CVFED FLO 2D Project Guidelines

FLO-2D Software, Inc.

CVFED FLO-2D Guidelines i

GUIDELINES FOR APPLYING THE FLO-2D MODEL TO THE CENTRAL VALLEY FLOODPLAIN EVALUATION DELINEATION PROGRAM

Submitted to: CA Department of Water Resources

Sacramento, CA

Submitted By:

FLO-2D/Riada Engineering, Inc. Nutrioso, AZ

Wood Rogers, Inc.

Sacramento, CA

October 9, 2009


CVFED FLO-2D Guidelines i

Table of Contents

Introduction .................................................................................................................................................. 1

FLO-2D Model Guidelines ............................................................................................................................. 1

1. Hardware and Software Requirements ................................................................................................................. 1 Hardware and System Requirements .................................................................................................................... 1 Software Recommendations ................................................................................................................................. 1

2. DTM Data and Elevation Interpolation ............................................................................................................... 3 The LAS Binary and ASCII Format ..................................................................................................................... 3 DTM Data Interpolation ....................................................................................................................................... 4 Out-of-core interpolation algorithm ...................................................................................................................... 5 DTM point sampling procedure ............................................................................................................................ 5 New GDS tools to interpolate large LiDAR point data bases. .............................................................................. 6

3. Getting Started ..................................................................................................................................................... 9 Grid System Size and Elevation............................................................................................................................ 9 Grid Element Flow Directions ............................................................................................................................ 10 FLO-2D Grid System Project Boundary ............................................................................................................. 11 Flood Detail Components ................................................................................................................................... 11 Create a Project Folder........................................................................................................................................ 12 Saving data .......................................................................................................................................................... 12 Build the Project Files ......................................................................................................................................... 12 Run the FLO-2D model ...................................................................................................................................... 12 Data Input ........................................................................................................................................................... 13 File Management ................................................................................................................................................ 13 Graphics Mode .................................................................................................................................................... 13 Simulating Channel Flow ................................................................................................................................... 13 Modeling Sediment Transport ............................................................................................................................ 13 Units .................................................................................................................................................................... 14

4. Overland and Channel Flow Roughness ............................................................................................................ 15

5. Running the Flood Model ................................................................................................................................... 18 Basic Overland Flow Simulation ........................................................................................................................ 18 Volume Conservation ......................................................................................................................................... 19 Possible Causes and Corrections - NOFLOC’s Elements ................................................................................... 19 Possible Causes and Corrections - Hydraulic structures ..................................................................................... 19 Possible Causes and Corrections - Outflow elements ......................................................................................... 20 Numerical Stability ............................................................................................................................................. 20 How can WAVEMAX be used to speed up the FLO-2D model or be used to make the model more stable?.... 21

6. River Channel Component ................................................................................................................................. 23

7. Hydraulic Structures .......................................................................................................................................... 29

8. Levees, Levee Breach, Fragility Curves and Safe Storage ................................................................................. 32 FLO-2D Levee Introduction ............................................................................................................................... 32 Levee Profile ....................................................................................................................................................... 33 Levee Overtopping ............................................................................................................................................. 33 Levee Breach Failure .......................................................................................................................................... 35


CVFED FLO-2D Guidelines ii

Using Fragility Curves to Locate and Initial Levee Breaches ............................................................................. 39 Benefits of Applying the Levee Fragility Curves to Flood Hazard Mapping ..................................................... 42 Safe Storage Criteria ........................................................................................................................................... 43

9. Floodplain Urban Details .................................................................................................................................. 44 Buildings and Flow Obstructions ........................................................................................................................ 44 Street Flow .......................................................................................................................................................... 46 Embankments...................................................................................................................................................... 48 Interior Drainage ................................................................................................................................................. 50

10. Sediment Bulking .............................................................................................................................................. 52

11. Reviewing FLO-2D Results ............................................................................................................................... 53 Volume Conservation ......................................................................................................................................... 54 Numerical Surging .............................................................................................................................................. 54 'Sticky' Grid Elements ......................................................................................................................................... 54

12. Guidelines for Flood Hazard Tools .................................................................................................................. 56 Flood Inundation Maps ....................................................................................................................................... 56 Flood Damage Assessment ................................................................................................................................. 57 Flood Hazard Mapping ....................................................................................................................................... 60 DFIRM Mapping ................................................................................................................................................ 63 CVFED Review Procedures ............................................................................................................................... 64

List of Figures

Figure 1. GDS grid system display the extent of an 8 million DTM point tile for a 250 ft grid system ................... 4

Figure 2. Enlarged grid element display of the LiDAR points .................................................................................. 5

Figure 3. Conceptual octagon for the boundary widths and flow lengths ............................................................ 10

Figure 4. Grid System Boundary ............................................................................................................................ 11

Figure 5. VELTIMEC.OUT Example ......................................................................................................................... 20

Figure 6. Channel conveyance is a minor portion of the entire flood volume. ..................................................... 24

Figure 7. Overbank flooding is extensive with channel-floodplain exchange for the entire reach. ..................... 25

Figure 8. Multiple channels are exchanging flow with the floodplain. ................................................................. 25

Figure 9. Interior drainage channels contribute to the flow exchange with the main river channel. .................. 25

Figure 10. Levee overtopping or breach failure is extensive for the entire reach. ................................................. 26

Figure 11. Significant return flow to the channel downstream (potential overtopping from the landside of the levee). ..................................................................................................................................................... 26

Figure 12. GDS channel editor dialog box. .............................................................................................................. 28

Figure 13. Plugged Bridge with Sediment and Debris ............................................................................................. 30

Figure 14. Levee flow directions are depicted in red; the left bank of the river is shown in blue and the right bank of the river is shown in magenta. ........................................................................................................... 33

Figure 15. Levee Profile ........................................................................................................................................... 34

Figure 16. Levee Freeboard Deficit Plot in Mapper ................................................................................................ 34

Figure 17. Example of levee breach urban flooding................................................................................................ 35

Figure 18. Pipe breach failure ................................................................................................................................. 37

Figure 19. Overtopping and channel breach erosion .............................................................................................. 37

Figure 20. Example of Sacramento River Basin levee fragility curves ..................................................................... 40

Figure 21. Levee breach with Variable Fragility Curves .......................................................................................... 41

Figure 22. Safe Storage for Levee ........................................................................................................................... 43

Figure 23. Area (yellow) and Width Reduction Factors (as lines within the yellow grid elements)........................ 44

Figure 24. ARF value grid elements outlined in yellow (zoomed view). ................................................................. 44


CVFED FLO-2D Guidelines iii

Figure 25. Street are shown as green lines in the GDS. .......................................................................................... 47

Figure 26. A number of canal and roadway features that may control the floodwave distribution. ..................... 48

Figure 27. Discharge and Suspended Sediment Load for the Sacramento River at Freeport, California ................ 52

Figure 28. Numerical Channel Surging Example. .................................................................................................... 55

Figure 29. Urban Flooding with Channel. ................................................................................................................ 57

Figure 30. Mapper Damage Assessment Table with a Cost Per Foot of Depth for Each Building Type. ................. 58

Figure 31. Mapper Displayed Color Coded Assignment of Damage Costs to Individual Buildings. ........................ 59

Figure 32. Interpolated Damage Inundation Cost for Individual Structures Computed by Mapper. ...................... 59

Figure 33. Flood Hazard Levels Based on Flood Frequency and Intensity. ............................................................. 60

Figure 34. Flood Hazard for Adults (Bureau of Reclamation). ................................................................................ 61

Figure 35. A typical flood hazard map delineating high hazard (red), medium hazard (orange) and low hazard (yellow). .................................................................................................................................................. 62

Figure 36. Alluvial fan flood hazard compared with an actual rainfall flood event. ............................................... 63

Figure 37. Typical DFIRM Panel ............................................................................................................................... 63

List of Tables

Table 1: Computer Specification Guidelines ............................................................................................................. 1

Table 2: Grid System Size .......................................................................................................................................... 9

Table 3: English Metric Conversion ......................................................................................................................... 14

Table 4: Overland Flow Manning's n Roughness Values1 ........................................................................................ 15

Table 5: Channel Flow Manning's n Roughness Values2.......................................................................................... 16

Table 6: Froude Number ......................................................................................................................................... 17

Table 7: Cohesive Strength and Friction Angle1 ...................................................................................................... 38

Table 8: Area/Width Reduction Factor .................................................................................................................... 46

Table 9: Criteria for Simulating Street Flow ............................................................................................................ 47

Table 10: Output files and Uses ................................................................................................................................. 53

Table 11: Flood Hazard Definition ............................................................................................................................. 61

Table 12. Definition of Water Flood Intensity ........................................................................................................... 61

Table 13. Definition of Mud or Debris Flow .............................................................................................................. 62

Table14: Guidelines for the Review of Project Submittals ....................................................................................... 64


CVFED FLO-2D Guidelines 1

Guidelines for Applying the FLO-2D Model to the Central Valley Flood Evaluation and Delineation Program

Introduction Guidelines are presented to apply the FLO-2D model to the California Central Valley Flood Evaluation

and Delineation Program (CVFED). These guidelines will assist the CVFED AEC project teams to have

consistent approach to the hydraulic modeling and flood hazard delineation of the Sacramento and San

Joaquin River floodplains. This document includes:

Outline of basic model tasks and procedures;

Guidelines to facilitate hydraulic modeling by the AEC Teams;

Standards for selecting and applying modeling components and attributes;

Overview of FLO-2D modeling system tools to enhance flood hazard delineation.

The FLO-2D model was selected along with Corps of Engineers HEC-RAS model to conduct the flood

hazard delineation analyses and mapping for the CVFED Program. The HEC-RAS model would be applied

to the Sacramento and San Joaquin River channels and the FLO-2D model would be used to simulate

two-dimensional unconfined flow on the floodplain in response to overbank flooding and prescribed

levee breaches. There may be reach with complex flooding that may require integrated channel flow

exchange with the floodplain (including return flow to the channel) for accurate prediction of

downstream floodwave attenuation and in these cases, the FLO-2D model can be considered for the

combined river channel and floodplain routing.



Guideline Limitations: These guidelines are presented to help the CVFED AEC project teams make decisions regarding the application of the FLO-2D model to the Central Valley river systems. A few of the guidelines may not be appropriate for some project conditions. Using these guidelines will require engineering judgment and ultimately the AEC project teams are responsible for the FLO-2D components and parameters selected in a model. The guidelines do not represent a step-by-step method or procedure for applying the FLO-2D model. The FLO-2D manuals, tutorials and lessons should be used for that purpose.

FLO-2D Model Guidelines These guidelines are organized according to the order of the modeling task that would be required to

develop a CVFED FLO-2D model. The guidelines first discuss hardware and software requirements and

then address project tasks starting with compiling and assimilating the digital terrain model (DTM) data

bases and corresponding aerial images. The document is divided by tabs that represent the FLO-2D

model channel component or feature that will be simulated in the flood hydraulics analysis. The

following sections (tabs) are presented in this document.

Hardware and Software Requirements

DTM Data and Aerial Images

Getting Started

Overland Flow Roughness

Running the Flood Model

River Channel Component

Hydraulic Structures

Levees and Levee Breach

Floodplain Urban Details

Interior Drainage and Embankments

Sediment Bulking

Reviewing FLO-2D Results

Creating Flood Hazard Maps



1. Hardware and Software Requirements

Hardware and System Requirements

FLO-2D can be run on any MS Windows operating system. As a general rule of thumb with most

computationally intensive software programs, bigger and faster is better when it comes to computer

hardware. For the FLO-2D model, flood simulation runtimes can range from a few minutes to several

hours. Runtimes for large projects and long duration floods may be eight hours or more. Suggested

minimum hardware requirements are at least 4 Gigabytes of RAM and as many processors as can be

cost justified. Computer hard drive storage is both plentiful and cheap and there is no need to specify

minimum hard drive space. FLO-2D Version 2009 will be optimized for 64-bit multiple processor

computers and both 32-bit and 64-bit FLO-2D programs will be available. While network servers are big

and fast, a typical 64-bit multiple processor computer with 8 Gigabytes of RAM off-the-shelf computer

will be sufficient for most FLO-2D projects. The following general guidelines are suggested.

Table 1: Computer Specification Guidelines

Type of Project Grid System Elements Flood Duration

Minimum Computer

Specifications

River & Floodplain Flooding <100,000 < 10 days 4 Gigs RAM, 32-Bit

Off the shelf

Limited Detail >100,000 >10 days 8 Gigs RAM, 64-Bit

Multiple processor

Many Features >200,000 > 30 days Server

Floodplain Only <200,000 < 30 days 4 Gigs RAM, 32-Bit

Off the shelf

Many Features (Levee Breach)

>200,000 > 30 days 8 Gigs RAM, 64-bit

Multiple processor

Alluvial Fan >100,000 < 2 days 4 Gigs RAM, 32-Bit

Off the shelf

Software Recommendations

The FLO-2D model and processor programs constitute a standalone system and no other software is

required. All the input data files are ASCII space delimited format. The FLO-2D modeling system has

both pre-processor programs to facilitate graphical input data assignment and post-processor programs

to plot and map output data. It also has a graphical user interface (GUI) to assist in the data input.

There are several ASCII data editor programs that can be used to data or reviewing output. These

include:

MicroSoft WordPad©

TextPad© (www.textpad.com)

UltraEdit© (www.ultraedit.com)

Microsoft VistaTM Excel©

http://www.textpad.com/

http://www.ultraedit.com/



These programs have some unique features for editing large data files including column operations

(delete, insert columns or add to columns), advanced sorting options, and search functions. TextPad©

and UltraEdit© are also noteworthy for their fast assimilation of unlimited file sizes.

In addition to text editor programs, a computer aided design and drafting (CADD) program or graphical

information system (GIS) program such as ArcGIS® can be useful. Specifically ArcGIS 9.2 or higher may

be valuable because the FLO-2D graphical processor programs Grid Developer System (GDS) and the

mapping software (MAPPER) are integrated with ArcGIS components and features such as shape files.

For example, MAPPER automatically generates shape files of all contour plots including contour plots.

Essentially all of the FLO-2D output data files can be imported into ArcGIS® for spatial or temporal

graphical displays. For advanced ArcGIS mapping and integration the Mapper_NET processor program is

also available with features such as breaklines and more flood contour controls.



2. DTM Data and Elevation Interpolation

The LiDAR DTM data base for individual AEC projects could be very large perhaps as large as 1 gigabyte

or more for some projects. To reduce the computer time required for loading data, creating layers, and

interpolating a large DTM data base, three programming tasks were outlined as part of the Guidelines

scope of work. The Large LiDAR DTM data bases associated with CVFED Program areas necessitated an

investigation of methods to streamline the process of loading and analyzing the DTM data in the Grid

Developer System (GDS). The GDS is the software system used to create the FLO-2D grid system and to

enter and graphically edit data for the FLO-2D model. The following tasks were completed:

1. Expand GDS Capabilities to Read Various LiDAR Data Formats. Both the published ASCII DTM

data and LAS binary data can be read by the GDS program. This will eliminate the need to

reformat the data into the space delimited x-, y- and z- format previously required by the GDS.

2. Speed Up GDS DTM Point Interpolation. A GDS integrated routine was developed to perform

the DTM interpolation outside the GDS. This will eliminate the need to load large DTM point

data sets into the GDS. DTM point filter criteria and the assignment of the grid element

elevations will occur in a subroutine called by the GDS that will save computer time and

resources associated with loading large volumes of data, assigning array space, creating layers

and swapping RAM and hard drive computer memory. The AEC project team effort in

processing the DTM data will be significantly reduced and will allow a virtually unlimited number

of DTM points to be used to in the grid element elevation interpolation.

3. Develop a DTM Point Sampling Routine. A point sampling routine was created to limit the DTM

points used in the grid element elevation interpolation. A statistic analysis was conducted to

insure the sampled points accurately reflect a mean DTM elevation for the grid element. This

will routine will reduce the computer runtime of the assigning grid element elevations to a large

grid system.

The LAS Binary and ASCII Format

DTM LiDAR data for the CVFED Program will be provided in the LAS data public exchange format. It is a

binary file format proposed as an alternative to proprietary data systems that are extensively used.

These files contain the X Y Z data for each LiDAR point but also include a substantial amount of

information related to data collection that is not required for the interpolation to the FLO-2D grid. To

avoid processing this extraneous data, it is more efficient to convert the LAS binary files to ASCII format

files that only have the X Y Z data. This approach is faster than directly reading the binary files with the

additional data.

In order to convert LAS binary format to ASCII format, we used LASTools: a set of programs developed

by Martin Isenburg and Jonathan Shew of the Department of Computer Science of University of North

Carolina at Chapel Hill (http://www.cs.unc.edu/~isenburg/lastools/). LASTools provide C++ classes and

executable programs that implement reading and writing of LiDAR points from and to the binary LAS

format 1.0/1.1/1.2 (as described in the April 2008 specification). This tool has been verified and is used

extensively to manipulate LAS files.

http://www.cs.unc.edu/~isenburg/lastools/



Recommendation: Use the LASTools to convert LAS binary format to ASCII format prior to using the DTM data in the GDS program.

The concern that ASCII files are larger than binary files is mitigated by the fact that the extraneous data

in the LAS files is not included in the ASCII files. Tests performed with the provided CVFED data showed

that the resulting ASCII file size is not substantially larger than that of the LAS binary file. For example, a

typical 8 million point tile has a size of 241,925,971

bytes in binary format and 257,398,271 bytes in ASCII

format, an increase of only 6%.

DTM Data Interpolation

Due to memory limitations in most off-the-shelf computers, the maximum number of DTM points that

could be previously loaded into the GDS was approximately 15 million points. The LiDAR data will be

provided in tiles of 5000 x 5000 ft, each containing more than 8 million points in LAS binary format. For a

typical grid system consisting of 250 x 250 ft square elements, each grid element will contain more than

20,000 points or a point density of approximately 0.3 points per square foot. Present memory limits

would permit just a few tiles to be imported for a particular project. Interpolating this LiDAR data to

assign the elevation to the 250 ft grid elements using the complete data base is both computationally

demanding and unnecessary. Figure 1 shows a 250 x 250 ft element grid and a boundary around a

typical LiDAR tile.

Figure 1. GDS grid system display the extent of an 8 million DTM point tile for a 250 ft grid system



Zooming in to one grid element shows the LiDAR point density:

Figure 2. Enlarged grid element display of the LiDAR points

The grid element elevation is represented by a single value based on an interpolation of all the DTM

points within a prescribed area whose diameter is a multiple of the grid element side (in the case one

grid element is the prescribed area). Using all DTM points within the grid element in the elevation

interpolation is represents extraneous and repetitious computations, and a small subset of points would

result in an accurate interpolated value. A new computational procedure was developed and

implemented in the GDS program to handle large LiDAR point data bases while maintaining reasonable

interpolation accuracy.

Out-of-core interpolation algorithm

To facilitate the GDS grid element elevation interpolation using the large CVFED DTM data bases, an out-

of-core algorithm was developed. This new routine will read and process the LiDAR data base without

requiring the GDS to import the data files. The algorithm was divided in two steps: 1) A data sampling

procedure will determine the number of DTM points inside a grid element that would constitute an

accurate approximation of the element elevation as measured by a user defined error. The optimized

number of points will be significantly smaller than the actual LiDAR points inside a grid element. 2) The

LiDAR tile of data will be processed outside the GDS environment avoiding the memory size limitations

that may exist if all the DTM points would be imported at once. Combined these two steps will reduce

the interpolation computational time by about ten times.

DTM point sampling procedure

The GDS point sampling procedure will reduce the number of DTM points processed for the

interpolation of the grid element elevation. Using fewer DTM points to perform the interpolation could

results in the deviation from a true representative grid element elevation. To optimize the computing

speed and concomitantly limit the potential interpolation errors, a sampling algorithm was developed as

follows:

1. Select at random a number of target locations inside the area covered by the LiDAR DTM tiles.

2. Compute the interpolated elevations at the target locations using a gradually reduced number of points (NP), for example:



a. NP = TNP. (TNP are all available points on a grid element located at each target point)

b. NP=TNP /1000

c. NP=TNP/100

d. NP=TNP/10

3. Compute the interpolation error Err(NP) for each case. The case of NP = TNP is assumed to be error free.

4. For each NP, compare the computed errors Err(NP) with a user selected error limit Eu.

5. If the computed error Err(NP) is less than the user selected error Eu, then use NP number of points will be used to compute all interpolations.

6. If Err(NP) is always larger than Eu , then the total number of points TNP will be used for the interpolation.

The interpolation algorithm was implemented in through a set of Fortran 95 routines in a MS-Windows

dynamic link library DLL called FLO2DLIDAR.DLL that is installed with the FLO-2D system. This library

contains subroutines that perform the following tasks:

1. Read DTM points from any number of LiDAR files.

2. Determine the optimum number of points to use in the interpolation.

3. Compute the interpolated elevations on the FLO-2D grid elements based on multiple LiDAR files.

New GDS tools to interpolate large LiDAR point data bases.

To implement the out-of-core interpolation algorithm, new dialog menu items and dialog boxes were

developed for the GDS. To perform the interpolation from multiple LiDAR files, the following steps are

necessary:

1. Use the GDS to create the FLO-2D model grid in the usual form.

2. Select the new Interpolate from LiDAR ASCII Files in the Grid Menu:



3. Select the LiDAR files that are going to be used to perform the interpolation:

Any number of LiDAR files can be selected from the dialog box.

4. By clicking the Open button, the routine will perform a preliminary evaluation of the files and

report the results in the interpolation dialog box:



Note that there are two radio buttons. The first button is an option to assign the maximum

number of DTM points used in the elevation interpolation of each grid element, regardless of

the potential error that may result in the grid element elevation. The maximum value of points

that can be used in the interpolation is project dependent, but a typical value for most

interpolations is 1,000. The second button provides the option to limit the Maximum relative

error in interpolation Eu to a user prescribed value that will define and may increase the number

of DTM points used in the interpolation. A typical value for the maximum relative error used in

the interpolation is 0.2 ft.

5. Clicking OK will perform the sampling evaluation and interpolation resulting in an elevation for

each grid element.



Recommendation: For the CVFED project, it is suggested that a 250 ft grid size be used unless the number of grid elements is greater than 200,000, then a 400 ft grid element size is recommended.

3. Getting Started

Grid System Size and Elevation

Consistent selection of grid size is important for potential FEMA submittals, future map use by the

communities and project t integration between the AEC Teams. Most of the CVFED flood projects will

involve major flood events (100,000 cfs to 400,000 cfs for the 200-yr flood based on the Corps 2002

Comp Study) and 100 to 500 ft wide grid elements will be appropriate. The flood delineation map

resolution and the computer flood simulation runtime should be balanced. Once the overall project

area has been identified, estimate the grid system size area (as a rough rectangle) and determine the

approximate number of grid elements that would be required for different size grid elements such as

100 ft, 250 ft, etc. The grid elements are square and estimating the maximum number of grid elements

is relatively easy. Refer to Table 2 to determine the relative flood simulation runtime.

Table 2: Grid System Size

Number of Grid Elements Model Simulation Speed

1,000 – 15,000 Very Fast (minutes)

15,000 – 30,000 Fast (~ hour)

30,000 – 60,000 Moderate (several hours)

60,000 – 150,000 Slow (< 24 hour)

> 150,000 Very Slow (a day or more)

Selecting the grid element size will control how fast the FLO-2D flood simulation will run. Often a grid

element is chosen for a project that is smaller than necessary. A small grid element combined with a

high flood discharge can result in long flood simulations times. To select the grid element size, the

following criteria are suggested: The estimated peak discharge Qpeak divided by the surface area of one

grid element Asurf should be in the range:

1.0 cfs/ft2 < Qpeak/Asurf < 2.0 cfs/ft2

or in metric:

0.3 cms/m2 < Qpeak/Asurf < 0.67 cms/m2

The closer Qpeak/Asurf is to 1.0 cfs/ft2 (0.3 cms/m2), the faster the model will run. If the Qpeak/Asurf is

greater than 2.0 cfs/ft2 or 0.67 cms/m2, the model should be expected to run more slowly.

As a general rule of thumb, for peak discharges on the order of 300,000 cfs or higher, a minimum grid

element size should be 250 ft. If a project application requires the analysis of a range of return period

floods, it is suggest that the 100-year flood be

used to select the grid size. The larger flood

events will run slower, but ultimately this will

balance the map resolution for the more

frequent return period floods.



Recommendation: Use the DTM points low filter (3 ft or 1 m) to interpolate the grid element elevations when it is apparent that a significant number of DTM points are inside the channel top of banks.

The GDS has an option for filtering both high and low DTM points when interpolating the grid element

elevations. Given the number and accuracy of available DTM points for Central Valley, the grid element

elevation assignment in the GDS will be representative. The LiDAR will also be pre-processed to exclude

DTM points that reflect buildings, bridges, trees or other features that do not represent the ground

surface. For these reasons, the application of the GDS high filter to assign grid element elevation is

unnecessary.

A low DTM point filter is necessary when the DTM data includes the river channel below top of bank. If

the bathymetric data is included in the DTM point file in sufficient numbers, it may lower the assigned

channel bank grid element elevation. This would cause some of the floodplain elevations to be

depressed along the bank and they may not compare favorably with the cross section survey top-of-

bank. To exclude the channel bed DTM elevations when interpolating the channel bank element

floodplain elevations, it is advisable to use a lower filter. For

example, if a low filter of 3 ft or 1 m is assigned in the GDS grid

element elevation interpolation dialog box, all those points 3

ft below the mean grid element elevation will be eliminated

and the grid element elevation will be re-interpolated.

Grid Element Flow Directions

FLO-2D routes flows in eight directions as shown in Figure 3. The four compass directions are numbered

1 to 4 and the four diagonal directions are numbered 5 to 8. Some components such as levees are

placed on boundaries of the grid element. The grid element boundaries create an octagon in this case.

The simplest FLO-2D model is overland flow on an alluvial fan or floodplain. It requires only the

topography files and a hydrograph along with the two control files CONT.DAT and TOLER.DAT. The

conceptualized grid element octagonal geometry is primarily important for the flow width across the

grid element boundaries and diagonal flow lengths. The grid element surface area is still a square.

Figure 3. Conceptual octagon for the boundary widths and flow lengths

The FPLAIN.DAT file defines the potential flow surface and the contiguous grid elements. It contains the

data that identify the grid elements and their neighbors, hydraulic roughness and elevations. The

horizontal position of the grid elements is defined by the CADPTS.DAT file that lists the grid element



Recommendation: A minimum of 5 grid elements should exist between the flow domain boundary and critical project features. If the flow contacts the grid system boundary during the first overland flow simulation, expand the grid system. To have the outflow nodes function as normal flow at the boundary, make sure that the grid system boundary is not located downstream of a hydraulic control.

number and x- and y-coordinates. With these two data files all the coordinate geometry (including

elevation) of the entire grid system is defined. If the FPLAIN.DAT and CADPTS.DAT file were created

with the GDS processor, these data files will be error free and no further modifications to these data

files are necessary to start a simulation.

FLO-2D Grid System Project Boundary

When the DTM points are imported to the GDS and a grid system is overlaid, the grid element fills the

screen window. At this point there is no project boundary or project area. The user selects the grid size

and then outlines the boundary using a polygon. When the polygon is completed or closed, the project

area is outlined by red boundary elements (see Figure 4). The area inside the polygon represents the

project flow domain. The red boundary elements are

not included as part of the project grid system. The

boundary should be created with the aerial image in

the background to avoid creating a boundary to close

to the project area. The boundary acts like a solid wall

and no flow leaves the project area unless channel or

floodplain outflow elements are assigned. The

following guidelines for constructing the flow domain

boundary are very important:

i. Provide enough distance between the boundary and the important project features to eliminate any potential boundary affects on the flow through the project area.

ii. If the flow reaches the boundary in locations other than the proposed outflow locations, then either re-establish a larger grid system or assign outflow nodes. Flow against the model boundary will result in flow containment and artificially higher flow depths.

iii. Assign outflow nodes along the boundary where the flow is expected to leave the grid system.

Figure 4. Grid System Boundary

Flood Detail Components

The first flood simulation for any project will be a simple overland flow model which will constitute a

base model upon which a more detailed flood simulation will be built. A suggested order of component

construction is as follows:



Inflow and outflow nodes Rainfall Infiltration Channels Levees and levee breach Streets Buildings Hydraulic structures (culverts, weirs and bridges) Multiple channels (rills and gullies) Mud and debris flows/sediment transport Evaporation and groundwater

Floodways

As new components are added to a model and tested, other components switches can be turned off in

the CONT.DAT file. The six data files necessary to conduct a simple overland flow simulation are:

FPLAIN.DAT CADPTS.DAT CONT.DAT

TOLER.DAT INFLOW.DAT OUTFLOW.DAT

The INFLOW.DAT and OUTFLOW.DAT files are optional, but almost all project applications will require

these two data files with inflow hydrographs and outflow grid elements respectively.

Create a Project Folder

Start by creating a subdirectory for the project data files and import the DTM data base files, map

images and aerial photos. This folder should be the main GDS editing subdirectory. By keeping the

editor data files and map data together, the time spent browsing for the files needed to create and edit

FLO-2D project will be minimized.

Saving data

When you are creating or editing the data files, it is suggested that you save the data files frequently and

use one folder for testing your project and one for editing your project.

Build the Project Files

Use the GDS to build a grid system. Most data files can be graphically created in the GDS. You can follow

the GDS “Getting Started” lesson to initiate a project. For easy access, put the GDS icon on the desktop.

Run the FLO-2D model

Once the six required basic data files have been created (CADPTS.DAT, FPLAIN.DAT, CONT.DAT,

TOLER.DAT, INFLOW.DAT and OUTFLOW.DAT), an overland flood can be simulated. You can run a FLO-

2D simulation by:

1. GUI - click on the “Execute” pull down menu.

2. GDS - click on “Run FLO-2D” command in the File menu.

3. Copy the “FLO.EXE” into the project subdirectory and double click it.



Data Input

When the data format seems confusing, review the example project data files provided in the Example

Projects subdirectory of the FLO-2D folder.

File Management

The output files in the project folder will be overwritten during subsequent model runs. To save any

output files that might be overwritten, rename the file or create a new folder, copy all the *.DAT files

into it and then run the new flood simulation in that folder.

Graphics Mode

To view a graphical flood progression over your project flow domain, follow these steps:

1. Click the GUI (FLO-2D icon) to turn ‘on’ the graphics switch (LGPLOT = 2) in the CONT.DAT file form. Set Graphics Display to ‘Detail Graphics.’

2. Assign an update screen refresh time (GRAPHTIM) in the lower left hand corner of the CONT.DAT file form to 0.05 or 0.10.

Simulating Channel Flow

To add a main channel to an overland flood routing routine, follow this procedure:

Review workshop lesson 4 and 5 and the section on channel modeling in the Pocket Guide.

If surveyed cross section data is available, create the XSEC.DAT file first. Then generate the CHAN.DAT file in the GDS.

Interpolate the cross section data in the GDS or in PROFILES.

Set the ‘Main Channel’ check box switch (ICHANNEL = 1) in the CONT.DAT file.

Prepare any channel inflow hydrographs for the INFLOW.DAT file.

Select a channel inflow hydrograph to be plotted (IDEPLT) in INFLOW.DAT file.

Assign channel outflow node(s) in OUTFLOW.DAT.

Review the “Channel Hints and Guidelines” section.

Modeling Sediment Transport

Mobile bed simulation is complicated and should be attempted only after a rigid bed model is fully

functional. The following procedure is required for sediment transport:

Turn ‘on’ sediment transport switch (ISED = 1) in the CONT.DAT file form.

Turn off the mudflow switch (MUD = 0) and set XCONC = 0.00 in the CONT.DAT file form.

Assign the SED.DAT file sediment parameters.

For channel sediment transport, set ISEDN = 1 for each channel segment in CHAN.DAT.

Read the ‘Sediment Transport – Total Load’ section in the FLO-2D Reference Manual.



Units

The following units are used in the FLO-2D model. These are the units that are entered in *.DAT input

files.

Table 3: English Metric Conversion

Variable English Metric

discharge cfs m3/s (cms)

length (depth) ft m

hydraulic conductivity (infil) inches/hr mm\hr

rainfall and abstraction inches mm

soil suction inches mm

velocity fps mps

volume acre-ft m3

viscosity poise (dynes-s/cm2) poise

yield stress dynes/cm2 dynes/cm2



Recommendation: The typical range of channel roughness for the mainstem river reaches in the CVFED project area is 0.035 to 0.065. A suggested generic n-value for overland flow is 0.085. For crops and cultivated fields a n-value of 0.125 and higher is recommended. Note: The GDS floodplain n-value default is 0.040.

4. Overland and Channel Flow Roughness

Spatially variable roughness will support realistic floodwave movement as well as improve numerical

stability. Typically Manning’s n-values that represent steady, uniform flow conditions are assigned to

represent variable flooding such as unsteady flow, expansion and contraction, variable flow directions,

bed forms, variable topography and vegetation. The Corps Comprehensive Study (2002) applied 0.06

floodplain n-values and 0.035 channel n-values with adjustments based on observed field conditions.

The n-value data presented in this chapter encompasses a more realistic range of flow resistance.

Additional tools (limiting Froude numbers and depth variable roughness) are also available in the FLO-2D

model to assess n-value variation during a flood event.

Typical n-values for overland flow are

shown in the following tables. For the

Central Valley much of the floodplain is

cultivated throughout the year even

during the midwinter high flood season.

Table 4: Overland Flow Manning's n Roughness Values1

Surface n-value Dense turf 0.17 - 0.80

Bermuda and dense grass, dense vegetation 0.17 - 0.48

Shrubs and forest litter, pasture 0.30 - 0.40

Average grass cover 0.20 - 0.40

Poor grass cover on rough surface 0.20 - 0.30

Short prairie grass 0.10 - 0.20

Sparse vegetation 0.05 - 0.13

Sparse rangeland with debris 0% cover 20 % cover

0.09 - 0.34 0.05 - 0.25

Plowed or tilled fields

Fallow - no residue

Conventional tillage

Chisel plow

Fall disking

No till - no residue

No till (20 - 40% residue cover)

No till (60 - 100% residue cover)

0.08 - 0.12

0.06 - 0.22

0.06 - 0.16

0.30 - 0.50

0.04 - 0.10

0.07 - 0.17

0.17 - 0.47

Open ground with debris 0.10 - 0.20

Shallow glow on asphalt or concrete (0.25" to 1.0") 0.10 - 0.15

Fallow fields 0.08 - 0.12

Open ground, no debris 0.04 - 0.10

Asphalt or concrete 0.02 - 0.05 1Adapted from COE, HEC-1 Manual, 1990 and the COE, Technical Engineering and Design Guide, No. 19,

1997 with modifications.



Spatial variation in floodplain roughness can be assigned with the GDS processor program. There are

several ways to edit the grid element n-values. In the GDS, individual floodplain n-values can be

modified by ‘point and click’ selection and editing. Groups of floodplain grid elements can be selected

by polygon and assigned a global n-value. Finally, the GDS can interpolate and assign n-values from GIS

shape files and the corresponding tables. Also any ASCII editor can be used to edit the n-values in

FPLAIN.DAT or CHAN.DAT files.

The FLO-2D model has options to adjust n-values at runtime including depth variable roughness and

limiting Froude numbers. Shallow overland flow roughness (less 0.2 ft or 0.06 m) is defined by the

SHALLOWN value in the CONT.DAT file. To improve the timing of the floodwave progression through

the grid system, a depth variable roughness can be assigned. The basic equation for the grid element

roughness nd as function of flow depth is:

nd = nb *1.5 * e-(0.4 depth/dmax) where:

nb = bankfull discharge roughness depth = flow depth dmax = flow depth for drowning the roughness elements and vegetation (3 ft or 1 m)

This equation prescribes that the variable depth floodplain roughness is equal to the assigned flow

roughness for complete submergence of all roughness (assumed to be 3 ft or 1 meter). This equation is

applied by the model as default and the user can ‘off’ the depth roughness adjustment coefficient for all

grid elements by assigning AMANN = -99. This roughness adjustment will slow the progression of the

floodwave. It is valid for flow depths ranging from 0.5 ft (0.15 m) to 3 ft (1 m). For example, at 1 ft (0.3

m), the computed roughness will be 1.31 times the assigned roughness for a flow depth of 3 ft. The

depth variable roughness will reduce high floodplain Froude numbers. When using the combined

shallow roughness value and the depth variable roughness, the following values are computed:

0.0 < flow depth < 0.2 ft (0.06 m) n = SHALLOWN value

0.2 ft (0.06 m) < flow depth < 0.5 ft (0.15 m) n = SHALLOWN/2.

0.5 ft (0.15 m) < flow depth < 3 ft (1 m) n = nb *1.5 * e-(0.4 depth/dmax)

3 ft (1 m) < flow depth n = n-value in FPLAIN.DAT

Table 5: Channel Flow Manning's n Roughness Values2

Channel Type n-value Main Channel – Shallow Slope 0.020 - 0.045

Tributary Channel – Moderate Slope 0.030 - 0.065

Lower Watershed 0.040 - 0.075

Upper Watershed – Steep Slope 0.065 - 0.15

Alluvial Fan 0.06 - 0.15 2Adapted from USGS Water-Resources Investigations Report 85-4004, Determination of Roughness

Coefficients for Streams in Colorado, 1985 and the USGS Water Supply Paper 1849, Roughness

Characteristics for Natural Channels, 1967 with modifications.



A similar equation can be used for the channel only the user can both control the adjustment factor

(ROUGHADJ) and its application on a reach basis.

A limiting Froude number for the channel, floodplain and streets can be assigned to modify the flow

roughness values. The Froude number has several physical implications; it delineates subcritical and

supercritical flow, it is the ratio of average flow velocity to shallow wave celerity and it relates the

movement of a translational wave to the average stream velocity. If there is a mismatch between these

physical variables in a flood routing model, then high velocities can occur that may result in flow surging.

Establishing a limiting Froude number in a flood routing model will help maintain the average velocity

within a reasonable range and help sustain the model numerical stability.

To use the limiting Froude number, estimate a reasonable maximum Froude number for your flood

simulation as shown in the following table and assign the value to either FROUDL (floodplain), FROUDC

(channels), or STRFNO (streets) variables. When the computed Froude number exceeds the limiting

Froude number, the n-value is increased by a small value (~ 0.001) for the next timestep. This change in

n-value will evolve a better match between the slope, flow area and n-value during the simulation.

When the limiting Froude number is no longer exceeded, the n-value is gradually decreased to the

original value. The changes in the n-values during the simulation are reported in the ROUGH.OUT file.

Table 6: Froude Number

Flow Type Reasonable Limiting

Froude Number

Main Channel 0.4 - 0.6

Lined Channel 0.8 - 1.2

Floodplain 0.4 - 0.8

Steep Floodplain 0.6 - 1.0

Street 1.2 - 1.5

Alluvial Fans 0.9 - 1.0

A review of the increased n-values in ROUGH.OUT will identify any trouble spots where the flow velocity

exceeds a reasonable value. For the next FLO-2D simulation, the maximum n-value adjustments in

ROUGH.OUT can be made permanent for subsequent simulations by renaming the *.RGH files to *.DAT

for the next flood simulation (e.g. FPLAIN.RGH = FPLAIN.DAT). The final n-values used in a simulation

should be carefully reviewed for reasonableness. In a practical sense, the final ROUGH.OUT should not

report any n-value changes, but engineering judgment on the final n-values used in the project is critical.

The limiting Froude numbers can be set to “0” for the final simulation to avoid any additional

adjustments in the n-values.

In summary, it is common modeling practice to underestimate flow roughness. Avoid using n-values that

represent idealized prismatic channel flow conditions. Poor selection of n-values and failure to provide

spatial variation in roughness can result in numerical surging in the model. By using the depth variable

n-values adjustment and limiting a Froude number assignment, n-values modification during a flood

simulation will result in more realistic floodwave movement through the river system.



5. Running the Flood Model

Basic Overland Flow Simulation

The CVFED Program will delineate the area of inundation on the floodplain for various return period

flood events. A basic overland flow simulation will require the following steps.

1. Compile the DTM and aerial images for the project area.

2. In the GDS, overlay a grid system and interpolate the grid element elevations.

3. Identify inflow nodes and assign inflow hydrographs/rainfall.

4. Locate and assign outflow nodes.

5. Run the basic overland flow simulation.

When the first overland flow simulation is completed, review the area of inundation. When river

channels are added to the model, the base area of inundation will be revised. The area of inundation will

be an indication of where floodplain details such as buildings or streets are necessary. Recognizing the

base area of inundation at the outset will save time and effort when adding floodplain detail.

Engineering judgment plays a role in determining which conveyance and containment features on the Central Valley floodplain need to be simulated. The focus should be on those features that are going to affect the area of inundation. These features may:

i. Have a significant design discharge;

ii. Involve a large storage volume;

iii. Impede flow to a substantial portion of the floodplain;

iv. Redirect flow to other portions of the floodplain not inundated.

These hydraulic control or drainage features may include: tributary channels, drainage and irrigation

canals; flood by-pass channels; streets or highways with curb and gutters; levees and berms; road,

railroad and highway embankments; bridges and culverts; flood detention facilities; diversion and

impoundment structures; and natural features such as oxbow lakes.

The CVFED flood areas are extensive with many potential floodplain hydraulic structures and large urban

areas. One of the project tasks will be to select those floodplain features for the model to accurately

delineate the area of inundation. For the large CVFED flood project areas this is a substantial task when

buildings, streets, and levees are considered. Guidelines for floodplain modeling are:

If the area of inundation is covered by three feet or more of flow depth, minor embankments, streets, and drainage canals can be ignored.

First focus on the features with the greatest potential impact on distribution such as levees and by-pass or other channels.

Initially work on only those areas within the initial flooded area and then determine what additional areas need to be addressed from subsequent flood simulations.

Consider modeling the largest return period flood first.

Simulate only the largest streets and avenues with curbs as conveyance features. The remaining streets can be represented by reducing grid element roughness values.

For shallow flow areas, buildings and internal drainage should be considered.



IMPORTANT NOTE: Review the SUMMARY.OUT file (for channels CHVOLUME.OUT) to determine how well the model conserves volume for a given flood simulation. If a simulation does not conserve volume, the user should determine the location of the data problem by alternately turning off components and rerunning the model.

Volume Conservation

Volume conservation is a measure of model accuracy. Failure to conserve volume is an indication of

data input errors. The inflow volume, outflow volume (including infiltration and evaporation losses) and

change in storage are summed at the end of each time step. The difference between the total inflow

volume and the outflow volume plus the storage is the volume conservation. Overall model volume

conservation is reported in the SUMMARY.OUT file and channel volume conservation is reported in

CHVOLUME.OUT file. By reviewing the difference between the two files, it is possible to tell if there are

data errors in the channel or on the floodplain. Data errors, numerical instability, inappropriate or

inconsistent simulation techniques will cause a loss of volume conservation.

It should be noted that volume conservation in any flood simulation is not exact. While some numerical

error is introduced by rounding numbers, approximations

or interpolations (such as with rating tables), volume will

generally be conserved within a fraction of a cubic meter.

The user must decide on an acceptable level of error in the

volume conservation. Most simulations are accurate for

volume conservation within a few millionths of a percent.

Generally, volume conservation within 0.001 percent or less

will be sufficiently accurate.

Possible Causes and Corrections - NOFLOC’s Elements

Channel elements that are contiguous but do not share discharge must be identified with the NOFLOC

variable at the end of the CHAN.DAT file. NOFLOC pairs must be identified near confluences, along

parallel channels, or where channels make a sharp bend. List each pair of non-sharing contiguous

channel elements only once. Review the CONFLUENCE.OUT file to make sure that you have identified all

the appropriate NOFLOCs. This file should have exactly the right number of confluence or split flow

locations. If there are more grid elements listed in this file than there are confluences or split flow

locations, more NOFLOCs are necessary. If an insufficient number of NOFLOCs pairs are assigned,

volume conservation may not be observed. It should be noted that the GDS automatically identifies and

assigns all NOFLOCs and it is only necessary to review these to make sure that only the correct number

of confluences or split flow locations are listed in CONFLUENCE.OUT.

Possible Causes and Corrections - Hydraulic structures

Each hydraulic structure (bridge, culvert, weirs, etc.) can be assigned a rating curve or table. The stage

discharge data is based on the headwater depth above the bed for either a floodplain or channel

element. Both the inflow and outflow elements must be assigned. If the hydraulic rating table or

equation is assigned incorrectly or if the data contains errors, volume conservation may not occur.

If the hydraulic structure can be isolated as the source of the volume conservation problem, check the

assignment of the inflow and outflow nodes in the HYSTRUCT.DAT file. If the structure nodes are

correct, review the rating curve or table for the lower depths. A linear interpolation for shallow depths

can result in an unreasonable high discharge and if this is the case, simply expand the table to account

for the nonlinear table or curve at depths less than 3 ft.



Possible Causes and Corrections - Outflow elements

Inappropriately assigned outflow nodes are a possible source of volume conservation error. Outflow

elements should not be doubled-up along a boundary. Each outflow element must have access to a

floodplain grid element upstream that is not an outflow element to estimate a uniform flow depth at

the boundary outflow nodes. This will be a possible cause of volume conservation error. Unless a

channel terminates on the grid system, the channel will also require an outflow node to predict the flow

off the grid system. Channel outflow nodes should also be assigned as a floodplain outflow node to

avoid overland flow to be forced into the channel to become outflow discharge off the grid system.

Numerical Stability

Numerical surging or model numerical instability are inherent in all flood routing models as a product of

having timesteps that are too large for the discharge fluxes in or out of a grid element. Numerical

stability criteria are used to the control the timestep. It is possible for volume to be conserved during a

flood simulation and still have numerical surging. Numerical surging is the result of a mismatch between

flow area, slope and roughness variables. Typically surging occurs because the modeler applies an n-

value representing the grid element or channel roughness that is too low. Low n-values can result in

maximum velocities that are too high resulting in spikes in the channel hydrographs. There are several

ways to identify numerical surging:

The maximum velocities can be reviewed in the MAXPLOT or Mapper post-processor programs;

The CHANMAX.OUT file lists channel element maximum discharges;

Every channel element hydrograph can be reviewed in the HYDROG program;

The VELTIMC.OUT (channel) or VELTIMFP.OUT (floodplain) files list maximum velocities sorted in decreasing magnitude (Figure 5).

Figure 5. VELTIMEC.OUT Example

Surging can be reduced or eliminated by adjusting the individual floodplain or channel element

roughness, slope (topography) or flow area. For floodplain flow, adjustments can be accomplished in

the GDS. For channel flow, the PROFILES program can be used to make adjustments to the cross section



data. Increasing the flow roughness will generally reduce or eliminate flow surging. Abrupt transitions in

channel cross section may be a source of numerical instability. Setting a lower limiting Froude number

for a channel reach may help to resolve a surging problem. For the floodplain, the global limiting Froude

number (FROUDL) is found in the CONT.DAT file. On occasion, the grid elevations assigned by the GDS

may not be representative of the field condition. In this case, change the grid element elevations to

make the channel or floodplain slope more uniform.

The FLO-2D model uses the Courant criteria and the dynamic wave stability criteria (WAVEMAX) to

control the magnitude of the computational timestep. For most river flooding projects, the Courant

stability criterion is usually sufficient to avoid numerical instability. This Courant criteria is hard-wired in

the model using a coefficient = 1.0 and there is nothing for the user to adjust. The dynamic wave sta-

bility criteria is an extension of the Courant criteria used and includes a slope term and specific

discharge for more complex flows:

Δtw < ζ So Δx2 / qo

where:

Δtw = computation timestep (seconds) ζ = dynamic wave stability coefficient (WAVEMAX) So = bed slope Δx = grid element width or channel length within the grid element (ft or m) qo = specific discharge

When the model timestep exceeds the WAVEMAX timestep Δtw, the model timestep is decreased. The

purpose of the WAVEMAX parameter is to provide more strict control of the timestep when analyzing

complex and rapidly varying flow in channels such as channel transitions, confluences and split flow

reaches. Lowering the WAVEMAX value decreases the timesteps. A detailed discussion of the WAVEMAX

assignment is provided in the Data Input Manual in the TOLER.DAT file description. For a floodplain,

channel or street grid element, the WAVEMAX value will increment or decrement based with the

numerical stability of the model.

How can WAVEMAX be used to speed up the FLO-2D model or be used to make the model more stable?

There are three options for applying the dynamic wave stability criteria:

1) Dynamic wave stability criteria controls the model timestep when WAVEMAX is set within the range 0.10 to 1.00 (typical value = 0.25). This makes the model run slower, but it will be more numerically stable.

2) Assign WAVEMAX as a negative number using same range of values -0.10 to -1.00 (typical value = -0.25). The floodplain Manning’s n-values will be incremented when the dynamic wave stability criteria is exceeded, but the timestep is not decreased. The runtime changes in the n-value will be written to the ROUGH.OUT file. The n-value variation occurs according to the following relationships:

i. n = n + 0.0006616 e (-10.9 n) when the limiting timestep is exceeded.

ii. n = n – 0.00005 when the limiting timestep is not exceeded.

This approach uses WAVEMAX to identify problem elements and uses increased n-values to resolve the numerical instability instead of reducing the time step.



Recommendations:

For the CVFED project, if the Qpeak/Asurf < 1.0: Set DEPTOL = 0.2 WAVEMAX = 1.0

For the CVFED project, if the 1.0 < Qpeak/Asurf < 2.0: Set DEPTOL = 0.2 WAVEMAX = 0.5

For the CVFED project, if the 2.0 < Qpeak/Asurf Set DEPTOL = 0.1 WAVEMAX = 0.25

These recommendations can be adjusted based on the project numerical stability results.

3) The dynamic wave numerical stability criteria are turned off when WAVEMAX is assigned a value of 100 or more. Assign WAVEMAX = WAVEMAX + 100. The timesteps are varied only by the change in depth (DEPTOL variable) or the Courant stability criteria.

The guidelines for applying these options are as follows:

Run the model with WAVEMAX = 0.25 and an appropriate limiting Froude number (e.g. FROUDL

= 0.9 subcritical flow on an alluvial surface). This will calibrate the model n-values for

reasonable Froude numbers. Review the maximum velocities in MAXPLOT or MAPPER to

determine the location of any unreasonable high velocities related to numerical surging and

increase the n-values of the grid elements in the vicinity.

Review the n-values in ROUGH.OUT. Make any necessary n-value adjustments in FPLAIN.RGH or

CHAN.RGH for high n-values reported in ROUGH.OUT. Also make roughness adjustments for

any observed high maximum velocities, and then replace FPLAIN.DAT with FPLAIN.RGH or

replace CHAN.RGH with CHAN.DAT.

Run the simulation and continue to replace FPLAIN.DAT with FPLAIN.RGH until ROUGH.OUT is

essentially empty. A few incremental n-values changes in ROUGH.OUT will not affect the

simulation. Adjustments can be made to WAVEMAX and FROUDL to decrease the number of

reported n-value adjustments.

Set WAVEMAX = WAVEMAX + 100 and run the model again. The model will run faster. Check the

maximum velocities for any inappropriate high velocities and make n-value adjustments. If the

model has numerical surging, set WAVEMAX = 0.25 and run the model. The model will run

slower, but should eliminate the numerical surging.

What are the results of applying different options?

You should notice an increase in model speed when selecting either option 2 or 3 above. Choosing

option 2 has the effect of improving the spatial distribution of reasonable n-values and reducing the

number of choke points associated with numerical instability. Varying n-values will help to control the

grid element discharge flux to accommodate the movement of the floodwave. Choosing option 3

essentially results in the timestep being controlled only by the Courant criteria and the model runs much

faster. The result of following this procedure will be:

A calibrated model for overland and channel roughness values which will more accurately simulate the movement of the floodwave.

The floodwave speed will be bounded by a reasonable limiting Froude number.

The model will run fast without numerical surging or oversteepening of the floodwave.



6. River Channel Component

In the Sacramento and San Joaquin River Basin Comprehensive Study (Corps, 2002), the FLO-2D model

was applied primarily for two-dimension flood distribution on the floodplains. For the Sacramento River

Basin, the report indicated that the channels were “clearly defined” and overbank flooding was

infrequent. Conversely, the channels in the San Joaquin River Basin had less conveyance capacity and

overbank flooding was more common. For this reason, the FLO-2D model simulated both the channel

and floodplain routing for selected reaches of the San Joaquin River Basin (Corps, 2002). There may be

reaches where the integrated channel and floodplain routing is appropriate. These include:

Channel return flow to the river;

Water surface elevations in the river channel that limit return flow;

Overtopping of levees from the land side;

Interior drainage connected to the river channel;

Multiple flood by-pass channels on the floodplain;

Floodplain storage that reaches equilibrium with the river flow between the levees;

Multiple locations of levee failure.

In these cases, it is necessary to have a fully integrated flood routing model to accurately track the flood

volume as it progresses downstream. This coupling will ensure that the prediction of levee breach

failures and overbank flows accurately reflect river and floodplain water surface elevations as the flood

is distributed on the floodplain and is correctly timed with other upstream and downstream levee

breaches. Since the HEC-RAS model is not capable of predicting integrated channel and floodplain

exchange (including return flow) on timestep basis, the FLO-2D model should be used for both river and

floodplain routing for these complex flooding reaches.

In a typical FLO-2D river flood application, the channel is defined by the same cross sections that are

used or would be used in a HEC-RAS model (steady or unsteady). The channel routing is performed with

a one-dimensional solution to the full dynamic wave momentum equation similar to the UNET model

(HEC-RAS unsteady). Also similar to UNET, the FLO-2D model uses vertical slices of the cross section to

generate a channel geometry rating table. Whereas the basic channel routing unit is the distance

between cross sections in the UNET model, the FLO-2D model uses a grid element to assess the channel

storage volume on a smaller scale. It has been verified that the UNET and FLO-2D models will predict

similar hydraulic results for in-channel flows.

The primarily advantage of the FLO-2D channel component over UNET, is the channel-floodplain flow

exchange which occurs on a channel element (bank element) basis. Both overbank flow and return flow

to the channel can be simulated during computational timestep. This channel-floodplain exchange

discharge is computed with the diffusive wave momentum equation (acceleration terms are ignored). In

the HEC-RAS unsteady model, the overbank flooding occurs as lateral weirs at prescribed locations with

estimated floodplain storage units. In FLO-2D, the flow between the channel and floodplain is

analogous to breathing for a more accurate simulation of the floodwave attenuation due to floodplain

storage than HEC-RAS.



Similar to the HEC-RAS unsteady model, FLO-2D can simulate levees and levee breaches, simulate

hydraulic structures and have imposed downstream stage control. FLO-2D also has a number of other

features that constitute an enhancement over the HEC-RAS channel routing. These include:

Depth variable roughness

Limiting Froude number – roughness variation

Unlimited confluences and split flow channels

In those reaches of the Sacramento and San Joaquin Rivers where the channel storage is a relatively

minor portion of the flood volume (~10% or less), the potential floodwave attenuation due to floodplain

storage will be significant. In this case, the accuracy of the flood hazard delineation will be based on the

model’s ability to compute the flow exchange between the channel and the floodplain including the

return flow to the channel over the levees from the landside. As noted by the Corps (2002), use of the

FLO-2D model is more appropriate in these instances. Channel routing using the FLO-2D model should

be applied for the conditions shown in the following figures:

Figure 6. Channel conveyance is a minor portion of the entire flood volume.



Figure 7. Overbank flooding is extensive with channel-floodplain exchange for the entire reach.

Figure 8. Multiple channels are exchanging flow with the floodplain.

Figure 9. Interior drainage channels contribute to the flow exchange with the main river channel.



Figure 10. Levee overtopping or breach failure is extensive for the entire reach.

Figure 11. Significant return flow to the channel downstream (potential overtopping from the landside of the levee).

Each of the above figures represents a complex flood condition requiring volume tracking. These cases

can be accurately simulated with the FLO-2D model. Modeling these flood cases with HEC-RAS requires

a number of subjective assumptions and decisions such as prescribing the location of the levee breach,

assigning the floodplain storage area, and assuming level pool storage with no floodwave timing. These

HEC-RAS assumptions also breakdown when it is necessary to predict return flow to the channel or have

internal drainage flood routing.

It is recommended that the FLO-2D channel component be developed in conjunction with the HEC-RAS

model in the event that it is necessary to account for the both the flood volume and timing of the

floodwave movement over the floodplain. This will enable the option of running the FLO-2D model for

the channel-floodplain exchange at a late time to accurately assess water surface elevations if return

flow to the channel downstream is required for the flood hazard mapping.



Recommendation: Use the FLO-2D model to conduct the channel-floodplain exchange flows. If necessary, calibrate the FLO-2D channel model to the HEC-RAS model for a discharge less than bankfull.

The following procedure is recommended for channel flow simulation for the CVFED Program:

Where a HEC-RAS model for the main river channel exists or is developed, the FLO-2D channel

should also be prepared from the HEC-RAS data.

Calibrate FLO-2D in-channel flows to the HEC-RAS model by spatially varying the n-values with

the limiting Froude number, depth-variable roughness and stability criteria.

Run the FLO-2D model with the complete channel-floodplain flow exchange as well as the levee

overtopping or levee breach to define the area of inundation.

Failure to have the option of using the FLO-2D to assess the channel-floodplain interaction may result in

having to do this task during the mapping phase of the project. For example, a consulting firm was

tasked with predicting the area of inundation in Sacramento for the American River flooding through a

prescribed levee breach. The Corps provided the breach hydrograph predicted with HEC-RAS at the

selected location and the floodplain flood routing was predicted with the FLO-2D model. Flooding for a

given return period flood was predicted to reach the levee near the confluence of the American and

Sacramento Rivers. The predicted water surface on the landside of the levee exceeded the levee crest.

Without the combined channel and floodplain flooding being routing in the FLO-2D, there was no

opportunity to assess the potential return flow to the channel. As a result, two FLO-2D model scenarios

were considered: 1) No return flow to the river; and 2) All the return flow over levee into the river was

contained in the river (river was a sink). The difference in the areas of inundation was approximately

70,000 acres (no return flow) compared to 40,000 acres (return enters a sink) indicating that the

combined channel and floodplain model was indispensible. The actual maximum area of inundation the

flood was between the two extremes, but determining the flooded area in this amounted to little more

than a guess. The FLO-2D model in this case

should have simulated the channel and

floodplain exchange.

The method for developing a FLO-2D main

channel component requires using the GDS to set up the channel data file, interpolate the cross sections

to the grid elements, and calibrate the water surface elevation. The procedure would be to use the GDS

manual, follow the Channel Lessons and review the FLO-2D Pocket guide to complete the following

steps:

1. Import the HEC-RAS cross sections to in the GDS.

2. Edit the cross sections to the top of the bank stations in the PROFILES program.

3. Outline the channel location in the GDS.

4. Establish the channel element data (channel length, n-value, cross section number, Figure 12).

5. Interpolate the cross section shape and slope to the channel elements

6. Assign reach segment limiting Froude number and depth variable roughness coefficient



Figure 12. GDS channel editor dialog box.

Several guidelines in using the FLO-2D model channel component are highlighted. Typically, HEC-RAS

average n-values for 1-D long reaches between cross sections are relatively low. For example, the n-

values assigned for the Corps 2002 Comprehensive Study were 0.035 for the channel. FLO-2D channel

reach n-value variation can be significant to account for cross section variation, flow in bends, flow

acceleration and deceleration, and bed forms. A reasonable range of CVFED channel n-values is from

0.035 to 0.06. The n-value variation can be determined by assigning a reasonable limiting Froude

number as previously discussed. Other guidelines include:

The channel should be continuous and organized from upstream to downstream in CHAN.DAT.

At a channel confluence, the downstream main channel grid element must be lower in elevation than the confluence element.

Eliminate channel elements with a channel length (XLEN) less than 50% of the grid element side width. Connect the channel elements across the diagonal instead.

Create a positive bed slope at channel inflow and outflow nodes.

Refer to the Pocket Guide for additional hints and guidelines, comments on channel surging and

numerical stability and a discussion on troubleshooting.



7. Hydraulic Structures

Hydraulic structures may include bridges, culverts, weirs, spillways, outlet works, lateral weirs, or any

flood conveyance facility that controls or affects the water surface elevation. These structures could

control flow conveyance in the channel, from the floodplain to the channel or from one portion of the

floodplain to another. The hydraulic structure can be combined with roadway, levees or other

embankments or with flood detention release facilities. Modeling hydraulics structures in the CVFED

area will assist DWR to identify potential additional structural and nonstructural facilities that could be

incorporated into the State Plan of Flood Control.

In FLO-2D hydraulic structures are simulated by specifying either discharge rating curves or rating tables.

The hydraulic structure in the model can be any hydraulic facility whose discharge can be specified by a

rating curve or table. Backwater effects upstream of bridges or culverts as well as blockage of a culvert

or overtopping of a bridge or any deviation from upstream normal depth are simulated through the

application of the full dynamic wave momentum equation. (Figure 13). Hydraulic structures can be

simulated in channels or on floodplains by appropriately identifying the structure location in the

HYDRSTRUCT.DAT file. A hydraulic structure can control the discharge between channel or floodplain

grid elements that are not contiguous and may be separated by several grid elements. For example, a

culvert under an interstate highway may span several grid elements.

A hydraulic structure rating curve specifies discharge as a function of the headwater depth h:

Q = a hb

where: (a) is a regression coefficient and (b) is a regression exponent. More than one power regression

relationship may be used for a hydraulic structure by specifying the maximum depth for which the

relationship is valid. For example, one depth relationship can represent culvert inlet control and a

second relationship can be used for the outlet control. In the case of bridge flow, blockage can

simulated with a second regression that has a zero coefficient for the height of the bridge low chord.



IMPORTANT NOTE:

HEC-RAS cross sections for establishing the bridge rating curves or tables should be limited to the bridge abutments. The rating curve or table should only reflect the discharge through bridge in the channel (as free surface or pressure flow) or over the top of the bridge within the vertical extension of the channel banks at the abutments. FLO-2D will compute the floodplain flow with the two-dimensional component.

Figure 13. Bridge Plugged with Sediment and Debris

By specifying a hydraulic structure rating table, the model interpolates between the headwater depth

and discharge pairs of data to calculate the discharge. A typical rating curve will start with zero depth

and zero discharge and can increase in non-uniform increments to a discharge higher than the maximum

anticipated flood discharge. The rating table is typical more accurate than the regression relationship

(rating curve) if the regression is nonlinear on a log-log plot of the depth and discharge. Flow blockages

by debris can be simulated by setting the discharge equal to zero corresponding to a prescribed depth.

This blockage option may be useful in simulating a worst case mudflow scenario where alluvial fan

bridges or culverts become plugged with debris. Each bridge over an alluvial fan channel can be

simulated for blockage forcing all the discharge to flow over the fan surface.

Rating curves or tables for all structures are created outside the FLO-2D model environment. The rating

curve or table data can be entered in the GUI or GDS, but must be generated through the user’s

knowledge of the type of structure. For example, culvert rating table data can be generated from

culvert tables or programs. Bridge rating table data can be developed using HEC-RAS and 4 cross

sections; two upstream and two downstream of the bridge. Spillway or weir rating curve data can be

determined from hydraulic reference manuals.

Mismatching the rating curve or table with the upstream channel conditions is one possible cause of

numerical surging in the FLO-2D model. For example, if normal depth in the channel for a discharge of

1,000 cfs (30 cms) is 3 ft (1 m) and the rating table at the same depth has a corresponding discharge of

only 500 cfs (15 cms) or conversely 2,000 cfs (60 cms), the resulting backwater effect or acceleration of

the flow would not only significantly slow the model down, but may cause numerical surging if the slope

and n-value did not correspond to such a severe change in flow velocity. Realistic backwater effects or

channel contraction can be simulated with rating curves or tables without numerical surging.

As with other attributes, the model should initially be run without adding any hydraulic structures.

Before adding the hydraulic structures, the baseline model should conserve volume, and be free of any



“sticky” grid elements that may be slowing the model down. This baseline model can be used to

estimate the area of inundation and determine what hydraulic structure may impact the flood

conveyance or distribution.



HINT: The levee is created and edited graphically in the GDS program. When the levee is situ-ated diagonally across a series of grid elements, use one side of one grid element and the opposite of the next grid element so that there is an equal amount of the floodplain on each side of the levee.

8. Levees, Levee Breach, Fragility Curves and Safe Storage

FLO-2D Levee Introduction

Levees represent a major flood control feature in the project basin bordering most of the major

channels within the valley. The levees both confine the extent of floodplain inundation and force more

of the floodwave downstream. Historically unconfined flooding along the river attenuated the

floodwave as it moved downstream. Limited flood storage between levees now forces more of the

flood volume downstream. As levees were developed, levee overtopping and breaching occurred

frequently during the large floods. A map showing the extent of levees in the Sacramento River system

was presented in the Corps 2002 Comprehensive Study. Along the Sacramento River levees range from

20 to 30 feet high with 3 to 5 feet of freeboard and are set back from the riverbank. In the San Joaquin

Basin, levees are only 6 to 8 ft and were designed for snowmelt flooding instead of the winter storm

runoff for the Sacramento River. Flood bypasses in the San Joaquin system also have levees. Riprap

bank protection is discontinuous along the rivers as noted in Comprehensive Study.

Accurate levee modeling is critical to the CVFED Program. A major portion of the flood volume could be

confined to the river corridor between levees or stored on the floodplain with levee breaches. The flood

storage on the interior floodplain could return to the channel or be limited by internal levees from

reaching outlying areas of the floodplain. There are numerous levees in the CVFED Program area that

extend into the interior floodplain. These levees may protect urban areas or structures and may or may

not be accredited. River levees have to be considered as flood retention structures in both directions

(riverside and landside) and the analyses needs to include the water surface elevations on both sides of

the river channel for potential overtopping of the levee or levee breaches. Levee overtopping and

breaches could return flow to the river from the floodplain resulting in the need for an integrated

channel and floodplain model.

Any interior floodplain embankment can simulated with the levee component. Embankments may

include roadway, railroad, berms, canal spoils piles and levees. These embankments may control the

distribution of floodwaters across the floodplain including agricultural areas. Embankments that are not

considered to be levees are addressed in a later section of this report.

The FLO-2D levee component confines flow on the floodplain surface by blocking one of the eight flow

directions. Levees are designated at the grid element boundaries (shown by the red lines in Figure 14).

If a levee runs through the center of a grid element, the model

levee position is represented by one or more of the eight grid

element boundaries. Levees often follow the boundaries along a

series of consecutive elements. A levee crest elevation can be

assigned for each of the eight flow directions in a given grid

element.



Figure 14. Levee flow directions are depicted in red; the left bank of the river is shown in blue and the right bank of the river is shown in magenta.

The levee data requirements include the crest elevation and the blocked flow direction. The levee

length within the grid element is based on the length of the side of the octagon that may be modified by

the width reduction value WRF. This length should be close to the levee length with the grid element if

the levee is overtopped and the overtopping discharge is computed. It should be noted that the model

assumes that the levee does not eliminate any storage in the grid element containing the levee. If the

levee is very large, then the area reduction values ARF can be assigned to reduce the available flood

storage in the grid element to account for the levee.

Levee Profile

A profile of the levee elevation can be viewed in comparison to the floodplain grid element elevation

using GDS Levee Profile tool. Figure 15 displays a levee profile comparing the levee crest elevation with

the grid element ground elevation. This tool identifies locations where the levee crest elevation or grid

element elevation need to be adjusted along the levee.

Levee Overtopping

The model will predict levee overtopping without failure. When the flow depth exceeds the levee

height, the discharge over the levee is computed as weir flow with a 2.85 coefficient. Weir flow occurs

until the tailwater depth is 85% of the headwater depth. At higher flows, the water is exchanged across

the levees using the difference in water surface elevation across the grid boundary. The levee output

file, LEVOVERTOP.OUT, reports the discharge hydrograph overtopping the levee element. The discharge

is combined for all the levee directions that are being overtopped. Levee overtopping will not cause

levee failure unless the failure or breach option is invoked.



Figure 15. Levee Profile

FLO-2D spatially identifies the loss of freeboard so that the user does not have to search and compare

water surface elevations with the levee crest elevation to determine the remaining freeboard. The

LEVEEDIFIC.OUT file lists the levee elements with loss of freeboard during the flood event. Five levels of

freeboard deficit are reported:

0 = freeboard > 3 ft (0.9 m)

1 = 2 ft (0.6 m) < freeboard < 3 ft (0.9 m)

2 = 1 ft (0.3 m) < freeboard < 2 ft (0.6 m)

3 = freeboard < 1 ft (0.3 m)

4 = levee is overtopped by flow.

The levee freeboard deficit can be displayed graphically in MAXPLOT and MAPPER as shown below.

Figure 16. Levee Freeboard Deficit Plot in Mapper



Levee Breach Failure

Since 1900 there have been over 160 levee breaches in the Sacramento Basin including 17 since 1990 (As noted in a Poster Prepared by URS and DWR, 2007). The list of potential levee failure modes is extensive:

Overtopping leading to a breach channel; Underseepage resulting in internal erosion; Slope stability failure; Levee structural collapse due water force or high pore water pressure; Piping; Wave attack; Animal burrows, cracking, or other structure defects; Earthquake soil liquefaction.

Historically, most of the Central Valley levee failures are initiated by slope instability or piping including

underseepage. These failures occur rapidly whereas levee overtopping failures tend to progress more

slowly. For the CVFED Program, the levee breach can initiate from flood storage on the landside as well

as the riverside of the levee and flow through the breach can occur in either direction.

FLO-2D can both locate and simulate levee breach failures (Figure 17). There are two failure modes; one

is a simple uniform rate of breach expansion and the other predicts the breach erosion. For both cases,

the breach timestep is controlled by the flood routing model. FLO-2D computes the discharge through

the breach in either direction, the change in upstream storage, the tailwater and backwater effects, and

the downstream flood routing. Each failure option generates a series of output files to assist the user in

analyzing the response to the dam or levee breach. LEVEE.OUT contains the levee elements that failed.

Failure width, failure elevation, discharge from the levee breach and the time of failure occurrence are

reported. Additional output includes the time of breach or overtopping, peak discharge through the

breach, and breach parameters as a function of time. Output files that define the breach flood hazard

include the time-to-flow-depth output files that report the time to the maximum flow depth, the time to

one foot flow depth and time to two foot flow depth which are useful for delineating evacuation routes.

Figure 17. Example of levee breach urban flooding



For the simplified levee failure method, the breach can enlarge both vertically or horizontally. Rates of

breach expansion in feet or meters per hour can be specified. A final levee base elevation that is higher

than the floodplain elevation can also be prescribed. The levee failure can occur for the entire grid

element width for a given flow direction. Discharge through the breach is based on the breach width

and the difference in water surface elevations on the two sides of the levee. Levee failure can also be

initiated by a prescribed specified water surface elevation for a given duration. The flow through the

levee breach is computed as broadcrested weir flow using a weir coefficient of 2.85.

The breach erosion component was added to the FLO-2D model to combine the river-floodplain

exchange and unconfined flooding components with a realistic assessment of a levee or dam failure.

The BREACH code developed Fread in 1988 that is publically available from the National Weather

Service was the basis for the breach component (NWS Breach model). The code extensively was

revised, corrected and enhanced. The basic mechanisms of levee breach failure are overtopping, piping

and slope stability failure by sliding, slumping or collapse. In the FLO-2D model, a dam or levee breach

can fail as follows:

Overtopping and development of a breach channel;

Piping failure;

Piping failure and roof collapse and development of a breach channel;

Breach channel enlargement through side slope slumping;

Breach enlargement by wedge collapse.

The user has the option to specify the breach element and breach elevation or to assign global

parameters and the model will determine the breach location. During a flood simulation, the flood

water ponds against the levee until the water surface elevation is either higher than the levee crest

(overtopping) or exceeds a prescribed breach or pipe elevation. The global breach elevation can be

specified as a depth below the crest elevation. When the water surface elevation exceeds the breach

elevation for a user prescribed duration, piping is initiated. If the pipe roof collapses, then the discharge

is computed through the resultant breach channel. A breach channel is also simulated if the levee is

overtopped. A description of the breach enlargement routine follows.

If the user specifies a breach elevation and duration, then piping will be initiated when the upstream

water surface exceeds the pipe bottom elevation for a cumulative duration. The breach discharge is

computed as weir flow with a user specified weir coefficient. The discharge is then used to compute

velocity and depth in a rectangular pipe channel. With the pipe hydraulics, the sediment transport

capacity is computed using an assigned sediment transport equation. The pipe walls, bed and roof are

assumed to uniformly erode (Figure 18). When the pipe height is larger than the material remaining in

the embankment above, the roof of the pipe collapses and channel flow through the breach ensues.

The channel discharge is also calculated by the weir equation and similar to the pipe failure the eroded

sediment is distributed on the walls and bed of the rectangular channel (see Figure 19). As the channel

width and depth increases, the slope stability is checked and if the slope stability criteria is exceeded,

the sides of the channel are assumed to slump and the rectangular channel transitions to a trapezoidal



channel. The breach continues to widen until the top width of the channel equals or exceeds the

octagon width of the grid element. At this point the breach is assumed to be stable.

Figure 18. Pipe breach failure

Figure 19. Overtopping and channel breach erosion

Breach enlargement is also possible by sudden collapse of the levee. The collapse is represented by a

wedge shaped mass of embankment material. This collapse is caused by the water force on the

upstream side of the wedge exceeding the friction forces of shear and cohesion that resist sliding.

When the breach collapse occurs, it is assumed that the breach enlargement ceases until all the wedge

material is transported downstream.

Flow through the breach is accounted by the volume conservation routines in the FLO-2D model that

tracks the storage along with the discharge in and out of every grid element according to the FLO-2D

timesteps. The breach component also assesses the sediment volume conservation and the water

discharge through the breach is bulked by the sediment eroded during the breach failure. Routing water

through the breach continues until the water surface elevation no longer exceeds the bottom breach

elevations, the upstream and downstream water surfaces at equilibrated or until all the ponded water is

gone.



Recommendation: Use high n-values for the breach pipe or

channel in the range from 0.10 to 0.25.

One of the reasons for selecting the NSW BREACH model is that the program had sufficient geotechnical

detail to mathematically represent the physical process of dam breach failure. The breach model

includes the following features:

The embankment can have an impervious core and a non-cohesive shell with different materials;

Embankment material properties include sediment size, specific weight, cohesive strength, internal friction angle, porosity and Manning’s n-value;

Breach channel initiation through piping failure;

Enlargement of the breach through sudden structural collapse;

Riprap material or grass on the downstream face;

Sediment transport for different size sediment in the embankment core or shell.

The list of levee breach parameters in the FLO-2D model is extensive, but only few parameters control the rate of breach erosion such as sediment size, cohesive strength, slope and Manning’s n-value.

Of these parameters, those which are the most difficult to evaluate are the cohesive strength, internal

friction angle and the breach pipe or channel roughness n-value. The flow through breach will be highly

turbulent. The breach channel will not be a uniform, prismatic flume-like channel. There will be blocks

of sediment obstructing the flow and

frequent wall or roof caving resulting in

a high roughness n-value.

The cohesive strength is highly variable depending on the size fraction, percent clay and water content

and pore water pressure. As general guide, the following range of values can be considered:

Table 7: Cohesive Strength and Friction Angle1

Soil Type Cohesive Strength

(lb/ft2)

Friction Angle

(degrees)

Gravelly or Poorly Graded Sand

20-100 38-46

Silty Sand 250-400 34-36

Silty-Clayey Sand 190-400 30-36

Clayey Sand 100-360 30-34

Silty-Clayey Fine Sand 100-250 28-34

Sandy Clay 100-360 30-34

Silty Clay 230-310 28-32

1Bureau of Reclamation, 1974. Design of Small Dams, Washington, D.C.



It should be noted that cohesive strength for consolidated, undrained clayey soils in varying degrees of

stiffness can range as high as 5,000 lb/ft2. In the absence of site data, a sensitivity analysis can be

performed using a range of cohesive strength from 100 to 300 lb/ft2 and a friction angle of 32 to 38o.

There are several important assumptions have been hardwired into the breach model. These are:

Initial breach width to depth ratio (BRATIO) – if the assigned breach width to depth ratio is 0., then BRATIO = 2.

The initial pipe width is assumed to be 0.5 ft (0.15 m).

The minimum and maximum Manning’s n-values permitted for the breach flow resistance are 0.02 and 0.25, respectively.

The downstream pipe outlet at the toe of the dam or levee is the grid element floodplain elevation plus 1 ft (0.3 m).

Breach discharge is computed if the upstream water surface elevation exceeds the upstream breach pipe or channel bottom elevation plus the tolerance value (TOL ~ 0.1 ft or 0.3 m).

If the specified initial breach elevation in the BREACHDATA.DAT file is less than 10.0 ft (3.0 m), then the initial piping breach elevation is assumed to be the dam or levee crest elevation minus the assigned breach elevation (Initial Breach Elevation = Levee Crest – BRBOTTOMEL).

Using Fragility Curves to Locate and Initial Levee Breaches

Fragility curves are used in a number of different scientific fields of risk and uncertainty failure analysis

ranging from seismic geotechnical structure failure to pipe network damage. While the levee fragility

analysis is not currently within the scope of the CVFED Program, it could be used in the future to refine

the flood hazard maps. An algorithm using the Corps’ levee fragility curves has been coded in the FLO-

2D model to automatically predict breach failure anywhere in the levee system. This FLO-2D levee

breach component with fragility curves represents a link between a geotechnical risk model and the

prediction of the water surface elevations. It links levee geotechnical performance and flood routing

hydraulics. It also implicitly defines failure probability for hazard mapping.

Levee core and shell material may vary significantly in levee systems especially where older levee

reaches do not conform to existing construction standards. As a result, levee failure is difficult to predict

in both time and space. Levees often fail before the flood waters reach the levee crest elevation due to

geotechnical flaws that initiate piping. Often a piping failure may expand to a complete breach in a

relatively short period of time while the flood is still rising. The Corps uses a levee fragility curve as a

failure risk model that incorporates the probability of levee geotechnical failure as function of flood

water surface elevation (Figure 20).

The levee fragility curves specify the relationship between the probability of levee failure and the water

surface elevation (or likely failure stage) below the levee crest. For example, the likely failure point (LFP)

can be selected corresponding to a 50% failure probability and this would establish the water surface

elevation at which the levee failure would be initiated. The fragility curves are based on a geotechnical

investigation of the existing levee system involving construction methods, levee soil composition,

foundation conditions, and other factors. At the present time, existing Central Valley fragility curves are



based primarily on engineering judgment and represent a qualitative approach to evaluating levee

integrity. Future fragility curves should be based on levee geotechnical investigations including field

inspection, in situ (borings), and laboratory testing. The extent of in-situ testing and laboratory testing

(cohesive strength, compaction and other tests) will determine the reliability of the fragility curves.

Figure 20. Example of Sacramento River Basin levee fragility curves

The FLO-2D model was uniquely suited to utilizing the levee fragility curves. It has a levee breach

erosion component that can initiate breach failure anywhere in the levee system based on water surface

elevation. When the water surface reaches a prescribed distance below the crest for a specified

duration, pipe failure will initiate. Through the physical process of sediment transport, the pipe breach

will expand to a channel breach and collapse as the levee erodes. When these fragility curves are

applied with the FLO-2D flood routing model using the levee breach erosion component, levee failure

location and time of occurrence can be predicted.

The levee fragility curves shown in Figure 20 are read directly by the FLO-2D model in a discretized

rating table format. In the data presented below, the first column is the curve reference name (two

fragility curves are listed); the second column is the probability of failure (x-axis Figure 20); and third

column is the distance below the levee crest in feet or meters (y-axis Figure 20):

FS1 0.03 6.0 FS1 0.15 3.5 FS1 0.50 2.5 FS1 0.85 1.0 FS1 0.95 0.0 FS2 0.03 9.0 FS2 0.15 5.5 FS2 0.50 4.0 FS2 0.85 2.0 FS2 0.98 0.0



Recommendation: A relationship between the inundation duration (above the LFP) versus the LFP should be considered for the levee geotechnical investigation.

Once the levee fragility curve data is enter, the user has the option of assigning a global or individual

fragility curve to levee grid element. The following levee fragility curve data assigned by the user is:

Global Levee Data: Line ID, Fragility Curve ID, Probability of Failure (e.g. 0.50)

Individual Levee Data: Line ID, Grid Element, Fragility Curve ID, Probability of Failure

Example: C FS3 0.5 (global data) (individual data)

P 3450 FS1 0.5 P 3558 FS1 0.9 P 3559 FS2 0.7 P 3669 FS3 0.5 P 3670 FS4 0.5 P 3782 FC1 0.3 P 3783 FS1 0.5

By assigning fragility curves to the individual levee elements, the user is essentially specifying the

potential beach. In addition, the user can assign a duration of inundation prior to breaching. This

saturation time is a key parameter in the modeling levee breach. The cumulative time of the water

surface above the fragility curve elevation (elevation below the levee crest) is tracked and the levee

breach is not initiated until the duration is exceeded. The duration might range from 0.5 to 24 hours or

more. For example, if the selected duration is 0.5

hours, the time that the water surface exceeds the

LFP elevation must be greater than 0.5 hours (not

continuous, but cumulative) for the breach to start.

When the levee fragility curves are applied to a levee system, the results show that the levee breach can

occur anywhere in the system at varying times depending of the selection of the fragility curve and the

prescribed probability of failure. After piping initiates, the pipe erosion expands to a breach channel

when the pipe roof collapses and escalates to a full breach when breach the channel side slopes

collapse. The breach expands in the FLO-2D model until the rate of sediment transport from the breach

decreases. Figure 21 shows a FLO-2D simulation of levee breach progressing at two locations at the

same time based on variable assignment of the levee fragility curves. In this figure, the levee

embankment extends from the red FLO-2D grid system boundary to the urban area in the image.

Figure 21. Levee breach with Variable Fragility Curves



Benefits of Applying the Levee Fragility Curves to Flood Hazard Mapping

For the CVFED Program, the AEC teams are required to estimate of the risk of levee failure. This is levee

failure could occur from river to floodplain or floodplain to river. The FLO-2D fragility curve method

combines geotechnical levee probability and flood routing for a spatial assessment of levee failure

corresponding with the floodwave progression. This represents a significant step in accurate hazard

mapping. The procedure for using the levee fragility curves in the FLO-2D model to locate breaches and

map the flood hazards is as follows:

1. Develop the FLO-2D with the channel and levee components.

2. Run the various return period flood simulations (e.g. 2-yr, 5-yr, 100-yr etc.).

3. Generate the levee fragility curves from the geotechnical data being collected for the CVFED Program t for the given project river reach.

4. Enter the fragility curve data for the levees in the BREACH.DAT file.

5. Enter the selected global or individual levee grid element fragility curve assignments.

6. Select different LFP such as 10%, 50% and 90% and run the FLO-2D model for the return period flood simulations.

The series of FLO-2D simulations for the various return periods with the failure probability assignments

(e.g. LFP = 10%, 50% and 90%) will identify the area of inundation associated with the geotechnical

confidence of the levee. If there are 6 return period flood events, there will be a total of 18 flood

simulations with the FLO-2D model. Clearly, the area of inundation will be much greater if the likely

failure point LFP was assumed to be 10% than 50%. For some of the more frequent flood events (2-yr or

5-yr), it is possible that there were would no levee breach. For the less frequent flood events, a breach

of the levee upstream might result in significant floodwave attenuation due to the floodplain storage

reducing the potential for a downstream levee breach in a reach of levee that was in poor condition.

Through volume conservation in the FLO-2D model, the flooding through a levee breach in one location

may reduce the potential for a complete levee breach or even breach initiation elsewhere in the levee

system where the levee may be weak. The different return period floods and different LFP scenarios

would result in significant different areas of mapped flood hazard. The area of predicted area

inundation could then be evaluated for flood damages.

Central Valley communities need to rehabilitate levees to provide both FEMA-level protection and

protection against the 200-year return period flood. The 200-year flood protection is required by State

Bill 5 to allow urban areas to continue to develop in the floodplain. The proposed Interim Levee Design

Criteria Modified Corps Approach stipulates that the upstream and downstream levees would be

modeled to the 1955/57 Corps design profiles and would not be allowed to overtop or breach in the

hydraulic models. By using the fragility curve methodology to achieve this level of protection:

DWR will have hydraulic model with a risk and uncertainty analysis to predict the water surface for the ‘Interim Levee Design Criteria for Urban and Urbanizing Area State-Federal Project Levees’.

The Modified Corps Levee Design Water Surface Elevation (DWSE which may be higher and require more freeboard), would be minimized for the appropriate upstream flooding conditions. The floodwave attenuation associated with a fragility curve 90 or 95% LFP provided by the FLO-



2D model will provide more confidence for the 90 or 95% assurance water surface elevation used in the design. It may eliminate the need to consider adding height to DWSE.

The flood hazard mapping with the 90 or 95% LFP can be used to establish the flood damage cost to justify levee restoration.

Safe Storage Criteria

The concept of unaccredited levee removal is conservative from a flood water surface perspective and it

can significant impact flood storage volume and floodwave movement through the river system on large

floodplain. It can also create significant problems for floodplain communities where the flood potential

may be non-existent. If the flood volume controls the area of inundation, the relationship between the

volume in the channel and the volume on the floodplain is critical to accurate hazard assessment. The

concept that the levees will melt away when the water contacts the levee is also conservative. The

volume of water represented by a water depth of two or three feet between setback levees can be

substantial in some locations in the Sacramento and San Joaquin River systems.

All embankments, even levees in the poorest condition, will store some water safely. This safe storage

behind levees may be only a foot or two of water above the toe of a levee (Figure 22). The concept of

safe storage is applicable to both the river and landside of the levee. On a large floodplain area, this

could represent a large volume of water that would not contribute to downstream flooding. The actual

safe storage water surface elevation on the levee would have to be determined by geotechnical

investigation of the levee, but could be related to the LFP for the levee fragility curves. For water

surface above the LFP, the levee breach would initiate. For water surface below the LFP, the levee

would be assumed to have safe storage. Levees in reasonably good condition, could store water to the

50% LFP, whereas levees in bad condition may have a 10% LFP (perhaps only a foot above the levee toe).

Figure 22. Safe Storage for Levee

Developing levee safe storage criteria would require input from both agencies and consultants, but at a

minimum, a levee safe storage of 1 or 2 ft might be considered which may have a significant impact on

the flood hazard mapping. Combined with the levee fragility curves, safe storage may result in more

accurate hazard mapping and reduce levels of flood uncertainty. The concept of safe storage can also

be used on alluvial fan embankments. On Tortolita alluvial fan near Tucson, Arizona the safe storage

provided by the spoils pile created by the excavation of the Central Arizona Canal crossing the fan stored

the entire flood hydrograph from some small watersheds. This resulted in FEMA approved DFIRM maps

that eliminated some urban areas from the flood hazard.



9. Floodplain Urban Details

Buildings and Flow Obstructions

One of the unique features the FLO-2D model is its ability to simulate flow through urban areas with

flow obstructions and loss of flood storage. Area reduction factors (ARFs) and width reduction factors

(WRFs) are coefficients that modify the individual grid element surface area storage and flow width

between elements respectively. ARFs can be used to reduce the flood volume storage on grid elements

due to buildings or topography. WRFs can be assigned to any of the eight flow directions in a grid

element and can partially or completely obstruct flow paths simulating floodwalls, buildings or berms

(Figures 23 and 24). The ARFs (less than or equal to 1.0) are specified as a percentage of the total grid

element surface area. It is possible to specify individual grid elements that are totally blocked (ARF = 1.0)

from receiving any flow. Width reduction factors (less than or equal to 1.0) are specified as a percentage

of the grid element flow width (one side of the octagon = 0.41412 x grid element side). A wall might

obstruct 40% of the flow width and a building could cover 75% of the same grid element.

Figure 23. Area (yellow) and Width Reduction Factors (as lines within the yellow grid elements)

Figure 24. ARF value grid elements outlined in yellow (zoomed view).



Note: Completely blocking a grid element flow direction (WRF = 1.0) creates a obstruction that cannot be overtopped. If there is a potential for the obstruction to be overtopped it should be modeled using a levee not a WRF value.

Recommendation: Only esti-mate the reduction values within plus or minus 10%.

Initially the model should be run without any ARF or WRF values to identify where the flooding may

occur. This initial simulation will determine the approximate area of inundation for assigning the ARFs

and WRFs. ARFs and WRFs can be assigned to grid elements either by grid element, polygon, or painting

a series of elements. The user should then identify the various obstructions to the primary flow paths.

For large urban areas, significant flow obstruction and storage loss detail may have to be added to the

model to accurately define the flood distribution. For example, subdivisions, industrial parks, and

shopping centers can significantly impact the flood mapping.

It is usually sufficient to estimate the area or width reduction on a

map by visual inspection. A detailed measurement is not

necessary because each building will have a relatively minor

impact on the flood area of inundation. Visualizing the area or

width reduction factors can be facilitated by inspecting grid system

over an imported background image in the GDS to locate the

buildings and obstructions. Only four width reduction factors

need to be specified for the eight possible flow directions. The

other four flow directions are assigned automatically by grid

element correlation. Two of the specified width reduction factors

are for flow across the diagonals.

The key issues regarding buildings and obstructions are:

When is it necessary to simulate storage loss and flow obstruction;

Which type of building should be simulated;

How much details should be used in modeling buildings?

Following a review of the area of inundation with the buildings, it can be discerned which urban areas

are important to simulate loss of storage and flow obstruction. Rural areas with barns or single

residential structures can be ignored as their effects on the storage loss are negligible. It is not necessary

to simulate buildings in areas where there is no flooding or the flooding is minor or just ponded shallow

flow. Not every building in a flooded urban area needs to be modeled. Buildings that are essentially

shells such as warehouses or storage units can be ignored (see the table below). Dense urban areas with

significant flooding such as a downtown city blocks should be assigned ARF values. Flood prone building

construction that should be considered for loss of flood storage would include both the foundation

system and the superstructure that are linked together for structural integrity. Consideration should

also be given to buildings that can effectively avoid inundation such as elevated buildings, flood proof

structures or large concrete structures. Irregular topography (mounds), dense vegetation (trees and

agriculture), subdivision walls (with and without pressure relief openings), and landscaped features

should all be reviewed when adding area and width reduction flood detail.



Table 8: Area/Width Reduction Factor

Building Type Range of Potential Loss

of Flood Storage Manufactured Buildings: Trailers, Temporary Housing, Sheds 0.0 - 0.1

Wood Frame, Open Foundations with Basements/Crawl Space 0.1 - 0.5

Slab on Grade Building 0.2 - 0.5

Wood Frame Buildings on Solid Foundation Walls 0.3 - 0.7

Masonry (Brick or Concrete) Residential Buildings 0.4 - 0.8

Commercial Warehouses, Storage Units at Grade 0.1 - 0.4

Large Commercial Masonry Building at Grade 0.6 - 1.0

Masonry Buildings with Raised First Floor 1.0

Elevated Buildings on Fill 1.0

Floodproof Structures 0.9 - 1.0

By initially running the model without a lot of urban detail, the area of maximum inundation can be

outlined. Within this outline, the level of urban detail can be established based on flow patterns and

development characteristics. Urban neighborhoods or subdivisions that are flooded but are not

considered in the floodplain path can be model for flood storage loss as a unit. In this case, loss of flood

storage is more important than obstruction of the floodwave path.

Street Flow

Streets can be important conveyance features that distribute flow to the outer portions of the floodplain

increasing the area of inundation. Street flow is simulated as flow in shallow rectangular channels with

a curb height. The channel routing algorithm is used to compute the street discharge. The data input

file (STREET.DAT) is organized by street. The user specifies a street name followed by the number of grid

elements that constitute a given section of street. A given grid element may contain one or more streets

and the streets may intersect.

The street segments within a grid element are assumed to emanate from the center of the element and

extend to the element boundary in the eight flow directions (Figure 25). For example, an east-west

street across a grid element would be assigned two street segments. Each segment has a length of one-

half the grid element side or diagonal. The flow direction, street width, curb height and roughness are

specified for each street segment within the grid element and can be modified by the GDS program.

Street and overland flow discharge exchange is computed in the channel-floodplain flow exchange

subroutine. When the flow exceeds the curb height, the discharge to floodplain portion of the grid

element is computed. Return flow to the streets is also simulated.



Figure 25. Street are shown as green lines in the GDS.

Similar to assigning other urban attributes such as ARF and WRF values, the project model should

initially be run without streets to determine the baseline area of inundation. The impact of adding the

streets to the project model can then be assessed. The baseline model should conserve volume and be

analyzed for “sticky” grid elements so that the simulation runs quickly. Streets that would distribute the

flow to the outer reaches of the model and expand the area of inundation should then be added. There

are a number of streets parallel to the channel in the Figure 25 that are not simulated because there

impact on redistributing the flooding from the channel is limited.

Table 9: Criteria for Simulating Street Flow Street Type Criterion

Unpaved or Paved Streets without Curbs (at grade) Do not model with street component. Reduce grid element n-values containing roadway.

Cul-de-Sacs (paved or unpaved) Generally cul-de-sacs and dead end streets can be ignored.

Streets with Curbs Perpendicular to the Primary

Flow Path If streets distribute flow to another portion of the grid system, then those streets should be simulated. Exercise judgment.

Neighborhood, Subdivision or Short Paved Street

with Curbs

May impact local flooding, but may not redistribute large flood volumes. Generally only a few limited subdivision streets are modeled in a project.

Main Avenues or Streets with Curbs Should be modeled for street conveyance if they are located in the area of inundation.

Elevated Streets and Highways Should not be modeled as street conveyance. Should be simulated for flow blockage as WRFs, levees, or elevated grid elevations.

Elevated streets, highways and agricultural roads should be considered for their potential flow

obstruction and perhaps modeled as embankments as described in the following section.



Embankments

The Central Valley floodplain outside of the levees has extensive embankment and drainage features.

Often the irrigation and drainage features (canals and ditches) have corresponding spoils piles or

constructed berms associated with roadways along canals. For that reason, the discussion of interior

floodplain embankments and drainage are combined in this section.

Embankments may include roadway, railroad, drainage canal spoils piles or other berms, and interior

levees. These embankments can control the distribution of floodwaters across the floodplain. In Figure

8 (p. 30) and 9 (p. 31), there are a number of embankments (roadways, levees and drainage canal spoils

piles) that are exposed above the flood water surface that should be included in the model for this reach

of river. To establish the potential impact on flood distribution by embankments, consideration should

be given to crest height, crest materials (pavement), embankment materials and design, and structural

integrity. Embankments that are unaccredited may have to be removed for FEMA mapping. If FEMA

flood insurance study maps are a product of this flood analysis, unaccredited levees and embankments

will probably not be simulated in the FLO-2D model.

Throughout the Central Valley there are canals, flood by-pass channels, irrigation drainage ditches or

conveyance control structures that will may affect the flood movement downstream and to other

portions of the floodplain. The DWR and the CVFED AEC teams have the responsibility to evaluate if

flood bypasses and floodways would reduce flood stage. The conveyance discharge may constitute

return flow to the river or redistribute the floodwater from the river to the interior portions of the

floodplain. The following figure illustrates the complexity of embankments, drainage canals and flood

by-pass channels.

Figure 26. A number of canal and roadway features that may control the floodwave distribution.

Roadway Embankments

Drains



When is it necessary to consider interior floodplain embankments? The following procedure is suggested:

1. Run the model for the two return period simulations that bracket the full range of potential flooding.

2. Identify those embankments within a given area of floodplain.

3. Initially ignore embankments less than 1 ft.

4. Start with the embankments that appear to have the greatest impact on the potential flood distributions. Run one or more of the selected flood hydrographs and note the difference with and without the embankments.

5. Based on the results discern whether additional similar embankments (in terms of crest elevation and location) are required to accurately simulate the flooding.

Possible embankment impacts on flooding include:

Changes in the maximum area of inundation (mapping). Flooding may be eliminated in some floodplain areas.

Increase in maximum and final flow depths in storage areas behind embankments.

Ponded storage may reduce flow velocities through a certain area.

Delayed flooding in certain areas initially protected by embankments.

Embankments including roadways, railroad grades, berms, spoils piles and even fencing can be modeled

with the FLO-2D levee component. The embankment (levee) data assignment for the eight potential

flow directions in any given grid element was described in a previous section of this report. Often

embankments in the interior drainage have a variable crest elevation which can be checked for accuracy

with the GDS levee profile tool (Figure 15, p. 38). The embankment crest elevation can be defined using

one of three methods:

1. GDS DTM elevation query

2. Assumed height above floodplain

3. Crest elevation survey

If detailed and accurate elevation data is available, the DTM points can be imported into the model and

queried where they coincide with the embankment using the GDS levee express editor. If the feature

has a relatively uniform height, the embankment elevation can be assumed as a specific height above

the floodplain elevation. If the elevation data is not detailed or accurate and the feature is not uniform,

a survey of the embankment crest elevation can be performed at intervals that are multiple of the grid

element size.

An embankment such as highway fill may be modeled by simply raising the grid element elevation. As

long as the extra loss of floodplain storage because the grid element is larger than the embankment, this

is a simple method to simulate embankments that are not anticipated to fail. If interior floodplain

embankment consists of spoils piles or poorly constructed berms, failure of the embankment during

flooding will add another level of complexity to the floodplain mapping. For large floodplain storage

areas, this may be necessary.



Interior Drainage

Guidelines for determining which drainage canal or facility should be included in the flood model are

presented. The primary focus is the flood volume moving across the interior flood floodplain and the

facility conveyance capacity. If the entire floodplain is inundated by 3 ft of flow depth, the impact of

small conveyance channels on the flood distribution will be negligible. If a given floodplain area has

shallow flooding, a drainageway and berm may capture and redirect the flow to back the river system.

When is it necessary to consider interior irrigation and drainage conveyance features? The following

procedure is suggested:

1. Run the model for the two simulations that bracket the full range of potential flooding (e.g. 2-year and 250-year flood hydrographs).

2. Locate drainage features in the area of inundation that have significant conveyance capacity.

3. Initially ignore canals or ditches that have a capacity of less than 100 cfs or whatever discharge capacity that might be considered negligible for the flow crossing the floodplain.

4. Start with one or more of the larger canals that appear to have the greatest impact on the potential flood distributions and run one or more of the selected flood hydrographs. Note the area of inundation difference with and without the canal.

5. Based on the results determine whether additional similar canals (in terms of conveyance capacity) are required to more accurately simulate the flooding.

Possible conveyance feature impacts on flooding include:

Changes in the maximum area of inundation (mapping). Flooding may be eliminated in some floodplain areas with increased return flow to the river system.

Decrease in the maximum and final flow depths in storage areas with conveyance features.

More flood volume in the river system downstream.

Canals and drainageways are simulated with the FLO-2D channel component. Most irrigation and

drainage canals and laterals have either a rectangular or trapezoidal cross section. These are relatively

straight forward to enter into the CHAN.DAT file using the GDS program. The channel course is graphical

outlined in the GDS, and the channel geometry data (width, depth and side slope) is entered in the

Window's dialog box. The channel length within each grid element is computed automatically. To

finalize the canal data set, the user should complete the following steps:

1. Add spatially variable n-values;

2. Confirm the canal reach channel length;

3. Check the canal profile slope and make adjustments.

To add detail to the interior irrigation and drainage network, conveyance hydraulic control can be added

to the model.

The hydraulic control in the canals, drainage ways and flood by-pass channels including gates, weirs,

spillways, pumps and other structures should be modeled if their operation results in the control the

water surface elevation or restricts the discharge to the river. If modeling the conveyance channel was

important to the distribution of the flooding on the interior floodplain, then it is likely that the hydraulic



structure will also have some flood impact. The hydraulic structures are simulated with a rating curve or

table (headwater depth vs. discharge) that is developed outside the FLO-2D model. The structures may

control return flow to the river or be blocked with debris resulting in local flooding during the

recessional limb of the flood hydrograph.



NOTE: A sediment load bulking factor (BF) is given by:

BF = 1./(1. - Cv)

where Cv = concentration by volume.

Recommendation: Use 5% maximum concentration by volume for a bulking factor of 1.053.

10. Sediment Bulking

Infrequent large flood events in major rivers carry sediment loads with concentrations ranging from 2%

to 20% by volume. A flood with a 20% concentration by volume represents an increase (bulking factor)

in the flood volume and peak discharge of 25%. A 25% increase in

flood volume may affect the flood area of inundation and impact

the operation of flood control structures and bridges. Limited

sediment records were examined to determine the potential range

of sediment loading (Figure 27).

Figure 27. Discharge and Suspended Sediment Load for the Sacramento River at Freeport, California

For discharges of up to 100,000 cfs in the Sacramento River, measured suspended sediment loads were

less than 1.0 percent concentration by volume. Higher sediment concentrations can be expected for

higher floods. The low measured suspended load may be the result of upstream storage and the

sediment load may be supply limited. Sediment

bulking may not significantly impact the flood

volumes. In the absence of data to justify a lower

or higher bulking factor, a conservative estimate of

5% concentration by volume is suggested.



11. Reviewing FLO-2D Results

Project review key questions are whether the flood simulation was accurately performed and whether

the flood maps represent a reasonable depiction of the flood of the flood hazard. The FLO-2D model

has a number of output files to help the user view the results. Floodplain, channel and, street hydraulics

are written to file. Hydraulic output data include water surface elevation, flow depth and velocities in

the eight flow directions. Several options are available to format the output files as either temporally or

spatially varied results. Discharge for specified output intervals (hydrographs) are written to various

files. Most of the critical FLO-2D model results can be reviewed graphically using the Mapper, Maxplot

or Mapper.NET programs which will plot area of inundation, maximum flow depth and velocity, and

other results over background aerial images.

The output files can be used to discern model performance. Before reviewing the project output files

the user should determine if the model is running fast enough and if the results are satisfactory. Table

10 lists the most important output data files when reviewing the model results. A mass conservation

summary table comparing the inflow, outflow and storage in the system is presented in the

SUMMARY.OUT file and at the end of the BASE.OUT file. Some output files are created by simply

initiating the various flow components (e.g. STREET.OUT is created when street flow is simulated).

Complete descriptions of all the output files are presented in the Data Input Manual.

Table 10: Output files and Uses

File File Use SUMMARY.OUT Verifies volume conservation. Percentage of volume loss should be less

than .001%

CHVOLUME.OUT Reports on the disposition of the channel volume.

TIME.OUT

Lists those grid elements that are frequently exceeding stability criteria.

The user should determine which grid elements are frequently exceeding

stability criteria. The user should review the top 2 to 5 grid elements in

the listing. If any grid element has time step decrements greater by an

order of magnitude than the following element listed in this file; grid

attributes such as topography, slope, or roughness should be adjusted.

ROUGH.OUT

The runtime Manning’s n-value changes when the maximum Froude

number is exceeded are listed in this file. Use the file to refine the n-

value spatial variability.

STREET.RGH

FPLAIN.RGH

CHAN.RGH

These files contain the changes in street, floodplain, and channel element

Manning's n-values when the maximum Froude number is exceeded.

After verifying that the n-value runtime changes are reasonable, these

files can be renamed STREET.DAT, FPLAIN.DAT, and CHAN.DAT. This will

overwrite all n values in the *.DAT files.

VELTIMEFP.OUT Lists maximum floodplain flow velocity in decreasing order of magnitude.

Any floodplain velocities greater than 6-8 fps should be reviewed.

VELTIMEC.OUT Lists the maximum channel flow velocity in decreasing order of

magnitude. Channel velocities greater than 8 fps for unlined channels

and greater than about 15 fps for lined channels should be reviewed.



There are several output files that identify modeling problems. The most important one is volume

conservation file (SUMMARY.OUT). The FLO-2D results should be reviewed for volume conservation,

surging, and timestep decrements.

Volume Conservation

Volume conservation is a key factor in the accuracy of any flood routing model. The SUMMARY.OUT file

will display the time when the volume conservation error began to appear during the simulation.

Typically a volume conservation error greater 0.001 percent is an indication that the model could be

improved. The file CHVOLUME.OUT will indicate if the volume conservation error occurred in the

channel routing instead of the overland flow component. By switching 'off' model components and the

volume conservation problem can be isolated. Some volume conservation problems may be eliminated

by slowing the model down (decreasing the timesteps) using the stability criteria. Volume conservation

problems are an indication of errors in the data files.

Numerical Surging

Numerical surging is the result of a mismatch between flow area, slope and roughness and can occur in

a simulation that conserved volume. It can cause an over-steepening of the floodwave identified by

numerous spikes in the output hydrographs (Figure 28). Surging can also be identified by excessively

high velocities in the VELTIMC.OUT (channel) or VELTIMFP.OUT (floodplain) files. Surging can be reduced

or eliminated by adjusting the stability criteria to reduce the timestep size. If the decreasing the

timesteps fails to eliminate the surging, then individual floodplain or channel element topography (cross

section), slope or roughness should be adjusted. Setting a lower limiting Froude number for a channel

reach may also help to eliminate the surging. For channel flow, the PROFILES program can be used to

make cross section adjustments. Increasing the flow roughness will reduce or eliminate most of the

numerical surging. Abrupt flow area transition between contiguous channel elements should be

avoided.

'Sticky' Grid Elements

‘Sticky’ grid elements (floodplain or channel elements) are those that are causing the most timestep

decreases forcing the model to run slowly. The TIME.OUT file lists the top twenty floodplain, channel or

street elements that caused the model to slow down. The file also lists which stability criteria is being

exceeded resulting in the timestep decreases. Adjustments can be made in the stability criteria to more

equitably distribute the timestep decreases. The model is designed to advance and decrement the

computational timesteps, so there have to be grid elements listed in the TIME.OUT file. If one or two

grid elements have significantly more timestep decreases (by an order of magnitude) than the other ele-

ments listed in the file, the attributes of the ‘sticky’ grid elements such as topography, slope or

roughness should be adjusted. The goal is to make the model run as fast as possible while still avoiding

numerical surging.



Figure 28. Numerical Channel Surging Example.



12. Guidelines for Flood Hazard Tools

Flood Inundation Maps

The FLO-2D modeling system has a number of flood analysis and mapping tools to enhance the flood hazard delineation and risk assessment. A brief overview of these tools is provided so that the AEC team is aware some of the tools for CVFED Program complex modeling issues. The FLO-2D Mapper and Mapper_NET post-processor programs can generate a diverse array of high resolution graphical plots and maps including:

Ground surface elevation Maximum water surface elevation Maximum depth (area of inundation) Maximum velocity Final depth Final velocity Specific Energy Impact Pressure Static Pressure Time-to-Peak Discharge (dam and levee break) Time-to-One Foot (dam and levee break) Time-to-Two Foot (dam and levee break) Temporally variable depth and velocity (flood animations) Flow depth over DTM points Flood hazard maps Shape files DFIRM’s

Maps can be generated for floodplain, channel and street flow and combined channel and floodplain

maximum depths. The maps can be displayed as either grid element plots, line contour maps, and

shaded contour maps. Shape files for importing results to GIS are automatically generated for most of

the Mapper plots. General guidelines to consider when developing flood maps are:

The flood hazard map resolution is only as good as that of the topographic data base. If the topographic contour map resolution is plus or minus one ft, then the flood contour map cannot be more accurate than plus or minus one ft.

Use background aerial images to enhance the maps;

Contour line width and shaded contour splash over floodplain features;

Interpolating and plotting flow depths over the DTM points to improve map resolution;

Using map resolution controls such as prescribed contour intervals, deleting the lowest contour, and contour smoothing can improve the map appearance;

Mapper_NET has advanced mapping features to improve map resolution including breaklines.

All CADD and GIS programs have to accommodate topographic data base resolution and contour splash

(e.g. flood contours that cover levees, buildings, bluffs or other features) when creating maps. Mapper

and Mapper_NET have options to address these mapping issues including maximum flood depth

computation over DTM points, shape file generation and breaklines and other mapping controls.



Maximum flood depth computation over DTM points. By importing the DTM ground elevation points into

Mapper and subtracting the ground elevation from the predicted maximum grid element water surface

elevation, flow depths are computed for every DTM point. Creating flood contours maps from the

interpolated DTM point flow depths will greatly enhance the flood map resolution. A file

(FLO2DGIS.OUT) of these DTM point flow depths can also be created for importing to GIS.

Shape file generation. Mapper and Mapper_NET automatically create shape files in the project folder

whenever a flood contour map is generated. These shape files can be imported to GIS or CADD

programs for further editing.

Breaklines and other map controls. Mapper_NET is integrated with ArcGIS Runtime Engine Controls. It

has a number of contour enhancements including a vast array of color combinations. There are

breakline options to limit contours from crossing topographic features.

Creating high resolution flood maps may require several rounds of review and adjustment of your

mapping controls.

To support the AEC Team cost estimates for flood damage and hazard mapping, the Mapper program

can generate shaded contour maps with background aerial images. This will enable the flood inundation

to be reviewed with respect to urban development, river channels, levees and embankments, streets,

hydraulic structures, internal drainage or irrigation water conveyance, and by-pass channels and flood

retention facilities. The flood contour maps may have to be adjusted and edited to reflect more

accurate flooding around these features. In the Figure 29 below the shaded contours are drawn over the

levee.

Figure 29. Urban Flooding with Channel.

Flood Damage Assessment

Levee



The important urban features in Figure 29 include the channel and levees, two main streets or avenues

running east to west, several north-south streets, and large buildings that reduce the flood storage.

Once the final inundation maps with these features have been completed, reviewed and edited, it may

be necessary to assign flood damage cost. This can be accomplished automatically in the Mapper if the

damage cost tables and building shape files have been created.

A true measure of flood risk is the actual damage cost resulting from the exposure of vulnerable

structures to a given return period flood. This flood risk is represented by linking the flood hazard map

and a damage cost assessment. A significant effort in the field is required to evaluate the potential cost

of inundation as function of flood depth. Similar to the Corps of Engineers Flood Damage Assessment

(FDA) program, a tool for estimating the total flood and individual structure damage cost has been

developed in the FLO-2D Mapper post-processor program. It can be used to estimate the cost of flood

damage for any type of structure or land use (e.g. agricultural crops). To apply this method in the

Mapper program, the following data must be available:

A polygon shape file where each polygon represents a structure or field.

A table file containing damage cost data as function of flood depth for each building type in the polygon shape file. The file will have a code that will correspond to a shape file polygon and cost data for damage per foot of flow depth.

The Mapper damage assessment table (Figure 30) is comparable to that used by the Corps FDA program

and can easily accommodate other depth-damage curves similar to the Federal Insurance

Administration’s (FIA) credibility weighted curves used in the HAZUS model. For each building identified

by a polygon ID, the table provides damage cost for up to 10 user specified depths (D1-D10). The

Mapper program will compute and interpolate the flood damage according the portion of the building

area covered by each FLO-2D grid element flow depth. It will also create a GIS polygon shape file of the

damage cost for each flooded building (Figure 31 and Figure 32).

Figure 30. Mapper Damage Assessment Table with a Cost Per Foot of Depth for Each Building Type.



Figure 31. Mapper Displayed Color Coded Assignment of Damage Costs to Individual Buildings.

Figure 32. Interpolated Damage Inundation Cost for Individual Structures Computed by Mapper.

To calculate the flood damage in Mapper, import the building polygon shape and cost table as shown

above and click on the appropriate commands buttons. Mapper will determine building (or crop)

polygon intersections with the FLO-2D grid elements and compute the total damage $$ using a weighted

average interpolation. The final product for the damage assessment module is the total flood damage

cost:



Typically the Corps damage cost assessment tables are established for conventional clear water

floodplain inundation. For the CVFED Program, the damage cost tables may have to reflect alluvial fan

flooding, mud or debris damages, or damages associated with high velocities, high impact pressure or

long duration inundation. The damage tables also have to be appropriate for the different types of

structures that may be designed to withstand debris flows including higher first floors, masonry walls,

and lack of windows and doors on the upfan side. The structure details represented in the damage

tables will increase the reliability of the risk mapping.

For the CVFED Program the primarily tasks that need to be completed for the flood damage assessment

are digitizing the structure shape files and creating the building damage tables. Each building within the

flood inundation areas would have to be digitized. It may be possible to burn the outline of buildings

from the pixel tones on digital aerial photographs into shape files. Methods to automate this digitizing

procedure should be considered. The field effort perform by the Corps in generating flood damage

assessment tables is extensive and time-consuming. This inventory of building flood damage as a

function of depth is required by the AEC teams to assess flood damage, so the automated damage

interpolation routine in Mapper will expedite the accurate computation of the total flood damage.

Flood Hazard Mapping

In the United States, most of the flood mapping is prepared to establish flood insurance rates. In many

communities this mapping is based on very inaccurate one-dimensional, single discharge analyses that

ignores the spatially variable floodwave attenuation associated with floodplain storage. The digital flood

insurance rate maps (DFRIMS) are also a very poor tool to communicate the flood hazard to the public.

Countries worldwide are adopting a mapping standard that accurately conveys the flood hazard to the

public when linked to a two-dimensional flood routing model. This mapping method is described below.

Flood hazard at a specific location is a function of both flood intensity and probability (flood frequency).

Flood intensity is determined by the flow depth and velocity. Flood probability is inversely related to

flood magnitude; i.e. large flood events occur less frequently. Flood hazard is defined as a discrete

combined function of the event intensity (severity of the event) and return period (frequency). This

approach follows European standards that delineate three flood hazard zones (Figure 33).

High

Medium

Low

High Medium Low

High Hazard (Red)

Medium Hazard (Orange)

Low Hazard (Yellow)

Inte

nsi

ty

Probability of Excedence

Figure 33. Flood Hazard Levels Based on Flood Frequency and Intensity.



These map colors translate into specific potential hazard areas as shown in Table 11. To define an

event’s intensity, most methods use a combination of flow depths and velocities. One method (Austrian)

uses a total energy defined as h + v2/2g, where h is the flow depth, v is the velocity and g is the

gravitational acceleration. The U.S. Bureau of Reclamation identifies hazard as a combination of depth

and velocity (Figure 34) with different relationships for structures and cars. Other methods define the

intensity in terms of a combination of h and the product of h and v.

Table 11: Flood Hazard Definition

Hazard

Level

Map

Color Description

High Red Persons are in danger both inside and outside their houses. Structures are in danger of being destroyed.

Medium Orange Persons are in danger outside their houses. Buildings may suffer damage and

possible destruction depending on construction characteristics.

Low Yellow Danger to persons is low or non-existent. Buildings may suffer little damages,

but flooding or sedimentation may affect structure interiors.

Figure 34. Flood Hazard for Adults (Bureau of Reclamation).

In the FLO-2D MAPPER program a distinction is made between water flooding and mudflows. Flood

intensities are defined in terms of the maximum water depth and the product of the maximum velocity

multiplied by the maximum depth. For the CVFED Program, it may necessary to reach consensus on the

hazard level thresholds in the following tables. These flow depth and velocities x depth thresholds can

be varied by the user but should be consistently applied for the entire CVFED Program area. For water

flooding, the flood intensities could be defined by the values in Table 12. Mudflows are more destructive

than water floods, thus the mudflow intensity criteria are more conservative (Table 13).

Table 12. Definition of Water Flood Intensity

Flood Intensity

Maximum depth h (m)

Product of maximum depth h times maximum

velocity v (m2/s) High h > 1.5 m OR v h > 1.5 m

2/s

Medium 0.5 m < h < 1.5 m OR 0.5 m2/s < v h < 1.5 m

2/s

Low 0.1 m < h < 0.5 m AND 0.1 m2/s < v h < 0.5 m

2/s



Recommendation: For the CVFED Program, the three return period floods are 100-year, 200-year events and 500-year return period floods.

Table 13. Definition of Mud or Debris Flow

Intensity

Flood Intensity

Maximum depth h (m)

Product of maximum depth h times maximum

velocity v (m2/s) High h > 1.0 m OR v h > 1.0 m

2/s

Medium 0.2 m < h < 1.0 m AND 0.2 m2/s < v h < 1.0 m

2/s

Low 0.2 m< h < 1.0 m v h < 0.2 m2/s

The resulting flood hazard maps reflect the probability of occurrence of a water or mudflow event for

three selected returns periods. This requires a FLO-2D simulation of each of the three flood frequency

events. The FLO-2D model will predict the maximum depths and velocities for each return period flood.

For each grid element, the event intensity for a given return period flood determines the hazard based

on the above criteria. An interpolated shaded color contour map based on the grid elements hydraulics

depicts the low, medium and high flood

hazards in Figure 35. A comparison of the

flood hazard with actual flooding from a

recent storm is shown in Figure 36.

Figure 35. A typical flood hazard map delineating high hazard (red), medium hazard (orange) and low hazard (yellow).

This mapping method represents a true measure of the flood hazard and has been effectively used in

other countries to communicate the hazard to the public. The advantages of this method is that is not

necessary to interpret the base flood elevations or flood contours from a FEMA flood insurance rate

with the building elevation to assess a potential flood hazard. In addition, the DFIRM flood elevation

alone is not necessarily an indication of flood hazard. The Mapper flood hazard maps provide a clear

depiction for the public whether a given building or neighborhood is within a high flood hazard area.

The floodplain manager can decisively plan, regulate and zone based on this flood hazard map and can

easily communicate the planning process and hazard to the community. The three sets of maps (area of

inundation shaded contours, flood damage assessment and flood hazard) developed with the FLO-2D

Mapper program and supporting FLO-2D modeling results provide the floodplain manager with all the

tools necessary to regulate alluvial fan development through zoning, mitigation or accurate flood

insurance rates based on actual damage cost risk.



Figure 36. Alluvial fan flood hazard compared with an actual rainfall flood event.

DFIRM Mapping

DFIRM maps can be prepared through the FLO-2D Mapper program using the add-on DFIRM tool developed by Anderson Consulting Engineers of Fort Collins, Colorado for the Colorado Water Conservation Board. This tool was embedded in Mapper program to facilitate the DFIRM mapping directly from FLO-2D results. The DFIRM tool has the commands to post process the FLO-2D results, perform QA/QC, create the panels and collars, annotate, and export the map (Figure 37). The FEMA guidelines for generating the DFIRM maps can be applied.

Figure 37. Typical DFIRM Panel



CVFED Review Procedures

Some general guidelines are presented for DWR and the Corps of Engineers to evaluate the accuracy

and map resolution of the AEC project team FLO-2D submittals. The following information should be

provided in the submittal.

Table14: Guidelines for the Review of Project Submittals

Topic Guideline

Project Purpose Statement of the project goal and objections, project location, type of flood analysis project (alluvial fan or river overbank flooding; channel or overland flow).

Review of DTM Data Base

Does the DTM data cover the entire project area? Was supplement topographic data used? Was the resolution of the DTM data? Was the DTM data suspect in certain areas and what was the data density? Were there corresponding geo-referenced aerial images?

Review of Hydrologic Data Base

The flood inflow hydrograph(s) and/or the rainfall controls the area of inundation. Was there consensus on the inflow hydrology?

Model Calibration Was the data base sufficient for the model calibration? Was the model calibrated to both water surface elevation and discharge hydrographs

Model Results

Did the model conserve volume? Did the model include a channel and was volume conserved for the channel (CHVOLUME.OUT)? What urban or floodplain details were simulated in the model? Was the channel flow modeled? Streets, levees, hydraulic structures, interior drainage canals – were any important flood features left out?

Mapping Results

Review the maximum flow depths in Mapper. Were there any significant depressions? If so, why? Are the depressions represented in the DTM data? Review the maximum velocities in Mapper and VELTIMEC.OUT and VELTIMEFP.OUT. Are the maximum velocities unreasonable? Are there only a few grid elements with unreasonable high maximum velocities or are there many elements with high velocities? Does the area of inundation contact the grid element boundary without outflow? If so, either the grid system should be expanded or more outflow nodes should be added. Were the DTM points imported in Mapper to improve the mapping resolution.

Model Error Checks

Check the ERROR.CHK file for any data input warnings or errors? Review the SUMMARY.OUT file for both volume conservation and model runtime. If the model had an extremely long simulation time, then the review the TIME.OUT file and determine which grid elements were slowing the model down and why? Is the model running slow because of only one or two grid elements listed in TIME.OUT?

Model Details

Were Manning’s n-values spatially variable on the floodplain and for the channels? Were the n-values adjusted during runtime using the limiting Froude number or numerical stability criteria (ROUGH.OUT)? Did the user just use the default n-values which would result in a poor simulation? Were the limiting Froude numbers reasonable (FROUDC in CHAN.DAT for the channels and FROUDL in CONT.DAT for the floodplain)?

Numerical Surging Review the channel or cross section discharge hydrographs in the HYDROG program for spikes indicating numerical surging. The CHANMAX.OUT file and the VELTIMEC.OUT file will also indicate discharge surging and high channel velocities.

Model Features

What model features were simulated and how did they affect the results? Review the hydraulic structures, streets, outflow discharge or stage controls, levees (embankments and berms), buildings (ARF and WRF factors), sediment transport, rainfall, infiltration, mudflow, and groundwater.

Documents

CVFED FLO 2D Project Guidelines