A storm tank, also known as spillway or rain tank, is a sanitation network infrastructure consisting of a tank dedicated to capturing and retaining rainwater transported to it by collectors, especially when there is very intense rainfall, to reduce the possibility of flooding in cases where the water runoff capacity is less than the volume of rain.

It also has the function of pre-purifying by preventing the first rainwater, which is the most contaminated - because even if the rain is very clean, it washes the asphalt - from being discharged directly into natural aquatic systems such as rivers, when a wastewater purification process is carried out, so that once decontaminated, the water can be discharged into streams or bodies of water for subsequent use.

These devices intended to laminate the maximum flows of an avenue "Avenue (hydrology)") are particularly important in areas where massive waterproofing of basins has occurred, for example due to urbanization. They are equally important in the event that the sanitation network is a unitary system, that is, one that conveys, mixing them, sewage and rainwater, as is the case of the famous sewers of Paris.[1]​.

The largest storm pond in the world is located in Madrid (Arroyofresno tank"),[2]​ dates back to 2009 and has a capacity of 400,000 cubic meters, ahead of another built in France.[3]​ Managed by the Canal de Isabel II, its surface is equivalent to that of 5 football fields, but with a depth of 10 meters.[4]​.