Enhanced Design method

An improved design method for pipe systems has been added to DRAINS in 2014, in response to overdesign occurring in some models. This procedure is applied for all hydrological models and rainfall patterns.

The first part of this method is the same as that applied previously to define pipe characteristics. It involves a downwards pass through the drainage system, establishing the flows for each sub-catchment, and determining the overflows from pits using their inlet capacity relationships. The overflows were then examined to check that they met the design criteria in minor and major storms, and pit sizes were increased to limit overflows to safe rates where necessary. The pipe system was also designed, selecting pipe diameters and defining invert levels.

The second part reviews the system and downsizes pipe sizes where possible. It provides a message saying how many pipes were able to be downsized. Pipes downsized by more than one size increment will only be counted as one pipe downsize.

It may still be possible to improve on a DRAINS design by manually downsizing pipes, although there is much less scope to do this than with the previous version. If you try to do this, some things to keep in mind are:

You should not make any pipe smaller than the biggest pipe upstream (i.e. if you have a run of say, 1200 mm pipes, you could try to downsize the one furthest upstream. If you can't downsize this, you will not be able to downsize any pipes in the run.

For major storms DRAINS will use the same freeboard criterion that is set in Project → Options. If you want to relax this criterion for major storms, you should also relax it in Project → Options prior to a design run in DRAINS.