What is the primary characteristic of detention in stormwater management?

The primary characteristic of detention in stormwater management is temporary storage with gradual release. Detention systems are designed to capture stormwater runoff during a rainfall event and hold it for a period of time before slowly discharging it into the drainage system or natural water bodies. This approach helps to mitigate the potential for flooding and erosion by moderating the flow rates of stormwater that would otherwise enter the system all at once.

By allowing the water to be released gradually, detention systems enable downstream water bodies to manage the increased flow more effectively, thus preventing issues such as sedimentation and water quality degradation. Moreover, this gradual release can help recharge local groundwater, optimize the use of water resources, and improve overall water quality by allowing particulates and pollutants more time to settle out before the water is discharged.

In contrast, the other options do not embody the function of detention. Permanent storage would not allow for the gradual release of water, while immediate drainage does not provide any storage, and complete absorption into the ground disregards the key principle of holding water temporarily.