A detention pond is an engineered drainage structure designed to temporarily hold a set amount of water while slowly draining to another location. They a best management practice installed for flood control when large amounts of rain could cause flash flooding if not dealt with properly. Additionally, detention ponds act as a trap where pollutants picked up by the initial surge of stormwater settle out before leaving the detention pond.
When an area is paved, or covered with a building, water runs off the property much faster than when it is in a natural state. The total amount of discharge is the same, but the discharge happens over a shorter amount of time. A hydrologist will design a detention pond to temporarily detain the water and keep the runoff to the desired rate. When the rain ends, though, the detention pond will be empty shortly afterwards.