Detention basins (retarding basins, compensation basins, balancing ponds)

Stormwater detention storages are often provided to reduce peak flowrates and to improve stormwater quality. They slow down flows and allow some sediments and pollutants to settle out. They can take many forms. Most commonly in Australia they have a (normally) dry storage area behind an earth embankment. Wet basins have a permanent pond with a storage area above this. Retention basins allow some or all of the water held to infiltrate into the soil.

DRAINS provides facilities for modelling many kinds of basin, allowing for low-level and high-level outlets and interlinked basins (where the outlet of a basin is submerged by the water stored in a lower basin). The complex calculations are based on culvert and weir hydraulics, with additional calculations for other complex relationships.

DRAINS does not assume direct rainfall on a Detention Basin, unless you specify an attached catchment. This allows the modeller the choice of either option.

Following a run, peak water levels upstream and downstream of the basin are presented in the Main Window. If multiple storms are used, the results for a particular storm can be selected from the pull-down menu that appears in the top-left corner of the drawing area.

Water levels upstream and downstream of basins can be examined using the Pop-Up (right mouse button) menu that provides options for plotting flowrates, storages and levels calculated during a run, or for displaying the numerical values. With older calculations using 1987 rainfall patterns, the pop-up menu provides these options:

With ARR 2019 calculations involving rainfall ensembles, the inflow and outflow hydrograph is not shown, but it is possible to see the water levels and volumes for the most critical storm and the summary of results.