Major/minor design for a small street drainage system at Gymea, NSW, using an IL-CL model and Lite hydraulic calculations

Major/minor design for a small street drainage system at Gymea, NSW, using an IL-CL model and Lite hydraulic calculations

The initial loss - continuing loss urban hydrological model recommended in ARR 2019 is applied in this model, along with the rainfall ensembles and median design procedure for ARR 2019.  

A DRAINS file for the example, named Gymea IL-CL Example - Lite.drn, is located in the folder C:\ProgramData\Drains among the Lite examples. If this folder is hidden, in File Explorer select the View tab and tick the Show Hidden Items box to make it visible. It can be brought into DRAINS using the Open option in the File menu. The DRAINS main window shown below displays a system of five pipes and six pits or nodes against the background of a street and cadastral (property) boundaries.  The land slopes from top to bottom.  A property sheet defining the inputs for a sub-catchment is also shown, along with a set of design rainfall patterns (10 patterns for each storm duration considered).



Initially, the names of pipes start with '??' because pipe diameters and invert levels have not been entered in the property sheet. This partial entry of data is permitted because a Design run will determine appropriate pipe diameters and invert levels.

The hydrological model is defined in the Hydrological Models option in the Project menu.



10% and 1% AEP design storms with durations from 10 minutes to 1.5 hours can be used in a Design run with IL-CL hydrology to establish pipe sizes and invert levels. The design procedure is based on a method from the Queensland Urban Drainage Manual, 1992 that defines the pit sizes needed to safely limit the flowrates along each overflow route.

The Design run is followed by analysis runs to simulate the operation of the designed system in the minor 10% AEP storm and the major 1% AEP storm. After an analysis run, the names of components change to colour-coded numerical results, such as peak flowrates through pipes and maximum water levels at pits shown below:


The ARR 2019 design procedure applied with this model is complex, running through eight sets of 10 design storms (for 8 storm durations), and selecting a median result for each duration. Results are displayed for the highest of these medians. The critical storms can be different for different system components. The results can be reviewed by right-clicking on a component (such as a pipe or sub-catchment) and opening a pop-up menu. Selecting View Peak Flow Chart or View Peak HGL Chart opens a chart of this form:



The HGLs (hydraulic grade lines) indicate the peak water level at each pit or node.  Running the mouse pointer over each vertical bar identifies the relevant and the peak flowrate or HGL level.

The significant concerns addressed in the design are avoiding flows over road low points at the two sag pits (shown blue in the main window), and preventing excessive flows along street gutters or channels. Results can be inspected using other options in the pop-up menu to display the median flow hydrograph, HGL plot and long-sections. To explore the example, you can:
  • Examine the Hydrological Model, Rainfall Data and other options in the Project menu, cancelling each dialog box or property sheet to ensure that the data is not changed.
  • Right click on a component to bring up the pop-up menu and select Edit Data to view its property sheet data.
  • Run the Design option in the Run menu and inspect the results from the 10% AEP storm. Peak flows for sub-catchments, pipes and overland flow paths and water levels at pits are presented as colour-coded numbers.
  • Select Analyse minor storms (Lite hydraulic model) and Analyse major storms (Lite hydraulic model) from the Run menu, and inspect the results from the 10% and 1% AEP storms.
  • Vary the Il-CL model parameters, such as the initial and continuing losses in the Hydrological Model. Lower losses or zero values will increase the calculated flowrates.
For individual pits and pipes, you can use the pop-up menu to examine HGL positions and flow hydrographs and to check the Long section option for the pipes.  You can also use the Export DXF Long Section option in the File menu to specify a route (such as Pipe A.1 to Outlet), and send a DXF drawing of a pipe long section to a CAD program, as shown below.

You can also view the overflow route flow characteristics by opening the pop-up menu for each overflow route and inspecting the Cross Section Data page in its property sheet. This provides a view of the nominated overflow cross-section and statistics of flow. Slope and the percentage of the contributing downstream sub-catchment area can be varied to determine flow characteristics all along the route.

From the Edit menu you can select the Copy Data to Spreadsheet and Copy Results to Spreadsheet options. Then paste this data into a blank spreadsheet and examine the tables, which provide documentation of the results of a run.

The background in this model was imported into DRAINS from a DXF file created in a CAD program, shown below, that was used to determine pipe lengths and sub-catchment areas.  To transfer a background to DRAINS you must open DRAINS as a new model, and from the File menu, select the options Import and Import DXF File and import the file Gymea Base Drawing.dxf from the folder containing the examples. 

You will be asked to nominate layers for pits, pipes and a background (in the original DXF file, pits are drawn as circles and pipes as lines). Both components and background appear, and the background colour can be changed using an option in the View menu.



In the LiteExamples folder within 'C:\ProgramData\Drains', this example has been saved with and without results, to demonstrate how results can be stored for future inspection using DRAINS or the DRAINS Viewer.