FLO-2D FAQ

Salvage after alluvial surge - Venezuela

1. What type of computer system do I need to run FLO-2D?
2. Is there any limit to the number of grid elements?
3. Is there a minimum grid element size?
4. What happened to the diffusive wave momentum equation?
5. For a simple flood simulation what data files do I need?
6. I have v2004.10. Can I download and use the FLO-2D v2009.06 model from the web site?
7. I can’t setup the model system from the CD, what can I do?
8. Can I use culverts in my channel with the HYSTRUC.DAT file?
9. What is going on when the model just stops in the graphics mode?
10. What is wrong with my graphics mode when I switch to the channel?
11. Why doesn’t the model run at all with the channel option?
12. Do I need to set the abstraction in the RAIN.DAT file if I am using the abstraction in the INFIL.DAT file?
13. Can I mix the channel geometry in the CHAN.DAT file?
14. Can I simulate a mudflow and sediment transport together?
15. I am interested in using another sediment transport equation. Can other sediment transport equations be used in FLO-2D?
16. What is the role of the limiting Froude numbers for channel and overland flow?
17. How is the SHALLOWN variable used?
18. What is the difference between the assigned n-value in FPLAIN.DAT file and the AMANN variable in the CONT.DAT file?
19. What is the purpose of XARF? What does it represent?
20. Can I use the TOL variable to simulate ponded water?
21. When should I adjust the minimum and maximum timesteps?
22. Can I adjust the topography in the FPLAIN.DAT file?
23. Can inflow hydrographs be assigned to the both the channel and floodplain nodes?
24. If a grid element is an outflow channel element, should it also be assigned as a floodplain grid element?
25. The cross section analysis is not providing the correct discharge results. What is wrong?
26. If the MUDFLOW option is initiated, is it necessary to assign sediment concentrations to the inflow hydrograph?
27. How is the floodplain outflow node flow depth calculated?
28. Is the hydraulic conductivity based on saturated conditions?
29. When a channel is extended into two or more grid elements can the floodplain overland flow cross the channel?
30. Can I set stage discharge relationships for the outflow nodes?
31. Why is the hydraulic structure rating table is not being correctly read by the model?
32. What does the levee error message involving floodplain elevations refer to?
33. My model stops and an error message occurs indicating the model’s failure to read Unit 9. What should I fix?
34. The error indicates that the channel is extending into a levee, how do I address this?
35. My error message refers to unit numbers, what are these?
36. Can I run the model longer than the last time increment in my inflow hydrograph?

Q1. What type of computer system do I need to run FLO-2D?
A.
Bigger and faster is better. The newest, fastest computer you can
afford is recommended. The operating system should be Windows XP or
Windows Vista. We have not addressed compiling the model for Linus as
yet. The model simulation is somewhat proportional to processor speed,
so take that into consideration when purchasing a new computer.

Q2. Is there any limit to the number of grid elements?
A. In theory, there is no limit to the number of grid elements in a FLO-2D grid system.

Q3. Is there a minimum grid element size?
A. 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 50 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.

Q4. What happened to the diffusive wave momentum equation?
A. 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?
A. The simplest flood simulation is overland flow (alluvial fan)
without channels, streets or other components. For this simulation you
will need FPLAIN.DAT, CADPTS.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.

Q6. I have v2004.10 or v2006.01. Can I download and use the FLO-2D v2007.06 model from the web site?
A. No, each model version requires the purchase of the update or
subscription installation CD. The various programs and dll’s need to be
registered on the computer. Once the purchased update CD 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. I can’t setup the model system from the CD, what can I do?
A. The model system setup is fairly simple. You probably have a CD that
can’t be read by the computer for some reason. Contact FLO-2D by email
or phone and request a new CD. If you are using a new operating system
and have any problems during setup, please let us know. We are not
aware of any conflicts with any operating systems at this time.

Q8. Can I use culverts in my channel with the HYSTRUC.DAT file?
A. 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.

Q9. What is going on when the model just stops in the graphics mode?
A. 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.

Q10. What is wrong with my graphics mode when I switch to the channel?
A. The most common error encountered in the graphics mode is the
plotting of the inflow hydrograph. The graphics mode requires that one
inflow nodes (channel or floodplain) or the rainfall be plotted on the
hydrograph. Only one inflow hydrograph can be plotted. The inflow node
(either channel or floodplain must be listed in the INFLOW.DAT file in
Line 1 (IDEPLT) and must be one of the inflow nodes listed in the
INFLOW.DAT. Line 9 in the CONT.DAT file must be assigned for the
graphics mode, INPLOT = 1 and LGPLOT = 2 must be set in Line 1 of the
CONT.DAT file. (See CONT.DAT Tab in Data Input Manual)

Q11. Why doesn’t the model run at all with the channel option?
A. 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)

Q12. Do I need to set the abstraction in the RAIN.DAT file if I am using the abstraction in the INFIL.DAT file?
A. 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.

Q13. Can I mix the channel geometry in the CHAN.DAT file?
A. 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.

Q14. Can I simulate a mudflow and sediment transport together?
A. 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.

Q15. I am interested in using another sediment transport equation. Can other sediment transport equations be used in FLO-2D?
A. 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.

Q16. What is the role of the limiting Froude numbers for channel and overland flow?
A. 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.

Q17. How is the SHALLOWN variable used?
A. 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.

Q18. What is the difference between the assigned n-value in FPLAIN.DAT file and the AMANN variable in the CONT.DAT file?
A. 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.

Q19. What is the purpose of XARF? What does it represent?
A. 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.

Q20. Can I use the TOL variable to simulate ponded water?
A. 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.

Q21. What happened to the minimum and maximum timesteps in v2006.01?
A. 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. .

Q22. Can I adjust the topography in the FPLAIN.DAT file?
A. Yes, it may be necessary to revise the grid element elevation in the
FPLAIN.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 FLOENVIR can graphical edit grid element elevations. You may
also edit the FPLAIN.DAT file directly using an ASCII editor.

Q23. Can inflow hydrographs be assigned to the both the channel and floodplain nodes?
A. 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.

Q24. If a grid element is an outflow channel element, should it also be assigned as a floodplain grid element?
A. 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.

Q25. The cross section analysis is not providing the correct discharge results. What is wrong?
A. 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.

Q26. If the MUDFLOW option is initiated, is it necessary to assign sediment concentrations to the inflow hydrograph?
A. 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.

Q27. How is the floodplain outflow node flow depth calculated?
A. 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.

Q28. Is the hydraulic conductivity based on saturated conditions?
A. Yes, the hydraulic conductivity in the Green-Ampt equation is the saturated hydraulic conductivity.

Q29. When a channel is extended into two or more grid elements can the floodplain overland flow cross the channel?
A. 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.

Q30. Can I set stage discharge relationships for the outflow nodes?
A. 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. .

Q31. Why is the hydraulic structure rating table is not being correctly read by the model?
A. 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.

Q32. What does the levee error message involving floodplain elevations refer to?
A. 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.

Q33. My model stops and an error message occurs indicating the model’s failure to read Unit 9. What should I fix?
A. A large number of subdirectories leading to the project subdirectory
results the path name that is too long for the Fortran language
compiler and code. You need to either reduce the number of
subdirectories to only 3 or 4 under the C:\ root directory or reduce
the length of the name of the subdirectories. An error message has been
introduced in the later releases of Version 2004.10 to identify this
problem. This type of error message could also be posed if the data
files are “read only.”

Q34. An error message indicates that the channel is extending into into a levee, how do I address this?
A. 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.

Q35. My error message refers to unit numbers, what are these?
A. 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.

Q36. Can I run the model longer than the last time increment in my inflow hydrograph?
A. 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.