(a) General
Design in DRAINS involves the determination of the characteristics of the components in a drainage system. This can be by trial and error, or by use of an automatic pipe system, such as that provided in DRAINS. The design is than checked using an analysis or simulation of flows through the system, involving hydrological and hydraulic calculations.
Different hydrological models can be applied to design, as described here. A new initial loss - continuing loss model has been presented in Australian Rainfall and Runoff 2019, but is still incomplete. Older models such as Horton (ILSAX), the rational method, extended rational method and storage routing models can still be applied.
To use the design procedure, first choose the relevant option in the Run menu. This runs the program and if no problems are encountered, a message appears advising that the process is complete and that you should analyse with minor or major storms to assess the results. You can do this by choosing from the available analysis options:
- for models set up with versions of DRAINS released prior to 2011, the superseded basic hydraulic model.
The basic hydraulic model is now obsolete, but can still be used to obtain results from earlier DRAINS models and to compare results from the hydraulic model alternatives.
The Design procedure selects suitable pit inlets from the sizes available for the selected pit type and chooses pipe diameters from those available in the pipe data base for the type of pipe specified. If a pipe system is to be designed, it is run using the Design option in the Run menu, and then followed by simulations of the system's behavious in various storms, using the Analysis options. It is not used when a drainage system is existing and pipe characteristics are fixed, or for open channels or detention basins.
An enhanced design method was introduced in 2014, and a more complex ARR 2019 design procedure involving ensembles of rainfall patterns was introduced in early 2017. This can also be applied in DRAINS.
(b) Further information
Design is the process of defining the basic characteristics of some object or system, for the purpose of creating this system. In engineering, it involves the use of a suitable model and calculations. For stormwater drainage applications, it determines the layout, position, type and size of the components of a drainage system. Design is one of the main modes of operation of DRAINS. Until 2011, there were two procedures available, the Standard Design method and the Advanced Design method. Now only the advanced method is available, simply called 'Design' in the Run menu.
If you were to apply DRAINS to a greenfields subdivision drainage system design, you might work out the plan of the system on a paper plan, or more likely, using a CAD or DTM program. You might also use the links between DRAINS and the DTM programs 12d, Civil 3D or Civil Survey Solutions’ Civil Site Design for this purpose. You would then set up a DRAINS model, probably with a background showing street layout and cadastral data. You would probably take this from a pre-existing base file that already contains the hydrological, rainfall, pipe, pit and overflow route information that you need, in data bases accessible from the Project menu. It is likely that the pipe layout and background would be imported from a suitable CAD file.
Following procedures in design manuals such as Australian Rainfall and Runoff (I you would set up a set of minor storms, which define the conditions that are to be addressed in sizing drainage systems. You would also set up a set of major storms, to be used to check that the system 'fails safe' in severe conditions. Minor storms may have average annual exceedance probabilities (AEPs) of 50% to 5% or average recurrence intervals (ARIs) of 2 to 20 years, while major storms are usually set at 1% AEP or 100 year ARI.
After running the DRAINS model using the design method, and checking this with an analysis using minor storms, you can alter the pipe system to refine the design (DRAINS provides designs that meet the design requirements, but there may be some overdesign). You could then transfer the system data (containing the data entered through property sheets, plus designed pipe sizes and invert levels, and x-y coordinates of pits and nodes), to a spreadsheet program using the option in the Edit menu, and copy this on to a worksheet labelled "Data". The results of the design run using minor storms could be transferred to another worksheet labelled 'Design' or 'Minor'.
After inspecting the design results, you could then run DRAINS with the major storms to check that the system operates safely in large storm events and transfer the results from these to another worksheet labelled 'Analysis' or 'Major'. The three worksheets provide the documentation for the design. You could then produce long sections of selected pipelines, and transfer these to a CAD program as a DXF file. If you were using a DTM program, you could transfer data directly from DRAINS and use the more elaborate drawing facilities that they provide.
Once a workable design has been obtained, you must check this by analysis runs from the DRAINS Run menu.
Examples 1 to 7, 9, 12 to 16 and 18 show various design situations and methods.