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This tutorial describes the process for modeling sanitary flows and combined systems. 

Sanitary sewers are designed to collect and convey sanitary flows. However, all systems collect additional flows to some degree. These flows include:

  • Dry weather groundwater infiltration through leaks in pipes and manholes (pits)
  • Additional infiltration during wet weather
  • Inflow of runoff during wet weather

These components of the total hydrograph are shown in the following figure:

Numerous techniques are used to model these non-sanitary flows. A popular method is the Rainfall Dependent Inflow and Infiltration (RDII) unit hydrograph. RDII is modeled with three triangular hydrographs, each have three parameters:

RFraction of rainfall that becomes runoff
TTime to peak hydrograph, hours
KRatio of base time to peak time

The procedure is also referred to as the RTK method. The three units and the total hydrographs are shown in the following figure:

Sewer models are calibrated using the mass balance described in equation (1). Dry weather flow is measured during dry conditions and is assumed to be constant (using the same hourly and/or daily variation patterns). Wet weather flow is calculated by subtracting dry weather flow from the total flow measured during storm events.

Wet weather + Dry weather = Total flow       (1)

In xp models, Runoff and Hydraulics modes are used to simulate wet and dry weather flows as per equation (2).

Runoff + Hydraulics = Gauged flow               (2)

This exercise begins with a model that is calibrated for dry weather flows in Hydraulics. The RDII unit hydrograph method is used to simulate wet weather flow. The accuracy of the calibration is assessed against gauged data.

  • Use the RTK Unit Hydrograph method to model wet weather flow in a sanitary sewer
  • Evaluate model flows relative to gauged data
Time1 hour
Data files
  • LWM01.xp (starter xp model)
  • 8CAM01.his (gauged data for link 88650601.1)
  • Wet weather flows.xlsx (spread used to analyze gauged data)

In this tutorial, you will complete five models as shown below:

The first model (LMW02.xp) will be the dry weather flow model using only the Hdr mode. In this model, you will simulate the dry weather flow from residents and compare the result with the observed (gauged) data at the link 88650601.1.

Using the model LMW03.xp, you will compare the flow during initial dry period with the gauged flow. You will use only the Hdr mode for this model as well.

In the model LMW04.xp, you will simulate the flow derived from rainfall (RDII) using only the Rnf mode. Note that you will only simulate the RDII in this model. This RDII will be transferred to the Hdr mode in the next two models for two different storm events. You will use the interface file option to transfer the RDII from the Rnf mode to the Hdr mode.

Using model LMW05.xp, you will simulate the dry weather flow + the RDII for the duration of Storm 1. Using LMW06.xp, you will simulate the dry weather flow + the RDII for the duration of Storm 2. You will compare both these simulated flows with the gauged data.

  1. Launch the program:
    1. At the opening dialog, navigate to the file LWM01.xp and open it. Save the model as LWM02.xp
    2. Set the mode to Hydraulics (Hdr). 
    3. Double-click any black node and double-click Dry Weather. 

    4. Select Default DWF and then click Edit. Note the Dry Weather loading. Click OK thrice to return to the network view.

  2. Add gauged data:
    1. Locate link 88650601.1 (You may use the Find Object command from the View menu or alternatively press<Ctrl>+F). 
    2. Double-click to open the Conduit Data dialog. 

    3. Click Gauged Data and then click Gauged Flow.

    4. In the Gauged Flow dialog, click Select File. Navigate to the file 8CAM01.HIS and then click Open.

    5. In the File Format section, check the Raf-Hydsys (standard) radio button. Click Edit to view the data.
    6. Alternatively, open the 8CAM01.HIS file using any text editor. Refer to the help files to know more about the Hydsys file format. Note the flow data begins at 11:40 on 7 Sept 2013 and ends at 12:15 on 7 Oct 2013 (row 8648). Time steps are five minutes and flows are reported in m3/s. The total flow over the period was obtained by numerical integration of the record and is 14,469 m3. The information in the file can also be viewed by clicking Edit in the Gauged Flow dialog.

    7. Click the Graph button to view a plot. 

    8. Click Close and then click OK four times to return to the network view.
  3. Determine the dry weather flows in the Hydraulics mode. 

    1. Go to Configuration > Job Control > Hydraulics. Enter the following settings:

      Job ControlHydraulics
      Start time2013 Sep 7 11:40:00
      Stop time2013 Oct 7 12:15:00
      Time step60 sec
      Save Results Every5 minutes

    2. In Configuration > Interface Files, uncheck all options.

    3. In Configuration > Mode Properties, only select Hydraulics.

    4. Solve the model by clicking the Solve tool  or pressing the F5 key. The solution will require several minutes for completion.

    5. Right-click link 88650601.1 and select Review Results

    6. Click the Properties tool from the File menu. 

    7. In the Properties dialog, under the Show section, select Flow and Gauged Flow. Clear all others and then click OK.

    8. With the cursor on the graph, right-click and select Customization Dialog.

    9. In the Style tab, set the color to blue and the line style to dashed for Flow. Click OK.

    10. Draw a rectangle over the graph to zoom in on an area of the plot. Note where the gauged flow exceeds the model flow. Right-click and select Undo Zoom and re-zoom to review the graph. Close the Review Results window to return to the network view.

    11. Open the output file. Examine Table E15 to find the total flow through link 88650601.1 (from node 88650601 to 88651702) = 12,943 m3.

    12. Save the model as LMW03.xp and rerun for the initial dry period (Sep 7 14:00 to Sep 10 12:10) and note the total flow in link 8865060.1. The total flows are compared with the totals obtained by numerical integration of the gauged data (see Wet weather flows.xlsx from the downloaded Getting Started Model Files). 


      Total Flow, m3

      Gauged data


      Entire record



      Initial dry period



  4. Add rainfall:

    1. Save the model as LMW04.xp

    2. On the Configuration menu, select Global Data.

    3. Select Rainfall as the database type in the left panel and Measured Storm in the right panel. 

    4. Click Edit.

      Note that the rainfall starts on Sept 10 at 1:45 P.M. In contains 7815 records at 5 minute time intervals giving a record length of 39,075 minutes or 27.14 days. Therefore, the end of the rainfall data is Oct 9 at 6:05 PM.  The year is 2013 not 2003.

    5. Click Constant Time Interval.

    6. Click Graph to display the data.

    7. Click Close. 

    8. Click OK three times to return to the network view.  

  5. Add the RDII record:

    1. In the Configuration menu, select Global Data.

    2. In the Data Base Type panel, select (R) RDII . Select WWF in the Record Name.


    3. Click Edit.   

    4. Select the box for All Months. Enter the data for the Short, Medium and Long Term Unit hydrographs as shown below. 


    5. Click OK twice to return to the network view.

  6. Enter subcatchment data:

    1. Set the mode to runoff (Rnf). 

    2. Double-click node 86648902 to open the Runoff Node dialog. 

    3. For Sub-catchment 1, enter 1 for the Area , Imp. (%) Width and Slope . Non zero values are required by the engine to complete the simulation. Even though non zero sub-catchment input parameters will generate small amounts of rainfall runoff, this runoff can be eliminated by using a sub-catchment infiltration record with high depression storage or initial losses.

    4. Double-click the Sub-catchments 1 button. 


    5. In the Sub-Catchment dialog, double-click the RDII button. 

    6. Select WWF as the RDII global database record and set the Use Defined Sewershed Area to 10 ha. Click OK.


    7. In the Sub-Catchment dialog, set the Rainfall to Measured Storm and Infiltration to Typical. Note that the Typical infiltration record uses very high depression storage to remove the small rainfall runoff, which occurs due to the use of non zero subcatchment input data. 

    8. Click OK twice to return to the network view.

    9. Enter the same sub-catchment data to node 88650601, make sure the Defined Sewershed Area for this node is set to 20 ha.

  7. Set the Runoff Parameters:

    1.  In Configuration > Interface Files, select Create New File in Runoff Layer.

    2. In Configuration > Job Control > Runoff > Time Control, enter the following settings.

      Simulation Start 2013 Sep 10 13:00:00
      Simulation End2013 Oct 9 23:00:00
      Dry Time step60 seconds
      Transition Time Step60 seconds
      Wet Time Step60 seconds

    3. Also in the Time Control dialog, select Use Simulation Start Time For Rainfall Event.

    4. Click OK twice.
    5. In Configuration > Mode Properties, under Solve Mode, select Runoff only. Click OK.

  8.  Solve the runoff. Solve the model by clicking the Solve tool or pressing the F5 key. The solution will require several minutes.

  9. Analyze the runoff results. Right-click node 88650601 and select Review Results. Click the Properties icon, and then select the Rainfall option as seen in the following:

    Note that there are two events over the simulation period. Storm 1 extended from Sept 10 14:35 to Sep 19 15:05. Storm 2 began Sept 23 18:30 and ended on Oct 3 2:45. If 12 hours are added to the end of the rainfall to allow RDII to drain though the network, then the two events will be examined for calibration.

    Table 2 shows the start and stop time for the rainfall and gauge data and the two events that will be used for calibration.

    Table 2 – Times for Data Files and Calibration Events




    Rainfall data

    Sep 10 13:45

    Oct 9 6:05

    Gauge data

    Sep 7 12:45

    Oct 7 12:15

    Storm 1

    Sep 10 14:00

    Sep 21 15:00

    Storm 2

    Sep 23 18:00

    Oct 5 15:00

  10. Simulate flows for calibration events:

    1. Save model as LMW05.xp

    2. In the Job Control settings, set the start and stop time to the values listed in Table 2 for Storm 1.  Set these for both the Rnf and Hdr mode time controls.

      Interface files

      Create New File in Runoff Layer

      Read Existing File in Hydraulics Layer

      Alternative to Interface FilesSolve Hydrology and Hydraulics Simultaneously: 
      Job Control>Hydrology>Run Hydrology/Hydraulics Simultaneously
      Job Control

      For both Runoff and Hydraulic Time Control:

      Start time: as per Table 2

      Stop time: as per Table 2

      Mode PropertiesRunoff and Hydraulics
  11. Determine the total flow in link 88650601.1 from Table E15 and enter the value in Table 3. Examine the Review Results graph for link 88650601.1
  12. Save the file as LMW06.xp and repeat for Storm 2.

    Table 3 –Wet Weather Flows for Storms 1 and 2.


    Total Flow, m3

    Gauged data


    Storm 1



    Storm 2




  1. Comment on the accuracy of the model in regards to sanitary flows. How could the calibration be improved?
    FYI, adjusting the sewershed area for the RDII to say 200 ha at node 88650601 makes a better calibration by increasing the wet weather contribution.

  2. Comment on the accuracy of the model in regards to wet weather flows. How could the calibration be improved?
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