FLO-2D Basic

FLO-2D Basic

FLO-2D Basic Free

FLO-2D Basic is a limited version of the PRO Model. It is a good tool for Demonstrations, students, training and FIS studies. A basic license is free but does not include training, webinars or technical support. FLO-2D is a combined hydrologic and hydraulic model so there is no need to separate rainfall/runoff and flood routing.

The Grid Developer System is fully functional and does not have any limitations. The limitations for this installation are as follows:

  • 100 ft or 30 m grid element size
  • 50,000 elements
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FLO-2D Basic a limited version of the PRO model. It is a good tool for Demonstrations, students, training and FIS studies. A basic license is free but does not include training, webinars or technical support.

FLO-2D Basic can tackle many diverse flooding problems including:

  • River overbank flooding
  • Unconfined alluvial fan flows
  • Urban flooding with street flow, flow obstruction and storage loss
  • Overland progression of tsunami and hurricane storm surges
  • Watershed rainfall and runoff
  • Flood insurance studies
  • Flood mitigation design

FLO-2D is a combined hydrologic and hydraulic model so there is no need to separate rainfall/runoff and flood routing. FLO-2D is a FEMA approved hydraulic model for riverine studies and unconfined flood analyses. For a brief overview of the model components For additional information download the Basic 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 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.

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.

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.

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

FLO-2D Software Inc. grants to the user, a non-exclusive, transferable, royalty-free License, for unlimited use and distribution of the FLO-2D Basic Model, processor programs and documentation commonly known and referred to as FLO-2D Basic Model. The License includes the right to copy the FLO-2D Basic model without restriction. The License granted above does not include the right to sell or receive monies or other payments for the FLO-2D Basic model.

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, of the FLO-2D Basic Model shall remain the sole and exclusive property of FLO-2D Software, Inc.

Terms of License

This License Agreement shall continue in effect until terminated. The term of the FLO-2D Bascic Model License shall be in perpetuity unless terminated by FLO-2D Software, Inc. Without prejudice to any other rights, this Agreement will terminate automatically if you fail to comply with any of the limitations or restrictions, commit default or fail to meet other requirements described herein.


The FLO-2D Basic Model and its accompanying software and processor programs may not be sold, resold, leased, lent, or rented to any other individual or organization. The FLO-2D Basic Model and accompanying software and processor programs cannot be 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 Basic Model and accompanying 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, Inc. and its employees do not warrant the performance or results that you may obtain by using the FLO-2D Basic Model or any results generated by the software. The user assumes the entire risk of using the FLO-2D Basic Model. 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 Basic Model or the inability to use the software.

The use of the FLO-2D Basic Model does not entitle the user to any technical support either by email, phone or mail, any training, webinars, short courses, updates or any help, support or assistance whatsoever in any manner without compensation. FLO-2D Software is not obligated to provide any technical support of any form or nature for the free Basic Model.


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 FLO-2D Basic Model 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, by sale, license, lease or other financial renumeration, 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 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 delete (remove) all electronic copies of the FLO-2D Basic model and documentation from any and all computers in the user’s 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.


© 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. The Desert R

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.

Q1. 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 2.4 GHz or higher. For Laptop’s i5 and i7 processors are good and processor speed is important. Use Windows 10 Pro or suffer the consequences. Just kidding but don’t get Windows Home because they throttle the processors.

Q2. 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 PRO grid system. The FLO-2D Basic is limited to 100ft element size and 30,000 elements.

Q3. 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, 100 ft elements or larger are suggested. See the Data Input Manual for detailed information regarding the grid element size.

Q4. 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.

Q5. 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, MANNINGS_N.DAT, CONT.DAT, TOLER.DAT, INFLOW.DAT, and OUTFLOW.DAT files. The GDS and QGIS Plugin programs create all the necessary files for a flood simulation.

Q6. I have an earlier 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 package. The various programs and dll’s need to be registered on the computer. Once the purchased update package has been loaded on your computer, you can then download the various programs with bug fixes and enhancements that are posted at the website throughout the year. These programs can be replaced in your FLO-2D subdirectory.

Q7. 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.

Q8. 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.

Q9. 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.

Q10. Can I mix the channel geometry in the CHAN.DAT file?

Yes, set SHAPE = “R”, “V”, “T”, or “N” in the CHAN.DAT at the start of each line representing a channel element. You can have rectangular, trapezoidal or natural shaped cross sections represented in any order. Try to avoid large variations in the flow area between contiguous channel elements and review the bed slope in the PROFILES processor to make sure that the slope is appropriate. See the Monroe Project Example CHAN.DAT file as an example.

Q11. Can I simulate a mudflow and sediment transport together?

No, these are two distinct physical processes. Mudflows are hyperconcentrated 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. Hyperconcentrated 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.

Q12. 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.

Q13. 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.

Q14. 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 supersedes the floodplain grid element n-values when the flow depth is less than 0.2 ft.

Q15. What is the difference between the assigned n-value in FPLAIN.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 FPLAIN.DAT file. AMANN is either positive or negative and is added to each grid element n-value.

Q16. What is the purpose of XARF? What does it represent?

XARF is a global assignment of flood storage loss on the floodplain. If you assign XARF =0.20, it means that 20% of the surface every grid element on the floodplain is eliminated from receiving flood flows. XARF can be used to represent dense vegetation or an urban area with numerous buildings over the entire grid system. Use XARF when modifying each grid element for ARF values may be unnecessary for the level of detail in the flood simulation.

Q17. What happened to the minimum and maximum timesteps in v2006.01?

The computational timestep incrementing and decrementing scheme has been further refined. The minimum timestep will continue to decrement until the user stops it. The minimum timestep at the start of the simulation is 1 second and the maximum timestep is 30 seconds. These are default values and are now hardwired in the model.

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 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 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. 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.

Q30. Can I run the model longer than the last time increment in my inflow hydrograph?

The question is whether the simulation time SIMUL in CONT.DAT can be greater than the last time listed in the INFLOW.DAT hydrographs. The answer is yes, but the model will just extend the last discharge listed in the inflow hydrograph because the model has nothing to interpolate to. It is recommended that you list the last hydrograph discretized time interval, larger than any simulation time you might considered in future runs. The discretized hydrograph time interval does not have to be uniform and you skip from 100 hours to 500 hours in one step if necessary. Please note that you should also use a 0 time and 0 discharge for interpolating between the first and second time\discharge increments.