FLO‐2DSWMMStormDrainModelGuidelines

FLO-2D Pro Model

Prepared by FLO-2D Software, Inc.

Reviewed by the Flood Control District of Maricopa, County, Arizona

January 2014

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FLO‐2DSWMMStormDrainModelGuidelines

TableofcontentsOverview ............................................................................................................................................................. 1

Storm Drain Model Approach ............................................................................................................................. 1

The type 4 inlet is discussed in the next section. ................................................................................................. 4

Storm Drain Model Controls ............................................................................................................................... 5

SWMM Development Guidelines ....................................................................................................................... 8

EPA SWMM Model Installation ....................................................................................................................... 16

Reviewing the Storm Drain Results Using the SWMM GUI and the GDS ...................................................... 16

FLO-2D SWMM Model Simulation Guidelines ............................................................................................... 20

Example Project: SWMM – FLO-2D Integration, Phoenix, Arizona ............................................................. 25

ListofFiguresFigure 1. GDS SWMMFLO.DAT Data File Dialog Box for Entering the Storm Drain Inlet Geometry Data .. 4 Figure 2. GDS Dialog Box for Entering the Storm Drain Rating Table Type 4 Inlet Data ............................... 7 Figure 3. Initiate the SWMM GUI .................................................................................................................... 8 Figure 4. SWMM Model for the Example Project ............................................................................................. 9 Figure 5. SWMM GUI New Project Tab ........................................................................................................... 9 Figure 6. Adding Junctions to a Project ............................................................................................................. 9 Figure 7. Adding Component Data to a Junction ............................................................................................. 10 Figure 8. Add a Subcatchment Area ................................................................................................................. 10 Figure 9. Add Subcatchment Data Dialog Box ................................................................................................ 11 Figure 10. Join the Subcatchment and Junction ............................................................................................... 11 Figure 11. Adding Conduits to Connect the Junctions ..................................................................................... 12 Figure 12. Adding Conduits Data to the Dialog Box ....................................................................................... 12 Figure 13. Adding a Gage ................................................................................................................................ 13 Figure 14. GDS Storm Drain Dialog Box Command ...................................................................................... 13 Figure 15. Example Storm Drain System Displayed in the GDS (showing inlets, outlets and conduits) ........ 14 Figure 16. Outfall Nodes Dialog displayed in the GDS is read from the SWMM.INP file ............................. 14 Figure 17. Create SWMMOUTF.DAT File ..................................................................................................... 15 Figure 18. GDS Menu of Storm Drain Display Options .................................................................................. 18 Figure 19. GDS Display of Storm Drain Node Inflow Discharge .................................................................... 18 Figure 20. GDS Display of Storm Drain Inflow and Pressure Flow Hydrograph ............................................ 18 Figure 21. GDS Display of Water Surface Head on the Storm Drain Inlet and Outlet .................................... 19 Figure 22. GDS Display of the Energy Grade Line on a Storm Drain Conduit ............................................... 19 Figure 23. Case Study Project Area .................................................................................................................. 25 Figure 24. SWMM Model Storm Drain System for the Case Study ................................................................. 25 Figure 25. Case Study FLO-2D Grid System Showing Buildings, Walls and Storm Drains ............................ 26

ListofTablesTable 1. Required Storm Drain Inlet Data in the SWMMFLO.DAT file ........................................................... 3 Table 2. Description of the FLO-2D – SWMM Conditions ................................................................................ 6

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Overview

The FLO-2D Model has been integrated with the Environmental Protection Agency (EPA) Storm Water Management Model Version 5.0.022 (SWMM) to simulate the exchange of surface water flow with a storm drain system. The two models will run simultaneously with FLO-2D being the host model. FLO-2D will calculate all hydrologic and hydraulic surface water flood routing while SWMM will only solve the conduit hydraulics and flow routing in a given storm drain network. The model integration was accomplished by revising the source codes to allow the models to share data on a timestep basis controlled by the FLO-2D model. The FLO-2D model will compute the storm drain inflow discharge based on the predicted grid element headwater depth and on the inlet geometry. This discharge will then be exchanged with the SWMM model to compute the storm drain system pipe discharge and the potential return flow to the surface through downstream manholes, outlets and storm drains.

StormDrainModelApproach

The model integration is set up so that the FLO-2D model computes all the surface water hydrology and hydraulics including the rainfall runoff, infiltration, and flood routing in channels, streets or unconfined overland flow. Consequently, the EPA SWMM hydrology and surface hydraulic components are not included in the integrated FLO-2D - SWMM interface. When the SWMM storm drain is developed, the user will input storm drain inlet/outlet location data, pipe geometry, and outfall data in the EPA SWMM graphical user interface (GUI). The SWMM model inlets/outlets will exchange water between the storm drain system and the surface water. Inlets and outlets (conduit discharge into a detention basin) will function identically based on pressure head in the storm pipe system compared to water surface elevation. Water can flow in either direction based on the pressure head differential if the dynamic wave routing is applied. The system can also use looped and dendretic network systems when asuming dynamic wave routing. Outfalls will discharge the storm drain water out of the SWMM model system. Once the storm drain network is established with the SWMM GUI, the user will assign the storm drain inlet geometry in a FLO-2D Grid Developer System (GDS) dialog box.

The model approach is to first develop the FLO-2D grid system by importing the digital terrain model (DTM) points, selecting a grid size, outlining the computational domain, assigning the grid element elevations and importing an aerial image. Other features of the surface water flooding such as inflow-outflow nodes, buildings, levees and others can then be added either before or after the storm drain system model is created. The SWMM GUI (activated from the GDS) can be used to assign the storm drain inflow and outflow locations. The GDS has several new functions to integrate FLO-2D and SWMM models:

Activation of the SWMM storm drain GUI;

Reads the data from the SWMM.INP file to assign the inlet location to the FLO-2D grid system.

Displays a dialog box to enter the storm drain inlet geometry data.

Displays the inlets and outlets and the piping network connections.

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In addition, the GDS has been expanded to graphically display the SWMM results including inflow and return flow hydrographs and the energy grade line of the drain system. FLO-2D SWMM results can also be displayed in the SWMM model GUI.

Some of the SWMM model data and functions have been modified to enable the flow exchange with the FLO-2D model:

Gages will work only as a switch that allows each sub-catchment area in the SWMM model to receive runoff information from FLO-2D and calculate storm drain inflow discharge. No additional data input is required for this modification. The gage name is optional.

Subcatchment areas will represent the surface area of the inlet grid element instead of the contributing watershed surface. For the subcatchment the required data input is:

‐ Name (Optional) ‐ Rain gage name ‐ Inlet or junction connected to the subcatchment ‐ Estimated area of the FLO-2D inlet grid element (sq. ft. or m)for reference only

Junctions can function as inlets (must be connected to a subcatchment) or as a connection between pipes. The junction will not receive inflow if it serves as a pipe connection. User must assign data for both cases. The required input data is invert elevation and maximum depth. Users can also input optional names and initial depth if needed.

Conduits will route the flow to the downstream node. Slope is calculated internally based on inlet and outlet node elevation data. Required input data is: geometry, length and roughness.

Simulation data includes date, timestep and routing control.

An inlet can be assigned to a floodplain, channel or street grid cell in FLO-2D.

An outfall can discharge to a floodplain, channel or street grid cell.

The folder path to SWMM Project (first line in the SWMMFLO.dat ) was removed.

Comparison between FLO-2D water surface elevations and the SWMM heads requires both models to have the same datum. Floodplain elevation (FPE) on grid cells is now compared/modified to RIM elevation.

When the SWMM GUI is called from the GDS, the storm drain system is created. When the GUI is closed, the SWMM data file is saved as “SWMM.INP”. The SWMMFLO.DAT file is then automatically generated by GDS using the SWMM.INP data to locate the inlet position and transfer it to the FLO-2D grid system. In the SWMMFLO.DAT file there are four storm drain inlet options:

1. Curb Opening Inlet at Grade (Type 1) 2. Curb Opening Inlet Depressed or Sag (Type 2) 3. Grate (Gutter) Inlet (at Grade or Depressed - Type 3) 4. Unique or unusual inlet geometry represented by a rating table. (Type 4)

The storm drain inlet data requirements are:

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Type 1 - Curb Opening Inlet at Grade.

Weir coefficient: 2.85 - 3.30 (suggested 3.00 English, 1.6 metric) Curb Opening Length Curb Opening Height

Type 2 - Curb Opening Inlet with Sag.

Weir coefficient: 2.30 (1.25 metric) Curb Opening Length Curb Opening Height Curb Opening Sag Width

Type 3 - Grate (Gutter) Inlet with/without Sag.

Weir coefficient: 2.85 - 3.30 (suggested 3.00 English, 1.6 metric) Grate Perimeter (not including curb side) Grate open area Grate sag height (zero for at grade)

Note: Orifice flow coefficient = 0.67 (hardwired) for all cases.

Type 4 - Variable area inlet not represented by Types 1-3 defined by a stage-discharge rating table.

Stage (depth) above inlet (ft. or m) Discharge (cfs) corresponding to a assigned stage (assigned in pairs of stage-discharge data with the first pair being 0. 0.

Table 1. Required Storm Drain Inlet Data in the SWMMFLO.DAT file

Node Type Length Width(sag) Height WeirCoeffX 1 X 0 X 2.85-3.20 X 2 X X X 2.3 X 3 X (wetted perimeter) X (Clear Area) X (sag) 2.85-3.20 X 4 - - stage discharge

where X represents a variable number to be entered by the user.

The following data is automatically generated from the SWMM.INP file:  

D    23    for i=1, n  storm drain inlets, FLO‐2D grid element number D  119 D    154 

... D is line character ID.

The user will then select the storm drain geometry dialog box in the GDS Tools pull down menu to assign the storm drain inlet geometry. The GDS SWMMFLO.DAT file dialog box is shown below:

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Figure 1. GDS SWMMFLO.DAT Data File Dialog Box for Entering the Storm Drain Inlet Geometry Data

The SWMMFLO.DAT file format is space delimited as follows:

Line ID Node Type Length Width Height Coeff Flapgate D 14292 2 13. 1.0 0.4167 2.3 0 D 14481 2 13. 1.0 0.4167 2.3 0 D 13785 2 20. 1.0 0.4167 2.3 0 D 14156 2 20. 1.0 0.4167 2.3 0 D 14156 3 11. 7.0 0.5 3.0 0 D 14156 3 86.5 10.75 0.5 3.0 0 D 14344 2 13. 1.0 0.4167 2.3 0 D 14537 2 20. 1.0 0.4167 2.3 0 D 13266 2 23. 1.0 0.4167 2.3 0

The type 4 inlet is discussed in the next section.   

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StormDrainModelControls

During a flood storm drain simulation, FLO-2D initializes all the integrated variables and flow conditions as follows:

Flow Condition Model Attributes

FLO-2D water surface elevation is greater than SWMM pressure head

Inflow discharge is computed by FLO-2D and exchanged with SWMM. An inlet can be assigned to a floodplain, channel or street grid cell. Only one (1) inlet can be assigned to one (1) FLO-2D grid cell. FLO-2D will use the corresponding water depth (floodplain, channel or street) as well as the inlet characteristics to calculate the inflow discharge in the SWMM_DISCHARGE subroutine. Floodplain elevation (FPE) is compared with Rim elevation and the FPE is modified if they are different and the changes are reported to a new file named as FPRIMELEV.OUT. The corrected FPE is not reported in the FPLAIN.DAT file. It is the user responsibility to revise the FPLAIN.DAT to the FPRIMELEV.OUT modifications if warranted. RIM elevations for the Inlets located in channel/street cells are not reviewed and verification has to be conducted by the user.

FLO-2D calls the SWMM_DISCHARGE subroutine.

The volume that enters the storm drain is calculated based on the inlet characteristics and on the FLO-2D water surface elevation.

Inlet conditions:

Curb opening atgrade no depression INTYPE=1;

Curb opening atgradewith depression or sag INTYPE= 2;

Grate (gutter) atgradeor depression (sag) INTYPE=3;

Stage - discharge rating table INTYPE=4.

Weir and orifice equations are used to calculate the discharge for the first 3 inlet types.

For the rating table inlet option, a relationship between grid element flow depth and discharge is assigned in the GDS. An additional file will be written by GDS (SWMMFLORT.DAT) for all the inlets with a rating table. Based on the FLO-2D grid element depth, the inlet discharge is interpolated from the rating table.

SWMM pressure head is greater than the FLO-2D water surface elevation

Return flow is computed from SWMM to FLO-2D.

The return flow discharge passes from SWMM to FLO-2D as a flooddischarge.

Return flow volume is distributed over the surface area in the FLO-2D cell as a flow depth increment.Return flow volume refers to ‘SWMM flooding’.

Inflow entering SWMM from FLO-2D is not allowed in this case.

FLO-2D water surface elevation is greater than the SWMM pressure head when an overflow condition is predicted in the SWMM model

No return flow from the storm drain is computed.

The SWMM overflow volume is set to 0.

Volume in the SWMM inlet node is set from overflow volume to full volume.

Volume conservation Inflow to and return volumes from the storm drain system are compiled and reported by the FLO-2D model.

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Reporting results

The FLO-2D file SWMMQIN.OUT reports the inflow and return flow discharge for each storm drain inlet or junction. This is different from the discharge values reported in the SWMM.RPT file that also includes lateral and water inside the inlet or the pipe inflow and outflow at a junction over the SWMM timestep.

Detention basin outfalls

Flapgates are used to stop flow going upstream into the storm drain system. Flow goes out of the storm drain system outlet node but cannot enter it. The SWMMFLO.DAT file was modified to include a variable switch (FLAPGATE). This variable takes the value of 0 (OFF) or 1 (ON) that controls outlet node discharge. The default value is 0 (off).

Free Outfalls

FLO-2D WSE and SWMM pressure head is compared. Outfall discharge from the storm drain will occur until FLO-2D WSE will be equal or greater than the SWMM WSE. Flow into the storm drain outfall depends on flapgate assignment (in SWMM.INP) and is based on the FLO-2D WSE. This procedure is available only for the ‘Free’ type of Outfalls. It does not apply to Normal, Fixed, Tidal or Time series type of Outfalls.

Table 2. Description of the FLO-2D – SWMM Conditions

The primary revsions to the SWMM code are:

The maximum non-flooded SWMM pressure head is transferred from SWMM to FLO-2D.

The SWMM pressure head is computed and passed to the FLO-2D model for each timestep and for each storm drain node. This variable is be used for comparison with the FLO-2D water surface elevation.

Determine the return flow and excess volume for the SWMM pressure head. This variable is used for comparison with the FLO-2D water surface elevation to calculate the return flow.

A switch was added to activate and deactivate a SWMM node's ponding feature. The ponding variable changes from 0 to 1 depending on the comparison between the FLO-2D water surface elevation and SWMM pressure head. If the switch is activated, node volume is changed from return flow volume into full volume and overflow is set to 0.

Flapgate option to simulate outfalls was coded as a switch to prevent water from entering the outlet node into the storm drain option. To use an outfall node as a ‘fake outfall’, set the flapgate variable to ‘ON’ and set the invert elevation to the ground level in order to represent the grid element ground level.

Rating table option (INTYPE=4) was added as type of inlet condition (Figure 2).

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Figure 2. GDS Dialog Box for Entering the Storm Drain Rating Table Type 4 Inlet Data

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SWMMDevelopmentGuidelines

These guidelines present an overview of the FLO-2D SWMM model integration. It is not the intent to present detailed instructions for a SWMM storm drain model development. It is left to the user to become familiar with the SWMM model GUI options and tools. For more detailed instructions for building the storm drain system refer to the EPA SWMM user manual. To create a FLO-2D SWMM model the following tasks are performed:

1. Create or open an existing FLO-2D model project. 2. Open the SWMM GUI from the GDS and develop the SWMM.INP data file. 3. Close the SWMM GUI and view the storm drain network in the GDS. 4. Generate the SWMMFLO.DAT file with the storm drain inlet data. 5. Run the integrated FLO-2D SWMM model.

Data Required: FLO-2D model project data files, storm drain data including inlet geometry, conduit sizes, and inlet/outlet locations. After the FLO-2D grid system is prepared, the storm drain model must be created to activate the SWMM switch in GDS. The steps are as follows: Create or open a FLO-2D model

1. Open the GDS and locate the Storm Drain folder.

2. Import any aerial images from the folder to help visualize the location of the storm drain system.

Open the SWMM GUI

3. From the GDS, initiate, the SWMM model interface.

Figure 3. Initiate the SWMM GUI

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4. If there is an existing SWMM storm drain model for this project, it can be viewed in the SWMM GUI. You can also view the SWMM.INP file in an ASCII file editor.

Figure 4. SWMM Model for the Example Project

5. To create new storm drain system, select a 'New' project in the SWMM 'File' prompt on the command bar. A few typical instructions are discussed below.

Figure 5. SWMM GUI New Project Tab

6. Use the Junction Button to add junctions in the project workspace.

Figure 6. Adding Junctions to a Project

Junction

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7. To add junction data, change the cursor to a pointer and double click on each junction.

Figure 7. Adding Component Data to a Junction

The question may arise at this point as to how to enter the coordinate data for each storm drain component. The SWMM GUI does not have the option to enter shape files. There are several options:

i. Import an image and click on the approximate location of the storm drain.

ii. Enter the coordinates manually in the Figure 7 dialog box.

iii. There is a program available from Geospatial Software Lab that will generate the SWMM.INP file based on shape files. The links are:

The open source GIS for software: 

http://www.mapwindow.org/downloads/index.php?show_details=62 

 The inp.PINS software: 

https://sites.google.com/site/inppins/ 

8. Select the Subcatchment button to add a subcatchment for each inlet.

Figure 8. Add a Subcatchment Area

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9. To add subcatchment data, change the cursor to a pointer and double click on each subcatchment.

Figure 9. Add Subcatchment Data Dialog Box

Note: Except for the name and the coordinate data, this data is for reference only. The subcatchment hydrology is not used in the FLO-2D SWMM interface.

10. Connect the subcatchment to the junction by adding the Junction ID in the Outlet item of the Subcatchment data block.

Figure 10. Join the Subcatchment and Junction

11. Select the Connect Junctions button to add a conduit.

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Figure 11. Adding Conduits to Connect the Junctions

12. To add conduit data, change the cursor to pointer and double click on each conduit to open the data dialog box.

Figure 12. Adding Conduits Data to the Dialog Box

It is recommended to keep the names of the various SWMM components simple and consistent with names such as I1, I2, I3, ...for inlets. Use O = outlets, C = pipe conduits, etc.

13. Select the Gage Button to place a gage anywhere in the project. The gage will work as a switch to activate inlets.

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Figure 13. Adding a Gage

The remaining steps for finalizing the storm drain input data are:

14. Set up the starting time and ending time by selecting Options/Dates. 15. Set up report time and routing timesteps by selecting Options/Time Steps. 16. Save the project. 17. Activate the GDS FLO-2D inlet geometry dialog box (Figure 14).

Figure 14. GDS Storm Drain Dialog Box Command

18. Input the SWMMFLO.DAT file storm drain inlet geometry (Figure 1). 19. Inlets to channel grid elements must be assigned to left bank channel element listed in

CHAN.DAT and the SWMMFLO.DAT file must be revised. 20. View the SWMM storm drain system in the GDS (Figure 15). 21. The inlet inflow discharge is reported in SWMMQIN.OUT.

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Figure 15. Example Storm Drain System Displayed in the GDS (showing inlets, outlets and conduits)

22. View the Outfall nodes dialog to create the SWMMOUTF.DAT file (Figure 16 and 17). This dialog reads the outfall nodes from the SWMM.INP file. The user has to select “Allow Discharge” to compute outfall discharges to the FLO-2D grid cell (Figure 17). If that option is off the outfall node will be treated as a regular outfall in SWMM.

23. Outfall Discharge to FLO-2D is reported on the SWMMOUTFIN.OUT file.

Figure 16. Outfall Nodes Dialog displayed in the GDS is read from the SWMM.INP file

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Figure 17. Create SWMMOUTF.DAT File

24. Run the project from GDS. 25. The SWMM results are reported in the SWMM.RPT file.

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EPASWMMModelInstallation

The SWMM Version 5.0.022 model can be downloaded from the EPA website or the FLO-2D website. The SWMM software program is free to the public. The FLO-2D PRO installer includes the installation of the EPA SWMM Version 5.0.022. This program must be installed on the computer in order to use the SWMM GUI to create the storm drain system. Once the SWMM storm drain model is created and the GDS is applied to generate the SWMMFLO.DAT file, the model is ready to run. Running the FLO-2D model with the storm drain interface can accomplished by using the GDS to activate the FLOPRO.EXE executable file or by placing the FLOPRO.EXE file in the project folder and double-clicking on it in an explorer program. The SWMM model results are published in the SWMM.RPT report file. The SWMM GUI can be utilized to examine the model results. The SWMM storm drain model interface is not available with the FLO-2D Basic Model, Version 2009 or previous versions. It is necessary to have the SWMM dll's (dynamic link library files) either installed in Windows with the FLO-2D installation or in the project folder (copy the dll's to the folder). VC2005-CON.dll is the dynamic library that links the EPA-SWMM with FLO-2D. The VC2005-CON.dll is copied on your computer on the following directories during the installation:

C:\Program Files (x86)\FLO-2D PRO, C:\Windows\System32 or C:\Windows\SysWOW64.

Using this approach, the model is able to find the VC2005-CON.DLL from any location. The User does not need to put the VC2005-CON.DLL on the project folder to make the dynamic link.

ReviewingtheStormDrainResultsUsingtheSWMMGUIandtheGDS

Once a successful FLO-2D SWMM model flood and storm drain simulation run has been completed, the user can initially review the storm drain results in the FLO-2D GDS program. The GDS can display the storm drain inflow and return flow hydrographs, the water surface head on the storm drain inlet and outlet and the hydraulic and energy grade lines of the storm drain system. A more expansive review of the results can be accomplished with the EPA SWMM GUI. A successful FLO-2D simulation will generate these files:

SWMM.INI. Contains information for the SWMM GUI about the model global settings and model output.

SWMM.INP. Contains specific information about the pipe network geometry, simulation length and hydraulic properties used by the SWMM model.

SWMM.RAIN. Not used during the simulations, but is required by SWMM to run the model.

SWMM.RPT. The output report file generated by SWMM at the end of a simulation and contains the pipe routing results for each time step.

SWMM.OUT. A binary file similar to the SWMM.RPT file, but one that is read by SWMM GUI to display results and network properties.

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The SWMM GUI is an excellent tool to read these files and to access the results in a user friendly fashion. The following steps help to access the results in the EPA SWMM GUI:

Step 1

Develop a FLO-2D model with the SWMM component and run a successful simulation. Take a note of the simulation folder location. Check that the following files are present: SWMM.INI, SWMM.INP, SWMM.RAIN, SWMM.RPT, SWMM.OUT.

Step 2

Navigate to the folder location and open the SWMM.INI file using a text editor software. This will be automatically modified by FLO-2D in a future enhancement. The user must find and set the following lines as follows:

[Results] Saved=1 Current=1

Step 3

Open the SWMM GUI and navigate to the FLO-2D simulation folder. Open the SWMM.INP file from the File|Open menu. This will access the model output and enable the GUI to display the results.

The EPA SWMM GUI has many capabilities for displaying the storm drain results at each node of the storm drain network. For example, the GUI can show the animation of the time varied values of depths, flows, velocities and many others variables for all the storm drain links. The GUI also plots of the results in multiple formats such as water surface elevation profiles, time varied plots, variable by variable plots and tables. Refer to the EPA SWMM GUI documentation on how to use the GUI tools.

Following a successful completion of the storm model, the FLO-2D GDS can display the storm drain node inlet or outlet discharge hydrograph including the return flow (pressure flow) to the surface. It can also plot the water surface head on the storm drain inlet and outlet as well as the hydraulic and energy grade lines (see Figures 18-22).

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Figure 18. GDS Menu of Storm Drain Display Options

Figure 19. GDS Display of Storm Drain Node Inflow Discharge

Figure 20. GDS Display of Storm Drain Inflow and Pressure Flow Hydrograph

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Figure 21. GDS Display of Water Surface Head on the Storm Drain Inlet and Outlet

Figure 22. GDS Display of the Energy Grade Line on a Storm Drain Conduit

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FLO‐2DSWMMModelSimulationGuidelines

Model Enhancements

Since the FLO-2D model computational timesteps are typically smaller than the SWMM model, the SWMM timesteps represent interface time interval. There may be several successive FLO-2D computational timesteps during which time the volume of water entering the storm drain system is accumulated. When the SWMM computational timestep is exceeded, the storm drain interface is activated and the SWMM models computes the storm drain system pipe discharge distribution. Potential return flow from the storm drain inlets and junctions to the surface water is based on the SWMM pressure head compared to FLO-2D water surface elevation. There are enhancements to the model dynamic integration that represent a significant improvement over the original SWMM model:

Inlet Geometry: In the original SWMM model, pipe discharge was based on the system conveyance capacity, ignoring the potential inlet control. In the integrated model, FLO-2D computes the storm drain inlet discharge based on the predicted storm drain inlet geometry and the FLO-2D water surface elevations. An inlet can be assigned to a floodplain, channel or street element. Three storm drain inlet option represent typical storm inlet designs. The fourth option enable a stage-discharge rating table to be defined as an inlet condition (INTYPE= 4). This allows unique or complex storm inlets to be simulated.

Storm Drain Return Flow to the Surface Flooding: SWMM model return flow from the storm drain was originally based on the ground elevation. The return flow is the excess discharge that overflows a storm drain inlet when the storm drain system capacity is exceeded or the pipe pressure exceeds the surface water condition. In the integrated model, the return flow is based on the relationship beween the FLO-2D surface water elevation and the SWMM pressure head at each inlet or junction. The general conditions that control the direction of the flow in the interface are:

For no potential return flow condition:

1. FLO-2D WSE > SWMM pressure head: Only inflow is calculated. Discharge is passed from FLO-2D to SWMM.

For potential return flow condition:

2. FLO-2D WSE > SWMM pressure head: There is no inflow or outflow calculation. Flow in the storm drain inlet is set as full volume.

3. FLO-2D WSE ≤ SWMM pressure head: Only return flow is calculated. The excess water volume under pressure is passed from SWMM to FLO-2D and distributed over the effective FLO-2D cell surface area.

Flapgate option: To simulate storm drain outfall that stops the surface water from entering the storm drain system, a flapgate switch was designed. The flapgate switch can be assigned to each outlet to prevent water from entering the storm drain network. This option can be used to simulate storm drain discharge into a ponded detention basin without the water flowing into the storm drain pipes. This feature overrides outfalls in the SWMM system without affecting the volume conservation of the storm drain model.

Outfall discharge:

1. GDS creates the "SWMMOUTF.DAT” containing the outfall discharge that are defined in the SWMM.INP file. The user can turn on ‘1’ or off ‘0’ the outfall

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discharge from FLO-2D to SWMM in the GDS dialog window (EPA-SWMM Storm drain | View Outfall Nodes Dialog).

2. If the outfall discharge from SWMM is off, the outfall will be treated as a regular outfall with no flow discharging from SWMM to FLO-2D.

3. The FLO-2D WSE and SWMM pressure head are compared and the Outfall will discharge until FLO-2D WSE is equal or greater than SWMM WSE. Backflow into the Outfall will be computed depending on flapgate assignment and the FLO-2D WSE.

4. Output Discharge from SWMM to FLO-2D is reported on a new file “SWMMOUTFIN.OUT.”

5. The invert elevation of Outfalls should be equal to or greater than the floodplain, channel or street elevations. SWMM outflow and inflow volume to FLO-2D channels are reported in the CHVOLUME.OUT file for volume conservation.

Storm Drain Data Guidelines

There are several suggested guidelines to follow when building the SWMM storm drain system swmm.INP data file.

1. It is recommended to keep the names of the various SWMM components simple, short and uniform such as I1, I2, I3, ...for inlets. Use O = outlets and C = pipe conduits. This will simply the graphics display of the storm drain system in the GDS.

2. Assign END_TIME for the model duration so that the END_TIME minus the START_TIME is equal to the simulation time SIMUL in the FLO-2D model CONT.DAT file (see the first lines of the swmm.INP file below).

3. Assign the WET_STEP and DRY_STEP to 1 minute as shown below. These watershed routing parameters are not directly used by the SWMM model, but can affect the storm drain routing if they are less than the ROUTING_STEP.

4. Set the ROUTING_STEP in the range of 1 seconds to 30 seconds.

5. Assign the VARIABLE_STEP to 0.5 to 0.75. If VARIABLE_STEP = 0, then the ROUTING_STEP will be constant. Otherwise, the ROUTING_STEP will decrease to improve the numerical stability.

[TITLE] [OPTIONS] FLOW_UNITS CFS INFILTRATION HORTON FLOW_ROUTING DYNWAVE START_DATE 09/28/2011 START_TIME 00:00:00 REPORT_START_DATE 09/28/2011 REPORT_START_TIME 00:00:00 END_DATE 09/28/2011 END_TIME 02:00:00 SWEEP_START 01/01 SWEEP_END 12/31 DRY_DAYS 0 REPORT_STEP 00:06:00 WET_STEP 00:01:00 DRY_STEP 00:01:00 ROUTING_STEP 0:00:20 ALLOW_PONDING YES INERTIAL_DAMPING PARTIAL VARIABLE_STEP 0.75 LENGTHENING_STEP 0 MIN_SURFAREA 0

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NORMAL_FLOW_LIMITED BOTH SKIP_STEADY_STATE NO FORCE_MAIN_EQUATION H-W LINK_OFFSETS DEPTH MIN_SLOPE 0

6. The report start time has to be equal to 0. Otherwise SWMM reporting time could not correspond to the time when FLO-2D calls SWMM, and nothing will be reported.

Storm Drain Simulation Guidelines

When a FLO-2D SWMM model ends prematurely, an error statement, code, and a description of the problem is written to the SWMM report file as well as in the FLO-2D ERROR.CHK. Check the SWMM Manual for more information. Even for successful simulations, the model results should be reviewed for reasonableness. The following checks are recommended after the storm drain data files have been created:

Number of inlets (SWMMFLO.DAT) is equal to the number of subcatchments (SWMM.INP). Each inlet has to be associated with a subcatchment.

The subcatchment is a virtual representation of the FLO-2D grid element. Water entering the FLO-2D grid element will enter the storm drain system based on the inlet geometry. The subcatchment area has to be equal to the FLO-2D grid cell area (ft2or m2). This will be hardwired in a future enhancement of the FLO-2D-SWMM Model.

Subcatchment width has should be equal to the FLO-2D grid side length (ft).

Floodplain elevation at inlet cell (RIM Elevation) has to be equal to floodplain elevation at the FLO-2D grid cell.

SWMM model numerical instabilities may be reduced or eliminated by:

Reducing the routing time step;

Utilizing the variable time step option with a smaller time step factor;

Increasing pipe roughness n-values;

Applying the dynamic wave routing option to account for backwater effects, entrance/exit losses; flow reversal, or pressurized flow;

Selecting the option to lengthen short conduits;

Reviewing the selection of timestep and the specification of the total simulation period;

Evaluating the correctness of the print and plot control variables;

Reviewing that the system connectivity was properly set up in the model.

The following are some recommendations and guidelines (FLO-2D PRO User’s Manual, 2012 and User’s Guide to SWMM5 13th Edition, 2010):

Continuity Errors:

When a model is completed successfully, the mass continuity errors (percentage) will be displayed in the SWMM report file (*.RPT). The FLO-2D volume conservation errors (acre-feet or cubic meters) are written to the SUMMARY.OUT file. These errors represent the difference between inflow and storage plus outflow volumes for the entire system. The storm drain system should be reviewed if the SWMM mass continuity error exceeds some reasonable level, such as 1 percent. The volume

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conservation should be much lower than this in a flood routing model and errors as high as 10 percent are often encountered in the SWMM model. The most common reasons for an excessive SWMM continuity error are:

Computational timesteps are too long;

Conduits are too short.

The SWMM report file (*.RPT) lists those nodes of the drainage network that have the largest flow continuity 

errors.  If the error for a node is excessive, then the user should first consider if the problematic node is 

important to the project.  In this case, further review is warranted to reduce the error. 

Timestep and Conduit Length:

Similar to the Courant criteria, volume conservation errors can occur if the timestep is longer than about two times the travel time through a pipe. This would comparable to the wave celerity being equal to about 1.5 times the average flow velocity in the pipe. The recommended minimum conduit length is 100 ft. Numerical stability constraints in the dynamic wave routine require that the timestep be not longer than the time (limit ~ 0.6) it takes for a dynamic wave (velocity plus wave celerity) to travel the length of the shortest conduit in the transport system. A maximum 5 second timestep is recommended for most model conditions.

Unstable Results:

Oscillations that grow in time are signs of numerical instability. Some indicators of unstable results are:

An unstable pipe usually is short relative to other adjacent pipes. The correction is a shorter timestep, a longer pipe length or combination of both. A careful check of the storm drain connections in all contiguous connections of the unstable pipe should be complete prior to timestep or pipe length adjustments.

A second indicator of numerical instability is a node which continues to dry on each timestep despite an increasing inflow. This is the result of a large timestep or excessive discharges in adjacent downstream pipe elements. The problem may usually be corrected by a smaller timestep.

Excessive velocities (over 20 ft/sec) and discharges which appear to grow without limit are evidence of an unstable pipe element.

A large continuity error is a good indicator of either instability or other problems. If the continuity error exceeds ± 10%, the user should check the pipe results for zero flow or oscillating flow. These could be caused by instability or an improperly connected system.

Other possible modifications to reduce instability include:

‐ Increasing pipe roughness

‐ Decreasing pipe slope

‐ Increasing or adjusting pipe geometry

‐ Reducing or eliminating pipe connections to isolate the unstable portion of the storm drain network.

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Typically SWMM results using steady state and kinematic wave are conservative when predicting water surface elevations. The dynamic wave option is less conservative. It is suggested that conservatively high n-values be used for storm drain pipes to reduced dynamic instability. Uncertainty associated with pipe material, obstructions, entrance and exit losses at junctions, and unstready flow warrant the application of conservative n-values. The SWMM numerical stability will improve with higher n-values than typically assigned to uniform pipes in a steady flow condition.

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ExampleProject:SWMM–FLO‐2DIntegration,Phoenix,Arizona

To demonstrate the integration of the FLO-2D and SWMM models, an urban area in Phoenix is selected as a study case (Figure 23).

Figure 23. Case Study Project Area

The test storm drain system was set up in SWMM GUI using 30 inlets distributed along 70 conduit segments leading to one outfall node to remove the flow from the system (Figure 24). The geometry of storm drain inlet was available.

Figure 24. SWMM Model Storm Drain System for the Case Study

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The surface water system was discretized in FLO-2D using 20 ft square grid elements (Figure 25).

Figure 25. Case Study FLO-2D Grid System Showing Buildings, Walls and Storm Drains

A 6 hour rainfall with a 100-yr return period was used to test the model. The infiltration, channel and levee components have been turned off to focus on the storm drain results. The model was run for a 2 hour simulation time. If the simulation time is modified in CONT.DAT, it must also be adjusted in swmm.INP file using a ASCII text editor or by using the SWMM GUI. The storm drain results are reported in SWMM.RPT file. A portion of the results from one storm drain inlet is shown below. The FLO-2D SUMMARY.OUT reports the volume conservation. Data and output files are included on the FLO-2D example folder located on the following directory: C:\Users\Public\Documents\FLO-2D PRO Documentation\Example Projects\SWMM Lesson.

<<< Node I14CP1 >>> -------------------------------------------------------------

Inflow Flooding Depth Head Date Time CFS CFS feet feet ------------------------------------------------------------- APR-23-2012 03:12:00 0.073 0.000 0.071 1246.801 APR-23-2012 03:15:00 0.081 0.000 0.075 1246.805 APR-23-2012 03:18:00 0.102 0.000 0.084 1246.813 APR-23-2012 03:21:00 0.128 0.000 0.093 1246.823 APR-23-2012 03:24:00 0.164 0.000 0.105 1246.835 APR-23-2012 03:27:00 0.212 0.000 0.118 1246.848 APR-23-2012 03:30:00 0.265 0.000 0.131 1246.861 APR-23-2012 03:33:00 0.359 0.000 0.151 1246.881 APR-23-2012 03:36:00 0.491 0.000 0.177 1246.906 APR-23-2012 03:39:00 0.687 0.000 0.208 1246.938 APR-23-2012 03:42:00 0.943 0.000 0.242 1246.972 APR-23-2012 03:45:00 1.285 0.000 0.282 1247.012 APR-23-2012 03:48:00 2.045 0.000 0.355 1247.085 APR-23-2012 03:51:00 5.503 0.000 0.596 1247.326 APR-23-2012 03:54:00 7.685 0.000 0.718 1247.448 APR-23-2012 03:57:00 11.065 0.000 0.897 1247.627 APR-23-2012 04:00:00 13.898 0.000 1.049 1247.779 APR-23-2012 04:03:00 15.843 0.000 1.169 1247.899 APR-23-2012 04:06:00 16.766 0.000 1.265 1247.995 APR-23-2012 04:09:00 17.090 0.000 1.405 1248.135 APR-23-2012 04:12:00 16.966 0.000 1.350 1248.080 APR-23-2012 04:15:00 16.740 0.000 1.332 1248.062 APR-23-2012 04:18:00 16.549 0.000 1.226 1247.956 APR-23-2012 04:21:00 16.208 0.000 1.194 1247.924 APR-23-2012 04:24:00 15.683 0.000 1.157 1247.887 APR-23-2012 04:27:00 14.833 0.000 1.103 1247.833 APR-23-2012 04:30:00 14.331 0.000 1.072 1247.802 APR-23-2012 04:33:00 13.906 0.000 1.047 1247.777 APR-23-2012 04:36:00 13.429 0.000 1.020 1247.750 APR-23-2012 04:39:00 12.898 0.000 0.991 1247.721 APR-23-2012 04:42:00 12.392 0.000 0.964 1247.694 APR-23-2012 04:45:00 12.181 0.000 0.953 1247.683