Long Term Storage is the term given to the volume of temporary storage which needs to be provided for the additional volume of surface water runoff that is generated by the development that is greater than the volume of greenfield runoff. The greenfield runoff volume is calculated using the 1:100 year 6 hour event. This volume is the amount that can be discharged at the 1:100 year greenfield runoff rate.
The additional runoff volume should be discharged from the site at a flow rate less than 2l/s/ha for this event. As critical duration events for the design of the site storage system will be much longer than 6 hours, the Long Term Storage volume is not calculated using the 1:100 year 6 hour event, but needs to be assessed using the critical duration event.