The purpose of a major drainage system is to transport the excess overland stormwater flow in a controlled manner. Uncontrolled overland flows erode properties, flood basements, damage foundations through the buildup of hyraulic pressures, and flood roads. Major drainage systems are now typically designed to handle the 100 year storm. They also provide the most cost effective method of expanding the drainage capacity of an area. Detention of stormwater runoff serves as short-term storage of stormwater until it is slowly released under controlled conditions.
Ponds are also an effective way to manage stormwater due to their large holding capacity.
Wet ponds are permanent water bodies designed to hold water until their capacity is exceeded, thus overflowing to a receiving water. They are long-term storage facilities and therefore provide long retention times. Wet ponds also serve as groundwater recharge sites. The purpose of rooftop, parking lot, street and subterranean storage is to delay the runoff or reduce the discharge rate to the major and minor systems. The basic requirements for the use of this technique include a containment location and an outflow release device to control the rate of runoff.