Flood storage is a common stormwater management approach in controlling the quantity or quality of stormwater runoff. It works on the principle of storing a portion of the surface runoff coming from the upstream and allowing a limited flow to the downstream of the catchment. In this way, the flow rate is restricted to stay within the capacity of the downstream drainage system.

Flood storage approach is often used when the capacity of the downstream drainage network is incapable to meet the increased peak flow rate arising from development upstream. Development usually propagates from the downstream. When the upstream areas are being developed, the downstream areas have already been fully urbanised. Upgrading the drainage network at the downstream areas will cause severe disturbance and is technically difficult due to lack of working space. The storage approach with construction works taken place at the upstream can be a suitable solution. 

Flood storage is finding its application in Hong Kong because the development is moving to the upstream and that there is a need to improve the standard of downstream drainage systems. A large underground storage pond has been constructed in Mong Kok area to alleviate the flood situation.

Schematic layout of an underground flood storage pond