The file for this example, Sydney OSD 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.  

This example involves a property drainage system for a dual occupancy development on a 800 m2 house lot with an on-site stormwater detention (OSD) storage at the outlet, shown below.



The land falls from west to east. A new house is to be constructed in a backyard. The detention basin is situated on and near the driveway running past the original dwelling, located to the right. Pipe flows into the basin are not specifically modelled, and the sub-catchment for the site area is attached directly to the basin. Invert levels must be specified for the pipe leaving the detention basin. 

Actually, two systems are presented – the pre-development situation, represented by a single sub-catchment and node located to the left, and the more detailed, post-development situation. DRAINS allows separated systems to run side by side with the same hydrological model and rainfall data, to make easy comparisons. Three storm durations are used in Analysis, considering AEPs of 10% and 1%. 

The storage is defined by an elevation-surface area table, which can be prepared in a spreadsheet and pasted into DRAINS. A DRAINS Utility Spreadsheet, downloadable from the 'Downloads' section of www.kustomengineering.com.au, can be used to prepare various inputs to DRAINS, such as rainfall patterns, hydrographs and rating curve (elevation vs flowrate) relationships.

The storages, orifice sizes, overflow weir heights and other factors can be varied to arrive at the most efficient design. Pre- and post development results should be compared storm by storm, as the ‘worst case’ result may be misleading.  Comparisons are easy to assess, using the drop-down menu in the top left corner of the screen to inspect results for  the various storms. Note how DRAINS deals with the high-level or overflow outlet from a detention basin. These are controlled by information specified for the overflow path, rather than for the basin itself. 

To assess basin routing results, it is necessary to run with individual storms, as was done in Example 2. In the file, Sydney OSD Example - Lite with individual storms.drn, the major and minor storms are set up with ensembles of ten 15 minute duration, 1% and 10% AEP storms. This allows inflow-outflow hydrographs to be compared, as shown below: