See CHAN.DAT Tab in Data Input Manual)
Q11. Do I need to set the abstraction in the RAIN.DAT file if I am using the abstraction in the INFIL.DAT file?
No, the ABSTR variable in the RAIN.DAT file is to account for the rainfall abstraction if you are not using the infiltration component. If you are using the infiltration component (INFIL = 1 in the CONT.DAT file), set the ABSTR = 0.0 in the RAIN.DAT file.
Q12. Can I simulate a mudflow and sediment transport together?
No, these are two distinct physical processes. Mudflows are hyper-concentrated sediment flows with sediment concentrations in excess of 20 percent by volume. The sediment transport component predicts conventional bed load and suspended load where sediment concentrations range from 3 to 10 percent by volume. Hyper-concentrated sediment flows such as mud and debris flows involve high viscosity, yield stress, buoyancy and hindered particle settling behavior and the fluid is treated as a continuum. In conventional sediment transport, water and sediment are as considered separate phases and sediment scour and deposition are simulated.
Q13. I am interested in using another sediment transport equation. Can other sediment transport equations be used in FLO-2D?
Yes, other sediment transport equations can be coded into the model. If a new sediment transport equation or other new component is required for a project, please contact us for special code developments. For components that will be useful in the model for other projects, the components might be coded without cost. For unique or single project components, a consulting fee may be required to add these to the model.
Q14. What is the role of the limiting Froude numbers for channel and overland flow?
The limiting Froude numbers for channel, street and overland flows will essentially reduce the velocity by increasing the roughness. When a limiting Froude number is exceeded, the roughness n-value is increased by 0.001 for the next timestep. This continues until the maximum Froude number is no longer exceeded. For certain physical environments, such as alluvial fans with sand-bed surfaces, there is practical maximum Froude that should not be exceeded. Generally, on alluvial fans supercritical flow (Froude No. = 1) does not occur because more sand will be entrained in the flow reducing the flow energy. Unless bedrock is encountered, it is reasonable to assume that flow on alluvial fans will be subcritical. A practical limiting Froude number on steep slope alluvial fans is 0.95. For most river channels at bankfull, the limiting Froude number can be calculated and will generally range from 0.4 to 0.6.
Q15. How is the SHALLOWN variable used?
For overland flow, the roughness n-value is generally assigned for peak flow hydraulic conditions. To more accurately simulate shallow overland sheet flow, a shallow flow n-value (SHALLOWN) can be specified for flow depths less than 0.2 ft. This will improve the time of concentration and arrival times for overland flow on alluvial fans. The SHALLOWN value supercedes the floodplain grid element n-values when the flow depth is less than 0.2 ft.
Q16. What is the difference between the assigned n-value in MANNINGS_N.DAT file and the AMANN variable in the CONT.DAT file?
The AMANN is a global value that increments all the n-values in the MANNINGS_N.DAT file. AMANN is either positive or negative and is added to each grid element n-value. See the CONT.DAT tab in the Data Input manual for a complete explanation.
Q17. Can I use the TOL variable to simulate ponded water?
No, the TOL value is a depth below which no computational routing is performed. The TOL variable is used so that the discharge routing algorithm is not performed on minor depths less than say 0.1 ft. A typical range of the TOL value is 0.1 to 0.25 ft. It should not be used to simulate storage ponding or rainfall abstraction. A spatially variable TOL is available to represent Low Impact Development.
Q18. Can I adjust the topography in the TOPO.DAT file?
Yes, it may be necessary to revise the grid element elevation in the TOPO.DAT. It is possible for the GDS or QGIS interpolation of the grid element elevation from the DTM points to result in an inappropriate elevation. There are number of ways to edit floodplain elevations. The GDS and QGIS Plugin can graphically edit grid element elevations.
Q19. Can inflow hydrographs be assigned to the both the channel and floodplain nodes?
Inflow hydrographs can be assigned to any number or combination of the channel and floodplain grid elements. Inflow hydrographs for the channel and floodplain should not be assigned to the same grid element.
Q20. If a grid element is an outflow channel element, should it also be assigned as a floodplain grid element?
It is suggested that an outflow element with a channel should also be assigned as a floodplain outflow element to permit any overbank flow to flow off the grid system.
Q21. The floodplain cross section analysis is not providing the correct discharge results. What is wrong?
When a grid element is listed in more than one cross section, the individual grid element hydrographs in the CROSS.OUT output file will not be correct.
Q22. If the MUDFLOW option is initiated, is it necessary to assign sediment concentrations to the inflow hydrograph?
Yes, if MUDFLOW = 1 in the CONT.DAT file, the inflow hydrographs in the INFLOW.DAT file must have sediment concentrations or volumes assigned to the hydrograph.
Q23. How is the floodplain outflow node flow depth calculated?
The outflow node flow depths are estimated using a normal depth assumption by calculating a weighted average of the flow depths in contiguous elements. The floodplain elevation of outflow nodes is automatically set to an elevation 0.25 ft or 0.1 m lower than the lowest upstream grid element unless it is already lower than all the upstream grid elements.
Q24. Is the hydraulic conductivity based on saturated conditions?
Yes, the hydraulic conductivity in the Green-Ampt equation is the saturated hydraulic conductivity.
Q25. When a channel is extended into two or more grid elements can the floodplain overland flow cross the channel?
No, the channel discharge exchange occurs between the channel and floodplain for each bank in separate floodplain elements. The floodplain flow is not shared between grid elements on the opposite side of the banks.
Q26. Can I set stage discharge relationships for the outflow nodes?
Stage discharge relationships can be assigned only for the channel outflow nodes in the OUTFLOW.DAT file. Time-stage relationship can be assigned for either channel or floodplain grid elements.
Q27. Why is the hydraulic structure rating table is not being correctly read by the model?
The first pair of rating curve or rating table data in the HYSTRUC.DAT file should be 0. and 0. to permit interpolation between zero depth and discharge and the first pair of nonzero data.
Q28. What does the levee error message involving floodplain elevations refer to?
If the levee crest elevation is lower than the floodplain elevation for contiguous elements, a warning message appears. The levee crest elevation should be higher than both of the two floodplain elevations separated by the levee. There is no value in putting a levee along a hillside.
Q29. An error message indicates that the channel is extending into into a levee, how do I address this?
A channel cross section can be wider than the grid element and the channel may extend through two or more floodplain elements (with both a right and left bank element). If the channel extension occurs on the inside of a bend the channel may extend into levees or even other channel elements. This can be viewed in the GDS or FLOENVIR by zooming in on the channel reach. After viewing the extension in the GDS or FLOENVIR determine how the channel left or right bank elements may be reassigned to eliminate the any channel extension problems. You may determine that the best approach is to shorten the channel width or length. This can be accomplished by reducing the channel top width (edit the cross section in PROFILES) or reducing the channel length (XLEN). If the channel extends through a levee element, it may be more practical to just set the levee back further away from the river.
Q30. My error message refers to unit numbers, what are these?
The unit numbers refer to the input or outfile file that is used by the model (e.g. TOLER.DAT = Unit 9). The cross reference list between these unit numbers and the file name are listed on the second page after the INPUT FILE DESCRIPTIONS tab in the Input Data portion of the manual.

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