Attenuation of stormwater flow peaks can be achieved by temporary storage of flood waves, thus reducing the risk of downstream flooding. This can be achieved by either dynamic channel storage, or temporary reservoir storage.

This section discusses reservoir type storage generally associated with retarding basins.

Reservoir storage can be achieved using retarding basins or retention basins. Retarding basins temporarily store part of the storm flow, whereas retention basins permanently store the flow until release by pumping or infiltration at a later time.

The main components of a retarding basin are an embankment, a normal outlet (generally a pipe or box culvert), and an emergency spillway. It is usual to design an uncontrolled normal outlet for a certain return period flood and allow floods greater than this to discharge over an emergency spillway.

Emergency spillways can take many forms including weir crests, glory holes and multi-level orifices and erodible embankments to name but a few.

Retention ponds are generally man-made or natural depressions. Flood gates or pumping arrangements may be necessary for eventual water release.

XPRafts allows for two basic retarding basin arrangements. These are discrete basins and hydraulically connected basins.Both retarding basins and retention basins have been used extensively throughout Australia as well as overseas to provide economical and practical solutions to a range of drainage problems.

Details of specific parameters used to describe the basin characteristics as well as details pertinent to the basin's general operation are illustrated in the figure below.